Just found this two old antenna drawings for a four and a seven elements yagi antenna by WB7PMP.
Looks interesting plans but have not yet attempted to build them.
The Double Bazooka Antenna is simple to build broadband dipole antenna
Someone consider the double bazooka antenna to offer a 3dB improvement over a common dipole antenna. The only caveat is that an antenna tuner must be used.
The design is easy to replicate and uses common materials. It is mostly made of RG58U coax, #16 automotive wire, rope suspension, and any plastic that can be cut into insulators.
This antenna can been used on 80 through 10 meters by cutting it to the center frequency.
Today during our Club annual “Filetto Day” , a parody event of the famous ARRL Field Day, I’ve been able to compare my Buddipole Antenna, with a home made 20-40 Wire Dipole antenna.
I want to share here my experience that should be considered just a simple personal test.
The Wire Dipole won the comparison. Signals were much more stronger compared to the ones received on the Buddipole tuned on the 20 meters band. QRM sounded higher on Buddipole.
The comparison has been conducted, in a mountain top, at 1500 meters a.s.l., by switching the antenna to a Yaesu FT-857D, while listining to stations calling and listening to several QSOs on 20 meters band only.
The Buddipole used is a commercia multiband antenna, self supporting thanks to the original tripod, and has a small dimensions.
The Wire dipole is easy to home made, is not multiband as the buddipole and requires a larger area to be setup. I used a fiberglass fish pole as supporting mast.
I decided to publish this website in order to pass on some insights about this antenna that I’ve garnered through extensive experimentation. A warning though, some of the combined design aspects of the antenna may be unique and unorthodox, a think out of the box antenna design. Note! I do not have a B.S. or M.S. in EE, which makes me a true “amateur” amateur radio operator not a “professional” amateur radio operator, so some of my antenna theory explanations may be incorrect.
I have gained DXCC on 160 meters with 145 confirmed as of October 1, 2005, using this antenna design, in approximately 4 years. 25% of the DXCC contacts were via CW and 75% via phone. 103 DXCC contacts were with 140 watts PEP, 17 with 800 watts PEP.
What I have done is to simply identify the basic inherent weaknesses of the average 1/4 acre city lot 1/4 wave inverted L with a 30-50 foot vertical section and a few 1/4-1/8 wavelength radials and have devised methods to overcome these weaknesses. This antenna design is not meant to be a rival to a 4 square vertical array but can compete with a full 1/4 wave vertical with 60 1/4 wave ground mounted or buried radial wires, if designed correctly.
First of all let me say that I’m not a professional broadcast radio engineer. My background is in the sciences, i.e., climatology, meteorology, oceanography and space plasma physics. I’m just a true amateur experimenter, antenna modeler and voracious reader of every book on antenna theory and design that I have been able to get my hands on, some 50 years old. As an avid antenna experimenter, I have spent approximately 10 years in the field experimenting with this antenna design and it’s variants (1/4, 3/8, 1/2 wave L/Tee Vertical), between 1993 and 2002 and have also done extensive modeling using EZNEC 5.0. My good friend K4TR Joe Dube of Brooksville, FL has also been experimenting with this design between 1997-2002.
Along the way I have come to the conclusion that some of present day antenna theory is just that theory, in general concepts not totally proven by controlled scientific experiment and/or overemphasized and therefore to be taken with a grain of salt in some instances! I have also concluded that allot of sound basic antenna theory and design has been lost to time and/or watered down, to the point that many Amateur Radio Operators are now grossly miss informed about the basics.
A Broadcast Radio Engineer may come along and poke holes in some of the following antenna theory and concepts, as I’ve explained them. I have been told repeatedly that I know nothing about antenna’s. Even if the theory of operation of the linear loaded voltage fed Tee Vertical as I explain it is flawed in any way, one thing that can’t be disputed is that the antenna is a proven performer.
The average city lot backyard 1/4 wave inverted L suffers from several inherent weaknesses to include high vertically polarized local noise pickup, absorption and pattern distortion of radiated signal due to surrounding ground clutter, high capacitive coupling signal loss between the antenna and the average backyard poorly conducting soil conditions, to include an inferior ground radial system and low radiation resistance, a measure of antenna efficiency, due to the typically short (30-50 feet) vertical radiating element section of a 1/4 wave inverted L.
The proper definition of radiation resistance is; the total power radiated as an electromagnetic radiation, divided by the square of the current at some defined point in the system. To put it in simplest terms, a measure of antenna transmitted signal efficiency.
A 1/4 wave radiator will focus it’s current field in the ground immediately around it’s feed point and as you extend the vertical section past 1/4 wave, the highest current point moves up the vertical section and outward on and in the ground surface. With much effort the near field transmitted signal losses can be reduced to a point that you improve antenna efficiency to maybe around 50-75%. However the average backyard 1/4 acre location makes it difficult to overcome signal losses in the mid field (200-500 feet) on 160 meters and signal losses in the far field (between 500 and 1000 feet for a 1/4 wave vertical and around 52,000 feet for a true 1/2 wave L/Tee Vertical) (Fresnel Zone) is out of reach for all of us.
