Saturday, November 14, 2009

TV detector van detector?

We are led to believe that TV detector vans work by detecting the local oscillator in a TV set.





If the receiver in the detector van is a superhet (and it probably will have to be, to pull in a signal of a few milliwatts coming from a sub-optimal antenna), it will have its own local oscillator; and if the I.F. is small, then its antenna should be good enough to radiate the L.O.





So would it be possible to build a "TV detector van detector" which cuts off the power to your TV set as soon as a detector van comes into range?





Alternatively, I guess you could build a TV receiver with a TRF frontend (like the very early 1-channel, 405 line sets) so there is no L.O.; this would be "undetectable" but not sure how well that would work in practice (625 line pictures are broadcast on UHF).

TV detector van detector?
I'm not sure that TV van detectors work in this manner. I think that you will find a spectrum analyser hooked up to a wideband yagi uda.


The operator would then hunt for peaks using the spectrum analyser.


Now you are correct in that the spectrum analyser is basically a superhet with a sweeping oscillator, but the problem comes in detecting the oscillator - as it's sweeping.





If you want to be TV detector proof, then





1/ ditch your CRT, this will get rid of the LOPTX and HF ccts that a detector van may look for.





2/ Replace your CRT with a LCD.





3/ Open up the LCD and remove power to the LO, disabling rf reception.





4/ Obtain a signal from cable, or via the internet. Satellite dishes are a bit of a give away.
Reply:What on earth are you talking about? Or better. WHERE are you talking about? I have NEVER heard of a TV detecting van.
Reply:Its my understanding that the detector van uses the fact that any receiver retransmits the signal (reason they don't allow radios on air planes). This is how they know what station you are watching.





This phenomenon can be seen by placing an FM radio on top of the TV and tuning it until the TV sound is heard. If you then turn over, the sound on the radio changes to that new station. I helped a friend with a bad TV sound one time like this.





You can experiment, if you want to take the risk to try and block it. They cannot tell if you have a black and white or color TV but I heard the (cheaper) black and white license is not available now.





You may want to look at the legalities more than the technology. There is something about them only coming into your house at a convenient time. I don't think they can barge in if you tell them to make an appointment, by which time, of course, you don't own a TV.


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