Yagis for 20m – 10m by Nathan A. Miller NW3Z
THE G5RV ANTENNA
The G5RV is a very popular antenna on the HF amateur band today. Despite it’s widespread use on the bands, there are some myths and misconceptions concerning the G5RV that seem to have a life of their own. Working with text from the ARRL “Antenna Compendium”, Volume 1, I would like to shed some light on this versatile antenna.
First, from Louis Varney, G5RV, of West Sussex, UK, here is some back- ground and insights into the G5RV. “The G5RV antenna, with its special feeder arrangement, is a multiband center-fed antenna capable of efficient operation on all HF bands from 3.5 to 28 MHz. Its dimensions are specifically designed so it can be installed in areas of limited space, but which can accommodate a resonably straight run of 102 ft for the flat-top.”
Louis further states that, “In contradistinction to multiband antennas in general, the full-sized G5RV antenna was NOT designed as a half-wave dipole on the lowest frequency of operation, but as a 3/2-wave center-fed long-wire antenna on 14 MHz, where the 34 ft open-wire matching section functions as a 1:1 impedance transformer. This enables the 75-ohm twin-lead, or 50/80-ohm coaxial cable feeder, to see a close impedance match on that band with a consequently low SWR on the feeder. However, on all the other HF bands, the function of this section is to act as a “make-up” section to accommodate that part of the standing wave (current and voltage components) which, on certain operating frequencies, cannot be completely accommodated on the flat- top (or inverted-V) radiating portion.
The design center frequency of the full-size version is 14.150 MHz, and the dimension of 102 ft is derived from the formula for long-wire antennas which is:
LENGTH (ft) = 492(n-.05)/f(MHz)
= (492 x 2.95)/14.15
= 102.57 ft (31.27 m)
where n = the number of half wavelengths of the wire (flat-top)
“Because the whole system will be brought to resonance by the use of a matching network in practice, the antenna is cut to 102 ft.”
As the antenna does not make use of traps or ferrite beads, the dipole portion becomes progressivily longer in electrical length with increasing frequency.
This effect confers certain advantages over a trap or ferrite-bead loaded dipole because, with increasing electrical length, the major lobes of the vertical component of the polar diagram tend to be lowered as the operating frequency is increased.
Thus, from 14 MHz up, most of the energy radiated in the vertical plane is at angles suitable for working DX.
Furtermore, the polar diagram changes with increasing frequency from a typical half-wave dipole pattern at 3.5 MHz and a two half-wave in-phase pattern at 7 and 10 MHz to that of a long-wire pattern at 14, 18, 21, 24 and 28 MHz.
Although the impedance match for 75-ohm twin-lead or 80-ohm coaxial cable at the base of the matching section is good on 14 MHz, and even the use of 50-ohm coaxial cable results in only about a 1.8:1 SWR on this band, the use of a suitable matching network is nessessary on all the other HF bands. This is because the antenna plus the matching section will present a REACTIVE load to the feeder on those bands.
Page 2 Thus, the use of the correct type of matching network is essential in order to ensure the maximum transfer of power to the antenna from a typical transceiver having a 50-ohm coaxial (unbalanced) output. this means unbalanced input to balanced output if twin-lead feed is used, or unbalanced to unbalanced if coaxial feeder is used.
A matching network is also employed to satisfy the stringent load conditions demanded by such modern equipment that has an automatic level control system. The system senses the SWR condition present at the solid state transmitter output stage to protect it from damage, which could be caused by a reactive load having an SWR of more than 2:1.”
In Part 2, I will discuss the theoretical operation of the G5RV antenna band-by-band…Keith, KE2DI
by Randall Thompson, K5ZD
Originally printed in the YCCC Scuttlebutt #119, October, 1995
I built my first Beverage this past year. It was so easy I can’t believe I lived without one. Here’s how to do it:
- Go to Home Depot (or other large hardware store) and buy a 500 ft roll #16 THHN or MTW wire. It’s available in every color you can imagine for about $15 per 500/ft roll.
- Go to local feed store and get some electric fence insulators. This cost me about $3.
- Call your favorite radio dealer and order an ICE Beverage matching box. Cost: about $40.