The linear loaded voltage fed Tee Vertical antenna places the highest current point at or near the top of the support structure gaining the following advantages. The elevated highest current point of the antenna is above allot of the local vertically polarized noise field. At my QTH my 1/4 wave inverted L noise level was always S9 to +5 over. With my linear loaded voltage fed Tee Vertical, the noise level has been reduced to S1-2. Of course the actual amount of noise reduction will vary from QTH to QTH. Another advantage of elevating the highest current point is, reduced radiated signal absorption and pattern distortion, away from omni-directional. In a sense you can say that the highest current point is getting a better omni-directional look at the radio horizon. Actually though it’s best to have the highest current point say approximately 25-50% below the flat top to assure vertical polarization. Remember the linear loaded voltage fed Tee Vertical is a DX antenna with a null overhead and therefore little high angle radiation close in for rag chewing.
Another advantage of elevating the highest current point, per the ARRL Antenna Handbook edition #15, is the reduction of capacitively coupled transmitted signal loss between antenna and lossy ground. Logic dictates that placing distance between the highest current point of the antenna and lossy ground possibly reduces capacitive coupling losses in the near field. Of course though due to the wavelength involved, the reduction in loss will be not the same on 160 meters versus say 40-10 meters.
The agreed upon standard for the number of ground radials for least near field loss for a 1/4 vertical antenna is 120 1/2 waves but you see a diminishing point of return after approximately 24 1/8 wave or 16 1/4 wave radials and there is virtually no difference (approximately 0.07 db) between 50-60 1/4 waves and 110-120 1/2 waves. Also basically your ground radials need not be any longer then the length of the vertical section of your antenna. An alternative to ground radials is an elevated counterpoise, which will be covered further into the text.
Radiation resistance, which as stated previously is a measure of transmitting antenna efficiency, is obviously a very important but difficult to accurately measure variable, basically the higher the value the better. Once again the proper definition of radiation resistance is; the total power radiated as an electromagnetic radiation, divided by the square of the current at some defined point in the system.
A 1/4 wave inverted L with a vertical section of 50 feet, will have a very low radiation resistance, around 15 ohms (very inefficient), increasing to near a theoretical 36 ohms as you approach a vertical length of 1/4 wave. Take this 15 ohms of radiation resistance and couple it with a poor ground radial system say 50% efficiency at best and you still have a very inefficient signal radiator. By the way, if you feed a 1/4 wave vertical at one end then the feed point impedance becomes the same as radiation resistance. However bend the radiator like an inverted L and the two are no longer the same.
Another method used to improve radiation resistance is to employ a capacity hat top loading system. A traditional capacity hat in the form of at least three flat top or sloping wires spaced approximately 120 deg apart and tied together at their ends in a ring shape, is employed to make up for the missing part of a short vertical antenna. Basically each top hat wire length should be at least the same length as the missing part of the vertical. On 160 meters an 1/8 wave vertical with an approximate length of 64 feet should have a three top hat wire lengths of 64 feet. This method of top hat loading increases the radiation resistance of the short vertical, (much like a linear load which is normally placed at the bottom of the vertical) only even better and moves the highest current point up the vertical portion of the antenna. The highest current point on my voltage fed Tee Vertical is elevated approximately 60 feet above ground using this method. If at all possible mount the top loading wires as high on the ends as in the center because dropping the wire ends effectively shortens the vertical section of the antenna. At my QTH the best I can do is to get the ends of the top loading wires 70 feet above ground versus 80 feet at center.
There are several methods that can be employed to reduce near field ground losses and in some cases increase radiation resistance and henceforth transmitting antenna efficiency, excluding the laying out of dozens of ground radials. One is to place 3-4 ground radial wires into an above ground counterpoise system (for a typical backyard 1/4 wave inverted L antenna). Four 1/4 wave wires approximately 15-30 feet off the ground, can rival 120 1/2 wave radials on the ground, as far as connection losses (which can 10-40 db) and lowest takeoff angle but not necessarily concerning near field ground losses (which has been measured at approximately up to approximately 5 db by W8JI). Unfortunately though raising radial wires into an elevated counterpoise also effectively shortens the vertical section of the antenna, similar to top loading wires.
It would seem logical that the linear loaded voltage fed Tee Vertical antenna would require a less extensive ground radial or counterpoise system in the near field at the antenna feed point, as the antenna is much longer than a 1/4 wave and has the highest current point elevated well above the ground surface and also well away from the feed point on the ground surface. However there will still be “some” losses in this nearer field but just further out from the antenna feed point. The problem though is that it’s difficult to get enough wire in the ground to overcome the ground losses at the further distance, on a typical 1/4 acre suburban lot.
