[TowerTalk] Relays for antenna switching?

[Tom Rauch]

> Does anyone have wisdom to share on selecting relays for home-brew legal
> limit remote antenna switches?  In particular, can anyone explain to me
> what an RF Relay is, as opposed to the common Power Relay?

Three things combine to cause SWR problems, the AMOUNT of 
impedance bump, how that impedance changes along the length of 
the bump, and the overall LENGTH of that bump.

The best relays either use short wide leads, or are a constant 
impedance that matches the transmission line.

Of course current rating and voltage rating are important, and the 
dc or low frequency AC current contact rating printed on the relay 
has little to do with the non-switched RF current capability of the 
relay.

Some 15 ampere rated relays will overheat at radio frequencies 
with 5 amperes, while other 5 ampere relays will handle 15 
amperes or more of steady RF current! 

In general the best relays have wide armatures with thick leads 
using non-woven strands, or better yet pull a shorting bar between 
two stationary contacts (like the RCS-8 relays do).

To give you an example, an RCS-8 style relay using the same 
contacts but with wire leads is good for about 2 kW ICAS to 30 
MHz, but that same relay as configured with the shorting bar 
(called a "form X" contact) will handle 10 kW ICAS at 30 MHz and 
work up to 250 MHz with reduced power!

Bottom line is look for a relay with good insulation and very thick 
leads, or better yet use a form X contact with big contacts.       

> Is there any reason that I can't use an off-the-shelf 30 amp power relay
> to switch RF?

Some will work, some won't. The low frequency AC rating is almost 
meaningless.



73, Tom W8JI
w8ji@contesting.com

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