Issues with segmented antenna elements

During the 2024 January VHF contest, the VSWR of my tower-mounted 6M5X went haywire while running high-power in a meteor scatter sked with K5QE. This destroyed the expensive LDMOS power transistor in the M2 solid-state kW amplifier (eventually repaired by John at Island Amplifier). After a few hours of troubleshooting, the source of the high SWR was isolated to the antenna and/or the brand-new flexible coax loop at the top of the tower.

With the antenna back on the ground, no obvious issue was found. The SWR as measured by the nano-VNA was erratic leading me to suspect something was amiss with the UHF connectors. Over the course of several iterations and many weeks, I replaced the rotor loop coax, T-match, and balun. The response looked decent at ground level, but when mounted up on the tower, the VNA sweep again revealed very intermittent SWR, jumping up into the range of 3:1--4:1. The odd behavior was independently confirmed with an antenna analyzer.

I could "cure" the poor SWR by connecting to the transceiver and applying a few Watts of 50 MHz RF power for a few seconds. The transceiver was instantly happy and even stranger -- re-connecting the VNA now showed a decent sweep. This good match would persist for perhaps 10 minutes to several hours, but eventually the SWR would go high on the VNA. In addition, rain would drop the SWR as measured by the VNA to an acceptable level in real-time. When the water evaporated, however, a poor SWR returned.

As long as I was pushing transceiver power into the antenna, there was no appreciable reflected power. But I couldn't risk using the amplifier without having a reliable load on it. I brought the antenna down again and carefully sprayed water on the individual elements while watching the VNA. When I got to the reflector, the SWR dramatically improved. This isolated the problem to the reflector element clamp. Taking it apart revealed what appeared to be significant oxidation or burning in the channels. The left photo below shows one side partially cleaned and the other with a blackened surface as I found it. This thin dielectric film was causing intermittent conductivity across the length of the reflector element. Note that the 1-inch 8-32 SS screws don't provide a reliable electrical path into the clamp as they are secured by nylon locknuts.

Not sure why this corrosion developed. I did not use any anti-oxidation compound when assembling it, at least not here in Florida. I don't recall using any paste when I first built it in 2007 when living in New Mexico, although that was so long ago I may have forgotten. There is no mention of it in M2's assembly instructions, so I'm pretty confident I wouldn't have applied any.

I removed all the elements, cleaned them with a wire brush along with the clamps, and re-assembled using Ox-Gard paste. I researched this subject a bit as there are many anti-oxidation products available. Ox-Gard appears to be a good choice as it has zinc particles for aluminum-aluminum connections and is also designed for outdoor use. After refurbishing the antenna, the VNA sweep showed excellent SWR, very close to the factory specification. I followed independent suggestions by N2CEI and WA4QLA and installed shorting straps at all 5 element clamps using stranded copper wire; the modified reflector clamp is shown in above photo. There is a bend in each strap to allow access to the boom mounting screw. The idea is that these shunts will maintain continuity if corrosion in the channels re-appears. W4IT recommended filling the exposed channel fillets with black RTV to discourage rainwater from accumulating there.

The modified antenna worked fine for about three months. Until it didn't.

The intermittent SWR problem returned in late summer 2024. At this point, I could never get a good VNA sweep in the hamshack. Applying Watts of power produced an acceptable SWR as displayed by the transceiver. This reproduced the behavior that existed prior to the refurbishing and shorting strap modification except that now the low-drive VNA indicated the antenna was chronically unhappy. Even heavy rain was unable to temporarily restore a decent sweep as had happened previously.

The antenna was lowered off the tower yet again. On the ground, it was discovered that the driven element had accumulated a large amount of built-up bird sh*t:

There was enough there to thoroughly coat the element clamp and penetrate into the critical contact surfaces including at the shorting strap. The weak probing signal from the VNA was evidently insufficient to push through the offending film. Once cleaned up, an acceptable SWR returned.

My conclusion is that the segmented element design is fine for temporary/portable use, but too unreliable for a permanent installation. Inverting the antenna to place the clamps under the boom might be a possible solution. Folks with crank-up or tilt-over towers may not mind ongoing antenna maintenance, but since I have to climb I really needed a more robust and reliable design. In November 2024, I replaced the 6M5X with the 6M6DX yagi that was acquired from Antennas-Amplifiers in Serbia. This company was highly recommended by the prolific EME operator Peter KA6U.