There was no rush to build the new 3-element 20 meter yagi since it can't be raised until the fall for several reasons. I worked on it over the summer as time permitted. It is now largely complete, tested and tuned. This fall I'll take down the TH6 and replace it with this antenna. I also plan a similar antenna for 15 meters. Both are to be fixed south, just like the TH6. They will be more efficient and less prone to multi-station interference than the TH6. Another for 10 meters can wait since the solar cycle is waning.
Although a 3-element yagi is a common directional antenna there are a few factors worth discussing. Those will make the article longer than you might expect. The details may prove useful to others, including how construction choices affect performance. No matter our experience and knowledge there is always more to be discovered even in antennas that we believe to be thoroughly understood.
Element taper and K-factor
The ratio of wavelength to wire diameter (λ/D) affect the SWR bandwidth of an antenna element. The higher the ratio the greater the change in reactance for a given change of frequency (dX/df). Therefore an antenna made from wire has a narrower SWR bandwidth than the same antenna made from tubes -- fat is good! The effect is greater at longer wavelengths (lower frequencies) for a given wire diameter. The effect is noticable at 20 meters, but small enough that it can usually be safely ignored.
Antenna modelling engines readily calculate the effect. It influenced my choice of a 7% spread for the parasitic elements of the 3-element yagi. For contesting I want a low SWR from 14.000 to 14.350 MHz. However, the model utilized fixed diameter elements, while the real antenna uses tapered elements. It is not obvious what the effective ratio is for tapered elements.
The steps for each half element are as follows:
- ~1.5": 3" long, calculated for the boom-to-element clamp, where director and reflector are electrically isolated from the boom; the estimated diameter for the DE (driven element) is not critical since it is accommodated by the matching network
- 1.315": 9" long for DE and director, and 21" for the reflector
- 1": 51-¼"
- ⅞": 51-¼"
- ¾: 51-½"
- ⅝: 21"
- ½": 24" (R), 13" (DE), 4" (D)
Small deviations of actual tube lengths were accommodated during construction so that the half-element lengths are 224" (R), 200" (DE) and 191" (D). The tubing from 1" to ¾" came from 20 meter yagis built by a long-gone Canadian firm almost 50 years ago and I saw no reason to take them apart to change their lengths.
When I ran the model with this taper schedule there arose a problem. The effective λ/D ratio was higher than that of the initial model with fixed diameter elements. The change of SWR at the band edges was enough that I increased the 7% spread to 8%. That achieved the same SWR bandwidth while sacrificing about 0.2 db of gain. Since this is a "multiplier" antenna pointed south the gain difference is not too concerning.
The lesson is that element taper is a significant design factor. The effective wire diameter more closely resembles that of the outermost thin tubes than the average or the large diameter found at element centre.
Boom
I was fortunate to have all the aluminum and attachments in my stock to build this antenna. That includes the boom. With the pieces I had there were several alternatives to make the boom. I finally decided to use a couple of length of 2"×1/16" tubes that came from old yagis. Hy-Gain used the same size tubes but neither of these appeared to come from a Hy-Gain yagi based on the wear marks.
To join the tubes I used a scrap 2' length of 2-⅜ OD schedule 40 6061-T6 pipe. Aluminum flashing fills the small gap since the ID of the pipe is a little more than 2". They are held together with ¼" stainless (304) bolts and nylocs. Lock washers are not effective on round surfaces so it was nylocs or jam nuts. I've used both techniques in my various antenna projects.
One end of the longer 2" tube has a press fit galvanized tube (left picture). Clearly this was the original boom's coupler to another 2" tube. I intended to use it to join the tubes but I couldn't make it work. There has been enough oxidation over the decades that making it fit would have taken a lot of tedious preparation of the surfaces. It wasn't worth the effort. I cut the steel flush to the end of the tube.
When I was done the boom was 282" (23'-6" or 716 cm). I reran the model with the slightly shorter boom (24' in the original model) and the performance difference was negligible, as I expected. The actual length of the yagi is several inches shorter, measured centre-to-centre from director to reflector, due to the boom-to-element clamps. One can be seen on the left being tested for its fit. The small clamp you can see on the right picture is one of the boom truss attachments. These were found in my junk box (source unknown).
Elements
Since the widest tubes of the old element sections mentioned above are 1", I slipped them inside 1.315" OD pipe. This is schedule 40 1" (IPS) pipe so the ID is a slightly over 1". For other antennas I've wrapped the 1" tube with aluminum flashing. The amount of flashing is easy to calculate since C = πD and D is 1" so the circumference is ~3.14". I rounded down to 3-⅛" so that the flashing didn't overlap itself.
It didn't work; no amount of forcing the pipe, flashing and tube would make them fit together. I measured the OD of one and the ID of the other with calipers and discovered that the ID of the pipe was a little less than expected. All of this aluminum was milled around 1980, so either the mill finish is responsible or it's a build up of aluminum oxide. Since I'm not equipped with a metal lathe I cut the flashing in half (to ~1-½") and it all fit together nicely.
