In the start of January, we wrote about our plans to deploy LA2SIX at Vassfjellet, where LA2VHF/4 and the other beacons are. We have an antenna which is resonant on both 50 and 70 MHz, so we just needed a diplex filter (diplexer) with the appropriate frequency response in order to connect both LA2SIX and LA2VHF/4 to the same antenna. The diplexer ensures that the output of the beacon goes to the antenna, and not to the other beacon.

To accomplish this, we made a simple PCB to hold a 5-pole lowpass/highpass style diplex filter. The project is rather simple, so it can be argued that a vero-board or similar would be a more appropriate solution. However, the PCB implementation makes for much better grounding, which in turn helps with heat transfer – increasing the power that can be run through the board. It also looks pretty. 🙂

The unpopulated diplex filter board.

If you want to use the board, it is available open source over at GitHub.

We populated the PCB with component values tailored for our 4m and 6m beacons. This is seen below. The realized board gives about 0.3 to 0.5 dB loss, and has excellent match for the desired band. Some tuning, i.e. pulling on the inductors, will probably be required for optimal performance.

Schematic sketch

Capacitors are all Johanson Technology S42E series (1111 EIA size high voltage RF caps). We used the 500 V rated variant for this design, which should allow for more than 100 W throughput. The coils are annotated with number of turns of 0.71 mm enameled wire around a permanent marker (15 mm diameter).

  • Soldering in progress
  • Overkill tuning setup
  • Final tuning of filter
  • Simulated response
  • Final tuning of realized hardware

With the diplexer completed, we are ready to deploy LA2SIX at Vassfjellet. Now we just need the snow to thaw. The weather has been nice lately, so hopefully we will be able to get up there in a month’s time.