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Re: [VHFcontesting] 902/3 filters

To: <vhfcontesting@contesting.com>
Subject: Re: [VHFcontesting] 902/3 filters
From: "MICHAEL SAPP" <wa3tts@verizon.net>
Date: Fri, 08 Aug 2008 13:09:58 -0400
List-post: <vhfcontesting@contesting.com">mailto:vhfcontesting@contesting.com>
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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