Antenna Selector with interlocking protection



As contesting is one of my playgrounds in our hobby I was thinking about the possibilities at our new QTH. I decided to go for a Single Operator 2 Radio (SO2R) or Multi Operator 2 Radio (M2) design. This means every antenna must can be switched to one of the 2 stations but with interlocking that e.g. Staion 1 cannot switch onto the antenna station 2 is already using. Of course you can buy all those devices 'ready built' on the market. But we are still working for the mortgage and of course you and me are homebrewers (shame on you if not) and this looked like a good project to me.

Since my former time as a service engineer I had a good source of 24 V relais used in our commercial devices. I took the relais from exchanged boards. These relais can handle 8-10 A at 250V AC and the contact system has very low inductance. This means no need to compensate anything on the hf-bands. For 2m or 70cm you need some small coils because the contacts are acting like a capacitor here.

The relais have only one switching contact. So it needed another relais in parallel to prevent that the other station can switch onto the same antenna. So for every antenna are 4 relais. Relais 1 to switch the antenna to station 1 and relais 2 switches the antenna to station 2. The 2 relais in parallel are opening the 24 V supply for relais 1 or 2 depending which one came first. Figure 1 shows the scheme of the relais. So we need 4 relais to switch 1 antenna between 2 stations. I have done this for 4 antenna cables coming to the shack from outside. But it can be easily extend for more antennas.


open antenna switch
The foto shows the open installation. The relais on the left side are the antenna relais and on the right side are the interlocking relais.

open antenna switch 2


Here is another view inside the switch board.


On the same bord will be added a BCD-Decimal decoder to switch all antennas and bandpass filters automatically.
The 4 antenna cables are leading to 4 different antenna locations. Each location or tower can have different antennas. Here are additional relais installed to change between the antennas. At the cable to Ant.1 (Mast/tower) is a box to choose between 4 antennas. This box is installed at the top segment of the tower, closed to the rotor and can be seen in this foto.

antenna switch at top segment before installation





--> page  top