wrote:
I recently acquired one of these cheap 2m handhelds from China, an
FDC-150 I think. It's great for the price (£30) apart from a problem
with QRM.

I use the radio with a 3 element beam from SOTA activations from hill
tops. It varies from location to location, but I often get strong
intermodulation effects (caused by pagers I think). I suspect the
radio, being wide band 136-174MHz, has insufficient filtering to
reject these strong signals.

The intermod is a real problem, as I am often unable to hear stations,
or only get half of what they are saying before they are wiped out. I
was wondering about building a 2m bandpass filter like the one at
http://www.arrl.org/tis/info/pdf/0005054.pdf

Does this look like a good bet?
Also any ideas where I can get the semi-rigid coax (UT-141 or RG-402)
in the UK?

Might have just what you need... it's somewhere in the back of the
mind...

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.

Some work with an optimizer was needed to produce the best design, which
proved to be quite tolerant of practical variations. We had an article
almost ready for publication since 2002, but didn't go ahead because it
seemed like "a solution waiting for a problem" - until this week.

A copy has been e-mailed to the OP, and if it works for him we will
publish it.




--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

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.
================================
Tnx Ian for the very useful info.

Frank GM0CSZ / KN6WH

On Feb 20, 8:56*am, Ian White GM3SEK wrote:


Might have just what you need... it's somewhere in the back of the
mind...

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.

Some work with an optimizer was needed to produce the best design, which
proved to be quite tolerant of practical variations. We had an article
almost ready for publication since 2002, but didn't go ahead because it
seemed like "a solution waiting for a problem" - until this week.

A copy has been e-mailed to the OP, and if it works for him we will
publish it.

--

73 from Ian GM3SEK * * * * 'In Practice' columnist for RadCom (RSGB)http://www.ifwtech.co.uk/g3sek

Thanks for the advice everyone has given.

I've decided to give Ian's solution a try when I get the relevant
lengths of coax and trimmers in a week or so.

I'll report back when I've built it.

73s de 2E0WNT