Another method is to lengthen the transmitting antenna. As mentioned earlier, in theory the radiation resistance measured at the end feed point of a 50 foot vertical section inverted L is around 15 ohms, a linear loaded 1/4 wave L is near 16 ohms, a full 1/4 wave vertical is 36 ohms, a full 3/8 wave vertical is 300 ohms and a full 1/2 wave vertical is 1000+ ohms, a very efficient figure indeed! Basically as you lengthen the radiating element the radiation resistance increases and it decreases as you shorten it, it also varies with the diameter of the radiator. Antenna input impedance varies according to where you feed it. The added length of the antenna can be placed in a linear load configuration.
As mentioned earlier, the average backyard 1/4 acre location makes it difficult to overcome signal losses further out in the near field (maximum concentrated ground current is approximately 3/8 wave length out from the feed point with a 1/2 wave vertical) on 160 meters. Reducing signal losses in the far field at the first reflection point (Fresnel Zone), which is around 52,000 feet for a true 1/2 wave vertical, is completely out of reach for all of us.
To recap the various methods of improving antenna efficiency and performance; lengthen the antenna past 1/4 wave using a linear load, add a capacity hat in the form of a three wire flat top, elevate the highest current point, use a radial counterpoise system.
So that’s it in a nutshell, the linear loaded voltage fed Tee Vertical can overcome most all the inherent weaknesses of the “average 1/4 acre city lot” backyard 1/4 wave inverted L.
Now let’s discuss the benefits of using the linear loaded voltage fed Tee Vertical on 80 through 10 meters, as a multi-band antenna. As the length of a transmitting antenna exceeds a full wave on the operating frequency interesting things begin to happen. Gain starts to increase and the radiation moves inward towards the axis of the transmitting wire, versus the 90 degree broadside you see on a half wave dipole at 1/2 wave height. As the transmitting antenna continues to become even longer in comparison to the operating frequency, multiple lobes of radiation form on the wire in response to the numerous highest current points that exist.
Using the Tee Vertical antenna as a multi-band antenna on 80-10 I’ve had very good results. On 17 meters I have worked 100 DXCC countries with minimal time and effort.
It is strongly recommended that a high voltage handling parallel network matching device be used to load up the linear loaded voltage fed Tee Vertical antenna. Also as a tuner will see at least 1,000 ohms of feed point impedance on 160 meters with a linear loaded voltage fed Tee Vertical, your average store bought Tee network tuner can’t deal with such a high impedance and voltage. My matcher is a parallel network consisting of high power components, one 700 pf split stator 5 kw variable capacitor and a 28uh 5 kw roller inductor.
It is also recommended that the parallel network tuner at the antenna end feed point be fed with a high quality run of Belden 9913/RG-8U or Belden 9258/RG-8X coax back to the radio shack. For 80 through 10 meter operation, it is recommended that you use 450/600 ohm ladder line from the antenna end feed point, to a “balanced” network tuner just inside of the shack.
Attaching one 1/4 wave radial for 80 through 10 meters, to the ground side of the tuner and tuning the radials for maximum current with say the MFJ-931 Artificial Ground removes 100% of any stray RFI in the shack to zero. I have found a minor amount of shack RFI on 40 through 10 meters using the linear loaded voltage fed Tee Vertical but have gotten rid of it easily using the above mentioned method. Also making up some stub lengths of wire that make the total length of the antenna on each band of operation an odd quarter wave multiple, moves the first highest current point at the matching network and removes all shack RF.
I’m constantly experimenting with different radiator lengths and layouts. As of 10/01/02 my configuration of the linear loaded voltage fed Tee Vertical/Doublet antenna is:
A linear loaded voltage fed Tee Vertical antenna with the entire vertical section and linear load section made out of 450 ohm ladder line. The vertical section is 80 feet high, with a 47 foot linear load horizontal section one foot above ground that terminates in the tuning doghouse, to a legal limit plus rated home brew parallel matching network and driven against one 1/4 wave radial on the ground, four 10 foot long ground rods and a 150 foot deep well casing. The capacity hat is comprised of three 144 foot wires using #12 stranded wire, spaced one foot apart and sloping down to 70 feet.
Of course the ground rods and well casing don’t do much if anything as far as reducing near field ground losses and are actually part of my DC lightning ground. My ground system is sitting over very wet and highly conductive muck soil with swamp and ponds in the near field and Fresnel zone of the antenna. I also have a near zero local QRN level even on the transmit antenna, lucky me!
I’ve also had similar good performance with a voltage fed Tee Vertical using three 200 foot capacity hat wires, a 52 foot vertical section, a 75 foot horizontal linear load one foot above ground, with nine 1/8 wave counterpoise wires 5 feet above ground.
Per the EZNEC 5.0 modeling program, my 80 foot Tee Vertical has a near perfect textbook circle radiation pattern, with 1.95 dbi gain at a takeoff angle of 20 degrees, a 3 db beam width of 51.2 degrees, F/B of 0.30 db, feed point impedance of 628.6-j19350, a 1 mile mV/m of 134.22 using 1000 watts, with the highest current point elevated at approximately 60 feet above ground. However for all intents and purposes the highest current is nearly equally distributed along the 80 foot vertical section thanks to the capacity top hat and 47 foot linear load horizontal section one foot above ground. See links below for model diagrams of the Tee Vertical antenna.
