Most common causes
- Antenna length is incorrect.
- Matching network values are wrong.
- Ground plane is too small or broken.
- Coax connector or cable is faulty.
- Antenna is tuned outside the final enclosure.
How to test
Start with antenna placement, cable loss, power supply noise and final enclosure testing. Then compare RSSI, SNR, packet loss or return loss before and after each change.
What to verify
Check antenna frequency, matching network, connector type, ground plane, cable routing, supply ripple and nearby sources of interference.