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Home-brewing your own NOAA weather satellite receiving station.

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Introduction:

There are many excellent publications which describe how to construct one’s own weather satellite receiving station, and with the different choices of receiver, antenna, computer and software, there are many ways in which it can be assembled. As a guide to an aspiring builder, the following notes describe how we put together our particular system (i.e. the one which produces the weather images displayed on our main web page). Although I regard the assembly of a NOAA satellite receiving station as a fairly straight forward task for a technically competent adult or enterprising schoolkid, it’s perhaps like riding a bicycle; easy when you already know how. Consequently, the following notes will explain things in straight forward not-too-technical language. Personally I found the assembly of our receiving system was really good fun, and I think the process makes an excellent educational project for students of any age. When a NOAA satellite receiving system is working properly, simply observing its routine day-to-day operation can be interesting and educational. Once a receiving station is displaying cloud imagery reliably and routinely, it can also be configured to automatically publish its up-to-the-minute imagery onto the internet, thereby also becoming a useful public service.

Thinking about it.

Before committing any funds to buying hardware, one can start to explore the world of Earth orbiting satellites by reading, browsing the internet, and also simply using one’s eyes to watch satellites pass across the night sky. It’s surprising what spacecraft one can see flying, if one knows exactly when and where to look. There are many bright easy-to-spot satellites such as the International Space Station (ISS), the Hubble Space Telescope (HST), the Space Shuttle (SST), Iridium satellites, NOAA satellites… to name a few… and of course numerous large luminous hunks of space junk. One excellent resource to start visually observing satellites can be found at Heavens Above. After registering at this website and logging in, one can obtain highly accurate forecasts for visible satellite passes, tailor-made for one’s particular geographic location. Particularly impressive are overflights of the ISS, and the spectacular flashes of sunlight reflecting off Iridium satellite antennas (so bright that they can occassionally be observed during daylight hours). The Visual Satellite Observer’s website also specialises in observing naked-eye-visible satellites, and is very good.

Another very useful tool for observing satellites, and for getting a feel for how orbits work, is a computer program called SatScape. With superb graphics and animations, this program is an excellent teaching tool for children and adults alike. It’s also free. The only minor problem with SatScape, is that it is written only for the MS Windows operating system. However SatScape may be run under Linux via the Wine utility, with some minor loss of functionality and speed.

Assembling the hardware

So you’ve finished thinking and watching, and want to get on and build a NOAA Weather Satellite receiving station. The three big items one must now consider buying or building are the radio receiver, the antenna, and the software to run it all. But before buying any of this, I strongly recommend the purchase of the Radio Amateur’s Satellite Handbook, published by the American Radio Relay League (ARRL). Like most ARRL publications, one gets a lot of bang for one’s buck, and this particular book is chock-full of useful information and practical construction tips for satellite receiver antennas and the like. Likewise the current edition of the ARRL Handbook and ARRL Antenna Handbook can be recommended.

The Receiver


The radio receiver is the most expensive item that one needs to buy, so careful consideration must be given. The imagery presented on our main web page, is NOAA APT imagery, which is transmitted by NOAA satellites at radio frequencies of 137.50, 137.62 or 137.9125 MHz, FM. You may have noticed that your broadcast FM radio tops-out at 108MHz. If you could keep spinning your radio dial beyond 108MHz, and on up to 137.50MHz, then several times a day you would hear this curious sound. This distinctive ‘tick-tock’ sound is typical of NOAA APT transmissions, with each half-second-long ‘tick-tock’, encoding a single line of the satellite image.

If a regular el-cheapo broadcast-FM receiver could be tuned up to 137MHz, it would be fine for the purpose of receiving NOAA APT signals, as the NOAA satellite radio signal is FM-modulated in much the same way as regular broadcast-FM radio signals. But modifying a broadcast-FM receiver to work at a higher frequency is outside the scope of most people, and buying a purpose-built unit is an easier option. As any listener to FM radio knows, the quality of FM sound is much higher than broadcast AM radio. This is because FM signals have better immunity from radio interference, and also because of the much higher bandwidth of broadcast-FM transmissions. On broadcast-FM, one can hear much higher pitched musical notes than AM radio. In fact broadcast-FM can potentially carry audio frequencies which are higher than human hearing can actually detect. And so it also is with the FM transmissions from NOAA satellites. Although many common scanning radio receivers can tune to 137MHz FM, the receiver’s bandwidth may be too narrow to receive the full signal, as indicated by poorer sound quality and absence of high-pitched tones. Narrow receiver bandwidth becomes apparent when the APT ‘tick-tock’ sound is converted into a picture by a computer.

So a wide bandwidth FM radio receiver is desirable, and so is one that can be controlled by a computer. Computer control is particularly useful if one wishes to have a fully automatic station which can periodically switch between the frequencies used by different NOAA satellites. So if one decides that wide bandwidth and computer control are essential, then the choice of available radio receivers becomes much narrower. Given my limited knowledge of how good various receivers are, and whether they represent good value for money, in honesty I can’t recommend any particular receiver. But I can direct folks toward the Hardware section of the WXtoImg website , which is a good place to commence researching suitable receivers. WXtoImg is the clever computer program which drives our satellite web page, and at the WXtoImg web site there is also good information concerning suitable 137MHz antennas. The receiver that we use for satellite reception, is an Icom PCR1000, and we have been very pleased with its performance. We purchased it from ATRC in Sydney, Australia, and we liked their sales service.

In early 2004, a new cheap kitset NOAA APT receiver has become available in Australia. The receiver was described in the Australian ‘Silicon Chip’ magazine, December 2003 edition, and the complete kitset is now for sale at Dick Smiths, stock number K3226. Recently I received an email from David Perry, who had just built one of these receivers. David has a web site through which he may be contacted, and he will repond to technical queries regarding the Silicon Chip receiver. Hopefully these queries, and their answers, will form the basis of a future FAQ page at his website, addressing a few of the problems that have been reported with this particular kitset receiver.

The Antenna


There is much misunderstanding about how antennas work, and how different types compare with each other. Section 10 of the ARRL Satellite Handbook has some good suggestions for basic antenna designs that can get one started with receiving NOAA APT signals. Likewise the Hardware section of the WXtoImg website has many good 137MHz designs, of which the Quadrafilar Helix antenna is generally acknowledged to be ‘king’ of 137MHz satellite receiving antennas (in fact the NOAA satellites themselves transmit their APT signal from the spacecraft using a Quadrafilar Helix antenna ). However for 137MHz LEO satellite reception, I have a personal preference for an old design of antenna (circa 1930’s), known as a Lindenblad Antenna. I have made a number of Lindenblads over the last few years, designed for various frequencies, and particularly like their simplicity, robustness and effectiveness for LEO satellite communications. They may not be the ultimate NOAA 137MHz antenna, but they’re pretty damn good in my opinion. If you wish to have a go at making a Lindenblad for 137MHz, please check out my construction notes.

The big advantage to using TV standard antenna equipment, is that it’s relatively cheap. Good quality low-loss coax cable, TV masthead preamplifiers and 75-ohm coax connectors are mass produced and readily available. NOAA satellites transmit their signal with around 5 Watts of radio power, and when a signal is being received directly from a NOAA satellite, the spacecraft will be at a distance somewhere between 800 to 3000km. So the received NOAA signals are generally weak, and easily interfered with by local radio noise generated by computers, TVs, VCR’s, broadcast-TV transmitters and the like. To avoid this interference, it may be necessary to locate one’s antenna well away from the sources of noise, and a long coax feeder may be required. When coax feeders longer than ~20m are used, it is advisable to also use a TV-grade masthead VHF preamplifier. In order to place our particular Lindenblad antenna at a relatively radio-quite site, we used 110 metres(!) of coax, along with a ‘Kingray’ brand of masthead preamp (commonly available at electronics stores in Australia).

The Computer


We perhaps take for granted the processing power of modern PCs, compared with computers of even a decade or so ago. These days any bog-standard computer now contains a ‘sound card’, or some device to process audio sound. For most of us, the sound card is just something that permits our computers to make occasional noises, or perhaps to play music. Few people use their PC’s sound card to routinely record sound, and generally that function goes unused. But even a basic quality PC sound card is equipped with a powerful analog-to-digital converter, which can digitally sample audio signals at rates of 10kHz and higher… in stereo! This facility to digitise audio signals is what allows the easy conversion of the NOAA APT ‘tick-tock’ sounds from a radio receiver, into numbers which can then be converted into a picture. If one goes back just a few years, reception of APT signals required a rack-load of expensive dedicated electronic equipment, whereas these days any old Pentium-class PC (or the like) equipped with a sound card, is easily up to the task.

So in selecting a computer which will be suitable, one doesn’t require anything particularly fancy. Just as long it contains a Pentium I processor or better, a sound card, and a reasonable amount of memory and disk space (say 64Mb+ RAM, and 2Gb+ disk space). As for computer operating system, I can’t really comment on MS Windows products as I’m a Red Hat Linux kinda guy, but Windows 95/98/ME/XP/2000/NT are all suitable. A standard feature of Linux is its ability to remotely control other Linux computers. I find this feature particularly handy when I need to restart WxtoImg from a distant computer. I really can’t comment on Mac computers as I’ve rarely used them, but I understand that MacOS X (or better) is suitable. With a computer’s half-life now around three years, there should be no trouble locating an older Pentium-class computer, and sparing it from the dumpster for another couple of years. But of course, one could choose to use a ‘good’ computer to run a receiving station, as the load on the PC’s processor is quite modest, and one will not notice a decrease in computer performance when WXtoImg is running quietly in the background. The only catch with using a ‘good’ computer, is that it really needs to run full-time in order to catch all available NOAA satellite passes, and non-stop operation may shorten a computer’s life.