Notice (top left) the obviously hand cut slits; most of the pipe ends didn't have slits. I didn't discriminate and all were joined in the same fashion as shown in the picture. The rear stainless screw head presses directly on the 1" tube for improved rigidity and electrical continuity (see the larger hole in the bottom left corner). Conductive grease was applied to all surfaces after sanding off imperfections and oxides.
The joints between tubes are peculiar. I don't think that was state-of-the-art a half-century ago! Metal strapping squeezes the slit tube onto the inner one. This would have required a special tool. Under tension the machine folds back and crimps then cuts the strap. It's a very solid if unconventional method of joining tapered yagi elements. I can only remember this method being used on shipping crates. A self-tapping screw further prevents slippage. While unusual I saw no reason to redo any of it.
The ¾" tube was the tip of the original yagi elements and the elements were roped with ¼" polypropylene twist rope. Those were not a good choice so the rope and plugs were removed. I don't anticipate much singing in this yagi but we'll see how that goes.
The smaller tubes were attached in ways I've done before. The end of the smallest tubes (¾") of these sections and the ⅝" tubes were drilled for #8 stainless screws and secured with nylocs. The ⅝" tubes were slit for hose (gear) clamps to secure the adjustable ½" tips. All except the longer tips on the reflector were pulled from my junk pile. They have holes and other imperfections from earlier projects which don't matter as long as the tubes are structurally sound. This isn't an art project.
Notice the black marker rings on the ½" tubes. The element halves were precisely measured to match the EZNEC model. From experience I know that the SDC (stepped diameter correction) algorithm is very reliable. For a 20 meter yagi I try to match the model to within ¼" -- try to do even better on higher bands. A lot of time and effort goes into the design and construction of a yagi so this is not the place to be sloppy. Don't cheat yourself by talking yourself into believing that precision doesn't matter.
Boom resonance
I reduced the interaction by raising the TH6 a little higher. When this yagi replaces the TH6 the problem must once again be dealt with. However, unlike the commercial antenna I have design options to nip the 40 meter resonance issue in the bud.
The director and reflector elements are electrically isolated from the boom. That prevents them from becoming capacitance hats with respect to the boom resonance. The stray capacitance between the 1.315" pipe and boom clamp is negligible in this application.
The insulators are cut from a flexible ABS pipe that was surplus to a property drainage project. They fit nicely and are strong enough to take the compression of the u-bolts to keep them in position. If you do something similar I will warn you that the insulators must be slipped on before the second half element is attached to the centre pipe, otherwise the tapered section joint hardware (screws, hose clamp, etc.) will block their passage. You'll would then have to cut the insulator, in one place if the insulator material is flexible or in two places if it is rigid. Those cuts can make it difficult to prevent insulator distortion and eventual failure.
The driven element is grounded to the boom. This is for the convenience of building the gamma match (see next section). Since an element near the centre of the boom does not electrically load it in the same way as the outer elements it is an acceptable choice.
I modelled this combination of element brackets to confirm that the 40 meter boom resonance was eliminated. Notice that placing the source at different places on the boom (top: near the DE; bottom: boom centre) effects its impedance at higher frequencies just as it would in any dipole. However, those are irrelevant to this scenario; you could feed the boom this way for real if you cared, but that is hardly typical! That peculiarity does not bear on the fact that the boom resonance on 40 meters is gone. Problem solved.
Gamma match
The reason is that both matching networks perform best when the self resonance of the DE includes -20 to -30 Ω of capacitive reactance, and both work well for the typical 20 to 30 Ω feed point resistance range typical of yagis. There is often no need to adjust the tips of the DE when a gamma match is used despite a beta being employed in the model.
The gamma match saved me the trouble of isolating the DE from the boom and splitting the DE at the centre as the beta match requires. That is, it's convenient and the gamma match works well, though often misunderstood. No, it doesn't cause the antenna pattern to be measurably asymmetric or promote common mode on the coax shield. A CMC (common mode choke) is advisable no matter the matching network.
The design of the gamma match differs in a few respects from those I built for my stacks of 10, 15 and 20 meter 5-element yagis. First, it is more rigid at the centre. A ⅛" thick aluminum bar ¾" wide is attached with a galvanized saddle clamp to the DE adjacent to the boom-to-element clamp. That is the minimum width to comfortably accommodate an SO-239 UHF panel connector. Hanging below it is a piece of rigid PVC that I pulled from my junk box. Have I ever mentioned that I'm a pack rat? I throw nothing out. Particularly astute readers will recognize that rectangle of PVC from a previous project.
The RG213 gamma capacitor is stripped of jacket and shield and inserted into the 7/16" gamma rod. The rod and shorting stub came from one of those ancient 20 meter yagis I scavenged so they are the correct dimensions for this antenna. When fully assembled the gamma match is stable and rigid, and should be up to the rigours of the elements. If not, since this is a 3-element yagi, the gamma match is easily accessed from the tower for service.
One addition I may add before the yagi is raised is an aluminum jumper from the bar to the element to bypass any galvanic incompatibility due to the interposition of the galvanized saddle clamp. It is fine as it is to conduct initial testing and tuning.
