Cushcraft A3S Antenna Repair

I bought a Cushcraft A3S antenna in October of 2020. It was delivered in good condition and I assembled it according to the provided instructions. The only hitch in assembly was that one of the brackets (part number 190143) that goes between the elements and the boom was made crooked, causing the director to be 17 inches closer to the driven element on one end than on the other. I contacted MFJ and they sent a new one but not before I put the antenna up. I should have checked it out on the ground, but we had a weekend of unseasonably good weather at the end of October and I was eager to have it up for November Sweepstakes. Only after it was up the tower and the weather had turned unpleasant did I put an antenna analyzer on it.

On twenty meters, the lowest SWR was 1.8 and was reasonably near the center of the band. On fifteen meters, the lowest SWR 1.6 but it was almost at the top of the band. On ten meters, the lowest SWR was 1.1 and near the bottom of the band. It was good enough to make some contacts over the winter, but I had to be careful about where I was in the band and — in some cases — use an antenna tuner.

Twenty meter band
FrequencyStanding Wave Ratio
14.00 MHz 2.6
14.05 2.2
14.10 1.9
14.15 1.8
14.20 1.8
14.25 1.8
14.30 1.9
14.35 2.0
Fifteen meter band
FrequencyStanding Wave Ratio
21.00 4.3
21.05 4.1
21.10 3.9
21.15 3.5
21.20 3.0
21.25 2.5
21.30 2.2
21.35 1.9
21.40 1.7
21.45 1.6
Ten meter band
FrequencyStanding Wave Ratio
28.0 1.2
28.1 1.1
28.2 1.2
28.3 1.6
28.4 1.4
28.5 1.5
28.6 1.7
28.7 1.9
28.8 2.0
28.9 2.2

Over the winter, I started seeing signs of intermittent connections in the antenna. With an antenna analyzer connected, I could watch the SWR and impedance meters jump around when the wind blew. The following summer was busy, but come fall we had some nice weather for tower work and I got the antenna down and disassembled it. Inspecting the traps, I found many of them had play in the aluminum ends. The cans all seemed to be reasonably tight, though.

I disassembled the traps and took a closer look. All of my traps have injection-molded plastic (low-density polyethylene maybe?) coil forms wound with bare aluminum wire. From pictures I found on the Internet, I think in the past MFJ used varnished copper. Going back further, it seems the traps were once made with aluminum (?) wire over a fiberglass rod, all covered in some kind of sealant.

The ends of the coils were attached to the aluminum tubing with self-tapping screws that went part way into the ends of the coil form. These also provided the mechanical connection. I didn't find any evidence of anti-oxidant compound on the aluminum-to-aluminum connections — neither on the coils nor the cans. It seems like maybe the lack of rigidity in the mechanical joints had led to the screws backing out a little, allowing for intermittent connections.

I made some measurements on my traps for future reference. I measured the distance from the inboard end of the tubing to the center of the can mounting hole on each trap, thinking the cans might have been hand-tuned. The distances did vary, but I'm not sure if that's the result of hand-tuning or production tolerance. The distances didn't seem to have a strong correlation with coil inductance.

All of the TA traps used a 2-3/4 inch coil form (not counting the stubs that went into the tubing). They had 11 inch cans with one spacer on the inboard end and one or two spacers on the outboard end. The length of the outboard tubing was 9-1/4 inches and the inboard tubing was 5-1/4 inches long.

All of the TB traps used 4 inch coil forms. They had 12 inch cans with one spacer at the inboard end and two at the outboard end. The length of the outboard tubing was 10-3/4 inches and the inboard tubing was 5-3/16 inches long.

All of the TC traps used 4 inch coil forms. They had 12 inch cans with one spacer at the inboard end and one spacer at the outboard end. The length of the outboard tubing was 10-3/4 inches and the inboard tubing was 5-3/16 inches long.

Trap physical characteristics
#TypeOutboard
spacers
Coil
turns
Can
distance
Coil
inductance
Corrected
inductance
1TA2151-11/16 inches1.912 µH
2TA2151-11/161.919
3TA2151-9/161.919
4TA1161-3/42.0931.874
5TA1151-9/161.905
6TA1151-9/161.912
7TB2251-5/83.533
8TB2251-7/83.544
9TC1262-1/43.704
10TC1262-3/83.4083.703
11TC1262-1/43.4093.694
12TC1262-3/83.3833.664

I'm not sure why some traps had an extra spacer in the outboard end. I saw some speculation on the Internet that it might be an attempt to transfer more of the bending load to the outer can because the plastic coil forms are less stiff than the fiberglass was.

I don't know why one of the TA traps had an extra coil turn. It didn't seem to be compensated for by the can overlap, leading me to suspect that the variance in can position is just down to production tolerances. I decided to remove the extra turn.

I also noticed that one of the TC traps had a higher inductance than the TB traps — as expected — but that the other three had lower inductances. Closer inspection revealed that each of these three had a turn shorted at the outboard end. Prying the shorted turns out of contact with each-other raised the inductance to the expected range.

To provide a better electrical connection, I removed all of the self-tapping screws that connected the coils to the tubing. I drilled the existing holes the rest of the way through with a close-fit clearance drill for a #6 machine screw. I applied anti-oxidation compound to each joint and installed machine screws with a flat washer over the wire and a star washer under the nut. After tightening each joint, I ground off the screws to be flush with the nuts.

To stiffen up the mechanical joints, I drilled holes through one wall of the tubing and into the coil form perpendicular to the machine screws. These holes were sized to be a slight interference fit with the self-tappers, which I re-installed. Constrained through two axes like this, the joints no longer wiggled.

