[TowerTalk] Relays

[Jir(í Šanda]

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" !?!?

The best are vacuum realays even more expensive than the coaxial ones.

Only usable contact material plating is GOLD, but unfortunately most of 
the relays capable of the power are silver or paladium plated according 
to my experience very bad, just look to the cathalogues of OMRON, 
MATSUSHITA etc.. what small signal each relay guarantees.......

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.
 
This mail is not an advice what to do  - it is just what to watch !!!

73 !

Jiri
OK1RI

Mikael Larsmark wrote:

>Hello!
>
>I wonder what kind of relays should I pick when building antenna 
>switches. The coax relays are to expensive so that is not an option.
>
>I have actually used car relays for a while and they seem to work 
>quite ok. I would like to know though what ratings should I suppose 
>to look after? How many amps, the width between the "connectors" 
>etc. should they handle? Of course this depends on how much 
>SWR and power I'm using but if I say that they should handle 
>1.5kW and an SWR of 2:1, atleast!?!
>
>//Mike (SM3WMV // SM3W)
>
>
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