> 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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