- Go to Radio Shack and get a 400-600 ohm resistor. I actually used four (4) 2K-Ohm, 2-Watt resistors in parallel!
- Roll out the wire in the desired direction. Mount the fence insulators to convenient trees (my Beverage is not perfectly straight) about 7-9 feet up. Connect one end of the wire to ground through the resistor. Connect the other end to the matching box. Connect coax. Enjoy!
I did follow the conventional wisdom of sloping the ends down. I used 4 foot ground rods at each end. I only have room for a 500 foot run. W3LPL has pointed out that 580′ might be a better length. It’s simple to solder some more wire on.
This antenna makes 80 and 160 enjoyable. Less than $50 to hear Europeans all summer on the LF bands seems like a good deal if you have the space!
When the antenna broke this summer, I used a split bolt connector to join the two pieces back together. You can find these for about $1 in the electrical aisle of the Home Depot (or hardware store). No solder required!
Note 2: You can also order an ICE matching-transformer from:
Industrial Communication Engineers, LTD.
Website:Industrial Communication Engineers, LTD.
About 1/2 the way down the above ICE webpage, you’ll see that ICE offers their Model 180A matching box for $39 (plus shipping).
The 180A has taps to select 50 or 75-Ohm coax feedlines, and taps to match 300/450/600 or 800-Ohm Beverage antenna loads.
The 180A also has dc blocking capacitors, and a gas-discharge lightning protection system.
The antenna was named for W4JRW who invented it and holds a patent on the basic principle and uses quarter wave stubs, which act as insulators at the frequency for which they are cut.
For example, the 6’11” stub (quarter wave times the velocity factor 0.8 of the feed line used) blocks RF for 28 mhz from reaching the rest of the antenna.
In the example shown in the diagram, tubular foam filled 300 ohm feed line was used, which has a VF of 0.8. Other feedlines may be used, for example, slotted ribbon and the length of the stubs worked out using the correct velocity factor
Building the lattin antenna
This will require some forethought and planning.
Avoid cutting the continuous top wire, which supports the whole system.
I wonder if it might be an idea to use a suitable polypropylene line to support the wire, which may be subject to breaks, especially at the solder points?
A suitable centre piece may be constructed and constructors may want to include a balun at the centre of this balanced antenna, which is fed with unbalanced line (coax).
A version of the Lattin could be designed for all bands, including the WARC bands – get snipping!
From a functional standpoint, the TS-2000 is two transceivers in one box. It is an HF transceiver that incorporates many of the features of the TS-570 and the TS-870 (while adding a few of its own), as well as a variation of the multifaceted TM-D700A. Its delineation is not quite that simplistic, however. The TS-2000 is a complex matrix of transceiver components that allows it to operate as two independent radios in one box. It can allow the user to operate on HF and either VHF or UHF simultaneously, which is something that recent offerings in the arena of 160m to 70cm all-mode radios cannot currently do. It also has the capability of operating as a full-duplex VHF and UHF transceiver for the purpose of working the OSCARs, or similar applications. What is truly unique is the on-board packet TNC that was borrowed from the TH-D7G, which has the ability to transfer data over the air at selectable rates of 1200 and 9600 bps. Tables 1 through 4 show the general specifications of the TS-2000.
Kenwood has listed a number of distinctive features in their promotional literature. However, it is my opinion that they have overlooked an important feature that definitely adds value from a consumer perspective: ruggedness. The box that houses the electronics is an aluminum casting. The fasteners are of a good quality, and the machine work is very high-caliber. When viewed in comparison to some radios that come in a thin-walled, spot-welded enclosure, the TS-2000 has extraordinary structural integrity.
The front panel is a stylistic ergonomic design. I found that everything was easily accessed, and functionally well organized. I appreciated the fact that the buttons were not too small for me to control, and that the markings were easy for me to read. Of course, the most important feature of any HF radio is how good the tuning knob feels. This one has all the feel of something superbly machined. There is no wobble that I could detect, and the motion is smooth and easy. The tuning rate is front panel menu controllable at rates of 500 and 1000 Hz-per-revolution. This tuning rate may be reduced by a factor of 10 by depressing the FINE button on the front panel. Did I mention that the tuning knob has a nice feel to it?
