[Elecraft] KAT500 antenna port isolation

[Erik Basilier]

Anyone who has looked inside a good coaxial relay or switch knows that for
that level of isolation the mechanical construction one needs to provide a
more or less complete shield around the chosen signal path, that re-forms
around the new path when the swich is operated. This is complicated and
expensive. I would not expect to find it in a normal HF amateur transceiver,
tuner, or amplifier. Maybe in some specialized equipment for multiradio
operation, SO2R or otherwise. It is certainly something to be hoped for in
future equipment. However, one should not be very confident that a switch
with high isolation numbers would always prevent the sort of signal leakage
mentioned in this thread. "Transmitting on a dummy load" is a common
expression, but I guiess it is seldom representationve of what is actually
going on. More likely, some signal is flowing as on the outside of shields,
and needs common mode chokes for suppression. 

This point is well known to high-end competition stations that depend on
separate receive antennas for the low bands. The job of such a receive
antenna is generally not to pick up as strong as possible version of the
wanted signal, but to pick up an adequate sampling of that signal along with
less noise covering up that wanted signal. This involves avoiding noise
pickup where the feedline runs through a high-noise environment (read: the
shack building), as well as long distance signals coming in from the back of
the receive antenna. (A conventional "front-to-back" number is not much
help, one must rather use a complicated measure of back side rejection over
all applicable "back" direction angles.) Noise pickup on the outside of coax
will generally nullify the performance of a well-designed receive antenna
that looks good on paper, unless the installation includes liberal
common-mode choking  and grounding of coax shields. I seem to remember one
report that said the grounding was so critical that a ground connectied only
to one side of the coax was markedly infererior to one that completely
encircled the coax. Installations normally include running the coax through
metal conduits, not just for mechanical protection, but for blocking noise
pickup. If it is not already obvious to the reader, I am referencing noise
pickup on receive antenna feedlines as something that corresponds to
transmit capability as well, allowing transmission "on a dummy load" even if
the dummy load is perfectly shielded. Bottom line: Leakage and crosstalk is
a system characteristic that results from more than the performance of a
given component such as a switch.

73,
Erik K7TV

-----Original Message-----
From: elecraft-bounces at mailman.qth.net <elecraft-bounces at mailman.qth.net> On
Behalf Of ANDY DURBIN
Sent: Saturday, July 07, 2018 3:54 PM
To: elecraft at mailman.qth.net
Subject: Re: [Elecraft] KAT500 antenna port isolation

"I suppose the only way to get good isolation data will be to do a sweep of
the KPA500 and Alpha Delta switch"


Sorry - that should have read "I suppose the only way to get good isolation
data will be to do a sweep of the KAT500 and Alpha Delta switch".


TMGS?


73,

Andy k3wuc



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