Inter-Digital filter for an ADS-B receiver

thingiverse

Update 2: I believe I've found an improvement for the 5-6 dB loss in the passband. It involves improving electrical conduction between the frame and covers. This requires a slight change in cover design as well as frame height. I'm also trying rectangular resonators to widen the bandwidth slightly. A rectangular resonator is easier to print smoothly lying on its side, where it's less sensitive to z-banding issues (if that's a problem). I still need to print updated revisions of new parts for testing. Hopefully, by the end of April, it'll be revision 1 of the filter. Update 1: I'm still trying to find a way to eliminate the 5-6 dB loss in the passband. Unless you have a cell tower nearby, this filter isn't recommended immediately after the antenna at this point. However, if you have a preamp and want to ensure only 1090MHz gets to your SDR by adding it after the pre-amp, then it should work. Just make sure your preamp doesn't have DC offset at its output since at DC it's a dead short. Inter-Digital filters aren't new and are, in principle, very simple to build. However, manufacturing one accurately at home can be a challenge, especially without access to precision metal cutting machinery. Making one from copper clad FR4 opens opportunities for DIYers, but precision cutting and assembling remain challenges. This builder had great success using FR4: https://keptenkurk.wordpress.com/2014/11/05/a-homebrew-1090mhz-ads-b-filter/. Please read the article so you're aware of what's involved when tuning the filter, since tuning could be a showstopper if you don't want to go as far. Having an entry-level 3D Printer, I decided to see if I could print parts with PLA and clad them with copper foil to create conducting enclosures and resonators. If you consider making this, get hardware parts before printing since the design may need to be adapted if parts don't fit. You might want to print a test block and take it to the hardware store to check if F-connector and brass nuts will fit. The filter needs these 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), and filament (~50 meter x 1.75mm). I've 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 haven't printed a test block, do so now. Check the fit of the barrel F-Connector and 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 it's slightly loose, it doesn't matter; if it doesn't fit, you might need to adjust STL files, do some trimming or drilling, or check your printer settings. Check that you can cut thread into the test block using M3 screws (remember it's plastic, so don't try to tow a truck with it).