Filter tests with the DG8SAQ VNWA

My 2m station is in a good location on the North Downs in Kent. It is also line of sight to several broadcast transmitter masts which present a few problems for my receivers. Mast head preamps are a non starter for 2m.

Almost every receiver that I have ever owned has suffered from strong signal problems from the band 2 transmitter at Wrotham. If I connect a microwave power meter to the 2m array (2x17ele) I can measure several mW of power from the anttanna when it’s pointing at Wrotham.  Most of the RF is out of band, predominately between 88 and108MHz.

Here is a screen shot with the antenna connected to the anaylser. The antennae are pointing directly at Wrotham. To the right of centre is GB3VHF on 144.43MHz, slightly left of centre is a pager signal probably from the same mast at Fairseat. GB3VHF peaks at around -40dBm if I beam that way.

Below is a closer look at band 2.

For the past 8 years or so I have used a simple notch filter to deal with this. However, it’s performance isn’t that great and lets through more than I’d like.

This is the response of the notch filter. The blue trace is insertion loss, green trace is return loss. Peak attenuation is -31dB at ~93MHz. Insertion loss is around 0.5dB

Years ago I purchased a helical filter manufactured by Aerial Facilities Ltd. It’s beutifully made and has a much better response than the notch filter. It was originally on ~137MHz. I am still trying to tune out the lump slightly HF of the bandpass and improve the insertion loss.