Measurement · Entry 07.3
What a contour means
A coverage contour is a statistical statement about a field strength, not a fence.
The line on a coverage map is a probability, not a boundary — inside it, the signal is likely usable; beyond it, reception degrades by degree.
The contour is a threshold, not a wall
A coverage contour marks the locus of points where the predicted median field strength equals a regulatory minimum — typically expressed in dB above one microvolt per metre. In the United States, the Federal Communications Commission defines specific contours for AM, FM and TV services, each tied to a different minimum field strength that corresponds to usable reception under specified noise conditions. Cross the contour line driving outward and the signal does not suddenly vanish; the median level simply falls below the threshold at which reliable service is expected.
| What | The consequence |
|---|---|
| Median field strength | the statistical middle of a distribution of readings at a given distance, not a minimum or maximum |
| Contour vs. coverage | the contour predicts where signal crosses a threshold; actual coverage depends on terrain, soil and interference |
| Regulatory use | contours define interference protection zones, not listening areas |
The word median is doing heavy lifting there. Field strength varies with terrain, soil conductivity, season and the hour of the day. Engineers derive contour predictions from established propagation models — for AM ground wave, the ITU-R P.368 curves are standard — and those models describe the statistical middle of a distribution, not a guaranteed minimum. On flat, highly conductive soil a transmitter can outperform its predicted contour; over rocky high ground or dry, poorly conductive soil, it may fall short.
What the contour cannot capture is what actually happens at any single fixed point. A house on the predicted 0.5 mV/m AM contour might receive a perfectly clean signal because it sits in a valley that shelters it from interference and has good soil beneath the ground-wave path. A house two kilometres inside that same contour might struggle because a ridge diffracts the signal away. Field strength measurement — a calibrated meter, a driven route, a grid of readings — exists precisely to replace the model with reality.
Regulators treat contours as planning tools and interference benchmarks: if a proposed new transmitter would place a defined contour inside a protected service's contour, the application fails on paper before a receiver is ever switched on. That is appropriate. But a contour is not a coverage guarantee, and it is not a silence guarantee either. It is an engineering estimate of where, on average, a minimum signal level is expected to occur — which is a useful and precise statement, as long as it is not mistaken for more than that.