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Doppler Systems

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Doppler shift systems

One system uses four quarter wave aerials mounted on a car roof.  They fix to the roof with magnets at their bases.  The aerials are arranged in a square or diamond formation on the roof.

The coaxial cables from the four aerials plug into a special box, which combines them into one aerial connector.  This plugs into a standard radio receiver's aerial connector.  The loudspeaker output plugs into another box.

The direction of the target transmitter is indicated on a display consisting of many LEDs arranged in a circle.  The accuracy of the bearing depends on how many LEDs there are.  The system is calibrated to show the direction relative to the direction faced by the car.

To get a bearing with a hand held directional aerial may take several seconds.  The Doppler system is instant, and the LED indicating the last known bearing stops on.

If the target station is transmitting permanently or frequently, the best way of using the system is to start with bearings from two locations.  This gives the rough area of the target.  After that, the car drives to the area.  As the target is approached, the car simply follows the roads and streets which go in the appropriate direction, left or right.

If the car drives at a normal speed on a straight road and the indicated direction sweeps rapidly e.g. from relative north east to relative south east, it means that the car has just passed the target.  Turning the car and going back slowly in the reverse direction results in the house (or transmitter aerial) being identified when the indicated direction is e.g. relative due west.

Doppler systems don't suffer from the close range signal strength swamping effect suffered by systems which use directional aerials.

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Page updated on 09 January 2017

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