The rear panel is also well designed. There is a minimum of clutter. The RF connectors are placed in such a manner as to minimize coax runs from their respective circuit boards. There are two selectable HF ports, as well as a handy RCA jack input for the ham who has a beverage or two that he or she would like to employ. The really nice thing is that the RCA jack is menu-selectable. There is no need to pull the cover off and manipulate a tiny microswitch.
There is a nice feature brought forward from (if I remember correctly) the TS-570. There are two separate CW interface ports. One is the standard stereo jack for the keyer paddles. The other is a direct keying jack that functions in parallel with the keyer. This is really nice when using your favorite contest logging software. You can operate the keying function of the logging software and the internal paddle simultaneously. And I might as well mention it now: There is also a menu item that allows the keyer to override the direct keying port if the operator so desires.
As I stated earlier, the TS-2000 operates by the carefully planned manipulation of a matrix of common circuitry, as well as a cadre of dedicated components. For example, the receiver front ends and transmitter power amplifiers operate as common assets for the two receivers. There are, in essence, two independent receivers. One is referred to as the MAIN receiver, and the other the SUB receiver. The MAIN receiver’s frequency is displayed in the prominent central position above the main tuning knob, while the SUB receiver is displayed in a half-sized font to the right. There is an exception to this, of course, as in the case of satellite operation, where the A and B bands may be switched back and forth, and when the MAIN unit is operating in SPLIT mode. It should be noted that even when something other than the SUB receiver’s frequency is being displayed in the right window, it continues to operate normally. This is to say that the SUB function is not suspended when the MAIN section is operating SPLIT.
The audio detection of the various operating modes (other than FM in the SUB receiver) is accomplished in the final IF using DSP. The ability to set the center frequency and width of the IF DSP filter on the fly means that there are no expensive crystal or mechanical filters to purchase. However, since both receivers cannot use DSP detection simultaneously, the SUB receiver only operates on FM and AM. There are a number of audio DSP algorithms available to the MAIN receiver (which will be discussed in a bit). DSP is used in the transmit path as well. This allows the user to program special RX and TX audio characteristics from the main menu. The frequency response of the RX and TX may be tailored to the specific tastes of the operator, or, if you are a kind-hearted soul, to the tastes of the listener at the other end.
There are three RF power amplifiers (four with the 1296 module installed). All three amplifiers run class AB, allowing linear operation on SSB and AM. They are quite rugged as well. The 2m and HF/6m amps shown in Fig. 1 are on the large board on the underside of the radio, and both are capable of 100 watts. The 440 amp is on its own board, and puts out 50 watts. Here is a case where the aluminum casting design comes in quite handy. It functions well as a heatsink, and is cooled by a very quiet fan that is controlled by temperature sensors.
I almost forgot to mention the superb automatic tuner that is included with the TS-2000. This is one of the better ones that I have used so far. While most of the automated tuners intended to drive coaxial antenna circuits are limited to around a 3:1 VSWR, this one is not. It has successfully tuned circuits with an indicated VSWR greater than 6:1. It has its limitations, though, and it will tell you up front. The limit appears to be in the 6:1 range, and will refuse to tune above that. It will also send you a polite “SWR” in Morse code to let you know that the tuning limit has been exceeded (a warning to you “slow-coders” out there – it’s faster than 5 wpm). I was very pleasantly surprised at the speed and range of this tuner.
Back to DSP
Part of the genius of this radio is the way in which it does signal processing. In this radio there is not one, but two independent DSP chips (see Photo J). They both run at a clock speed of 100 MHz, and actually communicate with each other when performing their individual tasks.
I have already mentioned that the operator may select preset frequency contouring for receive and transmit audio from the menu. What I did not mention is that the soon-to-be-released ARCP2000 radio control software will provide the ability to personalize one of those menu items. I hope to have a separate feature on that software in the near future. (I wrote this review during and after the big earthquake here in Seattle, so I didn’t get a chance to review the software.)
As for the IF filters, there are default settings that come up when a given mode is selected for the first time. You may then select the center and width of the filter for that mode according to your own tastes, and the radio will remember that setting from then on. You don’t need to re-enter these settings every time you turn the radio on. For your convenience, the center frequency and filter width are set using two vernier knobs in the lower left-hand corner of the front panel. The front panel shows both analog and alphanumeric displays of the filter settings.
