Rebuilding the 80 Meter Vertical Yagi: First Steps

I have been planning improvements to the 80 meter vertical yagi for a long time. Too many higher priority projects have kept it in the background for several years. Also, despite its complexity it has been one of my most reliable antennas. This fall I am taking the first steps to make it better.

I am proceeding in a set of carefully chosen steps. It is critical that after each step that the antenna work; I can't afford to have my best 80 meter antenna offline for long, especially now that the fall contests are rapidly approaching along with low band DX season.

The steps are as follows:

1. Install ground anchors to support a taller tower as the driven element and parasitic element support. These are set back further from the existing augur (screw in) anchors in accord with the planned tower's greater height. Since these are separate from the existing system there is no impact on the antenna.
2. Replace the radial plates for all 5 elements. My original design is not aging well.
3. Replace the parasitic element supports with more robust structures that repel wildlife and permit easy access for maintenance.
4. Replace the tower with a taller and stronger one. The intent is to eliminate the tall stinger at the top of the tower and provide robust support for an improved 160 meter mode for this antenna.
5. Replace the parasitic sloping T-elements with bent (sort of L-shaped) wire elements. The objective is to increase efficiency by raising the radiation resistance. This is the first step that changes to the electrical design, including the matching networks.
6. Add SSB to the yagi. The wiring and control structure has been present from the start but not used. Above 3650 kHz only omni-directional mode is available. There are alternatives I am considering to conveniently switch the yagi's operating range to 3650-3800 kHz with minimal loss of efficiency
7. More radials. Combined with step 5, from 0.5 to 1.0 db improvement can be achieved by doubling the radial count. The parasitic elements in particular need help since there are at present only 16 for each of them.
8. Improve the antenna's 160 meter mode. I'd like an effective 160 meter antenna available year round. The radials of the shunt-fed tower must be removed during farming season so I miss several months each year, from approximately May through September or October. The base loaded 160 meter mode of the 80 meter antenna is only minimally effective.

When complete, the antenna will be stronger, perform better, and be more useful. Although scrapping it entirely to replace it with a 4-square might be preferred by most, I really like the yagi. It is mechanically simple, takes up less space and is ripe for experimentation. I am willing to cede 1 db so that I can have some fun with this antenna project. If you've read the blog long enough, you'll know that I am more of a builder than a fervent competitor. 

The first step is now complete. Steps 2 and 3 are planned for this fall. The 4th step completes the mechanical rebuild, which I might not be able to accomplish this fall. The weather is getting colder, and sub-freezing temperatures will make concrete work difficult. The tower work would also have to be fit in between contest weekends. That's a tall order.

In this article I'll briefly review installation of the new anchors since that may be of interest to those with a similar challenge with small guyed towers. Then I'll address the other problems with the antenna that I hope to fix. Some things you only learn after the antenna has been in place for a few years. I've taken several temporary measures but, well, they're temporary and not good enough for the long haul.

The dead man anchors for this rebuild are identical to one that I've discussed before. There are a few differences. One is that with over 1600 meters of radials it is inevitable that one or more will be encountered when you stick a shovel in the ground. The picture shows one such case. I started digging with a hand trowel, carefully lifting the vegetation layer (it's a hay field) while checking for the presence of radials. Originally laid on the surface, they have become lightly buried over the years. Only one of the 3 holes didn't strike a radial. I worked around them rather than pulling them out of the ground during the work.

The angle and alignment of each anchor is critical to best performance. They are located the usual 80% of the tower height from the tower base and inclined to an angle midway between the two guy stations. For one guy station it should be inclined to point at that guy station. 

The angle in my case is 42°. I used an inclinometer to set the angle, approximately before the pour and then exactly after the concrete has been poured and is still wet. There is the possibility of anchor movement even if you're careful pouring the concrete. Don't use a quick setting concrete mix for this job!

The anchor must also point directly at the tower. A string or thin rope is tied to the tower leg that will be guyed and pulled taut to the anchor and a marker behind the anchor. The foreground steel rod was aligned with the bottom of the anchor using a level. It's easier to align the anchor this way than trying to run the string to the bottom of the excavation.

