40 Meter Dipole (aka Yagi Element) Installed

The experimental 40 meter yagi element is now complete and it is installed high in the sky. Before being raised a few changes and additions were required:

• Continuous centre pipe, replacing the split centre dipole feed
• Fitting the 2.375" schedule 80 centre pipe to the 1.9" OD pipe with a shim and a recessed bolt for a good mechanical and electrical bond
• Fitting the custom capacitance hat clamps and replacing the nuts with nylocs
  
• Gamma match built, fitted and tuned at a lower height
• Mast clamp made from an ancient aluminum plate and even worse muffler clamps
• The antenna was resonated at a higher frequency so that it would exhibit capacitive reactance, which eases gamma match adjustment
  

There are two reasons for installing the antenna. First, to test its robustness over a cycle of seasons, including wind and ice. If deficiencies are discovered I will correct them before constructing a 3-element yagi. The second is to have a high 40 meter antenna this contest season. Although it is not directional I want it for working distant DX multipliers and also on shorter paths as the MUF drops in the evening. 

 

Cycle 25 has arrived but for at least the next year 40 meters will continue to go long early in the evening. The XM240 at 21 meters height work well but sometimes it is too low for the best results. The broad pattern will come in handy for working stations when the XM240 is pointed elsewhere.

 

Gamma match

 

The antenna was trammed to 70' (21 m) to adjust the gamma match. It is far easier to do it there than at 150'. The rigging was already in place for another job so it was convenient. A dipole's impedance changes with height so I made allowances in the gamma match design. A final adjustment would be done when it was at its final height. The adjacent TH6 had only a small effect on the tuning.

 

I used gamma match software to determine the approximate gamma rod length and capacitor value. They were built longer and larger, respectively. Although all the software I've tried does poorly since gamma matched are fickle creatures it at least gave me something to shoot for. 

 

The rod is telescoping sections of ¾" and ⅝" tube, chosen for the ratio of ~4 between it and the parallel element sections. The straps holding the gamma rod (one conductor and one insulator) were made junk box parts. Gamma rod spacing is 12" centre-to-centre.

 

I started the adjustment with a variable capacitor to speed the process. I measured the capacitance for the matched position and made the final capacitor out of RG213 (shield removed) and inserted it into the gamma rod. Unfortunately that didn't quite work out. Inside the large tube the capacitance was only 1.5 pf/inch (0.6 pf/cm) versus 2.1 pf/inch when inside a ½" tube. A 180 pf capacitance (the estimated value +20%) far too long. 

I made another capacitor with an intact length of RG213, using the shield as the inner plate. That was much better at 4.8 pf/in (2 pf/cm). The PVC coax jacket is higher loss than the polyethylene inner dielectric but it is quite low at 7 MHZ. I added a layer of vinyl electrical tape for a higher flash over voltage, just in case.

A challenge with the adjustment is that the gamma match for 7 MHz is large. The strap is outboard at 7' to 9' (2.1 to 2.8 m) and the gear clamp for the telescoping rod is at the end of a 4' (1 m) tube. The latter can be reached with care but the former requires 90° rotation of the antenna and a way to hold it in position alongside the tower, and to do so with damaging the antenna and other tower attachments.

Adjustment of the match will be easier on the yagi. The element can be rotated on the boom to bring its full length within reach of the tower. All it takes is pointing the antenna where the rotation can be done without striking guys and other obstacles.

Rigging

A conventional tram line was used for lifting the antenna, with one important difference. The 40 meter antenna was to go to the top of the mast and the TH7 was in the way. The TH7 was turned so that the elements were parallel to the tram. The 40 meter antenna is given the same orientation, which is a little unusual (see pictures below). The challenge was to thread the antenna through the space between the TH7 elements and around other obstacles.

The tip to tip span of the capacitance hats is almost twice that of the opening between the TH7 driven elements. Further, there was no good way to attach a tag line for steering the antenna that would make it easy to remove it afterward. It would be too far out on the antenna to be reached and looping it around and down could easily damage antennas and tangle in the guys.

 

 

The solution was quite simple. As the upper tip approached the tower I ascended and manually steered it. I followed it up, directing it around the top guys, outboard prop pitch motor platform, TH7 elements, over the boom truss and past the mast. The last is critical since it's easy to forget and direct it to the opposite side of where the mast clamp must be. The antenna was rigged to be well balanced and it could be steered by the ⅜" tip with a light touch.

The difficult part was inserting the arms of the mast muffler clamps through the clamp plate. The antenna must be held level and flat on the plate long enough to get at least the first washer and nut on. Since the antenna was rigged to point upward to ease the threading operation the antenna was not level. Standing on top of the tower holding a 62' long and 55 lb antenna level against the weight imbalance and breeze while threading the clamp is, to put it mildly, not easy!

 

I recruited the tram line rope (now slack) to grip the "lower" side of the antenna to hoist it level and knotted it to keep it there. Although it rocked in the breeze it was stable enough to get the job done. The antenna was now halfway up the 10' mast, just above the TH7 boom truss. After descending for a lunch break I returned to adjust the gamma match (see below) and complete the lift. 

 

I stepped the mast and moved the pulley to the top. My friends pulled from below as I slid it upward. It went slowly because the clamps like to grab the mast. It's difficult to perfectly balance and align the mast clamps. I adjusted the gamma match, trying different antenna orientations, and then bolted it down and removed the rigging. 