I was concerned about the affect moisture, movement, and itinerant insects might have on the bare aluminum wire over time. I applied a good coating of home-made Q-Dope (polystyrene in ethyl acetate, 30% by weight) to seal it up. Polystyrene has a very low dielectric constant, so I don't expect it to change the interwinding capacitance much. Other sealants might hold up better or provide a better bond.

With the coils and mechanical parts taken care of, I re-installed the cans and spacers as I had found them. I set up my spectrum analyzer to check the resonant frequency of each can. I made a note of the resonant frequency as well as the distance from the inboard edge of the tubing to the inboard edge of the can as it had come from the factory.

Trap resonant frequencies
# Type Expected
frequency
Measured Corrected
Frequency Distance Frequency Distance
1TA28.60 MHz28.62 MHz3-3/8"
2TA28.6028.583-3/8
3TA28.6028.363-15/3228.62 MHz3-9/32"
4TA28.6029.253-9/3228.593-11/16
5TA28.6029.253-7/1628.613-11/16
6TA28.6029.083-15/3228.613-25/32
7TB21.5021.233-15/3221.503-7/32
8TB21.5021.523-7/32
9TC21.3021.832-3/421.303-3/16
10TC21.3021.742-11/1621.303-1/16
11TC21.3021.842-13/1621.303-1/4
12TC21.3021.832-3/421.323-1/4

The spread in values on the TA traps surprised me. More surprising was that although the TC traps were consistent in value, they were all much higher than the TB traps when the reverse should have been the case. I thought I could see the presence of an extra spacer increasing the capacitance of the TA traps, but the effect seems less apparent in the data when looking at the TC traps. Could just be a coincidence. I haven't done the math to see what the expected difference would be due to the change in dielectric constant.

Traps that were close to the expected resonant frequency I left alone, but the rest I adjusted before going any farther. I wanted to replace the self-tapping screws used to mount the cans with machine screws since I thought the would be less likely to back out when used with a star washer and properly tightened — time will tell. Where I used machine screws on the coil connections, the tubing was supported by the coil form inside so that the screws could be tightened without crushing the tubing. I made up some plastic inserts to support the tubing where I would put the machine screws for attaching the cans. With the supports in place, I drilled through the tubing and installed a machine screw with a flat washer and star washer to mount each can. Before installing the screws, I applied anti-oxidation compound to the electrical connection between the can and tubing.

With the cans refurbished (or just furbished?), I re-assembled the antenna to factory specifications. I used the new bracket on the director element and found that it improved the parallelism only slightly. Upon examination, it appeared that the circular cutouts for the boom were not perpendicular to the crease that the element sits in. I used a rotary tool with a coarse sanding drum to adjust the cutouts so the bracket would sit perpendicular to the boom. To replace the plastic caps I had cut off the traps, I used two-inch (un-shrunk) diameter 3:1 heat shrink tubing (the sturdy polyolefin stuff). I used a length of about 1-3/4" on each can-end.

I wanted to adjust the antenna to have its best SWR match in the middle of the phone portion of each band. I started out by standing it up vertically with the reflector on the ground and adjusted the element lengths to get the results I wanted. My first attempt at this was to lean the antenna against the side of the clubhouse, but I noticed that my measurements changed a lot if I moved the antenna left or right along the wall. I imagine this was due to flashing on the outside of the building or wiring inside the wall causing trouble inside the antenna's near-field. Moving out into the lawn away from the building seemed to give more repeatable results.

When it was time to put the antenna back on the tower, I raised it up first with a spare length of cable attached and checked it out with the antenna analyzer. The frequency with the best match had gone up about 200kHz on the ten meter band. We ran the antenna down and up a few times to adjust the ten and fifteen meter dimensions. Twenty meters came out about right after the other adjustments were made. Running the antenna up and down was easy with the tram-line rope arrangement we were using.

I think if I had it to do again, I wouldn't bother with adjusting anything on the ground. I think I'd still sweep it with the antenna analyzer to see that it was put together right, but leave the fine adjustment for up the tower.

I was hoping to get 1.5:1 or better SWR across the phone portion of each band, and came pretty close. I only adjusted the driven element, leaving the reflector and directory at the dimensions specified for the phone portion of the band in the assembly guide. I made a note of the final section length for each band and the difference from the specified length.

Final dimensions
BandFrequencySectionLengthDifference
10 m28.65 MHzA26' 8-1/4"-1.2%
1521.325B25-7/8"+2.2
2014.25C23' 1"-1.3
Twenty meter band
FrequencyStanding Wave Ratio
14.00 MHz 2.9
14.05 2.3
14.10 1.8
14.15 1.6
14.20 1.4
14.25 1.4
14.30 1.4
14.35 1.5
Fifteen meter band
FrequencyStanding Wave Ratio
21.00 MHz 2.2
21.05 1.9
21.10 1.6
21.15 1.5
21.20 1.4
21.25 1.2
21.30 1.1
21.35 1.1
21.40 1.3
21.45 1.6
Ten meter band
FrequencyStanding Wave Ratio
28.0 MHz 2.1
28.1 1.8
28.2 1.7
28.3 1.5
28.4 1.4
28.5 1.3
28.6 1.2
28.7 1.2
28.8 1.3
28.9 1.4

In the process of adjusting the antenna, I made some notes about the relationship between resonant frequency and element length. These figures aren't exact and I suspect they're not linear either. But they did help me get in the right neighborhood quickly. Your mileage may vary.

BandRatio
20m140 kHz/in
15m180
10m68

I hope you find this information interesting and helpful. If you have any questions or notice any mistakes, please drop me a line. If this is the kind of thing you're into, you may enjoy my other articles.

Aaron D. Parks
aparks@aftermath.net