Home > Antennas, M0CVO, M0CVO Amateur Radio, Making Waves, Radio and electronics > Making Waves – Double Trouble – a simple diplexer.

Making Waves – Double Trouble – a simple diplexer.

There are often times when a simple diplexer is required – perhaps you are wanting to try your hand at satellite communications or perhaps you are using separate antennas on a mobile dual band radio. Whatever you are planning you may find that commercial diplexers are costly so why not build one for yourself? Above is the schematic for a simple diplexer. The connectors, PL1, PL2 and PL3 can be replaced with BNC, N-type or whichever socket you prefer for coax connection. PL1 is for the coax run to your transceiver, PL2 for 70cm and PL3 for 2m. C1 and C2 are both 4.7pF and C3 is 10pF. L1 is 0.05uH or 1.5 turns of copper wire (1.0mm diameter) wound on a pencil, L2 and L3 are both 0.07uH or 3 turns of copper wire (1.0mm) wound on a pencil.

Use a small piece of PCB material / copper clad board as the earth an solder the components together – not the prettiest thing but you could put it into a small box.

For satellites you will need two small Yagis, one for 2m and one for 70cm at right angles to each other. on the same boom for handheld work and fed with a short piece of coax (50ohm) from the relevant output on the diplexer and then a short run of coax back to your transceiver. The capacitor type is optional but remember, the higher voltage rating on the capacitor, the higher power throughput you can expect however, for satellites you shouldn’t need more than 10W other wise you may cause unnecessary interference to other users or simply overload the satellite’s transponder.

For mobile setups with two antennas for a dual band radio, a short length of coax from the radio to the diplexer and then sufficient coax to each of the antennas from the correct port / connector. No external power source is required for the duplexer as it simply works as a high pass filter allowing the 70cm (and higher) signal through and a low pass filter allowing the 2m (and lower) signal through.

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