The Software

From my limited survey of the software available to run a NOAA APT receiving station, there is one program that is head-and-shoulders above the rest in my opinion, and it is known as WXtoImg. This one comprehensive computer program can take the raw NOAA APT ‘tick-tock’ sounds, and convert them into a variety of visually stunning images, and even automatically publish freshly received imagery straight onto the Web. The program is easy to use and pleasingly bug-free. But the thing about WXtoImg which impresses me most, is the superb user service provided by WXtoImg’s author. Minor bugs that I have spotted were promptly corrected, and the author is open to suggestion for how WXtoImg may be improved. The few minor suggestions of mine that were implemented have made the WXtoImg software somewhat more useful for our particular application. To be perfectly honest, I’ve never experienced software user service quite like it.

WXtoImg may be downloaded as freeware from the WXtoImg Website. There are versions for most popular computers and operating system platforms. The freeware version is quite powerful, has no use-by date, and will perform most tasks that people might wish. But once you have determined that this program does what you wish, I strongly advise payment the US$58.95 ‘Standard Edition’ licence fee. This makes the program even more versatile, and software and software support of this quality, needs to be supported.

Odds and ends

A cable between your receiver and computer sound card.
Most computers have a ‘line-in’ and ‘microphone’ sockets, which accept 3.5mm audio phone jacks. Most radio receivers have a 3.5mm audio socket which will accept headphones and the like. Some up-market receivers may also have a ‘line-out’ socket as well as a headphone/speaker socket. And this is where it can get a little complicated. Some receivers have a ‘mono’ headphone/speaker jack. Likewise the output from some receivers might overload the ‘line-in’ input to the sound card (even when the incoming sound is attenuated using the computer’s sound mixer controls). If the receiver’s output is too powerful for the sound card, an attenuating cable will need to be bought or made. These are fairly readily available at electronics stores such as Radio Shack, Tandy, or stores specialising in audio.

If one has a sound card which only has a microphone input (as is the case with many laptop computers) then one will need to severely attenuate the radio receiver audio signal before feeding it into a microphone jack. Moreover, the microphone inputs on most modern sound cards, are actually ‘hot mics’ (often labelled with the phrase ‘Plug in Power’). That is, DC electrical current from the computer is fed up the microphone cable in order to power the microphone. If one was to connect a radio receiver’s output directly to a PC sound card’s microphone input, then DC electrical power could feed back into the receiver, which is not good! So if one wishes to feed signals directly into a computer’s microphone jack, one needs to block this DC current with a capacitor, as well as severely attenuating the audio signal with some kind of resistor voltage-divider network. If you are not familiar with these terms, it’s time to hunt down your friendly neighbourhood electronics geek, or radio ham, for advice applicable to your particular equipment.

A GPS clock


Downloading NOAA satellite imagery requires good timing. The position of a satellite is defined by the so called Keplerian elements, a series of numbers which precisely define a satellite’s orbit and whereabouts. A satellite’s known orbit, coupled the very carefully controlled orientation of the spacecraft relative to the ground, mean that a computer program can precisely calculate what strip of the Earth is being observed by a NOAA satellite at any given moment, so long as the time is known very well. This is how a program like WXtoImg can accurately place a map overlay onto a freshly received satellite image. But how accurate does a PC clock have to be? Consider that NOAA satellites orbit with a ground speed of around 7km/second. Also consider a case where a PC’s clock was in error by 10 seconds. In this case the computer software would misplace the overdrawn map by 70km, which is not good. To get high quality images, where the map outlines are always correctly placed, requires accurate timing to ±1 second or better. WXtoImg permits misplaced map overlays to be manually corrected, but it’s preferable to not get it misplaced in the first place. One can periodically set one’s computer clock manually, but this gets a little tedious after a while.

For computers permanently connected to a LAN network, obtaining accurate time is fairly easy, by synchronising a PC ‘s clock to a local Network Time Protocol (NTP) server. But for people who only occasionally connect to internet via a modem, NTP isn’t an option. But there is another option, to use a GPS receiver. GPS receivers, as well as showing one’s geographic position very well, also generate accurate time. Many modern GPS receivers also have an output port which may be connected directly to a PC’s serial or USB port. There is a standard GPS interface protocol ‘language’ known as ‘NMEA-0183’, and technical folk may read all about it if they wish. But all one needs to know is that many GPS receivers can be simply connected to a computer, with the GPS antenna placed against nearby window. With the installation of a suitable program to read the NMEA data stream coming from GPS, one can then obtain very accurate PC time.

Yet another on WXtoImg’s many clever functions, is to manage a GPS receiver that has been connected to a PC’s serial port, and to use this data to periodically correct the PC’s clock. “Rather an expensive clock?!…”, you may say. Not necessarily. GPS receivers have been around for a few years now, and modern ones are getting smaller, cheaper and cleverer. So what’s happened to all those older bigger dumber GPS receivers? They’re out there somewhere, and if one’s lucky, one might come across a near-dead one that supports NMEA-0183. It doesn’t matter that the unit might be large and clunky, with a battered case and nearly dead display; if its NMEA port works, it’s a good one!

Then again, these days GPS receivers aren’t all that expensive. I synchronise my home PC clock using a ‘GPS Mouse’, a simple GPS receiver with no display, which connects to my PC via a USB port. One useful feature of this device is that it gets its power via the USB cable, meaning no extra power supplies are necessary. The ‘GPS mouse’ sits quietly by the window and my PC’s clock is always perfect (providing WXtoImg is also running). Such GPS computer clocks may also be run by a lot of good free/share-ware, available from websites such as tucows.

Antenna Masthead Preamplifier


These days one gets good bang for one’s buck with antenna preamps. The advent of surface mount electronics, low-noise high-gain GasFET semiconductors, and mass production, mean that even modestly priced units can still be good performers. In Australia I have been impressed by the ‘Kingray’ brand of TV masthead preamp, available at most TV/Radio stores.

Lightning Isolator


One of the nice things about NOAA satellite receiver antennas, is that they don’t necessarily need to be perched high on poles or buildings. In fact placing an antenna low-to-the-ground in one’s back yard may be preferable when located within high-radio-noise urban environments. But whether perched high or low, if one’s antenna is on the end of a long feeder cable, a nearby lightning strike may cause large ground currents and electric fields, which may induce a spike of electricity to run up the cable and zap the receiver. If one lives in a lightning prone area, a coaxial cable lightning isolator could be useful. With the advent of Cable TV, cable isolating devices are commonly available and relatively cheap (see image below left). Within Australia, coax isolators are sold at Dick Smith’s, stock number L4665. The following images show a coax isolator combined with a coaxial 50:75ohm impedance transformer. This arrangement gives our receiver a very high degree of protection from lightning induced nasties, as well as providing the correct impedance match to our receiver. I haven’t measured the insertion loss of this arrangement, but I have measured the impedance transformation, and it’s close to perfect.
Isolator Isolator
If one is using a masthead preamplifier which is powered by AC or DC current via the coax feeder, note that this isolator blocks DC and low frequency electrical current, and therefore it cannot be inserted in series with th main feeder. But whether a masthead preamp is being used or not, the best place for the isolator is directly connected to the receiver’s antenna input.

USB Sound Card


Some computers lack a sound card, or occasionally their ISA and PCI card slots are so full that there is no extra room for a sound card. In this case, a ‘sound card’ facility may be added by using a USB audio interface. Our particular Linux computer had no spare room for a sound card, so we added a Telex P-500 Digital Audio Converter. This gizmo works well and all our NOAA imagery is received through it. It has standard 3.5mm stereo microphone and headset jacks, and great care must be taken when connecting a radio receiver output to a microphone input (as explained above in the ‘A cable between…’ section above.

A friend of mine recently mentioned to me that he had purchased a USB sound interface device manufactured by Xitel, and was particularly impressed by the performance and the relatively modest cost. A device like this would also be quite satifactory for satellite recording purposes, and would require less fiddling than trying to send the audio signal into a ‘Plug in Power’ microphone jack.

Getting it together

Final interconnection of the radio, PC, antenna and preamp has been mostly covered above. Operation of the WXtoImg software is fairly straight forward in general, and you should not have much difficulty receiving a satellite image of some sort. And that’s where the educational bit usually starts; optimising WXtoImg settings, antenna location, audio levels, timing, interference minimisation etc… etc…

Putting together a system is fun and educational. Even with a hands-off fully automatic station, I still get a thrill actually hearing live NOAA satellite transmissions, and watching live imagery scrolling onto the computer screen as the satellite is passing overhead. At these times one can easily imagine being aboard the spacecraft, looking down on the Earth passing underneath. The fact that these images are ‘live’, and the cloud pictures can be directly related to real-time out-the-window cloud observations, allows one to become quite good at forecasting the local weather, or at least the local cloud cover. The ability to interact with live earth-observation satellites, using familiar looking equipment, brings immediacy and technical demystification to a student. Certainly I learned a lot, and still am…

Have fun!

Andre Phillips
UNSW
Siding Spring Observatory
Coonabarabran, NSW
Australia, 2357

References:

Radio Amateur’s Satellite Handbook (1998), by Martin Davidoff. Available from ARRL publications. In Australia on sale at Dick Smith’s.

ARRL Handbook for Radio Communications (current edition). Available from ARRL publications.

ARRL Antenna Handbook (current edition), edited by Dean Straw. Available from ARRL publications.

Page originally available at http://www.phys.unsw.edu.au/~map/weather/notes/growyourown.html

Building Lindenblad Antenna for 137MHz.

