Monday, November 23, 2020

KK1L 2x6 Antenna Switch

Completed relay unit.
I've been meaning to incorporate an automatic antenna switch in my shack for years. Indeed, I bought 20 RTB14012F relays 11 years ago just for such a project (and they were $2 each back then!). But, I'd never gotten around to designing and building one.

Somehow, I ran across a link to KK1L's 2x6 Antenna Switch board, and looked at the Relay Unit. I loved the elegance of the design, and the board was a beauty. I wrote to Ron asking him about the pricing for the board, and if my RTB14012F relays would fit.

Ron answered me promptly. He quoted me a price of $33 for each board, and he could mail up to five in a priority envelope for $7.15. He also informed me that my relays wouldn't fit -- that the pin spacing was incorrect. I was disappointed and set aside the idea for a couple of months. 

Board top.

While I was working on another order on, I couldn't get the KK1L relay unit board out of my mind, and checked out adding eighteen RTD14012 relays to my order. I decided I couldn't possibly do any better than Ron's design, so I wrote him back and ordered a board. I also used Ron's project, which included parts like the screw terminals, chassis, diodes, capacitors and resistors.

Electrically, this is a very simple design. There are six switching cells each with three relays, two diodes, two capacitors and one resistor. The "A" ports activate two relays, the "B" ports activate only one. 

Board bottom.

I had a lot of parts already. Besides the board, I ordered the relays, chassis, and screw terminals. I already had the SO-239 connectors, diodes, capacitors and some 1/2 watt 50-ohm resistors. 

[ UPDATE: I recommend using the 50 watt resistors specified by KK1L instead ]

Building the board was easy. The diodes and capacitors go in first, because they are under the relays. I used some 0.1 uF multi-layer ceramic caps I had on hand. These were smaller than the diodes and laid flat against the board. Relays went in next, and the screw terminals. I put the resistors in last. Then I had to remove them and put them in again, because I ran them through the wrong side of the board.

Now came the hard step - drilling the chassis. I'll be the first to admit I'm not a master machinist. I have a drill press, which helps. Precisely aligning all the holes necessary for the eight SO-239 jacks to pass perfectly through that beautiful circuit board was intimidating. I decided I only needed two screws on each jack to pass through the board. That limited the project to 24 holes, plus a couple of mounting holes, and a cable hole - 27 in all. 

I started with the printed template, but found that it didn't align perfectly with my board. I ended up using the board itself as a template and marked each hole on the chassis with a sharpie. 

The instructions say to use #6 hardware on the SO-239 jacks, but I have never seen a SO-239 that would pass #6 hardware. I used #4 hardware instead. I drilled all the holes with a #27 drill in any case, which gave me some leeway. The center holes were a little harder, I widened the pilot holes with two different drill sizes, then used a step drill to get to the final size. 

Chassis ready to install  board.
The instructions call for mounting the SO-239 jacks inside the chassis. I knew I had some hole alignment problems, so I opted to mount them outside the chassis. The disadvantage in doing this is that you can't remove the board from the chassis without unsoldering the antenna connections. But, it would hide all my ugly machining.

I used two short and two long screws for the SO-239s. For the longer screws, I used a jam nut and a couple of washers below the board, to give enough clearance from the chassis. I drilled the holes for the short screws after mounting the jacks and aligning to the board. I decided I only need two of the four mounting holes -- the two that were opposite the A and B connectors. The other end of the board is supported well by the three SO-239 jacks.

After everything was drilled and deburred, I took everything off the chassis and gave it a quick buffing with a wire wheel. This gives the chassis a little bit of sheen and hides any large scratches. The until will eventually be mounted on the wall as part of the single-point ground, so it didn't have to look too pretty.

Fully installed.
Once I had the board mounted, I realized I made a mistake. I mounted the screw terminals so the holes are facing the chassis (outside), which makes them difficult to access. Unsoldering would be a pain, so I decided I could live with the error. 

I did a little bit of testing. First test is to see that all antenna points read 50 ohms. Then I checked to see that all the relays activated correctly. Everything worked great. 

Next step will be to incorporate it into my station.

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