Roll your own Interdigital filter


A homemade interdigital filter for 1090 – how difficult can it be?

Following the results posted by TomMuc in this The Final Filter Shootout
post I thought I’d do some research on making my own filter(s). I’ve decided to try to make an interdigital filter as I have some of the bits already and they have a small insertion loss.

I found quite a few hacks had made filters for 23cm (1.2GHz) but many did not say how they worked out the dimensions. Further digging and I came up with some calculators.
These 2 are on-line

This one is a download

I decided to make a 3 stage filter following this example but with dimensions calculated from above. I’ve already hit a bit of a snag as the 2 on-line calculators come up with different dimensions for the box :roll_eyes:

I will report results once I get something working.

Anyone want to chip in with thoughts/experiences?

Do I Need A Filter?

maybe the link i posted in the other thread could be helpful for you because he did it with easy to find materials

the other thing in my opinion that is very important is to first setup two very good synchronized sites that produce over the week very close results in messages, range and aircrafts. only this basis then allows to see and document progress with new equipment e.g. filter.

i like your idea and curious about the results :slight_smile:


some more links that maybe are helpful for you:


@TomMuc - thanks for the links.


Happy Experimenting!
Waiting for results.


Watching with great interest.

A man with one watch knows what time it is.
A man with two watches is never quite sure.


If you are using double sided copper clad board, solder a nut to the outside only, keeping the inside of the cavity ‘clean’.
This isn’t so good (nuts on the inside)

This is best way to keep the interior clean (by soldering a nut to the outside only) - kind of like this (if you understand what I mean).

Best of luck.



Hi Nigel,

Thanks for the pointers about the nuts. It was mentioned as a problem by Keptenkirk in his blog above.

I’ve decided to go with 0.9mm copper sheet rather than the copper clad board. The sheet arrives on Saturday.


I think the copper sheet is a good choice.
Double sided PCB has always seemed to me to be prone to creating a capacitor between each side as well as unpredictable current loops.


This is as far as I’ve got.

I used a template for making paper/card boxes

Marked it out on the copper and started cutting. I used tin snips but found it difficult to keep to the lines.

I made a couple of formers from bits of wood to the exact length and width of the final box. I used them to bash the tabs to make the corners.

OK, the corners could be better. I won’t bother with the tabs next time but just solder the seams.

Next is to attach the SMA connectors and add the tuning screws.

I’ll be back :grinning:

[Edit] forgot to mention that I used 6mm copper tube and soldered them to the edges of the box. This doesn’t make a clean interior that Nigel mentioned above but will do for now.


Progressing well, but I see a small error.
It looks like you’ve drilled holes for the SMA connectors at the top rather than the bottom of the 1st and 3rd elements.


Hi Gecko, I drilled the holes whilst the copper sheet was still flat. Once I’d bent it to box shape the holes were not in the right place so I thought I’d fill them with solder and re-drill in the correct place at the bottom of the elements. The corner tabs also partially cover a couple of the holes.

Do you know if the bottom of the elements have to be closed/terminated? At the moment they are open.


You mean the ‘free end’? If so, yes it’s fine to leave them open. In fact it’s common for the tuning screw to be able to enter the tube (but not touch it) to increase the tuning range (tuning is achieved by varying the capacitance between the case and end of the element - the closer the screw, the higher the capacitance, the lower the resonant frequency).


No, the other end. I intended to use a bolt to hold the element vertical but found I could do it without the bolt with a bit of careful soldering. The bolt hole is still there in the side of the case.


hi guys

looks difficult to construct.
could any of these varicaps be used instead ? or an array of the blue ones ?

varicap 1


Easy way to eliminate that effect is to drill a small hole (or a few) through the board, insert a bit of copper wire and solder it on both sides of the copper forming a ‘dead short’.
Makes it easy to solder to the inside and the outside of the board to form an RF tight enclosure.

Another trick is to use copper tape (google slug tape) and solder that in various places.
Might be useful for plugging mis-drilled holes :wink:



The first one is both physically too large and the min. capacitance will be too high. It’s intended for a wide tuning range at low frequencies (HF). Just looking at the pic, it’s likely to have a tuning range from 20~100pF.
You’ll find a cap like this in your grandmothers AM radio where it’s used to tune across the band regularly and still have a long life.

Trim caps like the blue one are used on PCB interdigital filters, but in this case, the tuning screw is simpler, cheaper and provides a more appropriate capacitance range (maybe 0.01~0.5pF). In fact a tuning screw is quite similar to a ‘piston trimmer’ capacitor which are designed specifically for use at microwave frequencies.

The short answer is: Yes, if you pick the right part, it will work well, but there are (probably) easier ways to do it.

Variable air caps (some background)
Piston trimmers.


The 2-Gang Variable Condenser in Grand Ma’s radio.
No, it is not a typo. When Grand Ma was young, they used to call it condenser, not capacitor.



Well, much to my surprise, it works:grinning:. Winding the middle screw up and down increases or decreases the messages per second on Skyview.

Not the prettiest

I’ve run a couple of scans with rtl_power

Without the filter

and with the filter in

[Edit] Testing was done with a push fit lid. Once I get it tuned, I’ll make the lid permanent.


the scans look very encouraging! what kind of usb dongle did you use? would be interesting to see how scans look with the different filters you have - and whether the aircraft/range/message-rate correlates to what the scans show …

cool - well done!



Great improvement with filter.
Cutouts of scan 920M - 960M makes it easier to see the result at a glance.

Without Filter
with filter out (920M - 960M)


With Filter
with filter in (920M - 960M)