5 Antenna Measurements

If the conducted sensitivity and conducted output power are as expected, and the range is poor compared to the estimated range, the antenna efficiency is likely poor. The required tuning is a function of the ground plane size and the casing meaning that even if the antenna is copied directly from one of our recommended antenna designs the tuning could be not optimal. The Antenna Measurements section in the Antenna Selection Guide contains a more detailed description on how to measure antennas than the summary below.

• Preferred: Use a Network Analyzer (VNA) to accurately tune the antenna. Some mini VNAs are available on the market with a reasonable price tag if a full featured VNA is not available or cannot be rented.
  • Disconnect the radio and add a semi-rigid coax cable or SMA connector at the 50 Ω point towards the antenna matching network and antenna.
  • Compensate for the length of the semi-rigid cable or SMA with port extensions so they are not included in the antenna measurements.
  • Always measure the antenna in the final casing, otherwise the tuning will be invalid.
  • Always measure the antenna as it would be used in the application. If the device is placed on a metal shelf, the tuning must be made in a similar environment.
  • Measure the resonance of the antenna and make sure the matching is better than VSWR 2.
  • If the resonance is not correct or the matching is greater than VSWR 2, the antenna design has to be reviewed again.
• If measurements with a VNA is not possible then use an EM or Launchpad with a known good antenna.
  • Measure the resonance frequency by sweeping the frequency of the transmitter; measure which frequency the receiver reads as the highest RSSI.