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Re: [TowerTalk] Help need Vacuum Relay Isolation 1.8 to 30 MHz

To: "Jim Lux" <>,"RCARIELLO" <>,"Towertalk" <>
Subject: Re: [TowerTalk] Help need Vacuum Relay Isolation 1.8 to 30 MHz
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Wed, 5 Jan 2005 14:27:37 -0500
List-post: <>
> Did you happen to measure the impedance characteristics of
the cross talk?

There are multiple causes at work including ground loops in
a PC board. It seems many switches have more problems with
ground loops and lead dress than with relay capacitance.

> Here's a scenario..
> If you had several fairly directional antennas, pointed
> directions, with good isolation (say they were on opposite
sides of a hill..
> a situation mentioned on the list recently), and you
wanted to hook a
> transceiver to one antenna, and use a receiver to monitor
the other at the
> same time, on the same band, and then be able to switch
roles quickly.
> (Hey, maybe you're looking for your own signal after going
all the way
> around the world?)

Why wouldn't someone switch radios ahead of the amplifier in
a rare situation like that? It would be pretty foolish to
have two amplifiers on a single band. When we duplex on 160,
each radio shares a single common amplifier. The crossover
is done before the amps.

> I don't recall exactly what the original post was (it's on
> computer), but wasn't he switching multiple antennas to
either of two
> radios?  Antenna 1 goes to radio A or B, Antenna 2 goes to
Radio A or B,
> etc.  Antenna to the moving arm, radio A on one relay
contact, radio B on
> the other relay contact.

That would be like pointing a loaded gun at your radio! I
hope no one would be foolish enough to build a crossover
switch with multiple SPDT relays!!!

It should be relay armature to relay armature. Antennas to
either one stationary pole or the other pole of one relay,
and rigs to either one stationary pole or the other of the
second relay.

There is absolutely no way to do a safe crossover with two
SPDT relays. It is a recipe for disaster.

73 Tom


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