Q: How does a TV detector van work? Surely a TV, being a receiver, emits very little electromagnetic radiation and so is indistinguishable from any other electrical appliance. Do TVs emit their own special signal? Or are the detector vans simply bogeymen used to scare us into paying our licence fee?
A: The questioner is right to assume that some TV detector vans are 鈥渄eceptor vans鈥� but those that are not can detect TV sets with good accuracy. TVs do give off several types of electromagnetic waves and this can be tuned into with suitable receiving equipment. When switched on, a TV behaves like a low-powered transmitter, from both the tuner and the timebase circuits. The antenna used by the detector vehicle is highly directional so a rapid fix can be obtained on a suspected licence dodger.
More usually, the authorities receive details of TV sales from the dealer after purchase and they check them against their licence records. If you do not have a licence you may get a visit just in case you possess an unlicensed set.
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A: In common with most radio receivers, televisions are superheterodyne receivers, in which the incoming high frequency signal is mixed with a local oscillator signal to produce a lower, fixed-frequency signal 鈥� which is known as the intermediate frequency.
This is amplified, filtered and detected so that the signal processing circuitry does not need to be retuned to receive different channels and can be designed to have a precise gain-frequency profile and very high rejection of out-of-band signals. In the case of a TV, accurate separation of the sound, vision and colour signal components is also made possible.
Only the very first stage of the receiver has to accept very high-frequency signals over a wide band, and only the local oscillator needs to be tuned to receive different broadcast channels. It is tuned to the channel frequency plus the intermediate frequency, and the mixer generates the difference frequency, which is the intermediate frequency.
British colour televisions receive signals in the 470 to 860 megahertz UHF band, and their local oscillators operate between 510 and 900 MHz to generate an intermediate frequency of 39.5 MHz.
Although great care is taken to shield the local oscillator and mixer circuits, and to isolate the aerial from the mixer, some of this signal is radiated 鈥� mostly from the aerial 鈥� and can be detected by detector vans. Since the local oscillator frequency is always 39.5 MHz above the channel being received, the detector van can tell which channel you are watching.
A TV is also pretty noisy at other frequencies: some of the intermediate-frequency signal leaks out, and there is considerable radiation from the timebase scanning coils. These are driven by a pulsed signal at 14.625 kilohertz and so splatter characteristic higher-frequency harmonics into the ether (mostly as short-range magnetic signals); they can easily be detected with a long-wave radio near the TV.