Inter-Digital filter for an ADS-B receiver

Update 2. I believe I have found an improvement for the 5-6 dB loss in the passband. It involves improving the electrical conduction between the frame and the covers. It will require a slight change in the cover design as well as in the height of the frame. I am also trying some rectangular resonators to widen the bandwidth slightly. A rectangular resonator is also easier to print smoothly lying on its side where it is not as sensitive to any z-banding. (if that is a problem). I still have to print the updated revisions of the new parts to test though. Hopefully by the end of April. It will be revision 1 of the filter.

Update 1. I am still trying to find a way to get rid of the 5-6 dB loss in the passband. Unless you have a cell tower closeby this filter is not recommended for immediately after the antenna at this point. However, if you have preamp and want to ensure that only the 1090MHz get to your SDR by adding it after the pre-amp, then it should work. Just ensure that your preamp does not have a DC offset at its output since at DC it is a dead short.

Inter-Digital filters are certainly not at all new and in principle very simple to build. However, to manufacture one accurately at home, can be a challenge, especially if you don’t have access to some precision metal cutting machinery. Making one from copper clad FR4 does open some opportunities for the DIYer, but precision cutting and assembling still remains a challenge. This builder had very good success using FR4: https://keptenkurk.wordpress.com/2014/11/05/a-homebrew-1090mhz-ads-b-filter/
Please read the article so that you are aware of what is involved when tuning the filter, since the tuning could be a show stopper, if you don’t want to go as far.

Having an entry level 3D Printer, I decided to see if I could print the parts with PLA and clad it with copper foil to create the conducting enclosure and resonators.

If you consider making this, I suggest getting the hardware parts before printing since the design may have to be adapted if the parts don’t fit. You may want to print the test block and take it with you to the hardware store to check if the F-connector and brass nuts will fit.

The filter needs the following additional parts:
M3X12 screws ………………………………………………....… 23
M3 washers ………………………………………………………... 3
No 10-32 x ¾ Inch brass screws ……………………….....…....... 3
No 10-32 brass nuts……..……………………………………….... 6
No 10 brass washers ……………………………………………… 3
Mountable F-Connector barrels (with flat nuts)…....................... 2
Quality RG6 cable…………………………………………...….. 100mm
Copper Foil with conductive adhesive……………................. 1 Roll (~1500mm x 50mm)
Filament ………………………………………........................ ~50 meter x 1.75mm

I have found that this 2 inch wide foil allowed me to cover each individual resonator with one piece of foil.
https://www.amazon.ca/gp/product/B01I1XNY1E/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1

When you have all the hardware ready and you have not printed the test block, do so now.

Check the fit of the barrel F-Connector and the No 10-32 nut and screw. Also remove the core conductor from the RG-6 coax and test fit the wire in the small holes in the test block. If the fit is slightly loose it does not matter. If it does not fit, you may have to adjust the . stl files, do some trimming or drilling or check your printer settings.

Check that you can cut thread into the test block using the M3 screws. (Remember it is plastic, you will not be able to tow a truck with it ;-))