If you zig zag sections of wire, that can’t be placed in a vertical position, versus using a coil, it’s much more efficient then a coil and radiates to a certain extent. Actually, if the linear loaded sections are designed right, they can add to the current on the vertical section, of a 1/4 wave L. It’s an idea I borrowed from VE3DO and discussed in ON4UN’s book “Low Band DXing”.
Remember once again, the linear loaded voltage fed Tee Vertical is a DX antenna with a null overhead and therefore virtually no high angle radiation close in for rag chewing. Put your linear loaded voltage fed T antenna on a pulley and you can lower it at will, roll up one leg (100 feet) of the 200 foot flat top into a ball or place an isolation relay to electrically remove one leg, the antenna then becomes and inverted L electrically and performance wise.
However thanks to the creative ingenuity of Joe Dube, K4TR of Brooksville, FL., who owns D & G Antennas there is another option. Joe came up with the idea of turning our linear loaded voltage fed Tee Vertical into a ladder line fed all band doublet/dipole. By flipping a switch which actuates a SPDT 12 volt relay at the antenna feed point in the dog house, the Tee Vertical becomes a 160-10 meter horizontal doublet with lot’s of gain.
K4TR D&G ANTENNA MFG & SALES
At times due to the nature of propagation on 160 meters during propagation disturbances, a low dipole can outperform a Tee Vertical on DX and is also quieter. I also have the added benefit of switching to the regional big signal cloud warmer low noise dipole to overcome high summertime lightning induced QRN for rag chewing. I use the dipole antenna set up in conjunction with a good performing Time wave DSP-9+ for summer operation. Click on the link below to see a diagram of the remote relay switching arrangement.
Having field tested the K4TR’s doublet aspect of the antenna design on 80-10 meters during the summer of 2002 I can verify that it works very well as and all band rag chew and DX antenna. I use a homebrew Tee network matching box to tune out inductive reactance.
Also last but not least, a personal observation concerning short monopoles. When I model a 52 foot vertical with one 200 foot top hat wire using EZNEC 3.0 on 1830 kc, then add two more 200 foot top hat wires, the near electric field in V/m RMS increases, the total electric field at 1 mile increases and the feed point impedance increases a little. When I conduct the same modeling exercise on 180 kc I see the same results as at 1830 kc but cannot verify it in the field. With no top hat wires attached a 52 foot vertical antenna obviously has capacitive reactance and therefore inductive top loading wires are needed or a linear load or at a last resort a lossy coil. With the 52 foot vertical and three 200 foot top hat wires the antenna feed point becomes inductive and feed point impedance high enough for the necessity of a parallel matching network. When you feed a 90 degree monopole at it’s ground end the feed point impedance and radiation resistance are basically synonymous, lengthen the monopole to 135 or 180 deg and of course feed point impedance and radiation resistance become different but the added “electrical” length does seem to increase radiation resistance.
I’ve done extensive experimentation with radials on vertical antennas on 160 meters during the past 18 years.
Back in 2001 a MF broadcast engineer friend of mine using professional broadcast measuring equipment, took near field measurements of the electric field in V/m RMS. The antenna was a 1/4 wave inverted L with a 64 foot vertical section and (1/8 wave) 64 foot long radials laying on the ground surface.
I found the following:
There was little measurable difference between 0 and 4 radials, a small measurable difference between 4 and 8 radials, a medium measurable difference between 8-16 radials, a large measurable difference between 16 and 32 radials, a small measurable difference between 32 and 64 and no discernable measurable difference between 64 and 120 radials.
We then conducted another experiment using conventional (1/4 wave) 128 foot radials and found the data to be exactly the same as the 1/8 wave radials. To me this proved the theory that the radials need not be any longer than the vertical section is tall.
I have never had the opportunity to do the experiment with a full 1/4 wave vertical.
This statement will be controversial. Using a voltage fed electrical 1/2 wave tee antenna with a 64 foot vertical section and three 200 foot long top hat wires, in the near field we measured only a very small difference between 1 radial and 64 1/8 wave radials. We measured no difference between 1 radial and 64 1/4 wave radials.
The ground conductivity was pretty good at the location of the experiment. It was a typical Florida hammock swamp that had been filled in but always had black mucky soil and a high water table. The conductivity was approximately .03 S/M with a dielectric constant of approximately 20. I’ve always presumed that the results might be different over ground with poor conductivity.
Here are some modeling results for the linear loaded voltage fed Tee Vertical antenna using EZNEC 5.0. Click on the links below to see the results. Link #1 shows current distribution which is very similar along the length of the 80 foot vertical section but peaks at approximately 60 feet up, link #2 shows takeoff angle and total pattern.
article by W4HM originally available at http://www.wcflunatall.com/w4hm9.htm
I have been getting into software defined radio via RTLSDR and found the stock antennas woeful for reception and picked up a tonne of noise from my LCD and laptop – though it’s hardly surprising. So to improve the situation and spend as little as possible I decided to make a discone antenna.