Assembly
Once the boom and elements were built the antenna had to be assembled in the hay field. Although it is reasonably lightweight, an assembled 20 meter yagi needs a lot of room. I did this at the planned launch point for tramming it onto one of the guyed towers for testing and tuning.
Of the alternatives for getting it high enough off the ground -- preferably λ/2, or at least 30' (10 m) -- this seemed most convenient since for a 3-element yagi the DE is adjacent to the tower. It is easy to adjust the gamma match with minimum fuss. For yagis with any other number of elements (2 or 4+) the DE is out of reach from the tower and other tuning arrangements are necessary.
Since the 2" sections of the boom are different lengths and the longer one has the steel insert that I couldn't extract, the reflector was placed on the short section, since it is heavier, to help balance the antenna. With the DE closer to the reflector than the director the final balance point was almost exactly midway along the 2-⅜" coupling pipe. That's convenient even though the halfway mark by distance is on the longer 2" section.
I used my usual method to weigh the assembled yagi by placing an ordinary scale on a flat surface, then measuring my weight and the weight while lifting the yagi off the ground. The weight is 40 lb (18 kg), which is quite light for a yagi of this size. It will be up to 10% heavier when the boom-to-mast clamp and boom truss are added. Neither is yet built nor needed for initial testing and tuning.
Test & tune
The yagi sat in the field for two weeks while I travelled to Labrador. Back home with August racing to a close I really needed to get this project moving. The bugs were fierce before the trip and they were likely to subside later in the month. Despite that hope being dashed I ploughed ahead.
Yet there was no real reason to do this test. From long experience, models built with NEC2 and the SDC correction within EZNEC are remarkably accurate. It nevertheless gives me piece of mind and confidence that when the yagi is raised to its final position on the tower that it will work correctly. A fault at that time would be more expensive to fix. I judged that a few hours of work would be time well spent.
I set up a tram line between my two big towers to tow the yagi up above the lowest set of guys. A height better than ½λ is more than enough for precise tuning. The tram is at a shallow angle since the towers are 200' (60 m) apart. That makes it an easy one man job. Both the tram line and haul line are ropes so that they don't interact with the antenna. The biggest hassle was unravelling the long lines since no matter how well I fold them after use they still like to tangle.
Tram lines have featured so frequently in this blog that I see no need to say a lot about it. Put the term in the search box if you want to read more. One point to mention is the yellow rope from the tower to the link between the tram line and the winch-driven cable. It is a safety line so that if the winch fails the tram can't collapse. Another point is the height of the anchors and haul cable pulley at around 45' (14 m). The bottom guys of the destination tower are at 35' (11 m). That allows the element tips to clear the guys and place the yagi midway between the first and second set of guys to minimize interaction.
Hauling a 40 lb yagi at a shallow angle requires little effort. The penalty is that it takes a while due to the length of the tram line. It's a trade off I liked since it made the job easy to do myself. Once the yagi was up the tram line was slackened to bring the boom into the tower so that all parts of the gamma match could be reached from the tower.
The gamma capacitor was purposely made long. By the time I was done it as almost a foot shorter. I'll make it a little longer for the final version and add the support insulator for the gamma rod near the feed point. Tuning went quite well as you can see on the left. The final SWR curve will be slightly different and better since I stopped when I was close enough. Recall that this is a test to ensure that the yagi is performing as expected. So far so good since it closely resembles the EZNEC model.
Note the connection of the analyzer to the feed point with a male-male UHF adaptor. Although this is less convenient than a short jumper cable it has an important advantage: no need to compensate for the impedance transformation of the coax while adjusting the gamma match. Without it I'd need to guess at each step or use a tool like TLW to transform each measurement to the impedance at the feed point. This way is awkward but saves a lot of time and frustration.
With a successful test in hand I lowered the yagi and parked it in the hay field. It can stay there until I'm ready to raise it to its final location on the tower. Mechanical work on the gamma match, boom truss and other items are ongoing. I'll probably reuse the CMC that's currently on the TH6 or I may make a coax (solenoid) choke for it as I've done for my other monoband yagis.
Next steps
Obviously the next step is to raise the yagi. That will have to wait until the fall. First, it will take planning to arrange for the removal of the TH6. The 40 meter Moxon, which is not far below it, will have to be navigated around. I have a few alternatives to get it done. Aside from that I have other projects and obligations to deal with.
The second reason is that I'd like to have a 15 meter yagi ready to go at the same time. The TH6 covers 20, 15 and 10 meters, and although I can delay a south-pointing 10 meter yagi to the next solar cycle, 15 meters is more critical for the multiplier rich south. I have been assembling the parts but as yet I'm not far along. I will also need to bury more Heliax since I am running out of transmission line capacity. The 15 meter yagi will go on the tower with the 15 and 20 meter stacks, but low enough that it will not appreciably interact with the stack.
The 15 meter yagi will be a relatively easy project since the antenna is smaller. I will most likely keep it to 3 elements so that it can fit on a short boom while having similar or better performance than the TH6 which, although it has 4 elements on 15 meters, some use traps. I'll write about it when it's done.
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