Hi Folks: From time to time I've seen some 5 pole 900MHz interdigital
filters show up on ebay. They are about 3.5 by 2.5 x 3/4 inch in size and
are marked 890-905MHz. The bandpass is very flat and the loss is .8~1 db as
close as I can measure with my surplus test gear. One ebay seller mentioned
something about their use in railroad communication systems.
The response can be narrowed for a 902~903 peak response, it depends how
much patience you have for tweaking with a spectrun analyzer and sweep gen.
I would not try peaking one of those without instruments tho. The skirts are
pretty sharp below 890 and above 905, so they are usable without tweaking.
Another possibility is looking at the old F-204/U twin cavity filters that
were originally designed for 374~404 MHz. I cut these down for use on
1296MHz ala the WA5VJB pipe cap filter principle. On that band the cavity
cylinders are cut down to 2 inches tall and the inner guts pulled except for
the rotating tuning cap and I/Os replaced with #12 wire probes about 1.25 to
1.375 inch long for low loss (<.5~1db)/broader bandpass, or 1.0 inch range
for higher loss (3db) tight bandpass. I use one of these modified filters in
series with a Parabolic interdigital filter on 1296 to further tighten the
BPF response and have some adjustable tuning control for input matching. The
combined loss on both filters is .8db (12W in 10W out) on 1296. No reason
the same method could not be used on 900MHz. I'm guestimating I would have
to go with cavity cylinder heights about 3 inches tall and I/O probes around
1.8 inches long for a 900MHz version. If anyone is interested I can spend
an hour or two some evening putting one together for 902/3MHz and report
back on the results. It's been something I wanted to tinker with anyway. 73
Mike wa3tts
One side note: It's possible to use other F-series twin cavities for 23 or
33 cm BPFs but the ones with the smaller rotating tuner caps yield lower
throughput losses. Replacing a larger disc with a penny seems worthwhile, or
perhaps removing it altogether (shaft tuning only) and shortening the
cavity height to compensate.
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