Frequency, antennas, receiver sensitivity and environment jointly determine usable radio margin.

Transmit power is not range: four link-budget variables

A larger transmitter-power number cannot predict the usable distance of a video link by itself. Signal arriving at the receiver depends on the whole path and on whether the receiver can decode it with sufficient margin.

Account for gain and loss

A simplified calculation adds transmit power and antenna gains, then subtracts path and other losses: received power ≈ transmit power + transmit antenna gain + receive antenna gain − path loss − other loss. Distance and frequency affect free-space loss. Obstructions, polarization mismatch, feed-line loss and interference can make a real result worse than an ideal calculation.

Power is logarithmic: doubling transmit power adds about 3 dB, not twice the distance. At a fixed distance, doubling frequency raises ideal free-space path loss by about 6 dB. Antenna gain recovers margin only in the directions where that gain applies.

Sensitivity and margin

Receiver sensitivity is a threshold under particular bitrate and error conditions. The amount by which the received signal exceeds that threshold is link margin. Increasing bitrate or bandwidth may change the threshold. Extra margin helps a mobile link tolerate fades from motion or partial obstruction.

Four field questions

What frequency and bandwidth are used? What are the installed antenna patterns and cable losses? Which sensitivity figure applies to the target video rate? What terrain, height, foliage and RF noise lie on the path? Record these answers, then measure signal and throughput at repeatable locations before extrapolating coverage.

Before testing, record transmitter power, frequency, bandwidth, both installed antennas, receiver mode and sensitivity, path geometry and height, and local interference. A range result without these conditions cannot reliably be transferred to another route.