2m front end filter

(*** see updates here and here)

As you are aware one of the key features of the FCD is its wide frequency range of operation. While this is great, the compromise with wide band receivers are that they can suffer from desense from out of band signals.

Although the FCD has programmable and fixed front end filtering, at VHF the filters are quite wide and if you’re in a strong signal area, popping in a low loss front end filter will help here.

I’ve built three 2m band pass filters based on the design here.

What I’ve discovered is that you really do need to use the PCB layout with the through hole links, and you must use screening around the unit either with PCB material or for example some brass shim. Without these considerations I found the filter lossy and difficult to tune.

In addition, I found the inductors as specified made the filter LF at 116MHz, so a fair bit of stretching was needed. Luckily I have a VNA in the kitchen lab to do this although you could have a go yourself, tuning for maximum signal on a beacon.

I managed 1.2dB loss with about two or three minutes of coil massage. Probably I could tweak it a bit more given time.

The pictures show the filter and its response. The span is from 50MHz to 250MHz. Thus, in the UK, it might not be the best if you are suffering close in pager interference around 138 or 153MHz where notch filters will provide better immunity.

Howard