[TowerTalk] Relay Specs - Coax switch

[Keith Dutson]

Tom,

Thanks for the tips.  I particularly like the design of the RCS-8V box.
Those are custom made relays from Relay Service Company.  I called wanting
them to make some for me but the small quantity cost was too high.  Looks
like that design would handle well over the legal limit, and VSWR through
the unit is very low at 30MHz.

Keith

-----Original Message-----
From: towertalk-bounces at contesting.com
[mailto:towertalk-bounces at contesting.com] On Behalf Of Tom Rauch
Sent: Thursday, March 31, 2005 6:27 PM
To: keith at dutson.net; TowerTalk at contesting.com
Subject: Re: [TowerTalk] Relay Specs - Coax switch

> I am home brewing a coax switch for the shack.  I want it
to handle 1500
> watts of power, but not to "hot" switch.  I am looking at
PCB relays and
> found many that handle 10 amps, but only a few that
specify a high
> dielectric voltage.  What kind of voltage should be
expected out of a
> legal-limit amplifier such as an Alpha or Ameritron?  TIA

Keith,

Having been involved in this since the 80's, my advice would be to NOT use
DC or LF amperes, and you actually can't use DC or LF AC voltage either. To
pick a relay you'd have to test through dozens of relays. In some cases they
handle more voltage and/or current than the DC ratings, in many cases MUCH
less.

 I can show you nice looking large 30A relays that have terrible current
capacity at HF, some actually smoke at 1000 watts at 30MHz. Others rated at
2kV will have dielectric failure at voltages produced by a thousand watts or
less into a 50 ohm load. Some very small relays can handle over 10kW!!

I normally spend days or weeks testing new relays from vendors.
Unfortunately that time is paid for by others so I can't publicize results.
I can tell you what to watch for.

The things you have to watch for are:

The RF path must NOT be through braided or woven conductors, or through
contact materials that are exceptionally lossy at radio frequencies.
Beryllium copper is good, but watch out for plated steels that look like
copper or silver.

RF current and voltage must not be through the coil, or have a connection to
the pole piece of the coil

Watch out for small area conductors, you need wide smooth surface areas
carrying currents.

Most of the engineering time (cost/investment) is spent on picking a good
relay, and doing a good board layout. That care is why some switches like
the DXE switch or the RCS8V will blow away other switches that look like
they have nice jumbo relays. If you just pick a relay based on size or DC
ratings, you likely won't do well. Same for a board layout.
DON'T, whatever you do, make the common mistake of trying to use 50 ohm
traces over a groundplane on a double sided board. You'll wind up with a
horrible switch.

73 Tom

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