Interdigital filters for 23cms.

On several occasions I have tried to relay an ATV signal received at one end of the band and retransmit it at the other end. For example, I would like to be able to relay a signal that I can receive well, to a friend who is unable to receive that signal. I would also like to have 'look through' on a repeater (should I ever be so lucky to get one strong enough!).

Luckily Roy, G8CKN, came to my rescue with a tried design approach. This is based on a computer program for designing a fixed tuned filter, written by VK3UM. For amateur use it seems sensible to incorporate a trimmer on each tuned line in order to get the best performance from a handmade filter. To take this into account the rods should be made a little shorter than the program recommends. For my filter, centred on 1316 MHz, Roy suggested 51mm rods instead of the computer's 54.1mm.

Click here to see a typical design output from the program
pcb interdigital filterThis is my first realisation of the filter design, using PCB material for speed and simplicity of construction. The sides are 1.6mm double sided, but the base and lid are single sided 2.0mm thick for extra rigidity. To try to make the inside RF tight, I soldered copper foil to the inner surfaces and bent it over the top edges of the side walls.
This is my brass version of the same design. The brass I used was mainly governed by what I already had and what I could get cheaply and easily! However, the sides must be thick enough for tapped holes for fixing the lid and yet the end walls thin enough for the RF connectors' reach.
Brass InterDigital Filter
There should be several advantages of the brass over the PCB version. It can be constructed so that it is suitable for 'protected' outdoor use. Mechanical rigidity is better and hence also performance stability. The electrical contact of the lid with the body is better, provided lid fixing screws are spaced about every inch/2½cm.
A Simpler, Less Ambitious Filter.

This is my attempt at building the filter design of Steve, G6ALU. He describes it as, "An easy build 23cm look through filter". As you can see from the photo, all the component parts are standard items; the mechanical work and skill required to build it are basic so there is no excuse for not making one.

Please take a look at Steve's website Like all his projects he has made basic ideas into good practical performers while keeping in mind their ease of construction.

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