[VHFcontesting] 902/3 filters

[Joe Crawford]

N3EJT had some 900 mHz filters for sale on QTH.com a few months back.
                                                                    Joe 
W4AAB
----- Original Message ----- 
From: "Steve Tripp (K1IIG)" <stephen.tripp at snet.net>
To: <vhfcontesting at contesting.com>
Sent: Friday, August 08, 2008 3:41 PM
Subject: [VHFcontesting] 902/3 filters


> All,
> I see some great suggestions on how to deal with 902/903 problems. Here in
> the NE we are inundated with cellsites that can kill us on 903, not to
> mention all the other shared services in that band. FYI, as a retired
> Cingular/ATT engineer they do not have the best TX filtering and if you 
> live
> near one, you can have extreme overload. I happen to have one 150' from my
> ham tower with S50+ signals which tends to destroy my front end on 903. 
> AT&T
> is licensed to transmit on 880-894mhz and if you live within a mile, you
> will hear them on 902-3. Nextel is somewhere on 901 and they too will kill
> us. They, Nextel, has forced AT&T to place notch filters in their TX to 
> help
> with overload. I had them do so at my cellsite and it DID NOT HELP.
>
> I found some TX/RX single cans on Ebay for $20 and bought 4 of them. I
> carefully did a slope tune of them so 903 was on the lagging edge (not
> center tuned) creating  a sharp highpass filter. This worked but I had to
> except 4db insertion loss on Rcv. I tried a single large can with the same
> loss but was worse on rejecting the 880-894mhz cellular RFI.
>
> What a shame all this great equipment is going to the junk yard. If you 
> know
> an AT&T tech, buy him/her a beer and let them lead you to the sites where
> they leave this stuff outside. Focus on the AIF (antenna interface frame)
> for filters, preamps and lots of SMA jumpers.
>
> Steve
> K1IIG
>
>
>
>> 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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>
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