So, for a couple of months I’ve had a growing suspicion that I’m not receiving as well as I am transmitting. I first noticed the problem when observing the strength of signals from GB3VHF which after one winter storm seemed much weaker than it had been. However, there seemed to be no impact upon transmit as SWR remained negligible and stations some distance away where still receiving me well even when my transmitter was generating about 0.5W PEP!
Since first noticed I’ve worked many stations and the signal reports always seem to identify that I’m not receiving as well as the remote station. Establishing this pattern has taken some time. It should be said at this point that its the same antenna and feeder used for both transmit and receive.
Its been a wet Winter and not really suitable for removing weather proofing from exterior connections and outdoors work. But now Spring has arrived things are finally starting to get dryer and warmer. Alongside this I’ve taken down the mast and antenna for work to support an new PRO.SIS.TEL 641D rotator. This has enabled a detailed inspection and test of feeder and antenna.
Before starting any work I thought about and identified the following possible causes:
- failure of the receiver, possibly signal amplifer
- damaged feeder, possibly a connector problem or water ingress
- damaged feeder at the antenna feed-point. possibly also water ingress.
Problem (a) seemed possible but without a second receiver its very hard to be sure. But its a quality Transverter (Kuhne TR144H) and I’d be surprised if that had partially failed – remember signals are only 2 – 3 S points down on what I think they should be.
So, (b) or (c) seemed much more likely.
So with the mast down and everything removed back to a dry, warm environment I removed weather seals – no sign of water or even the slightest amount of moisture ingress. The coax was tested with a DC Ohm meter – no shorts or higher than expected resistances; with an antenna analyser and dummy load – again this looked perfect.
So attention shifted to the antenna feed-point, pictured before it went outside for the winter and before the connections where weather sealed. Note: Grey plastic tape overlays amalgamating tape. Again, there was no signs of moisture ingress into the coax, but there where signs of galvanic corrosion between the aluminium driven element and the stainless steel (A2 grade) machine screws to which the feeder connected. Actually, to be more precise between the aluminium driven element and the flat washers on each machine screw. It was clear that the liquid rubber sealant which I’d applied had not sufficiently covered these connections and thus water had been in contact between the different metals acting as an electrolyte.
To be clear, the machine screws where not showing any significant signs of corrosion, just the washers. So the feed-point was thoroughly cleaned and reassembled without using the washers then liberally coated in liquid rubber to seal against contact with water.
Now, at this point I don’t know if this was the problem, but it does make some sense. Any transmit signal would be of a much higher voltage/current than a received signal so would probably cross a mildly corroded junction with marginal attenuation. But marginal attenuation of a much weaker signal would be more noticeable. That’s my reasoning anyway.
The acid test will be when I erect the mast and antenna again in a few weeks and see if received signals such as GB3VHF have returned to their earlier strength. When doing this I will also use new feeder, so if signals are stronger I’m never going to be absolutely sure the problem was galvanic corrosion. Of course the problem may still be present and it could be another cause such as the receiver!
For information, the antenna is an 8-element LFA Yagi from InnovAntennas.
Andrew
G0RVM
