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From: tomb@hplsla.HP.COM (Tom Bruhns)
Newsgroups: sci.electronics
Subject: Re: How to switch high-voltage RF?
Message-ID: <5170137@hplsla.HP.COM>
Date: 22 May 91 18:32:13 GMT
References: <1991May21.042050.5040@ve6mgs.uucp>
Organization: HP Lake Stevens, WA
Lines: 15

The question about what sort of switching was used in the "smart" tuner
reminded me of working with a '419T' antenna tuner from Collins; it was
a mate to a '618T' HF transceiver used on aircraft (both commercial and
military).  It has vacuum variable caps and 'slider' inductors (kinda
like roller inductors, but the slider was driven by a motor shaft that
stuck up in the middle of the inductor).  Relays were small vacuum guys.
This thing, as I recall, guaranteed to tune in five seconds, but we knew
that if it took over two seconds, something was likely wrong.  One
of the failure modes was operation without an antenna connected.  It
tried to tune its output terminal, and generally ended up creating a
very high voltage node somewhere, one that would arc over.  It could
usually tune a 5" piece of wire in the mid-HF range.  --  But maybe
there's a lesson here about high RF voltages in tuners:  they can cause
nasty breakdowns, so it's best to avoid trying to tune things that will
be outside the range the tuner can handle.