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I like Lindenblads because they’re easy, cheap, robust, and good performers.  Section 10 of the ARRL Satellite Handbook presents good practical details for the construction of a Lindenblad antenna.  I have built my last couple of Lindenblads from lengths of 4mm-diameter galvanised steel fencing wire, some standard 300-ohm television ribbon cable, and a few bits of wood and plastic.  Another useful feature of a Lindenblad is that its 75-ohm impedance nicely matches into standard 75-ohm TV coaxial cable.  Keeping things ‘TV standard’ helps to keep costs down.

The only catch with using 75-ohm standard coaxial cable, connectors and masthead preamps, is that most 137MHz radio receivers have a 50-ohm input impedance.  A receiver should be connected to a coaxial cable with the same characteristic impedance as the receiver’s input impedance.  One cannot just connect 75-ohm coax into a radio’s 50-ohm antenna socket and expect optimal performance.  To mismatch impedances throws away a large proportion of an already weak signal, as well as providing a conduit for radio interference to enter the receiver.  Fortunately there is a simple transformer that can be made from a couple of short sections of coax, which can nicely match 75-ohm coax into 50-ohm coax, which can then be fed into a receiver’s 50-ohm antenna socket.  More on this trick below.

The dimensions for the Lindenblad presented in Figure 10.15 (on page 10-14 of the ARRL Satellite Handbook) are for the 2-metre and 70cm wavelength Amateur bands.  For a 137.58MHz-resonant antenna, the s,l,w and d dimensions presented in Figure 10.15, are 42, 926, 893 and 654mm respectively.  Note that 137.58MHz is half way between the two main frequencies used by NOAA satellites, 137.50 and 137.62MHz.  NOAA 137MHz NOAA radio transmissions are Right Hand Circularly Polarised (RHCP), meaning that the four folded dipole elements of the Lindenblad, need to be tilted clockwise from the horizontal by 30°, as viewed by a butterfly perched at the centre of the antenna.

Matching 75-ohm coax into 50-ohm coax

I first came across this useful technique on page 8.8 of the RSGB VHF/UHF Manual.  This type of impedance matching transformer is also known as a ‘twelfth-wave matching transformer’, and many references to it may be found on the Web.  For the technically minded, this article by Darrel Emerson is quite good.  But, all you really need to know is for 137MHz, the match can be achieved by soldering together two short sections of coax, one of 50-ohm RG-58 and another of 75-ohm RG-59, both measuring 117mm in length (pictured right and below, are a couple of 137MHz 50:75ohm transformers that I have made).  It is important to note that this match only works for a single frequency, and using standard RG-58 and RG-59 coax with a velocity factor of 0.66.   In fact any 50- and 75-ohm impedance coax must be used, but allowance must be made should the coax have a velocity factor different from 0.66 (see references below to standard coaxial cable specifications). 

Although it’s expensive and unnecessary for the average Lindenblad constructor, an antenna analyser such as the MFJ-269 is very handy for optimising the dimensions of a new antenna, as well as measuring coax velocity factors etc.  For those who own an MFJ-269, an RF noise bridge, or some other RF impedance measuring instrument, and who really want to understand antenna electrical behavior, The ARRL Antenna Handbook incorporates an excellent brief introduction to the use of Smith Chart paper for making antenna measurements, matching antennas to feedlines etc.

Connectors

Anyone who tells you that 50-ohm connectors can be used with 75-ohm coaxial cable, has a great future selling used cars.  Connectors are important, and particularly matching the correct connector to the coaxial cable type.  Of course the best kind of coaxial connector is no connector at all, so try and keep one’s main coax feeder in one long unbroken piece.  For inside-the-house coaxial connections, Cable TV style ‘F56’ and ‘F59’ connectors (for RG-56 and RG-59 type coax), are good, cheap and low-loss.  Likewise so-called PAL/Belling-Lee types.  They come in many varieties such as ‘twist on’ and ‘crimp on’ types, for various coax cable thicknesses, for indoor and outdoor application.  Common 75-ohm cables go by the names ‘RG-6’, ‘RG-56’ and ‘RG-59’.  When buying coax for NOAA satellite reception purposes, definitely buy “the good stuff”.  The sales people at you local Radio & TV store will know what you mean.  “The good stuff”, usually means ‘quad-shielded low-loss RG-6 or RG56/59’, with all-metal 75-ohm connectors to suit.  Specifications of ‘RG’ Series coaxial cables can be found in this useful chart, and similarly for British ‘UR’ Series coax cables.

The ultimate connector for this type of application is the ‘Type-N’.  They are very low-loss and very waterproof.  Type-N’s are usually found in professional applications where expense is less of a concern.  Type-N’s come in 50-ohm and 75-ohm versions, the 75-ohm being somewhat rarer and more expensive than the 50-ohm.  So if one should come across some Type-N’s and would like to use them on 75-ohm cable, make absolutely sure they really are 75-ohm connectors (this is usually stamped on them in small letters somewhere).  Even rarer and more expensive and than Type-N 75-ohm, is Type-N 75-ohm specifically made for RG-6 cable.  We use Quad-shield low low-loss RG-6 coax, terminated with 75-ohm Type-N connectors manufactured by Huber & Suhner.  These are outstandingly well made connectors, but rather expensive.  They are overkill for normal applications.

With 50-ohm cables, such as RG-58, Type-N’s or BNC’s are the best (but note that BNC does come in a 75-ohm version, although is somewhat rarer).  Most scanning-type receivers use a 50-ohm BNC connector for the antenna socket.  Under no circumstances do I recommend use of so-called PL-259/SO-259 plugs/sockets, also sometimes known (ironically) as ‘UHF Connectors’.  These are horrible performers at VHF frequencies and above, and not in the least bit waterproof.  But strangely, one commonly finds SO-259 sockets used on VHF Ham Radio equipment.  Using these connectors is a historical habit (dating back to WW2) that is way past it’s use-by date, and when I buy equipment that comes with these connectors installed, I strip them off and replace them with Type-N’s.  For outside cable connections exposed to the weather, it is most important to ensure that rainwater cannot seep into the inside of the coaxial cable.  Once inside the coax, there is no way for water to get out again, and over time the coaxial braiding corrodes and becomes electrically ‘lossy’ and a good absorber of the RF signals flowing down it.  If non-waterproof connector are used, they need to be thoroughly shielded from weather, and/or wrapped in self-amalgamating tape or the like.

Antenna location

Placing an antenna for receiving LEO satellites is a little different for placing antennas for TV reception.  With TV reception, the TV station transmitter is usually placed atop a distant hill, in which case a TV receiver antenna is best placed as high as possible, with a good clear line-of-site view toward the transmitter tower.  But an overflying satellite is usually at a considerable angle above the horizon, and the receiver antenna can easily ‘see’ the satellite even when it is placed relatively close to the ground.  As long as the antenna is at least a couple of metres above the ground, and likewise clear of electrically conductive objects and wire, and with a reasonable view of the sky, then this is usually fine for NOAA 137MHz reception.  Also being placed lower down reduces the risk of lightning damage.

The problem of radio interference should not be underestimated, particularly if one lives within a large city.  The advent of TVs, VCRs, PCs, microwave ovens, and a host of microprocessor controlled equipment, means that the VHF radio spectrum within a city is usually filled with electromagnetic crud, formally known as Electromagnetic Interference (EMI) or Radio Frequency Interference (RFI).  In general, national RF spectrum regulatory authorities protect the electromagnetic spectrum around the 137-138MHz space downlink band, but regrettably not in Australia.  Most distressing of all in Australia, is VHF TV Channel 5A, which has its vision carrier located smack-bang in the middle on the 137-138MHz band.  Tens of kilowatts of broadcast TV radio signal, easily swamps the 5 watts of radio signal from a 1000km distant spacecraft.  In such cases where local broadcast TV interference is a problem, placing one’s antenna low down in a back yard, or behind a large building, or in a small valley, may partially shield the antenna from the offending TV signal whilst maintaining a good view of the sky.

Antenna orientation

Lindenblad antennas have a deaf spot located directly above the antenna, and when a satellite passes directly overhead, the signal may be lost in the Lindenblad’s ‘cone of silence’ for a few seconds.  All antennas have directions in which they are more sensitive or less sensitive, and designing an ‘omnidirectional’ antenna involves compromises, as one cannot truly make it equally sensitive in all directions.  A Lindenblad maximises it sensitivity towards the horizon, where a rising/setting satellite is relatively far away and it’s signal correspondingly weak.  Lower elevations are also where LEO satellites spend most of their time during a typical satellite pass, so it makes sense to have this as the region where the antenna is most sensitive.

The Lindenblad’s overhead deaf spot may also be put to use.  My friend Colin has his house located in an area which is blasted by Australian TV Channel 5A (which has the channel’s FM video carrier dead-centred on 137MHz).  After weeks of trying every filtering and interference-reducing trick in the book, Colin tried tipping the Lindenblad antenna on its side (pictured right) and deliberately pointed the antenna’s deaf spot toward the offending TV transmitter.  To our surprise and satisfaction, the offending TV signal was largely removed, and now Colin receives acceptable NOAA APT imagery.

Our receiving site at Siding Spring Observatory (NSW, Australia) has a serious problem with radio interference at 137MHz.  For starters there’s nearby 22kV power lines, transmitting a broadband 100Hz ‘crackle’ across the entire VHF radio spectrum.  There’s also a very powerful broadcastTV/Radio transmitter tower (Needle Mountain, located 8.7km away).  There’s a mysterious broadband digital telemetry which shifts backwards and forwards in frequency, and which appears to be propagating along the electrical mains.  I have measured this interference many kilometres away, and always find it near power lines.  My suspicion is that it’s being transmitted by power generation utilities, to remotely control and monitor their equipment.  Finally, I’m sorry to admit, there is my own home-grown noise from our insufficiently EMI-shielded telescope control electronics.  In short, we’re swamped in a sea of radio noise.

