>From: Barry Fox <email@example.com>
>Sent: Mar 1, 2010 11:40 AM
>Subject: [TowerTalk] Budget 4-Pack?
>I have two HF radios and 4 different HF antennas. I would like to use
>each rig on any antenna, while obviously locking out that antenna for
>the other rig. The commercial products 6-Pack, 8-Pack, etc are way
>over my budget. Anyone have a schematic for a "4-Pack" or similar
>device they would like to share? Obviously relays are the way to
>go, and I can get the RF relay switching part of it on paper but
>not the lockout logic. Some sort of electro-mechanical exclusive-OR
>gate array / diode matrix? Anyone been there and done that?
I wouldn't trust lockouts in the relay drive logic (e.g. what happens if a
What's your port to port isolation requirement?
one way to do it is to use a single pole double throw relay connected to the
antenna that switches the antenna to one radio or the other. If a relay fails,
you can't wind up connecting one radio to the other, although you can wind up
with two antennas connected to one radio. (essentially it's a mechanical
If you need more isolation, you can put a SPST relay in series from the radio
to the port on the antenna relay (this would take 3 times as many relays).
One problem is finding a good SPDT relay for RF. Because of the way relays are
made, one set of fixed contacts is closer to the frame/coil than the other.
One strategy is to use a DPDT relay, short the two moving contacts together,
and then basically use the two halves of the switch in series. A failure that
makes one contact stick, but not the other, is unlikely, and even so, it just
"reverses" the sense of the switch.
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