Measurement · Entry 07.2
Proof of performance
The measured demonstration that an array is doing what its licence says.
The measured demonstration that a directional array is doing what its licence says — not what the engineer calculated it would do.
What the proof requires
A directional antenna array is licensed on paper: a specific pattern of radiation, with stated field strengths in favoured directions and minimum depths in the nulls aimed at other stations. The proof of performance is the measurement that confirms the real, installed system matches that paper licence.
| What | The consequence |
|---|---|
| Typical proof radials | 8 or more, covering all 360° |
| Near-field exclusion zone | typically the first several hundred metres from the tower |
| Null depth requirement in some protected-station geometries | 30 dB or greater suppression |
| Trigger for re-proof | any significant modification to the antenna or phasor system |
In the United States, Federal Communications Commission rules require a formal proof whenever a directional AM array is built or modified. The procedure involves driving radial measurement routes outward from the antenna — typically eight or more, covering the full 360° — and recording field-strength readings at intervals. The resulting figures are plotted against the theoretical pattern. If the measured values fall within the allowed tolerances, the licence is validated; if not, the phasor cabinet and coupling networks go back to the bench.
The measurement is taken with a calibrated field-strength meter at ground level, sampling the actual signal the array launches into the world. Distance is logged by odometer or GPS, and readings are corrected for terrain and soil conductivity variations along each radial. The data points closest to the tower — the "near-field" zone — are excluded from the analysis because the field is too complex there to compare cleanly with the far-field pattern the licence describes.
Operators are also required to monitor the array continuously through a system of base-current and sample-line readings. These "operating parameters" are logged in the station's records, and if any reading drifts outside the licence limits, the directional pattern must be re-established — which, after any significant work on the antenna, means another proof. The phasor that controls the phase and amplitude balance between towers is the usual point of adjustment.
The proof matters because a null is only as deep as the precision of the hardware driving it. A directional array protecting a clear-channel station at night may need to suppress its signal in one direction by 30 dB or more. That kind of suppression cannot be assumed — it has to be measured, documented, and defended on paper before the transmitter is allowed to run at full licensed power.