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[TowerTalk] Relays

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Subject: [TowerTalk] Relays
From: (Jim Lux)
Date: Tue Feb 25 13:09:40 2003
This brings up an interesting question... has anyone ever done some 
reasonable quantitative performance measurements on "non-RF" relays.  Sure, 
they aren't nice vacuum relays, and they probably have all sorts of 
capacitive and inductive parasitics.  However, those parasitics are 
probably reasonably small (and "tune-out-able").

For the run of the mill antenna switching application, you probably don't 
need 40 dB isolation for the unselected port, for instance.

Jiri brings up an interesting point, too, on oxide formation (as well as 
the related contaminant problem, for open frame relays) on the contacts.


Jiri wrote:
>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 +
>Mikael Larsmark wrote:
>>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!?!

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