Automatic Antenna Switch

This automatic antenna switch consists of two devices. Band decoder and antenna switch.

The band decoder identifies the band on which the transceiver operates and the antenna switch ensures the antenna switching.

I described the band decoder in this article: Kenwood-TS-890-Band-Decoder. Here I will focus more on the the antenna switch.

New version of my band decoder PCB has possibility to add SN7407 and it means to create ICOM CI-V interface. I only wait for my new IC-7300MK2 (hope it will come in January) to test ICOM code.

1. You will need a relay, for example RT424012, RTE25012 or similar. Both relays have the same size, pin connection, similar design but different contacts. RTE25012 has gold-plated contacts but its price is several times higher.
2. Another component you will need are connectors. They can be bought in China for a reasonable price but the quality depends on the seller. Links to verified sellers only work for a short time and therefore I will not list them here.
3. A very important part is also the correctly designed PCB. This is the third version of the PCB and I must say that I am satisfied with this one only. I tried two versions of double-sided PCBs where one side was mostly GND, but the results were not satisfactory even though I tried to design the dimensions so that the impedance was calculated to 50 ohm. This version is single-sided in the key part. Old but working principle - Other wires or GND as far away as possible. The PSB is designed to be used with a GND switched band decoder but also with a VCC switched one.
4. The connectors are connected to the top part which is actually also a PCB. It is a single-sided aluminum PCB that I ordered from China for a reasonable price. I chose this option because making the box otherwise would be quite difficult due to drilling large holes.I used a double-sided PCB for the side walls.
5. The last part is the blocking diodes and surge protection. Here you can use almost any diodes that fit on the PCB. I don't use surge protection because the control cable is only a few centimeters long.

Although the wiring is primitively simple, it is easy to make an antenna switch incorrectly. I had no idea it was so complicated and at every step you can make a mistake that will cause the switch to have a high SWR. I choose a SWR of 1:1.1 as an acceptable value. Previous versions had a SWR of around 1.15, which seemed quite high to me. I did not know why and found out experimentally after many hours of research. I will describe it in a moment.

The first choice and the only non-critical one is the choice of the logic type. Some Band decoders work with "GND switched" logic where the correct relay is selected by connecting its contact to GND. Other types use "VCC switched" logic where the correct relay is selected by connecting its contact to VCC. The board allows both logics using a single wire jumper.

The second option is to choose whether the unconnected antenna will be grounded or not. If you choose the option unconnected antennas will be grounded to GND potential, then you solder all relay contacts. However, this option will cause the resulting SWR to be just below 1.1. This is due to the fact that the contacts are structurally very close to each other and there is parasitic capacitance between the contacts. If you choose the option the unconnected antennas will be "open" then SWR will be about 1.06. In this case, it is necessary to modify the PCB a little. To ensure that the relay contacts do not really touch GND, it is necessary to enlarge the hole for the contacts to about 2mm. (Alternatively, cut off the relay contacts).

The third option is to use other relays that have a larger distance between the contacts.

The fourth option is to use two separate relays for each antenna. The first relay connects the selected antenna, the second relay grounds the unconnected antenna. Given the price of the relay, I ruled out this option.

I would like to add a note here that if you are going to conduct your own experiments you have to be very careful with evaluating the results. If you test a single relay on the board and connect the input and output connectors with it, you will get an SWR of 1.02, but by connecting each additional relay (which will be passive in the circuit), the SWR will deteriorate.

As I wrote, almost all case is made of PCB. There are 4mm spacers between the main and top PCB boards. After soldering the connectors, check the operation of the antenna switch. If everything works, you can solder the side walls. This will give you a mechanically solid structure that is inserted into the outer casing printed on a 3D printer.