1) Allow switching any antenna to either of two radios.
2) Allow using any antenna as separate receiving antenna or noise sensing source (for the MFJ-1025).
3) Allow using multiple antennas at the same time on the same radio (two yagis pointed in different directions).
4) Allow limited SO2R (single-op two radio) or multi-operator contesting.
5) Had to cost almost nothing.
The last item was a bit tricky but it didn't take too long to figure out the details. I had a box filled with surplus cube relays pulled from an old PLC unit and I mapped out all the RF paths based on what I could do with a pile of relays, my existing equipment, and the criteria above.
A few tweaks since 2012 but the basic layout has stayed the same. The antenna tuner always bypasses one radio or the other to the ALTERNATE OUT. The reject stubs will be a future addition.
I lined all the relays up on my desk to get an idea of the physical arrangement and that was more or less where it sat for a couple of years. In my head I would often put it all together and play "what-if" games, frequently going back to the drawings and making slight changes.
My relay arrangement plan from back in 2012. Some of my original paper drawings are visible in the upper left corner.
I had been slowly collecting all the parts I needed but when I finally started building the station last fall it ended up being a crash project to get on the air before winter and most of the antennas and the complicated stuff was put off until 2014. Now that winter is here again its time to get moving on building "The Matrix" because all the other additions depend on it!
The first step was prepping the relays. AC and DC ratings are meaningless for switching RF but what I did was eliminate the little wires inside the relays by soldering a solid wire across the two moving poles. This increased the current and voltage capability along with increasing the isolation. To further increase the isolation between relays, each was wrapped in aluminum tape and the tape was used to hold all the relays together. During one of my daydreams about operating this system I started thinking about how much RF current and voltage might be on the line going to the vertical antenna used for transmitting on 80 and 160m. I don't doubt that sooner or later a relay will end up getting burned and have to be replaced but "much later" is the preferred time frame for that so I added an open-frame, heavy-duty relay for the main vertical feed just to avoid this scenario.
Close-up of the taps on the LDF4-50A heliax cable during construction. Pieces of the copper-jacketed RG-402 came in very handy for this part, too!
The control wires for the relays will all enter the shack directly through a 25-pair telephone cable into a metal box containing back-EMF protection diodes, bypass capacitors, and ferrite beads for each wire. If there are any particularly stubborn antenna/frequency RFI problems then I can also add RF chokes as necessary.
The next step is putting the microcontroller together and writing some control software. That will be the subject of a post in the not too distant future. When I get around to it ;)