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[TowerTalk] Relay Contact rating for RF useage?

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Subject: [TowerTalk] Relay Contact rating for RF useage?
From: (Jim Lux)
Date: Mon Aug 11 14:41:17 2003
At 11:52 PM 8/6/2003 +0000, RCARIELLO wrote:
>Hello to all,
>How is the rating of a relay converted to RF usage?

It's not, really...

RF properties are sufficiently different that you basically have to measure it.

The big difference you see in voltage ratings for AC and DC more reflect 
typical usage (i.e. 110-220V AC power, 12-24 Battery power, so that's where 
the relay is designed, tested, and rated), and the "arc pulling" aspects of 
DC usage.

On both voltages and currents, you have to allow for VSWR (or ISWR), and 
for peak vs RMS.

All relays have some capacitance from contact to contact and from contact 
to frame/coil.  For an RF relay, this will be characterized in the data sheet.

RF relays (even non-constant impedance ones) are designed with skin effect 
in mind.  A  60 Hz or DC relay could use very small contacts as long as 
they can carry the current, which will be evenly distributed.  At 30 MHz, 
though, where the skin depth is <1 mil, tiny contacts may not be such a 
great idea.

>A company normally gives the voltage and current  rating for the relay
>250 v ac @ 2 amps  (assumed at 60 CPS)
>30 v dc @ 3 amps
>Also in some cases the maximum voltage from contacts to relay coil.
>How would this example rate for RF usage up to 30 MHz?
>What is the formula to be applied to other relays?
>What is the formula to be used for power in watts?

Use line Z, and a goodly safety factor for mismatch

>Will there be an impedance bump at the relay?

Yes, but it might be negligible, depending on frequency.. PL-259/SO-239s 
have an impedance bump too, but since they're short, it's not something 
folks obssess about.

Stray C is probably a bigger problem.

>If there is an impedance bump should it be considered a problem ?
>Should SWR in the line be a consideration?

You bet it should. has summaries of some of 
the stuff that's been published over the years.
(also try, as I'm in the process 
of moving, rearranging the web site...)

>Rich AA2MF
>See:  for "Self Supporting Towers", "Wireless 
>Weather Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with 
>any questions and ask for Sherman, W2FLA.
>TowerTalk mailing list

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