Another aspect of the power of this formidable DSP engine is the ability to reduce broadband noise and coherent interference. The MAIN receiver enjoys two types of noise reduction filters. The first, called NR1, is a linear adaptive filter that is similar to that found in many modern transceivers. What is noteworthy is that the threshold of NR1 mode is front panel selectable, or may be left in the AUTO mode. I have played with this a bunch, and found that leaving it in the AUTO mode works fine for me, especially when working SSB and FM. I should mention that the SUB receiver can employ this filter as well, but only this one.
The others are not available
The second mode is NR2, which is a correlation algorithm that has a variable duration of 2 to 20 msec. This is an excellent filter for CW use, but takes a little time to get used to. I have found that a setting of 8 msec is ideal for the type of CW operating that I enjoy, which ranges in speeds from about 18 to 30 wpm. It also took me a little time to get used to the mechanical artifact sound of the background noise. It sounds more like a babbling brook than the soft hiss of a Collins 73S3. Not to worry, though. It will sound normal to you in no time.
The TS-2000 has three, count them, THREE digital mechanisms for getting rid of those pesky 40m AM carriers and careless tuner-uppers. The first is an automatic notch filter with a variable threshold that can be controlled from the front panel. This is useful if there is some distortion or other modulation characteristics present on the unwanted carrier. There is also a beat-canceler, which leaves the IF passband alone, and removes the note from the audio. It is an adaptive filter that can handle more than one beat note, and will automatically shift frequency in synch with those tones that drift about. If you’re like me and have been frustrated by the efficiency of these types of unwanted tone removers, especially when they work so well at also canceling the station you want to listen to on CW, then fret no more. The TS-2000 also has a MANUAL beat canceler. This is great! I finally have the ability to notch out that nudnik who likes to tune up on me when I am in QSO on CW. What a blessing, and it works very well. And what’s better is that it doesn’t introduce a lot of distortion to the passband like the analog notch filters do.
There are some additional features associated with the DSP engine in this radio, like the vernier control of the AGC, that you can discover on your own.
This radio is so feature-rich that if I were to describe all of them, this article would cease to be an operator’s review, and become a rewrite of the operator’s manual. But there are a few that should be mentioned, the first of which is the memory and configuration management capability of the TS-2000. There are 300 memory channels available, which are easily programmable from the front panel. Let me tell you that this is a real blessing. These channels may also be programmed with an alphanumeric name tag (also from the front panel) that facilitates easy recall of just why-in-the-heck I saved each frequency and mode. These memory channels may also be grouped and scanned in 10 subgroups. This is quite handy for segregating the frequent- from little-used channels in the scanning process.
If you are in a hurry, and don’t want to fiddle with programming a specific channel, there is also the QUICK MEMORY function that may be used in the VFO mode, and provides convenient storage of 10 channels for quick retrieval. It has been very handy for me in contests, and in pileup management. It stores things like frequency, RIT settings, operating mode (CW, USB, etc.), and interference rejection modes.
There are also two generic memories for storing the basic configuration of the radio. If you operate the radio as both a mobile and base station, the entire configuration of the radio (all of the menu-controllable items) may be stored for those two operating environments. Another use would be simply to differentiate between primarily CW or SSB operation, or between contesting and rag-chewing. You get to choose, and it’s all commanded by a couple of front panel keystrokes. One of the things that I feel is commendable concerning Kenwood is that fact that they DON’T charge their customers for memory control software, and make it available in downloadable form on the Internet. If you go to the http://www.kenwood.net web page, you can download a program called MCP2000 that allows for simple programming, storage, and retrieval of these formidable memory functions. Figs. 1, 2, and 3 are screen captures of MCP2000. Fig. 2 shows an expanded control panel that allows detailed programming of each memory channel shown in Fig. 1. Fig. 3 shows how each menu memory setting may be programmed without having to go to a separate panel for providing the programming detail. This software is a must. I highly recommend it, and it’s FREE.
