[TowerTalk] Relays

[Doug Faunt N6TQS +1-510-655-8604]

   Date: Tue, 25 Feb 2003 18:45:56 +0100
   From: =?windows-1252?Q?Jir=28=ED_=8Aanda?= <jirka at jimaz.cz>

   Hi Mikael,

   you have two problems:

   1. - the smaller one - the relay should handle about 7-10A
   2. - THE MUCH bigger one - the relay should also be capable of 
   transfering 0.3 microVolt/50 Ohm - i.e. 0.006 microA . If you start to 
   play with this you find that this is THE PROBLEM. Most of the switches 
   work fine while on TX but you might be deaf on RX without knowing  - you 
   might have 10-30dB atenuation and how to find out, it might be easily 
   just the propagation. Just check up WX0B advice on all his gear. "Hot 
   switch time to time and the oxide on contacts and might burn out" !?!?

   Definitely if the relay is open frame and is silver plated you are going 
   to watch problems caused by oxidation thin noncoductive layer will broke 
   down while on TX so you will not observe any SWR deterioration , while 
   on RX it will be there and you will be listening just through the 
   capacity of the contacts and insulating layer ! You can expect the Oxide 
   layer to withtad up to hundreds of mV to couple of Volts than it brakes. 
    I have observed this phenomenon many many times and until we (me + 
   OK1RF) get of rid of most realys in the way. This phenomenon caused me 
   definitelvely a couple dozens of  decicive QSOs in various contest 
   during passed years.

I looked at the relays that Force12 uses in the relay boxes for
changing the center frequency of their 80M and 40M antennas.  They use
a standard 30A 250V DPST power relay.  I got some from McMaster-Carr
here in the US, which are exactly the same.  These relays have a
wiping action on closure, which should help some, and F12 recommends
turning the TX power to 10W and hot-switching them, periodically to
clean them, but warns against hot-switching at full power.  I've not
heard of any problems with them, but you can't really tell.

73, doug