HB VHF low-pass filter
 

Is it a single pager?
I once watched the local pager band with a spectrum analyser, and there
were at least a dozen pagers coming on and off at different times and on
different frequencies, the lowest of which was below 149MHz. (This is in
Australia) Several of them actually produced IM products between them
that were *not* from the spec an front end. I surmise that these are
actually produced locally. Apparently transmitter output stages can
produce these when there are antennas located close to each other
(within a few metres) on the same tower. Others have proposed a stub
configuration - you may have to consider beefing up the front end for
better strong signal handling as well if there are several signals.

Richard

wrote:
Hi gang,

And old chum of mine suffers badly from pager interference when he's
trying to listen on the 2m band. The interference is strong to him
(S9+) and takes out all his reception, despite the fact that it's quite
a way up-band at 153Mhz.
I've been looking into designing a filter for him to cut off by 153Mhz
to better than -20db but it turns out to be rather more tricky than I'd
imagined. Even with a seven element butterworth configuration, the
difference between the 146Mhz (wanted) and the 153Mhz (unwanted is an
insignificant 6db or so. I'm just not getting enough roll-off and need
something much sharper. Worse still, the roll-off I'm seeing is on a
computer simulation of the filter with ideal components and would no
doubt be even worse with real-world Ls and Cs. Any suggestions as to
how to tackle the problem, guys?
Thanks,
P.