Packet terminal node controller
One area where Kenwood has been out front in the development of technology for us radio amateurs is in the inclusion of packet terminal node controllers (TNCs) in their transceivers. They started with a handheld (TH-D7G), and quickly included their flagship dual-band mobile (TM-D700A). They have closed the product line loop with the TS-2000. The TS-2000 service manual states that the TNC is the same one developed for the TH-D7 by Tasco. It appears to me, at least, that this is the same product that has gone into the TH-D7 and TM-D700, and the Alnico DR135TP. Only minor variations in the command set for the TNC in each radio exist. Kenwood’s statement about the derivation of the TS-2000’s TNC appears to be right on the mark. Although there is no obvious way to connect a GPS receiver to the TS-2000, the GPS commands found in the TH-D7 instruction set can be observed when sending the DISP command to the TNC in the TS-2000. I am hoping that a future “blue-wire” mod will come forth from either Kenwood or the general amateur community that will allow GPS interface via one of the unused ACC ports (hint, hint).
The TNC itself is a modest performer, and has been well chronicled in other reviews that feature the radios mentioned above that also have it on board. Interface between the TNC and a PC or laptop is accomplished via a DB-9 serial port on the back of the radio. No high-priced level converters are required. In the case of the TS-2000, the TNC’s function is enhanced by its ability to access the DSP chipset to provide some prefiltering when operating AFSK at 1200 bps. At 9600 bps, the TNC has a direct analog route to the outside world via the FM modulator and discriminator. KISS mode for TCP/IP is included, and I can vouch for the fact that it seems to work quite well at both baud rates. I had an opportunity to test it on the local TCP/IP network that is run by Puget Sound’s WetNet Experimenters Group.
Although the APRS functionality found in the TM-D700 is not included in the TS-2000, there is yet one very unique and useful internal function that it can perform. It is called the Packet Cluster Tune (PCT) function. This is really slick, and it works like this. The user sets the SUB RX to the local DX packet cluster frequency, sets the SUB RX as the data band, and turns on the PCT function. When a packet cluster DX spot announcement is received, the frequency, callsign and other related data appear in the SUB window. The information is also automatically written to the QUICK MEMO pad for later retrieval.
The PCT function may be configured by front panel menu commands to do the following. First, it will provide an announcement to the operator in the form of a beep, a CW recital of the callsign, or (if you have the optional VS-3 voice synthesizer unit installed) a voice announcement of the same. That’s not all. You can also set this function to automatically set the radio to the frequency from the DX spot that is displayed in the SUB window. If that sounds like a potential inconvenience, the radio may also be configured to only change frequency when commanded to do so by depressing the SET button. I showed that function to some of the members of the Redmond Top Key Contest Club, and they got a big kick out of it. I have to admit that I have used that function quite a few times myself. It really helps to keep the traffic density down on the packet cluster channels, as this is a passive feature (meaning it doesn’t require any transmitting). With the dual radio personality of the TS-2000, the monitoring of the packet cluster channel is uninterrupted while carrying on a QSO on HF.
There is much more that I could write about the features and performance of this radio. I intend to write a separate review on the anticipated ARCP2000 remote control software that is soon to be released with their introduction of the “box” version of the radio. You heard me right. By the time you read this, Kenwood will have released the TS-B2000, which is a blank-faced version of the TS-2000 that may be controlled by the ARCP2000 software on a PC or laptop, and via the R2000 remote control head (borrowed from the TM-D700). In that review, I will also talk a bit about the following features:
- Sky Command II
- Crossband repeat
- Remote control
- Microphone control
- Direct FSK operation
- Satellite operation
- User-defined digital filtering of RX and TX audio
Until then, I encourage you to have a close look at this fine radio for yourself. My neighbor did, and went in and bought one for himself, and he is very critical of radios – their performance and features, that is. That in and of itself is a testimony to the impact that this rig can have on hams who come in contact with it.
The bottom line is that Kenwood has not just produced an excellent radio. What they have done is to further the state-of-the-art in affordable amateur equipment. And I will stand by my assertion that this radio provides a very high level of features and performance for its price, which, by the way, is currently about $2,270 over the counter. When I first heard about its impending introduction, I was certain that the price would easily exceed three kilobucks. Having said that, I believe that the price including the optional accessories such as the VS-3 voice synthesizer, the DRU-3A digital recording unit, the R2000 remote kit, and the soon-to-be-released UT-20 1296 module, will drive the total cost over that mark. However, these are optional items that can be purchased later based on a value-added decision that concerns your own operating needs and desires. The radio as it stands today is quite impressive.