You can see the existing screw (augur) anchor in the background. Notice that it is not quite in the correct position. I hit a rock when auguring it into the ground so I moved it a few inches. Several large rocks were found while digging the hole for the new anchor so I moved it back about 8". Small deviations from perfect anchor placement are allowed but don't use that as an excuse to be sloppy.

A block of wood and metal rods keep the anchor in its aligned position while the concrete cures. They can be removed a day or two later. That is also a good time to back fill the excavation. There isn't a lot of organic soil in this field, which is typical for the local geology. I put the soil and the lower material in separate piles so that while back filling the good soil is put back on top. The lower material displaced by the concrete and no longer needed is dumped elsewhere. Heap the soil high during back filling so that you aren't left with a depression after it settles. You might also want to tamp the material down as you refill the hole

While the concrete cures, let's review the remaining issues and steps that will come later.

There are two problems to be seen in this picture. The first is the preserved wood base. It's showing its age. It is slowly decaying and weeds are growing between the logs and at their edges. Replacement isn't urgent but must be done at some point since it supports the tower. The tower is isolated from ground and because it is not grounded there is no need for a concrete foundation. However, I will put in a 4' × 4' concrete pad when the tower is rebuilt (step 4) to keep the base clean and ease maintenance.

The radial hub is simple and easy to use but has proven not to be reliable. Electrical contact degrades over time for two reasons. One is that the copper oxidizes and the contact to the stainless hose clamp isn't strong enough to keep oxidation out of the contact area. 

The second is that the ring is "bumpy" so that a slight bend of a wire or unequal spacing of wires under the clamp causes unequal pressure on the wires. That leads us back to the first point since some of the wires are a little loose and there is no way to clamp them more securely. I've had to clean the wires and re-tighten the ring a few times. The same is true of the 4 parasitic element radial hubs.

All 5 radial hubs will be replaced with sheet aluminum and stainless studs for the radial wire connections. I will do that soon for the driven element (tower base) and later for the parasitic elements when I redo their bases. 

The bases of the parasitic elements are all protected as shown at right. It's ugly and makes maintenance difficult. But it's necessary. Deer and even coyotes love to gnaw on the antenna wires, control cables and the PVC enclosures. As currently constructed they are difficult to protect properly.

They will be replaced by wood posts in concrete and a small concrete pad. The PVC enclosures will be mounted on the posts and the antenna wires replaced by aluminum tubes up to at least head height. Those measures and a ring of galvanized mesh around the base will solve all of these problems while leaving the parasitic elements accessible for maintenance.

The tower sections have been stored for the past 3 years. When combined with the two top sections of the existing tower the total height will be over ~62' (~19 meters). It will need only a 2 meter mast to tune the tower to the same value as before, which should allow no change to the existing matching networks at the tower base. I will have two guy stations due to the greater height and a possible 160 meter stinger. The same considerations guided the design of the new guy anchors.

The stinger will no longer be needed. It has proven to be the weakest part of the 80 meter array. A short mast on the tower will be more than strong enough for the wire element support ropes. I will no longer have to be so careful when tensioning the ropes. Now when I periodically need to do that maintenance, the stinger and the insulating support pole above can bend, with the risk of excess stress leading to failure.

As mentioned, the tower will eventually have a stinger that will be used for 160 meters. There are electrical considerations to ensure the stinger doesn't strongly interact during 80 meter use and managing potential high voltages across relay contacts. I have a few alternatives to solve those issues when I am ready to proceed. But for now I intend the tower to be a plug in replacement with no changes to any of the matching networks.

Items 5, 6 and 7 will follow after the major mechanical upgrades are complete. I want to delay changes to the electrical design of the array until the mechanical rebuild is complete. It's a sensible step by step process.

Step 5 is quite simple. All that is required is to change the T-top wire parasitic elements with bent elements in the shape of an open L. Modelling suggests an improvement in efficiency due to the higher radiation resistance of the latter. It comes about because of the acute angle on the lower side of the T which causes field cancellation with the vertical section. Inverted vees with a small interior angle can suffer a similar fate.