In locations with high radio noise, and during times when a satellite’s signal is low, interference patterns will usually appear on the APT image.  Poor reception may be due to a large distance to the satellite (i.e. times when the satellite is near the horizon), a relatively weak signal transmitted from a satellite (as is usually the case with NOAA-12), or a satellite is passing through an antenna’s deaf spot (for a standard horizontally oriented Lindenblad, elevations > 80 degrees).  If the local radio interference should dominate the satellite’s signal, a strip of ‘salt and pepper’ noise pattern will usually appear across the APT image.  Moreover, when a NOAA satellite is passing directly overhead through a Lindenblad’s ‘cone of silence’, this noise stripe will likely pass right through the observer’s location displayed on the APT image.  Since we use our APT imagery primarily for local cloud cover forecasting, this interference stripe is particularly inconvenient.  On rare occasions a noise stripe will occur across an APT image, but due to extraterrestrial noise rather than any shortcomings of the Lindenblad.

After experiments with different antenna types, I finally followed Colin’s lead and simply tilted my tried-and-true Lindenblad by 90 degrees (April, 2004).  This orientation gives reduced reception of satellite signals near the horizon, but considerably improves the reception when satellites are passing overhead.  For our particular application, this is a more desirable outcome.  I have made and tested a few different designs of fixed polarised VHF antennas, but I am yet to find one which conspicuously out-performs a Lindenblad, whether horizontally or vertically aligned.  Incidentally, note that the tilting trick can only be performed with a non-conductive mast (a wooden one in my case).

Filters

If you are being driven to distraction by the presence of nearby powerful FM-radio or TV transmitters, and these signals are so powerful that they are breaking through the front-end filtering of your receiver, there are a number of commercially available in-line filters which may be inserted between the antenna and the coax.  The trick is to identify the frequency of the transmission, and then buy the appropriate TV-grade high/low pass filter to block the noise.  Interference on the 137-138MHz band is a common problem, and there are commercial band-pass filters available which are designed specifically for 137MHz space-downlink band.  Or one can build one’s own, such as this example of a ‘helical filter notch resonator’.

With our receiving station, we use a rather special kind of band-pass filter known as a cavity resonator.  These are the ultimate for filtering out noise, but rather expensive for a home constructor.  We are forced to use it because of our rather close proximity to a powerful TV and telecommunications transmitter tower (at Needle Mountain, located 8.7km away).  Our model C2A-6 single-stage cavity resonator was manufactured Polar Electronic Industries in Melbourne, Australia, and has these performance data.

Another way that unwanted radio noise can get into a receiver, is for it to be picked up on, and flow down the outside of the coaxial cable toward the antenna, where is it received, amplified, and fed back down the inside of the coax back to the receiver.  At our receiving site we are particularly susceptible to this type of noise due to the presence of many computers and poorly EMI-shielded electronics which are also located where our receiver is located.  Fortunately the signals flowing on the outside of the coax my be blocked by feeding the coax through cylindrical shaped ferrites, much the same as is usually done on computer video monitor cables and the like.  In Australia, cylindrical ferrites from Jaycar (stock no. LF1258) can be recommended.

References:

ARRL Handbook for Radio Communications (current edition).  Available from ARRL publications.

ARRL Antenna Handbook (current edition), edited by Dean Straw.  Available from ARRL publications.

Radio Amateur’s Satellite Handbook (1998), by Martin Davidoff.  Available from ARRL publications.  In Australia on sale at Dick Smith’s.

RSGB VHF/UHF Manual (fourth edition, 1994), edited by G.R. Jessop.  Published by the RSGB – Radio Society of Great Britain.  Since superceded by the VHF/UHF Handbook, edited by Dick Biddulph

Page last modified – 2005-03-17.   This page is still under development.  Please feel welcome advise me of any typos, broken web links, clumsy language, and/or suggestions for improvements.


Author : Andre Phillips – UNSW


Page originally available at http://newt.phys.unsw.edu.au/~map/weather/notes/buildyourown/lindenblad.html

The CobbWeb Antenna

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CobWbb

G3TPW CobWebb Antenna for the 14, 18, 21, 24 and 28 MHz Bands

Author and source: http://www.g3tpw.co.uk/

Read more: The CobbWeb Antenna

The main advantages of the CobWebb over other 5 band 14 to 28 MHz antennas are that it is small, lightweight, strong (made from fibre glass), only requires a single support, needs no rotator, gives full size dipole performance on all 5 HF bands (without the end on nulls that straight dipoles suffer from), is fed by a single 50 ohm co-ax cable (via an in built air core choke balun), and most important of all, it produces a pure horizontally polarised signal with a confined electric field. This results in much reduced coupling to nearby conductors, so that losses and interference problems are reduced to the absolute minimum possible.

The CobWebb can be easily mounted on a single 20 foot aluminium scaffold pole, fixed to the wall of the house with a couple of stand off wall brackets. The pole can then be pushed upwards to put the CobWebb up at 30 to 35 feet, or lowered so that it is at roof ridge height, to overcome any possible planning problems!

All the other small multi-band commercial antennas that are available are less than optimum. The “broad band” folded dipoles and verticals have lossy transformers or terminating networks in them. These may provide a fairly low SWR over the full 2 to 30 MHz band, but so does a dummy load! Some trap dipoles have lossy traps and baluns, so that a nice 50 ohm match is produced on all bands. Others have no balun at all, the feeder cable will then need to be a particular length to reduce feeder radiation and provide a match. Doublet Antennas may have some gain on the higher frequencies, unfortunately every 3 dB peak has a 20 dB null in other directions! Other small “magic maths” antennas are available, but their performance is so poor that you can’t even hear them on the bands, to measure how far down they are! The many types of vertical antennas are best avoided, unless you have no neighbours, because of all the interference problems.

The inherent losses and poor radiation efficiency of the mini-beams negates any so-called gain. Remember that if a beam has a gain of 4 dB over a dipole in free space, it will be DOWN on a dipole! It will also only “work” over a very narrow frequency range. A dipole or a CobWebb has a gain of about 5dB over a dipole in free space, (due to ground reflection) or 7dB over an isotropic radiator (7dBi) and will work equally well over the entire band. The CobWebb has no lossy components in it, so there are no power limit problems for the QRO operators. The low loss and consequent high radiation efficiency also makes it ideal for QRP!

The G3TPW CobWebb Specification

Covers all 5 Bands.  Gives a low SWR resonance on the 14, 18, 21, 24 and 28 MHz bands. The SWR at the band edges is mainly reactive, i.e. the resistive component is still near 50 ohms, so auto and simple ATUs can match it with low loss. Over 95% radiation efficiency on all bands! Have you ever seen this spec mentioned by other manufacturers?

Omni-directional.  Talk to all the world, without the need for a rotation system.

Absolute Minimum of EMC Problems.  Vastly reduced interference on both transmit and receive due to pure horizontal polarisation and confined electric field.

50 ohm Single Co-ax Feed.  Built in co-axial choke balun to prevent feeder radiation. Standard PL259 plug on end of short lead. Feedbox and resonators all pre-assembled.

No Compromise Performance.  Full size half wave dipole on each band, without nulls!

Fibre Glass Construction.  Flexible so no metal fatigue problems in windy locations.

Simple Assembly.  Fix fibre glass sections together. All screw holes pre-drilled. All elements pre-tuned, just uncoil them and fix to spreaders. No adjustments needed.

Small Size and Weight.  Only 2.6 metre (8.5 feet) sides and 6 kg (14 lbs) weight when assembled. 1 metre maximum length parts for low cost world wide delivery.

Easily Erected.  “V” bolt fixing to mast of up to 58 mm (2.25 inch) diameter. Can be fixed to 20 foot scaffold pole, which can then be pulled up to the wall bracket with rope.

160 Km/hour (100 mph) Wind Survival.  As long as the mast/support can take it!

The G3TPW CobWebb Design

The most important design point about the CobWebb is that it is a completely horizontally polarised, confined electric field antenna, which provides maximum radiation efficiency and the absolute minimum of interference problems. TVI fears have encouraged many people to shy away from the HF bands, in favour of VHF, UHF or 160 metres.

New stations often begin operating on HF using an end fed wire or a vertical antenna, a recipe for disaster, sometimes even on QRP. After starting in this way and getting involved with various EMC problems, it is often found that planning permission for a horizontal antenna is refused. You can’t really blame your neighbour for being concerned. If that small inconspicuous vertical or simple wire antenna causes so much trouble, what would it be like with the proposed mast and special horizontal antenna? The fact that the use of horizontal polarisation, particularly if it is from a confined electric field antenna like the CobWebb, would probably cure the breakthrough problems is very difficult to explain.

During experimental work on a 5 band beam, using full size resonators on each band, it was noticed how well the driven element worked by itself. It also became obvious that a full sized dipole, up in the clear, worked far better than the multi-band minibeams!

The CobWebb is a full size half wave dipole on each of the 5 amateur bands, 14, 18, 21, 24 and 28 MHz, for maximum performance. There are no lossy traps, stubs or loading coils, so there is no reduction of radiation efficiency on any band. Each dipole is bent round to form a horizontal square, to make the antenna omni-directional. Thus no expensive rotator is required. Each resonator looks like the “square halo” that is often used on VHF, for SSB mobile work using horizontal polarisation.

