Antenna Switch

Like most hams, I accumulate antennas over time.  I have one commercially made antenna switch, which can handle QRO operating, and so has some pretty robust switching.  However, it only has three antenna ports, and I never use more than 100W, usually much less.

Here's what my commercial unit looks like.  You may wish to note that a four port switch sells for anything from £60 - £90, depending on maker.  That's a lot of money for what is actually an extremely simple device.

Knife switch on a commercial, higher-power 3 port antenna switch.  Simple, but expensive.

As I have five antennas in active use at the moment, and always a couple under experimentation, I decided to make my own, seven port switch.

Now, a note of caution: if you use more than 100W output, you may want to select your switch unit with consideration to how robust the contacts are, and whether their arrangement might be too close for higher powers.  At 100W or below, the following should be more than adequate, but it's up to you and your training to check!

I obtained my 11-position, single pole switch from a well-known online auction site for about £2.50 each.

At the same time, I bought a smallish aluminium box from RS Components, who seem to have a more sensible size range than other outlets.  This was about £12.  I bought a lot of twenty SO239 panel-mount connectors for £8 - all the way from Texas - via the same online auction site.  They took a mere week to arrive.

The 11-position, single pole switch, and all the connections.  The SO239 on the left is attached to the common pole, to which the transceiver is connected.  The remaining seven are antenna ports.  Don't forget to add a grounding post as well.

I then bought a reamer, which cuts larger holes in metal to allow the SO239s to fit through.  This was a one-off tool outlay of £11.

Making the switch is easy-peasy, lemon-squeezy.  Just drill a number of modest sized holes where you want the connectors, and then use the reamer to widen the holes until the SO239s sit snugly against the box.  Then drill some smaller holes for attaching the SO239s to the box using M3 or thereabouts bolts and Nylok nuts.  Two is enough for each connector.

Drill a hole in the bottom of the box (NOT THE LID!), which is where you stick the switch through to attach it to the metal case.  The lid goes on last, and becomes the bottom of your antenna switch box.

The rest is just a case of connecting each antenna port to the switch.  You also need to attach a bolt through the lid somewhere out of the way so that you can ground the box directly (and not to another piece of equipment like an ATU, etc.)

And as I can hear the purist elders ask: "Ah!  But what about the inter-port isolation, my boy?", I ran a test using two rigs, one on a connected port, the other on a disconnected port.  According to this simple test, the isolation is very comparable to the commercial unit, being of the order of 60dB.

So there you go.  Apart from the fact that you rarely see antenna switches with more than four ports, this project saves you a lot of money.  Sure, you can do a nicer, less hurried soldering job than me, and maybe use a chunkier switch, but that's another story!