Can a radio transmitter somehow detect the number of receivers in its area?

Question

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.

Answer 1

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.

Answer 2

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

Answer 3

Sounds like complete and utter cr*p for all practical purposes. The actual energy extracted by a receiver is microscopic.

Though there is a story of a farmer who built a big tuned loop in order to extract free power from a nearby radio transmitter. Sufficient to distort the field pattern and be detected.

Answer 4

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.

Answer 5

Assuming the the field in question is the electromagnetic field, and all interactions are in the 'far field', then the question is 100% no, no you cannot sense increased load.

RF is just the production of light, albeit a frequency much lower than visible (WiFi runs at 2.4 GHz. Red light is ~400 THz).

Does a star experience more 'drain' because its light is being absorbed by my eye? Or a piece of silicon? Or a spherical cow?

Does a light bulb experience more 'drain' because its light is being absorbed by my office walls?

The is answer is absolutely no, once the antenna has produced photons, the energy is gone and all drain on that device to produce that photon has already occurred.

...

The answer is different if you consider near field - where inductive reactance dominates. This is how purely-passive, non-transmitting RFID tags mentioned in the comments work - they have an inductive circuit that is tuned to the frequency of the inductor that makes up the antenna, like a big open-air transformer. Here the antenna/transformer/inductor does actually sense increased load, because it is being coupled to the RFID's inductor.

Near field, however, only operates within about 1 wavelength from the transmitter. This is why near-field purely-passive non-transmitting RFID tags must use low frequencies, so that they can have reasonable operating distances.

A good reference is the following paper by two IEEE RF scientists: http://www.ee.washington.edu/faculty/nikitin_pavel/papers/RFID_2007.pdf

To quote:

Low frequency (LF, 125-134 KHz) and high frequency (HF, 13.56 MHz) RFID systems are short range systems based on inductive coupling between the reader and the tag antennas through a magnetic field. Ultra-high frequency (UHF, 860-960 MHz) and microwave (2.4 GHz and 5.8 GHz) RFID systems are long-range systems which use electromagnetic waves propagating between reader and tag antennas

Some wavelength calculations for those above frequencies, for the curious:

• 125 KHz == 2398.34 meters
• 13.56 MHz == 22.11 meters
• 2.4 GHz == 0.125 meters

Answer 6

This is explained here in detail:

So, in the optimum case half of the power absorbed by the antenna is immediately re-radiated. Clearly, an antenna which is receiving electromagnetic radiation is also emitting it. This is how the BBC catch people who do not pay their television license fee in England. They have vans which can detect the radiation emitted by a TV aerial whilst it is in use (they can even tell which channel you are watching!).

Answer 7

There is no way to detect the number of receivers from transmitting point. Once the EM wave leaves the near field of the antenna the wave becomes transversal electromagnetic wave and it has no influence on transmitter. However there is an interaction between sorrounding antennas at close distance (near field - half wavelength), but this is barely detectable.

Answer 8

Technically, it could be estimated. A known power level source will transmit to a certain distance before losing signal strength to half power (-3db). Every antenna and receiver between the source and that -3db distance will tap some of the power from the signal. If you have a receiver sensitive enough at the -3db distance, the number of interfering listeners in between could be estimated. Now, do that process in a circular pattern around the source and you could estimate the number of signal interceptors between the source and the known power level perimeter. A similar process can be used in cable transmission by determining the amount of signal power needed to maintain that -3db level at the end of the transmission line. (ie, each receiver requires 5 milliwatts to show the signal to its receiver, the end of the line will see a minus 5 milliwatts for each customer watching that channel between the source and the line end. If the end of the line experiences a half watt signal strength loss (500 milliwatts) it means 100 people are tuned to that channel.

That is doable physics. Whether the radio stations or cable providers do it is not known.

http://en.wikipedia.org/wiki/Transmission_(telecommunications)

Answer 9

Interessting question...

in an shielded labarotory, with a well defined receiver - maybe you can. In a real world scenario with all kinds of receivers from different manufaturers, aged, defect, and hundreds of enviromental distubance sources... Its a definitive No I would say.

I knew eg. sending towers powered nearby light chains in gardens without being pluged to a power supply (which was detected as it weakend the transmitters signal a lot). Other people reported a shovel in some special place and position to play radio. Everything matching some multiples of the signals wavelength drews power from the HF signal without being a valid receiver.

Answer 10

Superhet receivers are still the norm at present .They have a local oscillater and use standard intermediate frequencies .So it would give predicted lo radiation .For example a 1593 KHz AM station would make the receiver tune its lo to 2048KHz if it was picking up the station .Specailised narrowband equipment could pick up this carrier spouting out from the receivers antenna .If the receivers are not very good it is in theory possible to pick up this lo radiation .Very haed to do .Now if you quickly key the transmitter off and on so it is not noticed you could pick up a chirp in the lo due to pulling if the receiver is really cheap.

Answer 11

This question may be well answered by now but just to be sure - the load could be increased by a receiver in the near field, but once the signal is ‘emitted’ there’s no way to know what has happened to it. The RF energy will be absorbed one way or another and it’s impossible to tell whether a signal has been received and interpreted or just absorbed.