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
I decided to make a 3 stage filter following this Directory list 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
I will report results once I get something working.
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.
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)
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.
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
[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.
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
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.
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 …
