Thursday, February 12, 2009

Remote 3-position Antenna Switch

Part of the next phase of my station upgrade involves putting up some permanent antennas for 160, 80, and 40 meters. After a fair amount of research and staring at my QTH from across the street, I came to the conclusion that, at least for 80 and 160, my best bet is to put an inverted-L into my Ginkgo tree, and that I might as well keep the 40 meter vertical I currently have there.

When I went to model this using EZNEC, I tried to build a model with two inverted-Ls, one cut for 80, and one for 160, fed from a single feedline. Try as I might, I couldn't find a combination of lengths that resulted in resonance on both bands at once. So I decided against trying to feed all the antennas at once, and started planning a remote antenna switch.

Now, about a year ago, I did a bang-up job of making a trench across the yard, so I could run the feedline underground to the tree (otherwise the coax would need to lay on the ground and pose a tripping hazard). Unfortunately, I only ran a single feedline, and I am not digging that thing up!

So I started looking for a remote antenna switch where the power was fed to the switch via the coax. The hard part is finding one that can handle three antennas. With a DC circuit, you've got one "bit" of information - power on or off - to select an antenna, and that's only two. Every circuit I could imagine was fairly complicated - using voltage levels to select antennas, or some sort of pulse-counting system.

Finally, I happened upon a design in the ARRL Antenna Book that uses 12 VAC at the shack side. Imagine an AC-DC rectifier, with half in the shack, and half out at the antenna. The original design has two independent relays. One actuates when the positive-going AC is put on the wire, and the other actuates when the negative-going half is put on the wire. When a complete sine wave appears, both relays actuate. The design in the ARRL book is a little skimpy on details (e.g. the value of the RFC chokes aren't specified) but I found an article in the QST Archive that had a similar circuit for delivering DC voltage an a coaxial cable, and it gave recommended values (as well as instructions on how to modify a widely available Radio Shack choke to have the correct value).

I reworked the design a little so that with an SPDT switch with a center-off position, I can actuate no relays, one relay, or both relays, and then wired the relay contacts together so that I have:
  • Power off (center) = both relays off, antenna 1 selected
  • Switch in up position = relay 1 engaged, antenna 2 selected
  • Switch in down position = relay 1 and 2 engaged, antenna 3 selected.

I built the relay box into a waterproof plastic electrical conduit box, and sealed all the SO-239 connector holes with coax-seal. This thing should last a long time, I hope.

Now I'll be able to switch between 160, 80, and 40 for this weekend's WPX RTTY contest without going out in the rain!

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