50MHz/70MHz diplexer

By | July 10, 2020

I don’t have a huge garden for an antenna farm, so most of my antenna experiments are located in the attic. Also keeping the neighbours happy, and most importantly, the station manager happy, is a priority!

I have several runs of RG213 and RG58 into the attic, but wanted to be able to use 4m and 6m at the same time. Instead of running *another* length of RG213 into the attic, I thought i might try building a diplexer.

Building one for two close frequencies would be difficult as the cutoff would need to be sharp, but searching the net found https://www.la1k.no/2019/04/24/la1k-5-pole-diplexer-board/ which describes one such filter in use with LA2SIX and LA2VHF/4 based in Vassfjellet, Norway.

Although the author didn’t give actual values for the inductors, reading the article does describe them as “number of turns of 0.71 mm enameled wire around a permanent marker (15 mm diameter).”

Here’s my finished version:

So far it seems to work well, though the cut off seems to be about 56MHz rather than 52 for the low pass, and 68MHz rather than 70 on the high pass.

In actual use, I can’t hear any 6m RF on my Ascom SE550, but the Ascom does desense my FT450D listening on 6m. The FT450D does have the pre-amp switched on and probably has a much more sensitive receiver than the Ascom. Maybe an additional coax notch filter would help on the 6m side.