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Interdigital Filters

Sometimes you’ll come across a piece of commercial microwave gear, in this case it was a number of very nicely made interdigital filters out of Nokia microwave links. The filters were for frequencies around 1.8 - 1.9Ghz and salvaged by David ‘1FDMG.  As they came they weren’t going to be useful on any Amateur frequency, however by taking about 5mm off the six 30mm long interdigital elements and then re-tuning them they may be useful on 13cm.

This next photo is the Nokia filter being aligned using a directional coupler with network detectors connected to the analyser. The pickups on the filter input and output are simple wire loops, I didn’t try to optimise these at all, but left them as they came for 1800-1900Mhz.  The insertion loss at 2403Mhz is about 3dB.

These filters are an excellent example of what can be found in junk lying around, one of these would make the heart of a very nice 13cm transverter.

W6PQL opened my eyes to what can be achieved using simple construction methods with microwave interdigital filters and still achieve good results, his use of standard size diecast boxes for housings and then “designing” the filter to those case dimensions works surprisingly well.  By taking to these commercial Nokia units with a hacksaw I was able to move them up in frequency and with careful re-tuning keep their very good response.  Having said all that, these 6 section filters offer a very narrow passband of around 33Mhz at the 3dB points, making these more than acceptable in a 28Mhz IF transverter scheme for 13cm - quite extraordinary. The LO and image are both more than 50dB down.

You’ll notice very steep skirts on the filter response, the horizontal divisions are 14Mhz and the centre marker is 2403Mhz,

W6PQL - Interdigital Filters

W6PQL described some simple interdigital filters for 23 and 13cm using standard die-cast boxes as housings and designed the interdigital elements and spacing to suit. I needed a filter for the output of my 13cm transverter low power stage and thought I’d try replicate Jim’s idea, it works...

The picture above shows the basic 3 element filter, using 6mm alu rods and brass screws in the housing together with SMA connectors and thin strip brass coupling loops.  This filter is very quick and easy to fabricate and simple to tune for a passband response based on peaking the output.

In both plots, the horizontal divisions are 40Mhz and the vertical are 10dB, centred on 2403Mhz. The passband response to the left indicates an attenuation of 30dB+ for the LO and image using low side injection in a 144Mhz transverter scheme, the return loss at 2403Mhz is nearly 30dB.  This is a very good response for such a simple filter and entirely adjustable.