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Circuit to detect radar beam from speed detector?

Discussion in 'General Electronics Discussion' started by mountainman, Nov 25, 2013.

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  1. mountainman


    Nov 25, 2013
    I've searched the web for this topic but find a lot of confusion between detecting the radar itself and using the radar to detect speed. That and people going on about the legality.

    I use a professional, non hand held, doppler based 24.125 Ghz radar speed detector to measure the speed of cars.

    What I want to do is find out the limits of the radar beam so I can align the speed detector properly. I'd like to build a device that I can hold at certain points in the road and it will tell me if the radar beam is present there or not.

    Ideally I'd like it also to be able to reflect back the beam in such a way that the fixed speed detector is fooled into thinking it has detected an object moving towards it at 30 or 40 mph. That way I can not only align the detector but test if it is correcting detecting as well.

    Anyone know of a circuit diagram and build plans for a device that will detect radar as oposed to being a detector that uses radar

    First post here so be gentle!
    Last edited: Nov 25, 2013
  2. GreenGiant


    Feb 9, 2012
    What you are trying to build is IMMENSLY complicated, these radar detectors not only look for A signal, but a reflection of THE signal sent.

    You would have to create something that can take an incoming signal, read it, replicate it, then figure out based on the speed of your car what signal to send out.

    All of this while absorbing ANY and ALL signals generated by the source...

    I suppose the only real feasible way to do it would be to make it so that the radar sees you moving at some ridiculous speed which they will then discount as faulty equipment.
    In reality the only way to reliably do this would be to redo the exterior of your car to make it look like say an F-117 Nighthawk, flat radar absorbing paint, with sharp angular features, then you have the highest chance of making it work.
  3. davenn

    davenn Moderator

    Sep 5, 2009
    not quite that complex GG ;)

    welcome to the forums :)

    for your test receiver, you easiest way would be to use a radar detector unit relatively cheap to buy and you could not build it for that amount.

    Radar works for speed detection by using Doppler shift. so for every say 1km / hr of speed there will be an xx change in the frequency of the reflected signal
    for argument sake say 10kHz / 1km/Hr.
    You need to work out what that change is THEN you can produce a transmitter for the car with maybe several set freq's for several specific speeds.
    The transmitted signal will be far stronger than the reflected signal, and that will be what the speed detector sees.

    A bunch of us guys did this years ago back in the days when the radar units were on 10.525 GHz

  4. mountainman


    Nov 25, 2013
    @ Greengiant. I think you misunderstand my question, and are giving an answer similar to those that I found with all my Internet searches elsewhere. By detecting a radar bean I mean just that - detect the beam, just like a simple radio does with the same em radiation but at a different frequency. I don't want to detect the reflected beam and my car is not involved at all. I don't want to mount it in my car and I don't want to defeat the speed detector - quite the reverse in fact.

    I am already measuring the speed of other cars using a professional doppler speed detecter that is mounted at the side of the road. I simply want to find out how far on each side of it (or above/below) the beam is propagating so that I can ensure I am not detecting cars outside a certain position as measured from the speed detecting head. I don't want to measure the doppler shift / calcuate speed or anything like that.
  5. mountainman


    Nov 25, 2013
    Thank you, I did look around and yes you can get car mounted things for around £15 on ebay, the sort of things designed to warn drivers when they approach a speed trap. They might do the job instead of building one but I was looking forward to the challenge/ fun of making it!

    As for testing the professional unit by transmitting back a beam, yes, I understand the doppler shift principle and can calculate the frequencies needed but I have no idea how I would go about building a circuit to generate those frequencies. I have built some pretty complex circuitry in the past for my PhD to do with computer controlled detection of data from sensors but that was always by cobbling together/adapting bits of circuit diagrams I found elsewhere, not designing it all from scratch - and it was not to do with radar.

    Any hints where I might find decent circuit diagrams of what you suggest with device IDs and component values shown?

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