[NLRS] 900 Filters

[John P. Toscano]

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:

 >> Hi, Dave. If one of these cans was tuned to 902.6 MHz, would the
 >> passband include both 902.1 and 903.1 MHz? Can you give me any idea
 >> how narrow and how steep the passband falls off, and what the
 >> insertion loss at the tuned center frequency would be? If I'm not
 >> mistaken, one could re-tune for lower insertion loss and wider
 >> passband with a second adjustment control on the can.

Here is his response:

> I have no equipment to test the filters out with. My buddy has a IFR-1600
> in his shop but I can only get to use it when he stays late and hes 100
> miles round trip. Thats how I know the center freq, the high & low for
> the cans. All I know is that I've had reports back from hams that have
> used six cans for a 900mhz rpt and others that just used 2 or 4 cans for
> keep out local intermod.
> 
> At this time I have about 40-50 cans left along with some 4 can 19" rack
> mount plates and cables.
> 
> Sorry I cant ans your tech questions, Dave

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.

   See: 
http://www.txrx.com/product/product_view.aspx?UID=57494EDF-7B0D-4412-96A7-D426A4124508

There is a bandpass plot, showing one of these units tuned to 883 MHz. 
At an insertion loss of 0.5 dB at the tuned frequency, the passband is 
down just slightly better than -10dB (maybe -11 dB) at +/- 5 MHz.  If 
re-adjusted for maximum sharpness with an insertion loss of 3.0 dB at 
the center frequency, the passband is down about -28 or -29 dB at +/- 5 MHz.

You might be able to get to that plot with this URL:
http://www.txrx.com/product/product_image.aspx?CUID=512f1c7f-0d64-4a5e-9d91-785dc064755f&UID=976fec60-376c-4894-b2da-576ff9067487
   ...or this one...
http://www.txrx.com/product/product_view.aspx?UID=57494EDF-7B0D-4412-96A7-D426A4124508

So, if one of those was re-tuned to 902.6 MHz (for maximum passthrough 
of 902.1 - 903.1 MHz), you'd be looking at 897.6 to 907.6 for the +/- 5 
MHz points.

Hope this helps somewhat.
73 de WØJT