The main benefit of the higher radiation resistance is that the series ground loss (radial system and soil) becomes smaller relative to the sum of radiation and loss resistances. Each new parasitic element will be tuned to the same director and reflector self-resonant frequencies as the existing array. It will require experimentation to determine the length of the upper segment of the element. Once the first parasite is tuned the rest should be identical. The length of the vertical segment will remain the same so that I can use the existing element anchors. In any case, geometry (tower height and antenna field perimeter) doesn't allow much variation of the wire angle.

Higher radiation resistance and a lower ground loss by doubling the radials for each parasitic element will change the feed point impedance of the array when used as a yagi. The matching networks will have to be adjusted or replaced. That will be determined by measuring the impedance after the changes are made.

Adding SSB to the yagi can be done with an additional series reactance at the base of each parasitic element. This can be a coil (shorted for the 3650-3800 kHz range) or a capacitor (shorted for the 3500-3650 kHz range). The latter can be more efficient but is perhaps more difficult in practice since suitable capacitors are expensive and unusual to run across at flea markets these days. 

I'll close by mentioning the help I receive from a couple of friends. Chris VO2AC/VE3FU transported almost a half ton of concrete mix with his pickup truck. The 30 kg bags are cheap but not easily transported in bulk with a passenger vehicle. Dave VE3KG come by one morning for the mixing and pouring of the concrete for the anchors.

The bags in the picture are what was left over after the anchors were done. It may be enough for the 4 parasitic element bases. More bags will be needed for the tower base. The rebar has tape markings at 8" (20 cm) and 30" (75 cm) for convenient measurement of anchor width, concrete depth and excavation depth. 

40 Meter Reversible Moxon - Complete

The antenna on the tower and working. Finally. I've been using it for the past few weeks and my overall assessment is that it is a very good antenna, better than I predicted. In this article I'll review the design, the repairs and what I've learned about its performance. Instead of repeating what has come before I'll sprinkle the article with ample references to earlier ones. The antenna has been a regular visitor to the blog for quite some time!

The reversible Moxon replaces the XM240 on the rotatable side mount. Despite the limited rotation, since it's reversible it covers 275 of the compass, from southeast to west (default direction) and from east to northwest (reverse feature enabled). The dead spots are between west to northwest and east to southeast, which from here are the least productive directions. The 3-element yagi at the top of the tower has full compass coverage, including over-rotation when needed.

The antenna default direction allows coverage of the Caribbean, South/Central America and almost all of the US. That leaves its big brother (the 3-element yagi) to focus on Europe and longer path DX where it shines. Even so, the lower antenna performs better in all directions before sunset and after sunrise, and during geomagnetic disturbances, when lower elevation angles encounter D-layer absorption. That's due to their respective heights, not their relative performance.

As previously noted, this antenna requires NEC5 for accurate modelling. Although it took a lot of segments to get sufficient accuracy, the built antenna tracks the model exceptionally well. It seems to be well within 0.5% of its design frequency -- about 25 kHz. That's even more impressive considering the close spacing of the capacitance hat tips (30 cm) which increases sensitivity to inaccurate modelling.

Another critical measurement is the inductance of the coil that makes an element the reflector. Careful tweaking of the design resulted in a value of 1.2 μH. Values between 1.1 and 1.3 μH would be acceptable, provided that the two coils are equal. Otherwise the frequency range will differ from the other direction, which is undesirable. The measured difference is a very close 25 kHz. That's the same as measured for the sanity check with the antenna on the ground.

The test setup at right was used to measure the inductance for both element switch boxes. To roughly account for the short leads from the analyzer, I set the inductance a little higher than 1.2 μH. The inductance of the internal leads and relays requires no compensation since it is present during use.

To test the opposite direction, a 50 Ω load is connected to the coax connector. The stray inductance of the leads and relay has an effect, however it is only a few ohms of inductive reactance. That is low enough to be considered negligible.

