915 MHz Matching Network

Designed and measured a π-network matching a LoRa PA into a PCB antenna, cutting VSWR from 3.8 to 1.3.

Approach & key decisions

Match design
Measured S11 on a NanoVNA, pulled the Smith-chart locus with a discrete π-network, and iterated two board spins with adjustable footprints.
Layout
50 Ω coplanar waveguide with a continuous ground return, stitching vias every λ/20, and keep-outs under the antenna clearance zone.

System spec

Band: 902–928 MHz ISM
PA: RFM95W module
Instrument: NanoVNA-H4 + field strength meter
Tools: KiCad, SimSmith

Build gallery

Rev B antenna clearance and match network with 0402 passives.

Early discrete lash-up used to bound inductor values before layout.

Debugging timeline

Rev A
Match looked good on the VNA but field strength barely moved.
Ground plane under the monopole was still shorting the near-field. Opened a full clearance and restitched only at the feed.

Results & final demo

VSWR improved from 3.8 to 1.3 at 915 MHz.
Received RSSI at 240 m improved by ~7 dB through the same path.

The problem

A prototype LoRa node radiated poorly because the PA saw a reactive antenna impedance. Link budget left almost no margin through a single interior wall.

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