Meshtastic (LongFast) Interdigital Cavity Filter
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3D printable interdigital cavity filter for Meshtastic
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- Printables
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- Electronics
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- Creative Commons — Attribution — Noncommercial
3D Printable Interdigital Cavity Filter
Description
This is an interdigital cavity filter designed specifically for the default LongFast channel for Meshtastic (906.875MHz). It is designed with a ~5MHz bandwidth. It is made using conductive copper foil tape (with conductive adhesive!) and a few other things. The 3-piece design makes construction easier.
It gets -33dB rejection at 900MHz and 915MHz. Insertion loss is about -2.4dB which is a little high for a cavity filter, but keep in mind, this is 3D printed and made with copper tape.
Extras Needed
- 2in (or 50mm) Conductive Copper Foil Tape
- 1in (or 25mm) Conductive Copper Foil Tape
- M4 Brass Hex Nuts
- M4 x 16mm Brass Machine Screws
- Female SMA Panel Mount Connector
- 18AWG Solid Copper Wire
- M3 x 8mm Machine Screws (M3 x 10mm works too)
- Whatever SMA pigtails you need for your setup
Tools Needed
- Wire Cutters
- Soldering Iron
- Pair of standard scissors
- nanoVNA for tuning
Construction
You'll need to print out the enclosure and two lids. Be mindful of where you'll be operating this. If it is going to get warm, use a material that can withstand the heat (don't use PLA).
Use the copper tape to tape the interior surfaces. Try and make sure the tape lays down flat and smooth. If you have any seams, they need to be overlapping (1mm-2mm is enough).
There are multiple lengths of the resonator rods. It's best to get the tuning as close as possible with the rod lengths than by using the tuning screws. Start with 73.5mm and 74mm rods. If you have to screw the tuning screws in a lot to bring the curves down, replace the rods with the next size up.