The DPDT relays behave oppositely in each switch box, with one connected to the element in its NO (normally open) position and the other to the coil. The relay coil is energized to test the switch box in its opposite direction. 

You can see from the alligator clips to the 13.8 VDC power supply that I use a common ground (return) path for both RF and DC. I do the same for all my switching systems. Therefore only one conductor is required to energize the relay coil. 

A stainless screw stud serves for the relay power. A 1N4007 diode in the centre box (see below) acts as the flyback (suppressor) diode for the relays in all three switch boxes. This is possible because to reverse direction all relays are energized.

One of the faults that I had to repair was the relay in one of the element switch boxes. I was surprised to find that the relay coil was essentially shorted: 5 Ω rather than the expected 340 Ω. I don't know how that happened since it worked fine on the ground. Repair of the relay isn't possible since it's a sealed unit. I've never had a problem with this line of Omron relays for RF switching. I tossed it into my NFG bin and replaced it with my last DPDT relay (I'll have to order more). 

The other fault was in the centre switch box. The stud for the relay coil loosened, allowing the internal lug to touch the enclosure wall and short. It's a tricky bit of hardware since the DC path has to pass through the metal enclosure. 

I discovered that the insulator was too high and could not be properly tightened. I replaced it with a plastic washer that has an inner rim to centre the screw within the hole. A thin layer of silicone caulk is used as a moisture seal.

Do you recognize the enclosure? I reused the one for the upper 20 meter matching network to compensate for a misbehaving gamma match. After I repaired the yagi's gamma match the network was removed. All it needed was a third coax connector and a relay stud.

The capacitance hat clamp hardware was tightened. I had forgotten to fully torque the nuts before its first raising in the spring. The clamps rotated when the tram line was drawn over them. While the yagi was on the ground I made a few other hardware adjustments to the antenna. It should now be far more robust.

The same crew from the earlier raising (and lowering) happily came over one more time -- it's good to have friends. Hopefully this is the last time we need to raise the Moxon for a long time to come! 

The only rigging change was to run the tram line under the trailing capacitance hats. As usual there were steering issues to fit the elements between the guys immediately below its mount on the tower and the TH6 above. We used tag lines and a lever on the boom to tilt up the leading element tips. It took a lot of force on the rope connected to the end of the lever to rotate the boom of such a heavy antenna. I did the same for tramming the 3-element 40 meter yagi to clear the upper guys.

It was a bit of bother to get the boom attached. The antenna is too heavy to push it into position with one hand while driving the saddle clamps home with the other and reaching around the back to thread on the lock washers and nuts. I gave my friends what were to them very confusing directions on how to use the tag lines to bring the boom flush to the plate. But it got done just the same. The little HT's VOX feature was a perfect third hand.

With the antenna secure I measured the SWR in both directions (a friend in the shack operated the reversing switch). The SWR was near perfect but about 25 kHz different between directions. That agreed closely with my sanity check on the ground. You can look at the forward SWR curve in an earlier article and imagine the reverse direction curve shifted to the left a bit.

Unfortunately I could not easily grab screenshots on the tower at the time and then the connector was weather proofed. The above plots were taken in the shack when I was back on the ground. Different but still excellent. There are two likely causes. One is the ~80 meters of transmission line and several antenna switching relays between the antenna and shack. Although I use good and tested coax, mostly LDF5 in this case, some impedance deviation is normal. The other less likely but possible cause is that there is no CMC (common mode choke).

I debated whether to put one at each feed point, at the element switch boxes, or just one at the rotation loop. Since the SWR is fine at the latter point I'll probably put the CMC there. It isn't urgent. 

Now we come to the important part: how it performs on the air. The overlaid azimuth plots at right, in 75 kHz steps from 7.0 to 7.3 MHz, set the expectation. Again, NEC5 is used in the model with the antenna at its actual height over EZNEC medium ground.

The free space gain peaks at close to 7 dbi at 7.0 MHz and gradually declines as we move up the band. F/B behaves the same. Gain, F/B and SWR are really quite good right across the band.

