Data analyst by day, but I’m laying out my first RF board for our network loggers, so expect naive questions. 4-layer, 60 x 40 mm. On it: an LTE-M module with a u.FL to an external antenna, an SX1262 LoRa section with a chip antenna, and a GNSS module with a small patch. Layer 2 is a solid ground plane. Data from the board ends up in Hermetry (via LTE, LoRa is the fallback).
My question is the top layer. The reference designs I found contradict each other: one says pour ground everywhere on top and stitch it, another says no copper near the antennas, a third splits the ground between sections with gaps „to isolate the RF“. Right now I have a full top pour with a few random vias. The GNSS fix takes ages on the prototype and LoRa range is about half of the dev kit’s.
Way over my head, but is the board in a metal box? Ours lost GPS completely in a steel enclosure.
Plastic box. Good thought though.
Random question, are your chip antenna and patch on the same edge? I read they shouldn’t face each other.
I did a tracker board for my RF module last year and got the same „slow fix, half range“ result on rev A, so this is mostly what my lecturer took apart on it.
Keep the top pour, but make it a real ground: connected to the layer 2 plane with stitching vias everywhere, especially along every RF trace and around each RF section. Space them about 3 to 5 mm apart, tighter near the RF parts. A pour with a few random vias is a floating patch that resonates, and that alone can cost you sensitivity.
Do not split the ground. Isolate the sections by placement (each radio in its own corner, the noisy LTE power stuff away from GNSS) and, if needed, by a stitched fence of vias, but keep one continuous plane underneath.
Under and around the chip antenna and the patch: follow each antenna’s datasheet keep-out exactly. No copper on any layer in the chip antenna’s keep-out, and the patch wants its specified ground area under it. Michael is right too: give the two antennas different edges and ideally opposite corners.
Run your RF traces as grounded coplanar waveguide at 50 ohms (calculate with your stack-up, the vias on both sides are part of it) and leave a pi-network footprint at each antenna feed so you can tune later.
Then measure: send the LoRa RSSI and SNR and the GNSS satellite count and time to fix as readings with every message, and compare rev A against rev B on one chart. That’s the easiest A/B test you’ll get.
Rev B is back: stitched top pour every 4 mm, antenna keep-outs cleared on all layers, antennas on opposite corners. Time to first fix went from about 3 minutes to 35 s, and LoRa SNR at the same test spot improved by 6 dB. The chart comparison was very satisfying for a data person.
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