Ian White GM3SEK wrote:

snip


Some years ago John Regnault G4SWX was experimenting with various kinds
of filters using coaxial stubs. Many of the most useful ones were
published in Radcom and found their way onto my 'In Practice' website:

http://tinyurl.com/g4swxfilters

One of John's ideas was a filter with notches just above and below the
2m band, specifically to knock down the strong carriers from pagers. It
only needs two pieces of coax and two small trimmers.

The idea starts with an open-circuit quarter wave stub which is produces
a notch on the pager frequency, above or below the 2m band. To make it
field tunable, the stub is cut a little short and a small trimmer
inserted in series with the hot end. The only problem is that such a
stub will produce a mismatch at 145MHz: a stub that is resonant above
the band will appear capacitive at 145MHz, while a stub resonant below
the band will appear inductive.

These reactances can be compensated by a shunt inductor or capacitor,
but G4SWX's bright idea was always to use *both* stubs - regardless of
where the pagers are - and let them compensate each other.
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Tnx Ian for the very useful info.

Frank GM0CSZ / KN6WH