After some research I happened upon VE3SQB’s site and a neat discone design program for Windows.
As a compromise between frequency and unwieldyness I settled on 130MHz as the lower bound. Discones are inherently wideband and I expect the antenna to be useful for reception in the 60MHz to 1700MHz band the E4000 tuner can work with.
The ingredients, all from Bunnings are:
From Middy’s Electrical Wholesaler I bought:
With no test gear I have no idea what the true parameters of the antenna are. All I can say that it has massively improved the reception on my EzyTV dongle for VHF and UHF transmissions in conjuction with placing the antenna in the far end of my backyard. I can also see distant ADS-B blips in HDSDR which I will get around to tuning into on GNURadio.
So go forth and make your own discone!
Download the Discone Notes file
Link originally available here http://helix.air.net.au
There are quite a few ways to make full-sized dipoles that can be rotated.
How you attack this mostly depends upon what band is to be covered (basically how big) and whether the wire is to be horizontal or is permitted to slope downward from a central post.
In the horizontal case, the wire is threaded through the spreaders and may extend out the ends. The ¼” tubes have an adequate ID that #16 wire is easily passed through the tube, so extenders can be added to the usual fiberglass ½” tubes that fit the hub.
In doing this, you should be aware that the velocity factor will be less than unity, so the physical size of the dipole will be slightly smaller than that of free space. In order to make connections to the feed line at the hub, two ¼” diameter holes have to be drilled at an outward slant into two opposite spreader sockets.
These should be drilled at about a 45 degree slant beginning about ½” out from the center. A ten meter dipole requires no extenders. Longer wavelengths require extenders, and the 20 meter dipole may require wire extending slightly beyond the extended spreaders.
In general, feed the wire through the extender first, then into the ½” tubing, then slide the extender into the half inch tubing and push the wire beyond the end of the spreader about 4″. Feed the wire into the hub and up through the access holes that you drilled. Then push the spreader into the hub.
Now, from the tip of the spreader pull the wire until there remains just enough wire at the hub to make the connections to the feeder. Adjust the length of the spreader extenders, and tighten the hose clamps.
Leave about a foot of extra wire beyond the extender. You will then need to trim this to get proper resonance once the structure is in the air. In the case of ten meters, you are done, simply mount the hub on the mast and put it up. No guy lines are needed if you don’t mind a bit of droop. In the case of 20 meters, there is much more to do. Here, the length of the spreader will be about 15′ if you have a 1′ overlap with the extender.
So you will need a central guy post 6′ long, i.e., use a full 8′ section of 1″ tubing with 2′ below the hub. You will need guys to both the inner spreader at 8′ and the outer extender at 15′ up to the central hub for both spreaders. You also may need rotational stability if you want this to settle down after rotation or gusts of wind.
The two unused socket holes are there for a reason, so, fit two 6′ or 8′ (if you have room) ½” spreaders in these sockets and guy them in the same manner as before at the 8′ and 15′ locations. Always set the guys from the inside first, then add the outer ones. This is still a fairly loose structure since only gravity holds it in the downward direction. If this structure is still not stiff enough, you can guy downward to the mast as well. The limiting tension is set by the point where the extenders begin to buckle. That turns out not to be a whole lot of tension, because a 7′ section of the ¼” tubing sets the limit.
The second procedure is to make an inverted V dipole, where the antenna is the upper guy lines from the center pole out to the spreader tips.
For the ten meter case, this is nearly identical to the ground/counterpoise discussed in the Quick Vertical section. In that case, there are no extenders, so the construction is very simple. In the 20 meter case, all the same problems are encountered as above except that the wire load is acting as the upper guy lines rather than being in the spreaders. We also suggest using light wire for the 20 meter version. In fact, # 18 or # 20 hook-up wire works well, and the insulation should be left on.
We prefer the un-tinned type that is commonly available at Radio Shack. Using, the 6′ center pole, the length to the tip should be just about correct, however, the insulation slightly reduces the velocity factor, so you can shorten the extender or use a small length of Nylon fish line to extend the wire.
Note, the 17 and 20 meter versions of these dipoles are fairly large structures and can not be built up in small spaces. They are also rather flimsy, and go through lots of distortion when being tipped up. These are better erected from a push-up mast with the rotator near the top of the mast. This Dipole antenna gives the same gain as all other dipoles, however, the Half Square is a much better DX antenna for a given elevation and may be worth the extra effort. All parts used for the construction of the dipole can be used to construct the Half Square, so there is no loss in investment if you decide to switch. Note that 6 and 10 meters require no extenders, but we do recommend that you use guys from the tips to the center post. The post should extend about 3′ above the hub.
Parts required for all 6 and 10 meters versions:
|1||1||HUB 4-050-100, Central Quad Hub (RFJ)|
|2||2||8′ 1/2″ OD fiberglass tubing, spreaders (MGS)|
|3||1||8′ 1″ OD fiberglass tubing, boom (MGS)|
|4||2||GT 4-050 1/2″ Guy Ties, for tips of spreaders (RFJ)|
|5||2||GT 4-100 1″ Guy Ties, for tips of boom (RFJ)|
Extenders are required for 12 , 15, and 20 meters.
