Can a radio transmitter somehow detect the number of receivers in its area?
Actually, yes, a receiver can affect the transmitter. Passive RFID is based on this principle.
However, RFID only works at very close distances, where the receiver is absorbing something on the order of 10-4 to 10-5 of the transmitter's signal. In other words, the transmitter is sending out hundreds of milliwatts, while the receiver is absorbing a few microwatts. Such changes are just barely detectable at the transmitter with careful techniques.
However, for general broadcast radio, the transmitter is sending out tens to hundreds of kilowatts, while the receiver is absorbing tens to hundreds of femtowatts, which is a fraction on the order of 10-18. This is completely undetectable at the transmitter. Furthermore, receivers absorb signal regardless of whether they're turned on or not, so even if it were detectable, it would tell you nothing about how many people were actually listening.
It's technically possible to detect radio receivers if they are Superheterodyne receivers that use RF mixing to downmix the received signal to a well known intermediate frequency. You can scan for this frequency using a directional antenna and count the receivers around you.
Though this doesn't sound like what you're inferring since the transmitter can't detect the receiver based on signal "load" or other factors, it requires a special detector that's separate from the transmitter.
This is how radar detector detectors work. Also, some billboards use this technology to determine what radio station drivers are listening to so they can tailor the ads to the drivers' preferences:
- Billboards That Know You
- High-tech billboards tune in to drivers' tastes
No. There is no way for an AM or FM transmitter to determine how many people are listening. They provide exactly the same power output at carrier whether there are a million receivers within 1 mile or zero.
Digital transmissions that require a subscription can on the other hand possibly know how many receivers there are, if there is a two way verification link. Or like WiFi, each 'receiver' is actually interacting with the transmitter, but in neither case does it affect the output power of the transmitter, or is it able to be sensed by monitoring the output power.