 

A short length of RG213 connected the antenna to the rotation loop (already present). The coax was tied to the mast and the joints sealed. Mission accomplished. This description of the process may be too much detail for many readers but for those contemplating similar lifts the details may be helpful.

Adjusting the match

The impedance of a 40 meter dipole is affected very little by higher band antennas and the segmented guys. This is the expected behaviour for interactions with shorter conductors. 

 

The ground is a greater concern since for a 40 meter dipole since, relative to wavelength, it is never far away. Yagi impedance becomes independent of ground at low heights because they are directional.

Using EZNEC I calculated the impedance shift from the lowest tuning height of 21 meters (λ/2) to 46 meters (1λ). Unfortunately the transformation of that shift via the gamma match is difficult to predict. I've had poor experience accurately modelling gamma matches with software tools. Also, a gamma match is not ideal for the high R of a dipole but I wanted to test this as well as part of the 40 meter yagi project.

I did get an SWR of 1.2 at 21 meters height although I had bottomed out the gamma capacitor. I should have made it longer. Since it was close enough I decided to go with it for the final lift. The SWR was worse up there since R ended up higher, moving the gamma rod strap was too difficult and there was residual reactance the capacitor wasn't large enough to correct. 

 

The SWR was no better than 1.5 and rose to 2 at 7.3 MHz. It could have been improved but it was late in the day and I was sore from all that hanging from the top of the mast. I can replace the capacitor later if I'm so inclined. I may not bother.

 

The match is different when the 40 meter dipole is parallel to the TH7 elements or boom. It is mostly unaffected by the elements, which are short compared to the antenna, but there is coupling to the boom and truss when parallel to the boom. This occurs because the TH7 in this orientation looks like a 40 meter dipole (the boom) with large capacitance hats (the outer elements). 

 

With a suitable matching network the TH7 could be used as a poor 40 meter dipole. Likewise a 3-element full size 40 meter yagi can be used on 80 meters with a suitable matching network. An omega match is a good choice. However it requires other changes that are beyond the scope of this article. 

 

I oriented the dipole parallel to the TH7 elements which is the sensible choice with regard to interactions and operating convenience.

 

Performance

 

Testing of the antenna's performance is just beginning. It's about more than traditional antenna metrics. "After all," I said, in reply to a friend who asked me how well it works, "a dipole is a dipole." This is what I want to learn:

• Wind survival
• Ice load survival
• Gamma match survival
• 15 meter interaction
• Height advantage or deficit
  

The wind surface area (cylindrical projection) is 9 ft², including the boom-to-mast clamp and gamma match. The gamma match alone is 0.65 ft². The capacitance hats are robust but they stress the element where they are attached. I put them at the outer edge of the 1" × 0.120" tube rather than farther out -- where they would have more effect -- on a tube with 0.058" wall thickness. The total wind surface area of both capacitance hats is 0.5 ft².

 

The SWR of ~1.5 I measured on the tower was replicated in the shack. Then it rained and the SWR improved. After drying out it got a little worse but was a perfect 1:1 a little below the band edge. Clearly something odd is happening. Next time I'm up there I'll do an inspection. It may be nothing more than rainwater in the gamma rod. An unexpectedly low SWR is no less worrisome than a high SWR.

Notice that the SWR is high on 15 meters and that the third harmonic is ~24 MHz. This agrees with the measurement during experimentation and meets the design objective. As noted earlier the 15 meter SWR of the TH7 below it barely budged. I have not as yet tested the 15 meter pattern and some deterioration would not be a surprise. That is acceptable since the 15 meter stack will soon be online; for the next year the TH7 will contribute more as a high 10 meter yagi.

The antenna is noisy on 40 meters because a dipole is not very directional. A yagi pointed north of an east-west line is typically quieter from here because northern latitudes experience less violent weather. Despite that the comparison to the XM240 has proved interesting. Keep in mind the XM240 has ~6 dbi gain which is 4 db more than the dipole. On many receivers that is 1 S-unit.

My on air testing was delayed by a geomagnetic storm that erased most signals and altered elevation angle behaviour of DX signals. When conditions improved I tried again. Really, there is nothing exciting to report. It's a dipole high in the sky, and that's how it behaves. Mostly it served as a reminder of how height affects antennas, and that different heights make a difference, though perhaps not always as you might expect.

The lower XM240 was always better before sunset and during the geomagnetic storm since signals at low angles are more attenuated due to the longer path through the absorptive D and E layers of the ionosphere. Under more favourable conditions the high dipole is roughly equal to the XM240 on intermediate paths. On long paths the high dipole is better. It is also better to Europe later in the evening when the MUF drops closer to 7 MHz and favours low elevation angles.

Comparing antennas with equal gain would more often favour the higher one based on these early observations. The poor directionality of the dipole came in handy to hear and work stations in diverse directions, but often required help of a Beverage on receive to improve SNR. While not ideal for general operating it can be an advantage during contests.

I was rudely reminded of the ferocity of precipitation static on high antennas. When it occurs you use another, lower antenna or you listen  with low band receive antennas. Lower antennas suffer less from the effect.

With the 40 meter antenna in the air I am free to finish the work on the 20 and 15 meter stacks. Winter is coming and there is much yet to be done.

Photo credit: The two pictures showing me on the tower were taken by Alan VE3KAE.