W5KI@aol.com wrote:
> Ken,
>
> AA4NU's input is the better one, and really the most cost-effective for a
> quality unit(s). On that Top Ten site, if you go to Application Note 4, you
> will see the "simple" crossover arrangement that uses two of their AB
> switches.
> _http://www.qth.com/topten/apnote4.htm_
> (http://www.qth.com/topten/apnote4.htm)
>
> There are some crossover surplus relays out there, but the prices shown are
> quite high. I was fortunate enough to find this exact Amphenol N Transfer
> relay for $10 at a hamfest a few years back. Doesn't help you now, but
> keep an
> eye out at the bigger events.
> _http://www.surplussales.com/Relays/rfcoaxialrelays/rfcoax_n.html_
> (http://www.surplussales.com/Relays/rfcoaxialrelays/rfcoax_n.html)
>
> 73, Steve, W5KI
>
Except that the dual spdt arrangement in the apnote doesn't guarantee
that each radio is always attached to an antenna (which I think was one
of the OP's requirements).
If one relay box loses power, then you have a radio that is not
connected to anything.
If one relay box mis-operates, then you have two antennas connected to
one radio, and one radio connected to no antenna.
The transfer switch has the advantage that there's always a connection
between radio and some antenna (or dummy load). Yes, it's possible to
have a partial failure of a transfer switch (or baseball switch, as
they're also called, for waveguide), but the internal mechanism is
usually arranged so that they're "fail safe", barring some major
mechanical problem.
the apnote and topten's site don't give the internal schematic for the
ABSS, but it says it uses two separate SPDT relays inside, so in the
transfer switch application, you've got 4 separate coil/mechanical devices.
It's probably not important for HF applications, but the purpose
designed transfer switch usually has better impedance matching and such,
as well. Might have better isolation, too (almost certainly at VHF)
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