To determine how much 1/4″ fiberglass to buy, you need to calculate the approximate length required for the dipole. If the wire is to be inside the fiberglass, the velocity factor is slightly less than 90%.
The size of a typical dipole is given approximately by 468/fMHz. This formula has a small correction factor for finite wire diameter and end effects.
When the wire is inside the fiberglass tubing, the appropriate factor is about 435/fMHz, so the lengths of the spreaders require for 12, 15, 17, and 20 meters is roughly as 8.72′, 10.25′, 12.02′, and 15.33′.
Assuming 6″ overlap and 8′ lengths of 1/2″ OD spreaders, the extenders will have to be 1.22′, 2.75′, 4.52′, and 7.83′.
Obviously, there is no compelling reason to cut the 8′ of 1/4″ OD tubing for the 20 meter spreaders.
You can get both a 15 and 17 meter extender our of a single 8′ length of tubing. 15, 17 and 20 meters require lateral guys to increase the stability. This requires two 4′ lengths of 1/2″ OD tubing inserted in the two remaining sockets.
Guys should be run to the tips of both the 8′ dipole tips and the extended tips. This is also true from the central Guy post. The guys can be either 50 lb. test Nylon fishing line or Kevlar thread.
The photos associated with the Half Square antenna show structures built with both fiberglass and PVC.
Article by KQ6RH
originally available at http://www.antenna-magic.com/antenna/dipole.htm
Please feel free to build or distribute the information on these antennas. G4DHF retain the copyright to the designs and ask that they are not manufactured for commercial gain.
This is a design for a 5 element 2 Metre beam with a forward gain of 8dBD and a front to back ratio of over 24dB. As with my fishing pole yagis, it has a feed impedance of around 50 ohm and so the Driven Element only requires a simple unbalanced to balanced feed. The antenna utilises flexible plastic coated wire for the elements, which are supported by fibre spreaders and kite flying cord. When assembled the antenna forms a rectangle of only 1 x 1.51 Metres. When not in use the antenna collapses into a very small package, which can be carried in a medium sized pocket or small bag making it ideal for backpacking. I have successfully carried these antennas as hand luggage during air travel and worked some quite remarkable DX.
The inclusion of the 30cm ruler is for visual reference only
This yagi is designed using highly flexible 16g plastic coated wire with an outer diameter of 3mm and a stranded inner conductor of 1.5mm and is of the type readily available from most electrical retailers. The plastic coating has a significant effect on the resonant length of the elements due to changes in the velocity factor when compared to free space values. The value of the velocity factor was determined by trail and error over a period of many weeks and several yagis. A correction factor of around 7% to 5% from the free space values gave repeatedly good results and a value of 5% has been applied in this design. If a different wire diameter is used or the wire is non-insulated changes to the length of each element will be required to compensate. As the elements are very thin it was expected that the operating bandwidth would be limited to around a few hundred kHz at 144MHz. As the operating frequency was designed around 144.1 MHz this should be of little concern to those operating in the European DX section of the band. In fact, the beam will give creditable performance up to around 144.6 MHz after which the 50 ohm feed impedance, gain and front to back ratio rapidly begins to deteriorate.
Cut the element lengths to these dimensions
The elements are held in position along a string frame, supported by four 6mm fibre spreaders. A number of string types were tried, ranging from 1.5mm polypropylene line to the more conventional household twine. Almost all exhibited an unacceptable memory affect, which turned the beam into tangled wire when the supports were removed. The ideal material was found during a visit to a kite shop when 2mm flying line was purchased. This consists of hundreds of fine nylon strands bound together in a woven cotton outer sheath. This cord is highly resilient to stretching, has minimal memory affect and is extremely strong. Once used there was no going back to the types used in the prototypes.
The end of each element is secured into lengths of 5mm plastic hollow tubing, obtained from model and craft shops. Each element end support has a different length in order to preserve the shape of the supporting frame. After the tips have been drilled each element is threaded along the length of the cord to their correct position. The end of each element is at a high RF voltage potential so care needs to be taken to ensure that surrounding objects and materials due not exhibit detuning effects. Four small lengths of 2mm plastic rod terminate each of the supporting frame by providing tie points for the cord.
The Driven Element consists of a simple dipole. The feed impedance for a conventional yagi is usually considerably lower than the desired 50 ohm feed and requires some form of matching network to compensate. This design sacrifices a few tenths of a dB of forward gain so that by careful attention to the length and position of the first director in relation to the dipole the feed impedance is raised to the desired value. This concept is not new. Indeed, at least one well-known commercial manufacturer of V/UHF antennas has used this technique successfully over many years, including matching UHF folded dipoles directly with 50 ohm cable. These parameters are, however, quite critical and so careful attention should be given to ensuring that the recommended dimensions are followed. The centre of the dipole passes through a small plastic support and is terminated with standard spade connectors, as weight at this point needs to be kept as light a possible to reduce element sag. The dipole requires an unbalanced to balanced balun, which is described below.