The parallel but anti-phase “sides” of the antenna cancel the radiation that would normally be wasted as high angle radiation from a straight dipole and fill in what would otherwise be the nulls off the straight dipole ends. The resulting omni-directional pattern has many advantages over antennas with directional effects. Unless an antenna with nulls in it’s response can be rotated, it will be found that certain parts of the world will be very difficult to contact.

The five “squares” are made from white PVC covered multi stranded copper twin cable which is supported by a horizontal cross, made from white fibre glass. Each element is folded and tapped for impedance matching, so that the antenna looks like 50 ohms on all 5 bands. The small size ensures minimum windage and the low weight means that TV type brackets and masts may be used for supports. There are no aluminium elements to corrode and cause high resistance joints, or snap off in the wind.

The antenna is fitted with a co-axial choke balun at the common feed point, to prevent any current from flowing down the outside of the co-axial cable feeder. This balun is absolutely vital, to prevent any radiation or pick up of signals by the feeder. EMC problems can be just as bad as with verticals if this balun is not exactly right.

The electric fields of the CobWebb are confined because the high impedance ends of each element are only a few inches apart. This reduces the coupling to nearby objects so the antenna does not need re-tuning for operation at different heights and locations.

Other Multi-band Antennas for the HF bands

The Horizontal End Fed Wire
This is the simplest and cheapest antenna of all. It will require a very good ATU to tune it up though! Very high voltages may be present at the feedpoint, dependant on the exact length and frequency in use. You will probably need to use a so called kilowatt ATU, when using 100 watts, to avoid arcs on the ATU tuning capacitors. Severe interference problems are common. The antenna is obviously unbalanced with respect to earth, so vertically polarised radiation will occur. This radiation will be either off the vertical feed wire connecting to the end of the horizontal span, or from the earth lead if the wire comes straight into an upstairs “shack”. RF in the shack problems often occur, RF feedback, RF burns to the operator if any metal is touched, and general EMC problems normally restrict these antennas to QRP operations. There is an obvious radiation hazard when using this type of antenna. The local field strengths produced can be way above the maximum permitted safety levels.

Doublet
Any long wire can be fed at it’s centre with a tuned open wire feeder. This should maintain balance to prevent vertically polarised radiation off the feeder and subsequent interference problems. Whenever a twin feeder system is used though, the antenna itself must always be exactly electrically balanced and symmetrical, any slight lack of symmetry will cause feeder radiation, with the dreaded vertical polarisation. It will require a high quality proper balanced ATU i.e. not an unbalanced one with a toroidal balun transformer added on the output! (The balun will only work efficiently over a limited range of impedances) As with any long wire antenna, the polar diagram will contain many deep nulls, in which directions communications will be difficult. This more than negates the few dBs of gain that doublets have on the higher frequency bands.

Note Large spaced open wire feeder should not be used in the shack, or anywhere where people or animals may be able to come into close proximity with it. The fields will not cancel in line with the feeder (they only cancel broadside to the feeder), unless the distance is many times the wire spacing. Unless the small spacing twin is used, the RF radiation problems in the shack can be almost as bad as those with end fed wires.

G5RV
This antenna is very cheap to make, but because it is only resonant on one band, it requires a very good ATU to tune it up on all the other bands. It is really too long to give consistent results in all directions on the higher bands, the polar diagram becomes very “petal” shaped with many deep nulls. It is far better to convert it into a doublet antenna and use open wire feeder all the way to a proper balanced ATU, rather than to use co-ax cable for part of the feeder. The SWR on this antenna is very high on all the bands (except 14 MHz, where it is only about 2:1 at resonance), so the use of co-ax can cause very high losses. The ribbon feeder normally supplied with commercial G5RVs is very poor mechanically, if it swings in the wind it breaks quite quickly.

Vee Beams and Rhombics
Great, if you have got the necessary real estate! Radial terminated Vee beams and/or Rhombics, with switching to beam to any part of the globe. Shack in the middle of the 10 acre site, which is a Polynesian island, with only local girls for company! Ahh, this is paradise, but would you bother going on the air? Dream on!!
 

Log Periodics
Too big and low gain for the size. Note the CobWebb has a “gain” of 7 dBi.
 

Trap Dipole
Trap dipoles use tuned circuits to isolate sections of a dipole, such that electrically it looks like a half wave dipole (with a low impedance feedpoint) on each band. Four pairs of traps would be required for a 5 band dipole. If the inverted “V” configuration is used, then the feed impedance should be about 50 ohms. However the traps act as loading coils on the lower frequency bands, so the antenna becomes shorter than normal. This should reduce the radiation resistance of the antenna on the lower bands. The radiation from an antenna is determined by the product of current and length. If the length is reduced then, for the radiated power to remain the same, the current must be increased. The increased current for the same power must mean that the voltage will reduce. The shorter length should therefore cause the feed impedance to be much reduced. This should cause the SWR to increase, if it does not the traps must be lossy!

Trap Vertical
This is in effect half of a trap dipole, fed against ground. They can give a good match, but being on the ground the signals will be attenuated by surrounding objects, particularly on the higher frequencies. If your ground has poor conductivity results will be very poor. Verticals can be elevated, using either radials or extra traps. They can then work very well, as long as you don’t have any neighbours. In an urban environment the EMC problems can be chronic.

All vertical antennas suffer from another problem. They require good conductivity soil for many wavelengths around them to compete with horizontals. Even verticals that do not use a ground connection i.e. vertical dipoles or ground planes etc. can still be as much as 8 dB down on a horizontal dipole antenna, due to reflection losses. The difference between sea water and very poor ground is up to 9 dB for a vertical antenna, but only 1 dB for a horizontal.

The Magnetic Loop
This is really just a short dipole that is bent and end loaded with a variable capacitor. There is no magic involved in the way it works, a standard “electromagnetic wave” is produced from them, as per any other antenna. It is not producing magnetic waves with special immunity to “Electric Field Interference”. It does have a confined electric field so that near field “capacitive” coupling to surrounding conductors will be reduced. There will be some reduction in performance on the lower bands, a well designed 5 metre circumference loop will normally work over the 14 to 28 MHz bands with a 50% efficiency on 14 MHz.

To keep the efficiency this high, the loop will have to be made out of large diameter copper or aluminium, NOT co-ax!! The resultant high “Q” will mean that the loop will have to be continuously re-tuned, as the receiver is tuned round the band. The remote control system that is needed to do this can be quite expensive, also the tuning capacitor will have to stand many thousands of volts, even when only running 100 watts. For high power operation a vacuum variable capacitor is normally used, which is very expensive.

A loop standing on the ground in the vertical plane will radiate a vertically polarised signal. The ground will absorb/reflect the horizontal component straight upwards. The vertical component at low angles to the horizon i.e. that required for DX operation, will have a sharp null in the response broadside to the plane of the loop. This can be useful for reducing interference, but it does mean that the loop will need a rotation system.

To prevent low angle signals from being absorbed by surrounding objects i.e. shrubs, fences, trees, houses and the myriad of conductors around the QTH, it is obviously far better if the loop antenna is mounted up in the air. If it is mounted about 30 feet high then the horizontal radiation will be able to be used for low angle DX operation. The loop can then also be mounted in the horizontal plane to eliminate the vertically polarised radiation, and so reduce EMC problems.

The small diameter loops made from co-ax or flat bar give very poor performance. They don’t need retuning as often as you tune round the band, because they are low “Q”, the problem is that most of your transmitter power will be dissipated as heat!

The main advantage of the loop antenna is it’s continuous frequency coverage over the entire spectrum, by remote controlled re-tuning. This is ideal for military users etc. but against this is the cost, especially if you only wish to transmit on the amateur bands.

Mini-beams
These antennas are a very big compromise. They generally only cover the 14, 21 and 28 MHz bands, because of interaction effects with 18 and 24 MHz. A reflector resonance for 28 MHz will act like a director resonance for 24 MHz, a reflector resonance for 24 MHz will look like a director resonance on 21 MHz etc. etc. The result is a very expensive trap dipole! The gain that they are supposed to have is only over a very narrow bandwidth. They are very difficult to set up, most people just give up and feed them with an ATU. They need to be rotated, not because of a good front to back ratio, but because of the nulls off the ends of the dipoles! They are often rated for high power use, presumably what this means is they don’t actually catch fire when used with QRO, because they certainly get very hot!

Broad Band Verticals
These antennas use most of the applied power to produce a low SWR reading. The  fraction of power that is radiated can still produce some DX QSOs on the higher frequency bands though, it’s amazing what you can do with QRP on HF! If the guy with a 100% efficient antenna is getting a 59 plus 20 dB report, then the guy with the 1% efficiency will still get S9!! On the lower frequency bands these antennas are very good dummy loads!

It always amazes me that if a 100 watt rig was only putting out 10 watts, most amateurs would be very upset, yet they will use a 10% efficient antenna and be quite happy. The reason is, of coarse, they can’t measure the radiation efficiency but they can measure the power/SWR.  Many radio amateurs seem to think that a low SWR means that all the power is being radiated, so you cannot really call the producers of these antennas “Con Artists”, they are merely producing what the radio amateurs say they want! They are even quite good for lack of TVI and general EMC problems, as they radiate such a small amount of the applied power!!! Caveat Emptor!

Broad Band Terminated Dipoles
These antennas also use most of the available power to produce a low SWR reading on the lower frequency bands. On the higher bands they only use about half of the power for this purpose, so about half of the power is actually radiated! Unfortunately, on these higher frequencies, the antenna will have lots of lobes and deep nulls as per a standard doublet antenna. The military have used this type of antenna in the past, because of its ease of matching over a wide continuous frequency band, but it really is just a waste of power to use it on the amateur bands.

