The M-RK Radios use a non-standard antenna connector, which means only their proprietary antennas can be used, and they are rather expensive. When opening up the radio, on the back panel there is a small circuit board with two spring contacts to the main board, and the connector for the top antenna as well as the side mounted connector for accessories. The PCB is mounted in a small metal housing for shielding and is attached with two screws. The side RF connector needs to be removed by unscrewing it from the outside. This was a pain, but possible to do with a screwdriver and pliers. From here the PCB and metal housing can be removed. The PCB can be desoldered from the metal housing.
Original PCB with Antenna Installed
Connecting Pads on Main Board
This board seemed like the easiest way to adapt to a standard RF connector and antenna. I took measurements with calipers and designed a simple board with KiCAD. I still was not sure the best way to fit a connector into the radio, so I just broke out to two pads for the first revision. The board is small and only about $1 for 3 from OSHPARK so I planned multiple revisions from the beginning. I also measured the spring contacts and found multiple options from Digikey, categorized as EM shield fingers. ordered some samples and choose the one that fit the best, as well as being inexpensive.
The first revision arrived a few weeks later, and surprisingly fit almost perfectly! It was very slightly too wide, but with some cutters I was able to fit the board in. The contacts aligned perfectly. I was a bit too generous with my cutouts and plan to reduce them in later revisions.
Custom PCB V1 installed
Custom PCB compared with Original
Next was figuring out an antenna connector that would fit. I initially was looking at either a BNC or SMA connector that would fit in the threaded hole for the old antenna. Neither of these fit without drilling the hole out which I wanted to avoid. I also looked at using some coax soldering with a SMA connector, but this would be fairly permanent. My next idea was using a small U.FL to SMA cable. This would allow the board and connector to be easily removed. My idea is to sink the base of the SMA connector into the body of the radio by heating up a same-size nut and melting the radio body. I updated the PCB with the connector and sent it off to OSHPARK to be manufactured!
I received the new boards as well as connectors, and short U.FL to SMA cables. I soldered them together and fit it all together. The results were not quite what I was hoping for. To get everything to fit and the cable plugged in, the cable ended up being bent fairly tightly. When I tried to put the case together the case ended up squishing the cable slightly. I wasn't happy with this solution so I went back to KiCAD and tried again.
My next design shrank the board considerably. I moved the U.FL connector to the back of the board under the ground contact. My thought was that by moving the connector further back the cable won't have to bend as much. One concern while designing was how hard it was going to be to plug the connector in. Turns out my concerns were right. It is not possible to easily plug the connector in while everything is soldered.
At this point I'm fairly frustrated, I'm three designs in and it seems like I'm not going to be able to get everything to fit like I wanted, while doing minimal modifications to the radio. Next I am looking into modifying the metal chassis the antenna connector attaches to as well as soldering directly to the radio.