[NLRS] 900 Filters

[Donn Baker - WA2VOI/0]

Hi John.

John (the other one) had a good find, and you done good to follow up.  Thanks !

There are several of us who can measure the cavities and/or tune them up.  I'll certainly volunteer.  I am in the 
process of trying to get ahold of 4 of them myself.

I've speen some time tring to make an interdigital filter for 902-903 that will block the 890 MHz carrier that seems to 
be problem.  Best I could do was get about 12 dB of suppression at 890.  I found a good write-up in the RSGB Microwave 
Handbook for filters and interdigital filters.  After a lot of pencil-pushing, it looks as if its just not possible to 
make an interdigital filter that will meet the requirements.  It can't be realizied... the coupling between elements is 
less than the diameter of the elements themselves !  (The fractional bandwidth is just too small... 12MHz/902.5MHz = 
0.013 or 1%.)

I was going to try "suck-outs" instead of filters, but if these cavities will work, they're cheaper than I can build 
something !  From the material you found, it looks as if one will do the job, and certainly two will.  The only 
"problem" is that they are a little on the large size, but, hey !  If that's what it takes....

73 Donn
WA2VOI/0

----- Original Message ----- 
From: "John P. Toscano" <tosca005 at tc.umn.edu>
To: <NLRS at mailman.qth.net>; <BadgerContesters at mailman.qth.net>; "Chippewa Valley VHF Group" <cvvhf at cvvhf.org>
Sent: Monday, October 29, 2007 11:36 PM
Subject: Re: [NLRS] 900 Filters


>
>
> J. Oehlenschlager wrote:
>
>> Just saw an ad on QTH.COM  ad number 625782  for 900 MHz filters  $20 each.  John O.  K0JO
>
> I found the ad, and it looks pretty interesting. For your convenience, here is a summary of the ad:
>
> > These are TX/RX Systems Inc., filters/cans and are now tuned at
> > 931.0 mhz, but will tune between 890-950 mhz. They are pass filters.
> > They are 4" round, 6 1/2" high, 9 1/2" total height including tuning
> > rod. $20 per individual can. I also have small amount of factory
> > marked TX/RX 19" rack mount plates for holding either 3 for 4
> > filters for $10, along with double shielded "N" connector jumpers
> > at  $3 each that go between the cans. Pictured is a 19" rack mount
> > that holds four cans with three cables.  All + shipping.
> > Dave N3EJT 301 233-0910 anyday till 11pm
>
> Not much detail about the passband width, insertion loss, or skirt falloff rate, so I wrote to the seller:
>

> Anyone want to volunteer to sweep one of these and see how it tunes up? Sounds like he has a boatload of them for 
> sale, and if they have appropriate filtering characteristics for weak-signal use, there might be significant local 
> interest...
>
> Now for the (potentially) bad news:  I went to Tx Rx Systems' web site, and looked for a cavity filter that seems to 
> match the details that were given. They show a current model 11-90-05, 4" diameter bandpass filter, which is tunable 
> over the range of 806 - 960 MHz. This sounds like it is quite similar to what he is selling, although it has BNC 
> connectors instead of N connectors. Otherwise, the dimensions etc. seem the same.
>