Every once in a while i bring my car down to a local mechanic to have some small thing fixed. The guy is kind of a burned out hippie, and he usually has cool stories and jokes that i listen to for a little bit.
One time i was noticing an old radio he had from the 70’s. This radio had the capability, it seems, to recieve special bands that were only to be used by the cops or paramedics and such.
He warned me not to put it on those service bands because they can track the signal or the reception or something. Several times he was listening to the police band for fun and a patrol car rolled up later on and told him to turn it off.
Is this possible? Is there any way the police could tell if he was listening in to this or that particular radio traffic? The guy is a bit out there, so i figured he was likely BS’ing me, but is there any truth here in general?
There is no way to tell if someone is scanning or receiving any particular frequency; however, a station transmitting can be found by radio direction finding. I used to do this working for the coast guard, when recreational boaters would accidentally leave an open mic on VHF marine band channel 16.
Secure communications are usually encrypted. As far as I know, you have the right to tune into (receive) any frequency you can find, but you are only legally entitled to transmit on those frequencies for which you are authorized by the FCC (or CRTC). This is why you can buy scanners with large frequency range capability, but transmitters are band specific.
This isn’t true. There are several bands of frequencies you are specifically prohibited from intercepting by the Electronic Communications Privacy Act of 1986, which include cordless and cellular telephone signals. The Act also states that should you unintentionally receive such communications, you are prohibited from disclosing their contents to anyone not privy to the original transmission.
In general, and in theory, yes. Older radios produced what’s called IF (intermediate frequency) when receiving. As the name suggests, it’s between the radio station’s frequency and audio, and I won’t bore you with how it all works with generating a frequency, mixing it with the incoming signal, getting a resultant lower frequency… Some radios did “leak” (accidentally transmit) the IF, so yes, it is possible to tell what station an older radio is tuned to.
In practice, not really. The cops would have to be actively searching for this oddball radio’s IF, which isn’t necessarily an easy project.
In a way, there is. I know that one plan to get more accurate Arbitron ratings is to scan cars in congested areas to see what stations they’re listening to. I can’t remember how this was to be done, possibly by reflecting a laser off of the windshield, and interpreting the results, but it sounded promising.
Or maybe the po-po have a bug in this mechanic’s shop and are screwing with him.
Radios still use an intermediate frequency. It is generated by combining a locally generated radio signal with the incoming station signal in a device that puts out the frequency difference between the two signals. This difference frequency is then amplified, detected for the program content, etc., etc.
TV receivers work the same way.
Some small part of the locally generated signal will be transmitted into the surrounding space by the receiver. So, by having a really sensitive receiver and a directional antenna it is possible, although difficult, to find a receiver that is tuned to a particular station.
However in a large city, or even a moderately large one, you really would have to have some idea of the general physical location to be searched in order to be successful, I would think.
The guy was probably BSing you, but detecting a radio in use is (or perhaps was) possible.
In Britain, the BBC used to have detector vans that would drive around neighborhoods (sorry, neighbourhoods) to detect people using unlicensed radios and TVs. (Owners of all radios and TVs had to pay an annual fee that went to support the Beeb, but some people tried to get around it.)
I think the vans operated in the 1950s and into the 1960s, but I don’t know when (or if) they stopped, or whether modern electronics make such tactics impossible. Q.E.D.?
BTW, this is what Monty Python were spoofing in the Fish License sketch, when they referred to “Cat Detector Vans.” “The man said that their equipment could pinpoint a purr at four hundred yards! And Eric, being such a happy cat, was a piece of cake.”