Topband: Blowing diodes on relay switches

[Shoppa, Tim]

The antenna end of mine, looks very similar to PG0A/PA3FYM implementation, drawn at http://remco.org/wordpress/wp-content/uploads/2008/01/pg0a-k9ay-small.jpg

Uses an isolation transformer (not an autotransformer) and provides galvanic isolation between antenna, antenna ground rod, and the coax.

I do not use LED's in mine, I just use regular diodes, and these are the diodes that are blowing. I'm starting to believe that it's during RTTY contest weekends when I'm on 10 a lot, that the diodes are blowing, and I think Tom's suggestion about bypassing effective for  10M (not just 160M) may be most relevant.

Tim N3QE

-----Original Message-----
From: kd9sv [mailto:kd9sv at comcast.net] 
Sent: Friday, November 15, 2013 7:43 AM
To: Shoppa, Tim
Subject: RE: Topband: Blowing diodes on relay switches

Tim can you share a schematic of what you are doing?  I don't see how it is possible to couple DC voltage through a transformer...de gary

-----Original Message-----
From: Topband [mailto:topband-bounces at contesting.com] On Behalf Of Shoppa, Tim
Sent: Friday, November 15, 2013 7:34 AM
To: 'topband at contesting.com'
Subject: Topband: Blowing diodes on relay switches

I have a homemade K9AY loop with the classic diode setup to pick direction remotely. Unlike the classic switching scheme with Bias-T's, I use isolation transformers at both ends to couple switching voltages into the coax. I started with 1N4001 because they looked more than beefy enough.

This past summer I lost the diodes several times, I assumed due to thunderstorm activity. Diodes failed shorted each time. Easy enough to fix.
This time I put in 1N4007's.

This past weekend I again lost the diodes, again failed shorted. No thunderstorm activity but maybe induced signals from my TX antenna? Big RTTY contest with the amp on.

I also had built a little switching matrix of transistors and foot switches inside the shack to let me pick direction with a footswitches. Those transistors have blown several times too. Again first time I assumed thunderstorm but not so sure anymore.

So obviously there is some voltage differential zapping these simple low voltage switching devices. Thing is, the switching network is completely galvanically isolated at the antenna end. Confirmed with ohmmeter and HV supply. Unlike the classic K9AY design, I use isolation transformers at each end to couple the switching voltages in.

So what is killing these simple semiconductors? Thinking about removing all semiconductors and running independent switching wires.

Tim N3QE
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Topband Reflector