This balun serves to provide an unbalanced to balanced 50 ohm match, which helps reduce RF currents on the outer of the feed. Omit this balun at your peril as the antenna may exhibit false resonance at the desired frequency or high SWR due to the presence of circulating currents. For RG-58U coax with a velocity factor of 0.66, cut a 34cm length and trim back the braid 5mm at en end. Note that the two lengths run parallel to each other and that the braid and inner are isolated at opposite ends.
The end of each element is cut to the corrected length and secured into plastic element spacers, which are drilled 5mm from the tip and threaded onto the two outer lengths of kite cord. The element positions are marked out onto a length of wood and the cord stretched between two sets of panel pins. Thread one end of the Reflector element first, followed by the plastic end cap that holds the fibre support. When the beam is assembled these caps hold the Reflector and 2nd Director tight against the end supports. Next thread the Driven element, Director 1, another end cap and finally Director 2. Repeat the process for the opposite side. When the fibre supports are inserted into the end caps the beam assumes it’s physical dimensions. The element ends can be glued along the cord, but leave Director 1 free until adjustments are complete.
The shape of the yagi is created when the four fibre supports are inserted into a central hub and the four plastic end caps at each corner of the beam. Lengths of 6mm fibre rod are available from a DIY stores and kite retailers. I have even successfully used the types that support bicycle flags. The total length of each support is around 865cm, which I cut into three sections for portability. There is an additional 4cm of “hidden” length to be added when they are fitted into the central hub. I placed small lengths of 8mm (6mm ID) aluminium tubing on each end of all the second sections to enable the rods to be compression fitted together. A word of caution is necessary when handling and cutting these rods as fibres can easily become embedded in the skin causing irritation so gloves must be worn. When the rods have been cut each end should be dipped in “Super Glue” to prevent the fibres from peeling. Furthermore, the addition of heat shrink sleeving makes for safer handling. It is not recommended to use wooden dowel for the supports as the tension required to form the structure is quite high and this will either distort or snap the wood.
This yagi has to be lightweight, strong and portable. Metal fixings are almost eliminated. The supporting plates consist of two 9cm x 7cm sheets of 3mm acrylic. The geometry of the structure is scored on one of the sheets to determine the centre. Mark the 67-degree angle created by the fibre supports. The sheets are held together with the markings on top so the centre-supporting hole can be drilled. If two stacked antennas are planned, supported by a 6 metre to 9 metre fibre pole, the top antenna has a hole diameter of 2.5cm while the second, located some two metres lower has a cut out of 3.5cm. The four 4cm lengths of 8mm aluminium tube that house the fibre spreaders are then pop riveted in place. Turn the support over and secure the reverse. Two small “L” shaped aluminium brackets (or square open section) are formed and secured either side of the plates. These support a length of ribbed rubber matting, which, with the additional of cable ties, increases the grip between the central support and the mast support. There may be concerns regarding the effect of sunlight on acrylic over time but because the yagi is intended for temporary use the trade off in weight, strength and ease of assembly more than compensates for this potential problem.
Attach the antenna to an insulated mast at a comfortable working height of between five to six feet above ground in a clear environment. Attach the ends of the coaxial balun to the Driven Element and connect a length of 50 ohm cable to either an antenna analyser or SWR meter connected to a low level (1 watt max) 2 Metre signal source. Note the impedance or amount of reflected power. If the dimensions of the beam have been followed closely these will be reasonably low and close to 50 ohm. Moving only the position of the first director in 2mm steps, usually towards the Driven Element, note the change in impedance until the desired match is achieved. The antenna can now be raised to a more suitable operating height to confirm alignment. When complete, the position of D1 is marked and secured. Lengths of lightweight plastic placed either side of the dipole centre help to maintain this distance, which is critical, when the elements sag slightly in operation. Once aligned and the distance between D1 and the Driven Element has been secured power levels of up to 300W have been used successfully.
As has been previously suggested, stacking two or more of these beams is perfectly viable due to their low weight. Stacking two antennas at a distance of one wavelength (6′) should yield a forward gain of about 11dBD, which makes for quite a potent one-person system.
Readers may be interested to know just how effectively this type of antenna can perform. While testing one of the prototypes in the garden of my QTH near Spalding (IO92UU) in May 2005 we had a Sporadic “E” event late in the afternoon. I worked CN8LI (IM63) at a distance of 2163Km, EA9IB (IM85) 1966Km and several EA7’s running only 20W. Given the high QRM levels and the number of G stations who were active, these results speak for themselves and should encourage potential builders to “get busy”. I would be interested in receiving comments and details of your operating experiences using this type of antenna.
