Initial Analysis

Analysing the source of the problem

In Quality circles, it's a old saying that what you can't measure you can't control. This applies equally well to this problem, so the first thing to determine was what was causing the interference.

Horizontal bands can be caused by lack of signal - particularly if you have a turnstile antenna as these have quite bad nulls. However, you can usually recognise this form of interference as, at first, there is gradual drop of signal over a few lines, followed by a sharp null confined to just a small number of lines, followed by a smooth recover back to full strength. A band of interference that starts and end suddenly is more likely to be man-made. An additional signature of pager interference is the digital modulation which results in a quite characteristic pattern on the picture. I have also observed two other forms of interference, both of which display as patterns of about 20-25 "clumps" per NOAA image - these are related to mains supply frequencies either from electrical interference, an arcing thermostat for example, or from faulty transmitters.

By using a scanning receiver I was able to determine that the interference sounded like a pager problem, so my thoughts turned to filters.

Trying a Filter

I had an old filter that I'd built some time ago for the 2m amateur radio band. I though that it would tune down to 137MHz, and indeed it would. Trying this seemed to reduce the interference quite a lot, indeed it almost completely vanished on the lower frequency channels, but NOAA 14 was broken up during the day, and Meteor (on 137.85MHz) was impossible.

Later measurements showed that this filter had a loss of about 2dB in the passband, and about 3dB at the pager frequency, so it was doing very little more than act as an attenuator!

Since this was first written, I have changed from the filter shown above to a Helical Resonator Notch Filter, which is described later.

Next - the problems with a new antenna