I wrote the following in response to a question on the AMPS list, and
though it might be useful to cross-post here.
G4SWX has designed a range of highly effective RFI filters based on coax
stubs - see:
www.ifwtech.co.uk/g3sek/swxfiltr/swxfiltr.htm
These filters fall into three groups:
1. Transmitter harmonic filters:
If your exciter (transceiver) is performing correctly, the only
significant spurious signals should be harmonics. All you then need is a
filter to notch-out those specific harmonic frequencies - and this you
can make from a few lengths of coax and a matching inductor. Designs are
given for 50, 144 and 432MHz.
2. FM Broadcast Lowpass Filter (144MHz reject):
This amazingly simple filter has low loss (<1dB) and a flat passband in
the FM broadcast band up to 110MHz, and then rolls off to give a deep
notch at 144MHz.
3. UHF TV Bandpass Filter (432MHz reject):
This is a real "brick wall" filter for the European UHF TV band! It has
low loss (<1dB) from 470-1000MHz, but it has 30dB notches at 432MHz and
1.3GHz. It also has a highpass response which gives excellent rejection
at 144MHz and below.
The FM and UHF TV filters are designed for 75-ohm antennas as used in
Europe, but they should be equally suitable for CATV distribution
systems.
Dimensions are given to cut the coax stubs from RG58 or RG213. The
matching inductors are easy to wind, and are not critical. If you can
use a ruler and wind a coil, no specialized test equipment is necessary.
Try them - they really do work.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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