[TowerTalk] Budget 4-Pack?

[Jim Lux]

-----Original Message-----
>From: Barry Fox <foxbw at comcast.net>
>Sent: Mar 1, 2010 11:40 AM
>To: towertalk at contesting.com
>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 relay sticks)..

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 interlock)

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.