BATC Forum

Hi,

Any 3 or 5 bay 23cm interdigital filters for sale

Brian G6HFS

I have a spare 3 element one. What freq do you want it on?
Gareth

Hi Garath,

for datv 131600mhz for local repeater

Brian G6HFS

OK, I just gave this one a tweak using Satsagen, looks like it might help you. The skirts can be made a few dB steeper but the insertion loss goes up too.
Its 500g packed up. Cover the postage required and its yours for a good cause
Gareth

No disrespect to Gareth who is offering this item gratis and contributes a lot to the cause, but the curve shown has no sign of the flat top to be expected from a three pole filter. I am wondering the item is based on the RSGB VHF/UHF Manual drawing.

I suspect that this drawing contains a draughtsman's error in the position of the resonator taps, and they should be 1/2" from the outside of the case, not the inside. I base this on the results of putting similar dimensions into a filter design application, and on my own experience with a crudely made version from 1970s.
Moving the tap to be about 6mm from the resonator base transformed it into a three pole filter rather than a single pole filter with the outside rods acting as little more than adjustable coupling elements. Admittedly, my plot shows that I went a bit far with this resulting in 2dB ripple in the passband.
However, anyone who has made one of these or is planning to do so may like to take the tapping point in the drawing as a starting point for experimentation.

Hi Garath,

Filter arrived and gave it a little clean up, inserted and working good so have them for tx/rx now.

73

Brian G6HFS

Glad it does what was required Brian and thanks to G3YKI for raising the issue of curve shape. The three filters I (had) came in a box of silent key stuff I bought a few years ago. Once Satsagen came out I was able to check if the filters were as written on them and capable of adjustment to current DATV frequencies.
I assumed that with three 'prongs' they were probably interdigital filters but comparing my curve with your curve its not as selective. This is as perhaps you might except with just one resonant element.
'Every day is a school day', if I ever need a steeper skirt I know what to do

Gareth

If anyone else is struggling with filters I have a 50 ohm network analyser which you may like to use, 9kHz to 3GHz. Aligent 8714ES, calibration is accurate.
I also have spectrum analysers to 3GHz which some may wish to use.

Email Davezawadi@yahoo.co.uk
All in Bristol.
David