During conversation, a colleague proposed that over-the-air television and radio broadcasters can determine the number of viewers or listeners based on the "load" on their signal. This seems to me like total bupkis, but he's piqued my curiosity and I've not been able to find a discernible answer when searching the web to prove him right or wrong.
Is such a thing even possible? Do the number of receivers within the broadcast range of a transmitter put any "load" on that signal? I always thought that the amount of power required for a transmitter simply determined the distance at which the signal could still be reliably received. AFAIK receiving a radio signal does not require any actual power at the listener's end, except to filter and amplify that signal into something useful, and that power is provided locally.
If this were true, it seemed plausible to me that one could place several signal monitors at a fixed radius from the transmitter and measure the signal strength at each. Monitors with weaker signal must have more receivers between that monitor and the transmitter, which could be used to extrapolate the number of receivers within that arc of the radius at, say, -3 dBm per receiver.
What I do know is that obstructions between the transmitter and receiver degrade the strength of the signal, so in that situation, one would have to account for buildings, trees, mountains, birds, precipitation, clouds, airplanes, helicopters, low-flying kayaks, large snowmen, and Santa Claus.