Congratulations and thanks to Kenwood for maintaining their vision, engineering and manufacturing skills, as well as the financial commitment required to continue to provide innovative products to the amateur radio marketplace.
Article originally available here http://elkel.ca/kenwood_a/ts-2000_review!.htm by d r.o l s e n.r i ck
All rights of their respective owners.
They say if it didn’t blow down it was not big enough, this one was big enough and it did blow, not off but up and over the top of the tower like an umbrella one very windy day in January 1974. I was at work and the XYL called and said the “thing” blew off the top of the tower- WOW, I imagined it in somebody’s living room. In fact it did not blow off it blew over the top, broke in two and slid down about ten feet an hung on the safety cable. When I originally built the thing I rigged this safety line of 3/8 inch aircraft cable from the tower to the four inch diameter boom just in case. I remember I used to laugh when I told people about the safety cable never thinking it would actually blow off the tower. The storm was really a bad one, very high winds with ice covering the boom and elements. In fact a drive in movie screen blew over just down the street from me. It was a very sad occasion, I was the one sad and the neighbors were glad. The obliging neighbors called the building inspector and he was waiting for me when I got down from the top after attaching a rope, disconnecting safety cable and cutting the coax cable and letting it down, smoothly. The inspector notified me that one of my neighbors said that it had blown down three times already this year. This beam worked very well for me for several years.
I apologies for the quality of the photo. Its the only one I have. What you see is what you get. Very narrow beam pattern, that’s not QSB man that’s my beam swinging in the wind.
The omega match was motorized because it was so far out on the boom.
Wound on a 3 foot length of PVC pipe, the long loopstick antenna was an experiment to try to improve AM radio reception without using a long wire or ground. It works fairly well and greatly improved reception of a weak station 130 miles away. A longer rod antenna will probably work better if space allows. The number of turns of wire needed for the loopstick can be worked out from the single layer, air core inductance formula:
Inductance = (radius^2 * turns^2) / ((9*radius)+(10*length))
where dimensions are in inches and inductance is in microhenrys.
The inductance should be about 230 microhenrys to operate with a standard AM radio tuning capacitor (33-330 pF). The 3 foot PVC pipe is wound with approximately 500 evenly spaced turns of #24 copper wire which forms an inductor of about 170 microhenrys, but I ended up with a little more (213uH) because the winding spacing wasn’t exactly even. A secondary coil of about 50 turns is wound along the length of the pipe on top of the primary and then connected to 4 turns of wire wound directly around the radio. The windings around the radio are orientated so that the radio’s internal antenna rod passes through the external windings. A better method of coupling would be to wind a few turns directly around the internal rod antenna inside the radio itself, but you would have to open the radio to do that. In operation, the antenna should be horizontal to the ground and at right angles to the direction of the radio station of interest. Tune the radio to a weak station so you can hear a definite amount of noise, and then tune the antenna capacitor and rotate the antenna for the best response. The antenna should also be located away from lamp dimmers, computer monitors and other devices that cause electrical interference.
Low profile operating — SMALL LOOP ANTENNAS
Magnetic loops are very effective small antennas. these 3 to 3 1/2+ foot antennas perform close to and in some cases (low mounting heights for one) better than even a small beam. The reason is that a magnetic field is much more concentrated than an electrical one, for example a small horizontal loop at 17 feet performs better (lower radiation angle) than a full size dipole at 35 feet, in fact better for DX than a beam at that height because of the much lower takeoff angle. If you place a beam 1 foot off the ground it will only radiate straight up while a vertical loop will still work DX stations quite well.
This smaller more intense magnetic field also has the advantage of greatly reducing TVI – RFI potential if the loop is more than 15 feet or so away from TV antennas, electronics ect. Another advantage of this magnetic field is the very low background noise heard on the loop because most man made noise is electrical fields. For example if you lived next to a Shopping Mall the loop would not hear all that lighting and power transformers. Also reducing interference is the loops “Hi Q” which means that it receives & transmits on a narrow band range compared to the “full size” antennas.
