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[TowerTalk] Relay source wanted

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Subject: [TowerTalk] Relay source wanted
From: (Tom Rauch)
Date: Wed, 27 Jun 2001 10:24:58 -0400
> Sounds interesting, Tom.  Can you suggest some specific relays that
> incorporate this contact design, or a reference that will list ones that
> do?  I know you've said in the past that they are used in the Ameritron
> RCS-8, but don't have one here I can open up to see.

There are dozens of off-the-shelf relays that use form-X contacts, 
so one would have to look through Web pages or catalogs. They 
are mostly power relays, and some that will easily handle 1.5kW 
are in the range of less than $2 each wholesale!

The RCS-8 relay is a custom relay. It is a good example of how 
well a form-X contact works at RF. If you don't hot switch that relay 
it will handle 10 kilowatts into a matched 50 ohm load up to 30 
MHz, and has negligible SWR below 250 MHz in a good layout.

The same style relay from the same source using conventional 
leads and normal contact arrangements is marginal at a few 
kilowatts and shows SWR problems at 30-40 MHz.

Relays are a lot like switches. The arrangement of contacts and 
the routing and positioning of voltage standoff and current carrying 
is more important that things that often impress us, like being big 
or having lots of ceramic.

Looking at the switch (or relays) and following the current path and 
looking at possible arc paths is the best way to pick a relay, along 
with eventual RF testing. The first thing I do with sealed is get a 
sample and look at lead lengths and surface area as well as 
potential arc paths, even if I have to cut a few sealed relays open.

Some very big and very expensive switches and relays handle 
considerably less power than some pretty small and inexpensive 
switches and relays. A small ruler and good eye will tell you the 
likelihood of success.
73, Tom W8JI 

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