LoRA mesh networking is gaining a lot of popularity, especially Meshtastic.
The available modules have very lax bandpass filtering. This can be fixed by a simple filter from a tinned still can and some semirigid coax. The construction is based upon the W1GHZ Altoids Tin Filters publication for 901MHz, but tuned a bit lower.
Construction
Please read the original W1GHZ publication, this construction is very similar.
The filter is housed in a steel candy tin, with dimensions very similar to an Altoids candy tin.
The resonators are made out of 53mm of semirigid cable: 55mm of cable was cut, and then 2mm removed to expose the center conductor so that some of it could be pulled from the cable. The center conductors are fed out of the case via 1.5mm holes.
The resonators are 17mm apart from each other and about 8-10mm from the bottom.
The signal is fed to the resonators via SMA sockets, with feedpoint 11mm from ground.
The box is cleaned from rosin using isopropanol, soldered shut, and then tuned via careful pulling of the center conductors, with the center conductors being bent by hand to touch the case. After tuning the center conductors are soldered to the case.
Results
On 868MHz the filter achieves -20.9dB return loss (SWR 1:1.2) and -0.63dB insertion loss. The insertion loss is lower that the author gives in his publication.
The markers below are:
1. 868MHz 2. 853MHz 3. 901MHz 4. 808MHz 5. 934MHz
The measurement was done via a NanoVNA v2 calibrated right before use.
VY 73 DE
Jacek / SQ5BPF