Crossed Field Antennas
These antennas don’t work! A little knowledge is a dangerous thing! The co-ax feeds radiate a little power, because they don’t have proper baluns on them! The metal bits radiate a little power because they are metal and have a small amount of RF current flowing through them! One guy on the internet has even offered a $10,000 dollar reward to anybody who can demonstrate one working! The so called “antenna design experts” who invented and patented the CFA have not claimed their reward!!!  A short length of wire and a parallel tuned circuit ATU will work much better! It’s a classic case of somebody inventing a theory and then trying to make the facts fit the theory!!
 

Parallel Connected Dipoles
These antennas work very well. The dipoles need to be spread out so that the high impedance ends of the shorter ones are not affected by the longer elements. They can be arranged “maypole” style round a central support, so that they can act as guy wires as well as antenna elements. When arranged in this way the feed impedance will be about 50 ohms so they can be fed with 50 ohm co-ax, via a choke balun. This system works very well when out portable, the only problem is the vertically polarised radiation off the ends. This radiation fills in the nulls off the ends of each dipole, so that no rotation is needed but it can cause the dreaded interference problems.

These antennas really need to be horizontally polarised for minimum EMC problems, but they then need a 75 ohm feed. They can be double gamma “T” matched to 50 ohm co-ax, as long as an effective choke balun is used. This is in fact what the G3TPW CobWebb antenna is! The dipoles are each bent into squares, so that they can be supported by a single horizontal fibre glass cross, rather than having separate supports for all ten dipole ends. Bending the dipoles into squares also eliminates the dipole end nulls, resulting in an omni-directional radiation pattern. Another major effect is that the electric field of the square dipole will not couple into the ground, or any other nearby conductors so losses are much reduced.

Verticals and EMC (Electro-magnetic Compatibility)

Whilst we have all heard that vertical antennas cause interference problems, I suspect that most people don’t fully appreciate why. I certainly didn’t, until I investigated the TVI problems that I had with my original 5 band vertical design. In fact I always used to say that verticals cause TVI because they put out such good signals at the low angles that we need for DX communication! It was only when I discovered that my vertical was causing lots of EMC problems with my neighbours, whilst my horizontal half wave dipole antenna at 30 feet was not causing any problems at all, yet giving a better signal in Australia, that I realised that it was the vertical polarisation that was to blame.

It is obvious that vertical antennas will couple more power into nearby vertical conductors than horizontal antennas will. This causes much loss of radiated signal and also causes many EMC problems. If more RF energy is being coupled into a vertical TV feeder co-ax cable, then TVI is going to be more lightly. Interference will also be picked up from the TV, producing an increased background noise in your receiver.

However I was not convinced that this was the whole story. My neighbours portable radio was also clobbered by the vertical but completely clear when I used my horizontal! The portable radio could be oriented in any plane, with no effect on the amount of interference received.

Tests with a calibrated field strength meter showed that the field strength levels near the ground were very low from the horizontal antenna, and very high from the vertical antenna! The field strength meter needed to be up at a height of  about 20 feet before the field strength readings from the horizontal antenna were similar to those from the vertical. Reports from Australia showed the horizontal to be 3 dB stronger though!!

The susceptible portable radio was substituted for the field strength meter. It became obvious that the ground was effectively “shorting out” horizontally polarised signals. It was found that at low heights the power fed into the horizontal antenna needed to be about 100 times (i.e. 20 dB) greater than that fed into the vertical antenna, for the same interference levels at a given distance.

It was realised that if any electronic device with an EMC problem is mounted close to the ground (in terms of wavelength) then it will not receive horizontal polarised signals very effectively. Up to a height of about a quarter of a wavelength the ground will act as a reflector, so that horizontally polarised signals will only be received from higher angles. Below a height of about an eighth of a wavelength the ground will also provide a lossy dielectric path for the horizontal electric fields, and thus tend to short them out and/or dissipate them. Thus the ground will protect electronic devices from horizontally polarised electromagnetic fields. Vertically polarised fields will however be received very well, even if the susceptible equipment is actually sat on the ground.

I decided to concentrate all my efforts on the design of horizontal antennas, to avoid all the hassle of  TVI, HI-FI and telephone breakthrough etc. I thought that it would be very anti-social of me to market a vertical antenna that would subject lots of radio amateurs, and their innocent neighbours, to EMC problems that could be avoided by the use of the correct polarisation.  Thus the CobWebb concept was born!!

G3TPW  COBWEBB  F. A. Q. s (Frequently asked Questions)

What is a CobWebb antenna and what is it’s gain?

It is a very efficient horizontally polarised omni-directional antenna for the 14, 18, 21, 24 and 28 MHz bands, with a gain of 7 dB over an isotropic radiator (7dBi). Note that this is the same as a standard dipole, although a straight dipole will have sharp nulls off each end. A standard straight dipole has a gain of 5 dB over a dipole in free space! I notice that the specification of my standard large 3 element tribander says that it has a gain of 8.5 dB over a dipole in free space. This means that it has a gain of 3.5 dB over a dipole at the same height!

How does it work?

Technically the CobWebb is 5 separate full size dipole antennas, each bent into a square. This makes it very small (only about 8 foot square) but it is still full size! It has 5 separate double gamma tee matches to match each element to a common 50 ohm co-axial feeder, and has a built in air cored co-axial choke balun, to prevent feeder radiation.

What is the secret ingredient. Why does the CobWebb work so much better than other antennas?

Simply because it radiates all the power fed into it, and it also radiates in an omni-directional manner so there are no nulls. There are no lossy ferrites,  traps or loading coils to heat up, and each element is full size and made from 84 strands of 0.2 mm diameter pure copper wire with a plastic covering.

The confined electric near field (caused by the high impedance ends of each dipole being close to each other) also ensures that the antenna does not couple to other electrical conductors i.e. telephone wires, power cables, television antennas or even the ground and lossy di-electrics such as trees and buildings. Thus the radiated power is not absorbed by nearby objects, it is all radiated into free space. Breakthrough and noise pick up are also reduced to an absolute minimum and the ground conductivity and height do not affect the antenna tuning.

What is the maximum power that can be fed into it?

Nearly all the power fed into the CobWebb will be radiated, so the antenna will not heat up and so limit the power rating. There are no ferrites used so no intermods are produced. It has been tested with 3 kilowatts of RF, above this level there could be a problem with corona discharge sparking at the ends of the elements as the air is ionised!

How is it constructed?

A single horizontal fibre glass cross supports all 5 elements. The feed-box is on the end of another solid fibre glass rod, such that the feed is in the centre of  one of the sides of the squares. Each rod is secured with “U” bolts onto a 0.25 inch thick aluminium plate. Another similar plate allows the antenna to be fixed to a vertical mast of up to 2.25 inches in diameter, with the “V” bolts provided. Plastic covered “figure of 8” section 84 strand copper conductor is used for the elements, which are secured to the fibre cross by the unique G3TPW system to prevent pre-mature  breakages!

I’ve not seen any of your adverts for ages. Why don’t you advertise the CobWebb more?

The last time we advertised in a magazine was in 1993! The CobWebb was last reviewed in a magazine in 1996 though. It is interesting to note that the magazines always asked us if they could review the CobWebb, we never had to ask them!! Of course the reviews in the magazines were very good adverts, but they only last for a month or two. We find that the 750+ antennas that we have sold are the best long lasting advertisements, literally being broadcast to the world every time somebody says, “And the antenna here is a CobWebb”.

So are your antennas sold purely by word of mouth?

Most of our sales do come from people who have received personal recommendations from existing users. However, the interest is mainly generated when people hear  stations using CobWebbs doing so well on the bands. The recommendations are normally received when the enquirer is trying to find out our address/telephone number!!

This is, of course, a scale model made specially for your lectures. How big is the full size antenna?

No, we only have full size CobWebbs! We don’t need a scale model of the CobWebb because it is so small. I must admit that even I am still amazed when I look at a CobWebb and realise that though it is so small IT IS STILL FULL SIZE!! A 33 foot dipole is a much bigger beast than a square with 8 foot 6 inch sides!

Can two CobWebbs be used as a beam?

Yes, but only by phasing them, i.e. they both have to have separate feeders. Parasitic elements will not work because the CobWebb does not couple to other nearby conductors! The maximum gain obtainable is less than 3 dB over a single CobWebb  and of course a beam rotator is needed. The phasing adjustments needed to provide optimum directivity across all the bands are quite complex and the phasing unit becomes expensive and difficult to operate. If you increase the antenna height by 50%, then the DX signals will increase by more than they would by phasing 2 antennas!

I’ve heard that different antennas suite different locations. Will the CobWebb work at my QTH?

YES! The ground conductivity for many wavelengths around an antenna will affect its performance. Vertical antennas that use a ground connection can be as much as 20 dB down on a dipole. Verticals that do not use a ground connection i.e. elevated feed vertical dipoles or ground planes etc. can still be as much as 8 dB down on a horizontal, due to reflection losses. The difference between sea water and very poor ground is up to 9 dB for a vertical antenna, but only 1 dB for a horizontal
.
The CobWebb is not affected as much as other antennas by trees, buildings, power lines, TV aerials feeders and telephone wires etc. The poorer or more cluttered that your QTH is, the better the CobWebb will perform, compared with other normal ants, including a straight dipole!

These are the reasons why CobWebb antennas so often out perform other antennas, they are not affected by the nearby environment so THEY WORK AT ALL LOCATIONS.

How do I mount it?

The CobWebb can be mounted on a vertical pole of up to 2.25 inches in diameter. It can be added to existing antenna installations such that the mast goes straight through the CobWebb. A 20 foot scaffold pole makes an ideal mast, this can be fixed to a wall with a couple of stand off brackets.

Do I need planning permission for it?