Moonraker supply a whole range of wire trap dipoles covering from 2 to 5 HF bands (MTD1; MTD2; MTD3; MTD4; MTD5; MTD6). Diamond also produce trapped wire antennas, the W-721, W-728 and W735. Comet and Diamond each produce similar interesting 5 band wire dipoles that utilize both traps and a fan arrangement – the Diamond W8010 and the Comet CWA-1000. If space really is limited then look out for KZJ Communications (dongo1950 on ebay) – he produces ‘Limited Space Inductive Dipoles’. These are inductively loaded and shortened dipoles so they will have reduced efficiency, of course, but are very nicely made, so might be very useful in a tight spot.
To obtain good efficiency and achieve a low angle of radiation, desirable for longer distance DX, a horizontal dipole needs to be installed at a good height – over 20 feet would be desirable and it is quite common to install horizontal dipoles at around 30 to 40 feet above ground level. This might be a problem at some QTH’s, it certainly is at mine!
Allan Copland, GM1SXX comments: “The dipole will operate well on the band it has been sized for , if placed at a suitable height, but will also operate as a’ three-half-wave’ aerial at three times the frequency and so on, so it’s not strictly a single band aerial. An 80M dipole (132 feet typical) will work nicely on 30 metres (three half waves) but not on 40m (two half waves)… because on 40M the feed-point is at a voltage node and not at a current node, for easy feeding. Most aerials are current fed.
The radiation pattern changes when a dipole is not used on its design frequency. The pattern will break up into multiple ‘petals’. This can be either a disadvantage or an advantage depending on what you expect from it. Since most of us use co-ax, an UN-BAL should really be used to connect the unbalanced feeder to the balanced aerial, but how many people actually bother? Not many I suspect. It’s possible of course to use a balanced feed-line system instead with a dipole and just have a delta match (no centre insulator… none needed). There are many choices and permutations, but in general, dipoles are centre fed at a point of current maximum (and minimum voltage).
A normal dipole is current fed but of course can be voltage fed instead. This is what’s done in the EFHWA or Fuchs aerial where a resonant half wave wire is fed at one end (max voltage / min current) from an L/C tank, against a very short counterpoise wire.
Original article is :W3DZZ Dipole Aerial design by the Maidstone Amateur Radio Society
Construction: I made and put this antenna up the Friday afternoon before the ARRL RTTY Roundup (1999)! It took me about 3 hours to make and install on the tower. Most of that time seem to be used untangling the wire..hi hi
I made the antenna 142 feet long. Use tiewraps at each of the three points to form a small loop at each end so that A 3/16″ rope could be used to support the antenna. I used a center insulator (Budwig HQ-1) that I had left over from an old dipole. The feed point was 12′ from the bottom of one leg.
This antenna requires a matching element of feedline made out RG-59 (75ohm) coax. The formula or the length is a 1/4 wavelength at the operating frequency times the velocity of the RG-59. It is pretty easy to figure, a 1/4 wavelength at 7.050 (234/7.050) = 33.19′ times 66 percent equal 21.9. I had RG-59 with the polyprolene dialectric therefore the length turned out to be approximately 22 Feet.
Since I had the Budwig HQ-1 with the SO-239 moulded in the unit, I put PL-259s on each end of the matching coax and then used a barrow splicer to put the regular 52ohm coax feedline to the antenna.. A little weather proofing and this setup has worked well for many years.
My mounting point was only about 45′ above ground, which is insulated from the tower by a 3′ fiberglass pole. The legs of the antenna were secured East and West, and North and South is broadside to the antenna. Because the mounting points was so low, the bottome portion of the antenna was sloped out several feets to help provide space under the antennea for cutting grass.. hi
Results: Actually, the antenna played better than anything that I have had previously. The SWR was flat from 7.0 to 7.1 only rising to 1.5 at 7.3. This was perfect because I wanted it to play RTTY and CW… I did find that the antenna appeared to be less effective broadside, to the North. Stations in New England were difficult to get, however, the Europeans and Western USA were answering on the first call. Also, a few days after the contest, I talked to K4HXW/Mr Tucker, he indicated that he thought the antenna was playing much better than the inverted Vee that I had previously.
Added 31 Aug 2002 Update: I put this antenna back up recently and found that the correct length was a little off on my original antenna. According to ON4UN’s Low Band DXing book, the proper length for a delta loop should be 1.05% to 1.06% wavelength. I used 1.05% and the correct formula to calculate this length in feet is to divide 982.8 by the frequency in Mega Hertz (982.8/7.150=137.45′). I also have changed where the antenna is feed to horizontal by feeding the antenna in the middle of the bottom. This is so that I could have better effectiveness in domestic contest. After several days testing, the antenna seem to work well. Only time will tell… hi hi
Added 17 Oct 2011: Been living in Alabama since 2005 and installed the Delta Loop with the apex at 65′ and the bottom slightly kicked out a bit (not more than 5 degrees). I also changed back to the feed point to 12′ from one of the bottom corners. I really have not been able to see any difference in it performace based on changing the feedpoint. It has worked marvelously both for DX and Domestic contacts. I truly think that it is the best single wire antenna I have ever had on 40m.
I am probably going to put one up for 80m when the WX permits.
Article by KC4HW originally available at http://kc4hw.net/deltaloo.htm