When I had the XM240 on the same rotatable side mount during DX contests I would often aim the 3-element yagi to Europe and use the XM240 to work the US, South America and other southerly directions. Since, like most yagis, the XM240 is uni-directional so it only covered west to southeast (~130°).

As already mentioned, the opening to Europe before sunset favours high elevation angles. With the reversing feature of the Moxon I can quickly switch directions to take advantage of its lower height in this circumstances. It also does well after sunrise towards the west while I point the 3-element yagi north for long haul contacts into east Asia.

Recent conditions turned the tables on my expectations. With so much geomagnetic activity, low elevation angles encountered high absorption. That meant the Moxon at its lower height outperformed the 3-element yagi on many DX paths, including the all important one to Europe. That shows the Moxon's value but it made comparisons difficult.

It is fair to say that the Moxon's gain is better than the XM240 with its coil-loaded element. That isn't a surprise since the latter is a high-Q antenna that also has a narrow SWR bandwidth. This is best I can do to assess gain on the air since it is very difficult to measure differences of a db or two, especially with the XM240 on the ground!

F/B is far easier to assess with the reversing feature. It is excellent on all paths within the CW segment and up into lower part of the phone segment. Once we reach 7.2 MHz the F/B becomes quite poor, possibly worse than the model predicts. I have more testing to do so this is not the final word. However, the excellent SWR means that the entire band can be worked without an ATU, even with my Acom 1200S solid state amplifier.

I have extra coax coiled up for the runs to each element that could be used to wind on a ferrite toroid to make a CMC. I used longer lengths of LMR400 for routing flexibility, not for a CMC; LMR400 is too stiff for that use. When I added the outer shield of the coax to the model it suggested that common mode may be present. A poor F/B (pattern distortion) is the most obvious consequence. However, the excellent F/B lower in the band is evidence that disfavours that explanation.

Another possible cause is the interaction due to the proximity of the capacitance hat tips to the guy wires in some directions. The distance isn't really very close except in comparison to the 40 meter wavelength. Again, that seems like another unsatisfactory explanation due the antenna's excellent F/B low in the band. The same can be said for the proximity of the TH6 above the Moxon, even after we lifted the TH6 higher earlier this year.

Clearly I have more testing to do. Even if it is the F/B is poor high in the band I am not too concerned. The reason is that as a contester I value gain and SWR more than F/B for the potential contacts to be made off the back. I can live with it. I will add a CMC later at the rotation loop (not at each element).

A picture further above shows another oddity: the rotator. That's a Hy-Gain Ham-M that I've owned it for 40 years -- I refurbished at least twice -- and it's at least 10 years older than that. It is under-powered for this large antenna. What saves the day is the symmetry of the reversible Moxon. High winds, even those that excite oscillations, place very little net torque on the mast. 

The only real problem is the 105 lb antenna's momentum. You typically have to wait several seconds for the antenna to coast to a stop before it stops turning so that the brake can be safely engaged. If it becomes a problem I'll swap in a rotator with a higher overturning moment. But not this year since I don't have one at hand.

To summarize the antenna after a years long process is, to put it simply, I like it. I could live with the reversible Moxon as my only 40 meter antenna. It's that good. I am not surprised that many contesters stack 2 or 3 W6NL Moxons on 40 meters rather than take on the far greater challenge of a 3 or 4-element yagi despite the latter's better performance.

NEC5 made this antenna possible. I still marvel at how closely the model agreed with the built antenna. It took many modelling variations and tweaking to make a symmetric and reversible yagi that performed to my satisfaction. While few will ever buy or build an antenna of this size, I believe that I've demonstrated what is possible with enough ambition. Perhaps one or more readers will be incentivized to do the same.

And the XM240? I might yet raise it again on the 20/15 tower and point it south as a multiplier antenna (similar to the how the TH6 is being used). The Moxon's reversing feature makes that job less urgent. I'll mull the idea over the winter, and whether to stack the Moxon and the 3-element yagi.

I look forward to working you with the Moxon in the upcoming CQ WW SSB and CW contests.