This effect is very pronounced on the lower bands that the loop will work. I have built several loops and along the way have learned quite a bit about these “little wonders”. Usually I use the parts from one to make the next but my “ugly loop” is one that I keep in service.
While looking thru old QST’s I read up on the Cushcraft R-3 vertical antenna. I made a mental note that the remote controlled variable capacitor looked like it would make an ideal loop component. I found this one on E-Bay for about $25, this was nice considering the most expensive part of the loop is the High Voltage Variable Capacitor and motor drive and controller.The only drawback to this capacitor was the metal mounting (which increases the stray capacitance) and the fact that it was a single stator type not the less lossy butterfly type.
To reduce the losses I directly connected (metal braid) the shaft on the capacitor to the loop itself and polished the capacitor plates to increase power handling. The range of this setup was around 30 thru 180 pf and it will handle 100 watts. I could of reduced the minimum value of the capacitor by eliminating the metal half (future project ?) of the mounting but instead used it as part of the loop. I wanted to test some of the current theories of loop design and this setup was ideal. I built up a PVC frame (easy change of dimensions) and used of all things roof flashing (very thin aluminum) for the loop itself. This cheap material made dimension testing easy, the final size of the loop is 42″ in diameter by 1.6″ wide. One thing that the loop programs (like mloop32) do not take into account is the fact that if the material is too wide it starts acting like a capacitor not just an inductor. This loop is completely assembled with mechanical (nuts & bolts) connections.
This is a big “no no” in loop construction which I used to make testing of different configurations easy and to test that “all connections must be welded” theory. On the air tests, field effects and bandwidth checks have shown that this loop is very efficient until you tune to 40 meters (around 7%), not that surprising since that band is below the theoretical range of a 42″ loop. It sure is fun to stretch it that far and to make 40 meter contacts on such a small antenna. The loop is feed with an 8″ diameter faraday loop not on the centerline (null) of the loop, this is because the capacitance circuit break is on the side of the capacitor assembly.
The pictures show how the control wires are routed “off center” but this is in the null of this loop, if you don’t do this the wires will couple with the magnetic field and detune the antenna. I found very little performance change with the feed loop off center so I left it there for mechanical simplicity. I learned the hard way about the high voltage’s (8,000 V +) in the loop when insulators in the path of RF flow started burning thru (bad) until redesigned. The R-3 controller does a good job of tuning the loop 19 thru 7 MHZ, its meter is a position feedback that lets you know where the capacitor is tuned. A loop like a commercial version like the MFJ “HI Q” Loop (below) or a home brew may have the unique properties that may help you out in your situation.
NOHC Article originally available at www.geocities.com/n0hc
The MFJ-940 VERSA TUNER II is a useful little antenna tuner for the HF-bands. However it suffers from a minor design error, which can be easily rectified.
As other antenna tuners may show the same kind of “weakness”, the modification described here can be used to improve other types.
The connection between the components in the tuner – coax connectors, switch, coils and variable capacitors are made of rather long pieces of tinned copper wire.
These wires act as small selfinductances. In normal operation stray inductances are absorbed by the tuning components, however when the tuner is switched into “bypass” mode, it affects the 50 ohm match between antenna and transmitter. This is worst on the highest frequencies.
You can check an antenna tuner by measuring the VSWR through of the tuner, when it is terminated by a good 50 ohm load. In my case I could measure a VSWR on 30MHz of 1.8:1 – not very good for a simple bypass!
The solution is to compensate the series L from the wires with parallel C’s. By doing this in the upper end of the frequency range a broadband match can be obtained.
In the MFJ-940 five 15pF capacitors are used. Four from each of the four coax centerpins to ground and one from the switch rotor to ground. This completely tunes out the reactance of the internal wirering. – see modified diagram.
The capacitors must be able to handle high voltages – I’m using 500V ceramic tubular types and have no problems at the 100W level.
This modification improves the return loss at 30 MHz from -12dB to -30dB and at the same time reduces through loss (attenuation) from 0.3dB to only 0.1dB.
By OZ2OE originally at hjem.get2net.dk/ole_nykjaer/oz2oe