RadioBanter - View Single Post - HB VHF low-pass filter

April 21st 06, 03:16 AM posted to rec.radio.amateur.homebrew

On 20 Apr 2006 15:24:34 -0700,
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.

A notch filter is the easiest thing to do. While a pass cavity will
probably work too it will be quite large and expensive.

Try a small box with connectors on either end and a wire passing
through. connect a series tuned coil / capacitor resonant at the
offending frequency. You can get up to 30 db of rejection with a
single notch. Two circuits connected side by side will improve around
another 10 db. A quarter wave length cable (and shielding) between the
two circuits will approach 60 db if needed.

The higher the Q of the circuit the narrower the notch will be and the
less loss of the wanted frequencies being passed through.

73
Gary K4FMX