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> www.unixservice.com.au/hamradio/repeaters/duplexers/23CMpix/
IMG_3809c.JPG

Hi all,

I've just attempted to retune this duplexer from Radio Frequency Systems P/L.
Model: CP1800-I42

Originally on 1710 and 1910 MHz. It tuned down to 1273 and 1293 MHz OK but
kept the same "channel" bandwidth of about 20 MHz. (the width of the pass band)

So, it's skirts only attenuate 45db or so the Tx carrier and/or the
Tx spurii.

I note the inter digital coupling screws do nothing and I mean nothing
to reduce the coupling and thus the filter bandwidth. (the three on the
left and right ends of the top cover)

Has anyone any thoughts on making it useful for our 20MHz split 23cm
repeaters?

We have a "parrot" 23cm repeater over in the Wollongong area and changing it to Full duplex will require
cavity filters. I acquired this one, at a good price a few years ago. Perhaps spares never used.
Using 70cm cavities in 3rd overtone is a great idea. 

Robin, 

My test gear here is the new VNA the SAA V2 and a Wiltron 640 from around 1985. What I'm asking is:
Noting that both sides are just bandpass. And, as this inter-digital design is common.

How does one reduce the coupling so as to reduce the bandwidth?

As mentioned, the top screws do absolutely nothing, an imperceptible change.
I need to achieve somewhere between 5 and 10 MHz, not 20MHz.

Has anyone here any experience, either practical or with simulation?

BTW: The SAA V2 is magic.

Also, I don't know who the designer was in RFS but it's a bugger to tune with those M3 brass screws
wobbling about inside the finger ends with only 1/2mm clearance or less.