I bought a NanoVNA to properly measure the 6m/4m diplexer. The results were mixed:
So, testing the 70MHz high pass showed that there was 22dB attenuation at 50MHz. Not too bad! I could try adjusting the coils for more attenuation, but there was no de-sense on the 4m radio So I’m happy to leave it at that.
An initial measurement of the 50MHz low pass shows it to have a 0.26dB insertion loss, but a terrible 5dB attenuation at 70MHz.
After adjusting (squeezing) the coils, this was the best I could achieve. 12dB attenuation at 70MHz. Most of the adjustment was the 9 turn coil, so the inductance need increasing on that component.
As a quick test I held the 9 turn coil together as much as I could; 23dB attenuation. Thats more like it! Although, notice there’s more insertion loss at 50MHz. That might be my fingers!
So, the 4m high pass filter looks ok. Maybe a few adjustments could increase the attenuation but for now it’s decent. The 6m low pass is a different story. After some adjustments I was able to get 12dB attenuation, which isn’t great. I might take the 9T coil out and replace it with one of higher inductance and see what happens.
Watch this space!