Most people don’t bother to apply, if they are just mounting a CobWebb on a 20 foot pole. The pole can be slid down through the brackets so that the CobWebb is below the roof ridge height. This meets the standard “not above the roof ridge” planning restriction; it can then be gradually pushed up to the optimum height of 33 feet!

Will it stand the high winds at my QTH?

CobWebbs are in regular use in the Falklands and on Ascension Island. They have stood up to gales that have destroyed many other antennas. Most of the breakages that have occurred have happened when either the mast has come down, or the antenna has been dropped. Even then the antenna can be quickly and cheaply fixed because the short 1 inch diameter fibre glass joining tubes break, and the antenna simply folds up. These joining pieces can be easily changed in a just a few minutes. If the wires do get broken they can be repaired with choc block screw connectors or crimp connectors, or we can supply new wires.

How does the CobWebb minimise the chance of TVI and breakthrough problems?

The pure horizontal polarisation and use of a choke balun reduces the chance of breakthrough to the absolute minimum possible. The confined electric near field (caused by the high impedance ends of each dipole being close to each other due to being bent into squares) also ensures that the antenna does not couple to other electrical conductors i.e. telephone wires, power cables or television antennas.

Are there any complicated adjustments to be made during installation?

None at all! The CobWebb is not detuned by nearby objects, including the ground, because of the confined electric field. Thus the CobWebb can be pre-tuned and matched during production, so that it works at any QTH and any height!

Home build a Kenwood PG-5G cable.

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Parts List

P1 9 pin D socket with shroud.
P2 8 pin mini DIN plug.
Some 4 or 5 core shielded cable.

Making Up

Following the pictorial guide (right), solder wires between the relevant pins on each plug, not forgetting to slide the Mini DIN plug casing onto the cable before you start.

The diagram is colour coded as well as having the pin numbers marked on it, for example the Red connection goes from Mini DIN pin 5 to pin 3 on the 9 pin D socket. Plug views are of the mating surfaces.

If you have a 5 core plus shield cable, solder the shield to each plugs’ metal casing. If the cable you have has 4 wires and a shield use the shield for the Mini DIN pin 4 to pin 5 on the 9 pin D (Ground)

After checking the soldering is good and there are no shorts or stray whiskers, assemble the shroud and cover around the plugs and you’re ready to go.

Connecting to a PC without a serial port

This cable should work with USB to Serial adaptors.

Home build a Kenwood PG-5A cable

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This lead is the audio data cable for some of the Kenwood Mobile transceivers. It is also available as part of the PG-5H kit where it is supplied along with a PG-5G lead.

Parts List

P1 & P2 3.5mm stereo jack plugs.
P2 6 pin mini DIN plug.
Some “figure 8” shielded cable or similar 2 core plus shield cable (An old PS/2 mouse lead could be used with a little ingenuity).

Making Up

Following the pictorial guide (right), solder wires between the relevant pins on each plug, not forgetting to slide the Mini DIN plug and the 3.5mm plug casings onto the cable before you start.

The diagram is colour coded and some method of telling the 3.5mm plugs apart is advised:
The Red connection is the Recieved Audio Signal.
The Green connection is the Transmit Audio Signal.
The Black connection is common ground.

After checking the soldering is good and there are no shorts or stray whiskers, replace the covers on the plugs and you’re ready to go. If the plug covers are the same colour you may want to label the leads accordingly.

The $4.00 Ham Radio Satellite Antenna

Posted in UHF Antenna, VHF Antennas Leave a comment

Article by Dave Tadlock

Simple, inexpensive and lots of fun! Here is an easy to make home brew antenna that can get you on the air working satellites or be built for use as a portable hand held antenna to extend the range of your HT.

It’s a dual band 2m/70 cm yagi antenna made with common materials and cost very little to make. Also, the antenna is fed with only one coaxial cable and does not use a duplexer.

For many decades radio amateurs have built antennas with wood and wire and have had great success using their home brew creations. This antenna was built in the same tradition and I am pleased to say that I made my first satellite contact using such an antenna.

To make this antenna I only needed to buy just a couple of items. Everything else I had on hand. I had to buy the wood for the boom, two 1-1/4″ long machine screws (although I bought 4 total) and a package of small wire nuts to place on the ends of the elements just for a bit of safety. Since I already had the screws, coax and connectors I spent less than $4.00 to make this antenna. I have well gotten my money’s worth out of it and have thoroughly enjoyed using it!

Construction & Materials
The antenna is made with a 1×2 pine/spruce furring strip for use as a boom and steel coat hangers for antenna elements. I used two trim screws to hold each parasitic element in place and stainless steel #6 machine screws with matching hardware for the driven elements.

Although the dimensions shown in the diagram are for use with steel coat hangers, you can experiment with other materials such as welding rods, stainless steel rods, etc.

The first step was to mark the boom for the elements using a tape measure and a carpenter’s square. Some planning ahead of time on paper allowed me to make room for an extra 70 cm director just past the last 2 meter director.

In starting from scratch I began by marking the 2 meter elements on the straightest 1 x 2 x 8′ long furring strip that I could find from the lumber yard. An equal spacing of 12-3/8″ is what I used for the 2 meter band so that I could add an extra director to give the antenna a little extra gain over a three element yagi and still have a fairly short antenna. That would make the 2 meter yagi antenna length just over three feet long not including the handle.

At first I marked five 2 meter elements on the boom instead of four but after thinking about it I decided that the extra mark was a good spot to cut off the boom. This made the wood boom 50-3/4″ (128.9 cm) long.

Starting from the director end of the boom mark the boom as follows. A mark at 1-1/4″, 13-5/8″, 26″, 38-3/8″, and 50-3/4″ (3.2 cm, 34.6 cm, 66 cm, 97.5 cm & 128.9 cm).

Use a carpenter’s square and mark a straight line across the boom at each mark. This will also help to square the coat hanger elements. Cut the boom off at the 50-3/4″ (128.9 cm) mark.

Also along each line, mark the center of the boom. Doing this will simplify adding the elements.

Flip the board over, and again starting with the director end of the boom, mark the 70 cm elements as follows. A mark at 1″, 6″, 11″, 16″, 21″, 26″ and 31″ (2.5 cm, 15.2 cm, 27.9 cm, 40.6 cm, 53.3 cm, 66 cm & 78.7 cm). After using the square to mark a line across the board, mark the center of the boom along each line.

The next step in preparing the boom is to mark the two holes for the feed-point. The feed-point is located at the 26″ (66 cm) mark on the boom. From the center mark on the boom, measure out 3/8″ (9.5 mm) in both directions and mark for drilling two holes. The spacing between the two marks should end up being 3/4″ (19 mm) apart. If you are using #6 hardware then drill two 9/64″ holes through the boom to attach the dipole elements.

Once the boom is marked and the holes are drilled the handle can then be shaped. At the end for the handle trim a little wood off the edges and/or use some sandpaper to smooth out and form a nice handle. Using 80 grit sandpaper will make short work of this.

Four coat hangers will have to be straightened to make the 2 meter elements. If you don’t want to straighten all of the coat hangers to make the 70 cm elements then you’ll need at least seven more coat hangers. Straighten the coat hangers and cut to the size shown in the diagram.

After cutting the directors and reflectors use a marking pen and mark the center of each element.

Cut the coat hanger wire for the driven elements a couple of inches longer than needed. Bend a loop in one end of the coat hanger wire large enough to wrap around the machine screw. Then measuring from the end of the loop to the end of the element, cut the dipole half to the length shown in the diagram. After making the four dipole halves use sandpaper or a file to remove any enamel or vinyl coating from the ends of the coat hanger elements where the leads are attached to the two halves of the dipole elements. The enamel coating on the parasitic elements does not need to be removed.

The dipole elements are mounted to the boom using #6 stainless steel hardware. A flat washer is first placed on a #6 x 32 x 1-1/4″ machine screw. Next goes on the 2 meter element, then the assembly is placed through the hole in the boom. On the opposite side the 70 cm element is placed over the machine screw, then the coaxial cable connection, followed by a flat washer, split lock washer and a nut.

If you cannot get the leads from the coaxial cable to stretch straight across the feed-point then use the shortest length possible. The leads can be connected using crimp on ring connectors or by wrapping the coaxial cable leads directly around the machine screws between two flat washers.

To mount the reflectors and directors to the boom I used some 3/4″ long self tapping trim screws. You may instead use any screws with a large head or a screw with a flat washer.

Line up the center mark on the parasitic elements with the center marks on the boom then fasten each element using two screws. You may pre-drill two partial holes 1/8″ from the line on the boom for the screws that hold the elements to the boom.

Add a plastic wire nut (twist on wire connector) to each end of all the elements.

The coaxial cable feeds the dipole driven elements at a 90 degree angle. The cable is run along the boom and brought back past the 2 meter reflector then secured with either a plastic tie or vinyl tape. The antenna will not work properly if the cable is allowed to hang down near any of the elements.

Tuning the Antenna

f the antenna is built as shown then it should not need much tuning if any. Tuning is of course by adjusting the lengths of the dipole elements making them either shorter or longer as needed. Check the antenna outdoors with an SWR meter or analyzer. If you notice a big problem then most likely it is the connection at the antenna feed-point or possibly the UHF connector (PL-259 or BNC connector).

Parts List

  • 1 each 1 x 2 x 8′ Pine/spruce furring strip.
  • 8 to 11 each Steel coat hangers.
  • 18 each Screws with large pan head (or screws and flat washers).
  • 25-pack Plastic wire nuts
  • 2 each #6 x 32 x 1-1/4″ Stainless steel machine screws.
  • 2 each #6 Stainless steel nuts.
  • 2 each #6 Stainless steel split lock washers.
  • 4 each #6 Stainless steel flat washers.
  • 2 each #6 Crimp on ring connectors.
  • 4 to 12 feet 50 ohm Coaxial cable with UHF or BNC connector.

