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Radar Detectors

  1. Feb 4, 2005 #1
    I was reading something today about it being stupid to own a radar detector in Australia because the police carry radar detector detectors with them. Is this even possible?

    I would have assumed that radar detectors don't send out a signal of sorts but only attempt to detect incoming radar waves. I of course don't have the knowledge to make a valid judgement though.

    Truth or scare tactic?
     
  2. jcsd
  3. Feb 6, 2005 #2
    I have also wondered this, perhaps the Electrical Engineers could easily answer.
     
  4. Feb 7, 2005 #3

    minger

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    Yes, it is possible. I did a quick search and it's called VG-2 technology. Still not sure how it works. Of course the high-end detectors are "immune" to VG-2. I have absolutely no idea how it works, I'm just glad radar detectors are legal where I'm at.
     
  5. Feb 7, 2005 #4
    Here :

    http://radar.757.org/VG2.htm

    Seems like a detector emits ever so slightly when it looks for radar. "Detector detectors" search for this emission. In other words, detection is not a "passive" process. I suppose detection involves the slight emission resonating with the incoming wave.
     
  6. Feb 7, 2005 #5

    chroot

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    Standard heterodyne radio receivers use a local oscillator for tuning. When you want to tune a particular radio station, you adjust the frequency of this local oscillator. The output of the local oscillator is mixed (heterdyned) with the incoming RF, which brings the desired signal down to baseband.

    Unfortunately, some of the energy from the local oscillator sneaks its way back out the antenna and actually gets broadcast by the antenna. (The electricity doesn't know which direction it's "supposed" to travel!)

    As a result, nearly all radio receivers actually broadcast a weak signal indicated what frequency they are tuned to. If you're listening to 105.3 FM on your car stereo, you'll actually be broadcasting a very weak 105.3 MHz carrier signal. Similarly, your radar detector is nothing more than a radio receiver operating in the GHz bands, and it also broadcasts some of its local oscillator energy. This leaked signal is not very strong, but it's strong enough for a cop to use a device like the VG-2 to catch you.

    There are other radio receiver topologies that use purely passive electronics and won't leak this way, but they're huge and expensive and fragile. You wouldn't be able to hang one from your windshield, but you might be able to tow one on a trailer. Of course, that'd just make detection even easier.

    - Warren
     
  7. Feb 7, 2005 #6
    What makes passive devices so huge? Radar dishes? antennas? the long wavelengths involved?
     
  8. Feb 7, 2005 #7

    chroot

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    Passive receivers would have to made out of a lot of filters -- tons of inductors, capacitors, and resistors of myriad values. They would have to be pretty precise, too, which means they'd be expensive. You'd also have to tune them regularly, as the seasons change.

    - Warren
     
  9. Feb 8, 2005 #8

    Cheers mate that's very informative.
     
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