Rich Place WB2JLR – March 12 2026

Recently I flashed my Heltec V3 to be a Meshcore repeater. Since there are several cell towers in the vicinity, and the V3 has no front-end filtering, I was concerned that the radio might be overloaded by strong signals. The reported noise floor on my radio was in the -112 to -118 dBm range, indicating little to no interference, but I thought it best to add a filter anyway.

I borrowed George KW4L’s TinySA spectrum analyzer to see what the spectrum looks like inside my Sun City Center house. Note the strong cellular base stations in the 870-890 MHz range, just below the 902-928 MHz band where Meshcore and Mestastic operate.

These plots were using a small rubber-duck antenna from inside my house. The first shows a 100 MHz span centered on 910 MHz and the second image covers 100 MHz to 1GHz.

https://www.changpuak.ch/electronics/interdigital_bandpass_filter_designer.php

Is a tool for designing interdigital bandpass filters. It provides the critical dimensions based on the frequency and other requirements that you enter. The larger the filter, the lower the loss and the sharper it will be. A typical 3-section filter looks

Not having a machine shop, I improvised building a filter from double-sided copper clad-circuit FR4 circuit board material. Around the top of the enclosure, I used copper tape to connect the inner and outer surfaces. That must be done because all of the RF currents flow on the inner surface of the enclosure, and one cannot solder that seam on the inside once the top is in place. Unfortunately, I could not hold the tolerances accurately enough for this filter to work acceptably, so I constructed a simpler 3-resonator design.

I used 3/8 OD copper tubing from Home Depot for resonators. They are just under one-quarter wavelength long at 910 MHz, and adjustable screws provide capacitance at the end, bringing the resonate frequency down. On the inside of the enclosure brass nuts are soldered to the wall, and on the outside another nut locks the screw in place.

I was concerned about the tuning screws possibly touching the copper tubing; they need to be close but not touching. So in the end of each pipe I inserted a plastic anchor, like those used to hang stuff on drywall.

This frequency response plot shows about 0.4 dB insertion loss at 911 MHz and 52 dB attenuation at 875 MHz

This zoomed-in plot shows 0.3 dB insertion loss at 910 MHz and over 37 dB attenuation below 890 MHz. The yellow trace show 14 dB return loss, equivalent to an SWR of 1.5:1.

With the filter between the antenna and the spectrum analyzer, the strong cellular signals are gone but weak signals in the 902-928 MHz ISM band are still present.

The Heltec V3 repeater and interdigital filter fastened to my garage rafters.