IT person drifting into hardware, so be gentle. I designed a small 2-layer board with an RFM95W (868 MHz) for tank level sensors. From the module’s ANT pin to the U.FL connector is about 6 mm of trace. Same firmware, same antenna, same spot: the dev board reaches the gateway at -98 dBm, mine at -115. In the frames log mine also has SNR around -4 where the dev board sits at +7.
Do I need impedance matching on a trace this short? I thought 6 mm at 868 MHz was too short to matter.
6 mm is electrically short at 868, matching shouldn’t cost you 17 dB. Look elsewhere.
Is the antenna actually an 868 one? The 915 ones look the same.
868, same one I use on the dev board.
Short helps, but 17 dB is not « short trace » territory, that’s a broken RF path. Did this on a sensor board for a cleanroom monitor and lost about the same.
Questions to check: is there solid ground under the trace on the other layer? Is the trace 50 ohm? On 1.6 mm FR4 a 50 ohm microstrip is around 3 mm wide, so a 0.25 mm signal trace is closer to 120 ohm. Over 6 mm that alone isn’t the whole 17 dB, but add no ground plane under it and the trace is half an antenna radiating into your own board.
What I’d do on the next revision: coplanar waveguide with ground on both sides and a row of vias, width from a calculator for your stackup, a pi network footprint between module and connector (0 ohm in series, others not fitted until you can measure), and the U.FL ground pads stitched straight to the plane. Keep the switching regulator away from that corner.
Then compare again in the frames log, it shows the best RSSI and SNR per uplink so you can check both boards on the same gateway without extra tools. See The frames log.
Bottom layer under the trace was empty, I had routed the battery line through there. Rev B: CPWG at 50 ohm, via fence, ground solid. Now -100 dBm and SNR +6 at the same spot. Thanks Laura.
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