Originally available by KG0ZZ at amateurradio.bz/4_dollar_satellite_antenna.html

DXSpotter by YO8SAW

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DX Spotter

I’ve been using DX Spotter by YO8SAW for several years.

The software, whose last built was 1.3.106 has now been dismissed and it is no more supported, but I still have installed on my PC. I Find it very essential and usefull to track latest spots with easy filtering features. Even if it now the best on it field, it is part of my toolbox.

Some of the main features:

  • Can connect to DX Spider and CC Cluster types of nodes
  • Can be minimized to the system tray and issue alerts for desired spots
  • Supports spot filtering based on DX band, callsign, country, active DXpeditions and worked/new countries
  • Supports three types of alerts: color change, sound alert, icon and balloon popup
  • Reconnects automatically if the connection is interrupted
  • Can be set to start automatically at Windows startup
  • Retrieves previous spots when connection is established
  • Send spots
  • View details about spotted callsigns, such as DXpedition information
  • Statistics – see how many spots have been processed, displayed or how many triggered an alert
  • CAT through Ham Radio Deluxe
    • Tune rig to spot frequency
    • Automatically detect split operation and set rig to split mode
    • Read rig frequency and use it when sending new spots
  • Automatic update of country database
  • It’s free!

https://drive.google.com/file/d/1l9ZV11vVRPXliVxb98uB5Q4NsWrZnHtl/view?usp=sharing

The Hentenna

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While browsing through my new copy of Simple and Fun Antennas for Hams one day in the Spring of 2003, I discovered an interesting looking design, with an interesting name – the Hentenna. The versions described were made of wire and cut for 6 meters, although it was suggested that the design could be used on other bands as well. l was looking for a 2 meter antenna to use for a second rig, and I thought I could adapt this design to 2 meters easily.



Some Hentenna Background

The Hentenna is a loop style antenna developed by Mr. Tadashi Okubo JH1FCZ, Mr. Someya, JE1DEU and others in Japan in the 70’s. It was first described in the US in a Feb 1982 QST article by Koji Sugihara, JJ1UMS and Shirow Kinashita, JF6DEA/KE1EO wrote about it in the ARRL Antenna Compendium Vol. 5. The gain is approx. equal to a 3 element tribander, JH1FCZ reported 5.1 dBd in 1972, yet it is small compared to a beam. 1

It looked like the type of antenna I wanted to try on 2 meters, small and with some gain. However it also looked like it was a little complex to build for what I had in mind.

DESIGN GOALS

I wanted it to be very easy to build, broad banded, and inexpensive, all good antenna attributes. It looked like a good candidate for a “plumbers delight” copper pipe project. A major consideration was to build it so it required no tuning of the feed point. So I scaled the dimensions for 2 meters and began to experiment.



Several prototypes were constructed of ½” copper water pipe and fittings purchased at the local hardware store, empirically arriving at the final dimensions below.

BUILDING IT

I recommend building the ½” copper pipe 2 meter Hentenna to these dimensions:

LEAD Technologies Inc. V1.01


Overall length = 40 inches Overall width = 12 ¾” Feed point = 7 3/16” to center line of T connector

You’ll need one 10′ piece of ½” copper pipe, four right angle elbows, two tee’s, and two end caps from the hardware store, total cost about $12. With the dimensions below you can just jam it all together, apply the heat and solder and it will work just fine. Or you can be a little fussier and adjust the dimensions perfectly before soldering. I used a tubing type cutter; it’s much easier and more precise than a hacksaw. Use a propane torch and apply the heat to the fittings so the solder wicks into the joint, sometimes referred to as “sweat soldering”.

Cut the pipe as follows:

2 pieces 31 13/16” for the long side pieces above the Tee’s 2 pieces 6 1/16” for the short side pieces below the Tee’s 2 pieces 11 1/2” for the two end pieces 2 pieces 5″ for the feed points.

Use lead/tin rosin core solder, and shine up the ends of the pipe and the insides of the fittings before soldering. I just laid it out on the garage floor to keep it flat and applied the heat. Be sure to wear safety goggles since concrete may “pop off” little pieces when overheated.

The caps go on the end of the feed tubes and the distance between them will be around 3/4″. Solder the coax braid to one cap and the inner conductor to the other; just tack them on, ugly style. There’s no tuning involved; the position of the tees takes care of it for you!

TO B OR NOT TO B (Balun that is)

A balun probably should be used to preserve the radiation pattern. But while experimenting with the prototypes, I discovered that any metallic objects inside, on or near the loop had major effects on the match. So I just stuck the coax on without a balun and it seems to work just fine, in a practical sense, at my QTH.


INSTALLING IT

Here are several things to consider when installing this antenna.

1. Use a non-conductor for the mast. I used 1 ¼” schedule 40 PVC pipe.

2. For the vertically polarized version, tape or cable tie the coax to the middle of the end piece nearest the feed point, NOT the mast. Let it hang over the end some then attach it to the mast below the antenna. Side mounting the antenna may be a better option.

3. Use non-conducting hardware to attach it to the mast. Steel or brass hardware has a really detrimental effect on the match. During the course of experimentation I’ve used nylon bolts, tape, cable ties, and wood dowels. Nylon boltsare by far the best mechanically and the easiest to install.

NOTE: The signal polarity is perpendicular to the axis of the feedline.

4. For a vertically polarized signal for repeater use, the 40″ dimension must be horizontal to the Earth. Typically this is the orientation for working FM mobile and repeaters. See the top picture at the right.

5. For a horizontally polarized signal for ssb or CW, the 40″ dimension must be vertical to the Earth. See the bottom picture at the right.

Article by WA0ITP originally available at http://www.wa0itp.com/hentenna.html

A Cost Effective Current-mode 1:1 Balun

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describes a 1:1 balun.

Introduction

A cost effective current-mode 1:1 balun can be constructed from a length of coax and a rod typically used for a broadcast antenna loop-stick, some electrical tape, cable ties, a length of PVC water-pipe and some connectors. The balun is formed by winding several turns of coax on the ferrite rod.

Principle

The operating principle is that the inner conductor and the inside of the braid act as two opposing bifilar windings with substantial inductance inserted in the outside of the braid. Differential current passes through such a transformer with little insertion loss as the opposing windings of the transformer mode effectively eliminate the winding inductance. If you want to run an unbalanced differential current through the transformer then substantial inductance will be present. Thus the current balun suppresses common-mode current. Since current flowing on the outside of the braid, is referenced to ground it must flow through the impedance resulting from the winding inductance formed by the outside of the braid and the core. This inductance will reduce the current if the impedance is high enough.

The same principle applies in the common-mode choke where two or more wires pass through a ferrite core. A typical example is seen in the ferrite chokes clamped on the monitor cable of computers.

High permeability cores can be used for current-mode baluns or common-mode chokes as there is not net magnetic field around the bifilar winding even though substantial currents are flowing.

Construction

A ferrite rod is easier to wind and cheaper than a toroid. At 160m I found that I needed 30 turns of RG58 U/C to ensure that I obtained equal, but opposite current, in each leg of an asymmetrically mounted dipole. To place 30 turns you will need to wind more than one layer across the core. The turns can be held by insulation tape and by applying two cable ties on the ends of the last layer. One end of the coax is terminated in a connector while the braid and centre conductor are split out and used as the balanced feed at the other end. You should use coax with adequate breakdown voltage to avoid damage when operating into mismatched loads.

Housing

The balun can be housed in PVC water pipe. Cut a section large enough to make two end pieces which can be flattened with the aid of the hot air from a hair-drier or heat-gun. The circular end-sections can be cut with tin-snips. I drilled a hole for a panel-mount connector in one end and used banana connectors for the balanced feed on the other end. The end sections should be inserted inside each end of the pipe and held in place with the PVC glue. I have found hot-melt glue adequate and easily removable. Extra protection is obtained for the ends if you leave an overhang by inserting the ends further into the pipe. My balun has survived several four-wheel-driving desert trips and is still intact and operating after five years.

Ralph Holland 1996

4m Slim Jim Antenna

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This 4m Slim Jim is cheap and easy to build yet it greatly out performs the more usual dipole due to its low angle of radiation. An SWR of 1:1 is obtainable across the 4m FM band with simple adjustment. The photograph and PDF diagram show the construction, however a brief description is also given. The spreaders are made from plastic knitting needles; two small holes are drilled about 10mm in from each end of the spreader to accommodate the aerial wire.

Suitable sized holes are then drilled in the fishing pole to mount the spreaders. The spreaders are then glued in position using a two part epoxy. The telescopic fishing pole sections are also glued to each other using epoxy. Once the glue is set the antenna wire is then threaded through the spreaders according to the diagram.

RG-58 is connected to the matching section but not soldered yet, just wrap the braid and inner around the aerial wire. Raise the antenna at least 3m above ground and clear of any objects (walls etc), apply a few Watts of RF and adjust the feed point to give a 1:1 SWR (slide the RG58 up and down the matching section). When the correct feed point has been found, solder the feeder connections.

I dabbed some yacht varnish over the connections and the first couple of inches of the coax to stop water ingress down the feeder.Mount the aerial as high as possible; due to the construction materials the wind resistance is very low so it should survive the worst of any storms. N.B. I did try making a 4m slim Jim out of 450 ohm ladder feeder from a diagram posted on the web, however i could not get it to work correctly (maybe the two conductors in the ladder line are too close for 4m use.

Barry Zarucki M0DGQ

Article originally available at http://www.m0dgq.co.uk