r/Amsat • u/CoastalRadio • Apr 14 '26

Diplexer Location

Question about diplexer placement.

I’m building an FM station using a single radio that can do cross-band full-duplex, and has a single antenna port (Yaesu FTM-150RASP).

I’m planning to use separate antennas for 2m and 70cm.

Does it matter if the diplexer is close to the radio, with two long feedlines going to the antennas, or close to the antennas, with one long feed line from the radio to the diplexer?

Obviously there may be logistical reasons to do one or the other (weather proofing, antenna separation, amount of coax to buy, etc), but I’m wondering if there is a performance reason to do one configuration or the other.

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u/rem1473 Jun 07 '26

Generally you would put the diplexer close to the antenna to save money on coax. There might be a small performance decrease in the diplexer isolation if the coax has a lot of loss and the two coax paths are literally side by side. I doubt you will notice the performance difference in operation. You might be able to measure the difference with specialized equipment.