How Speedar works.

The Speedar hand-held traffic radar equipment is based on the well-known and legally-accepted principle known as the Doppler effect. This is a fundamental law of physics first demonstrated by the Austrian physicist Christian Doppler in 1842. It applies to the alteration in frequency caused when a wave of energy of any sort is reflected by or transmitted from a moving surface. Everyone is aware of the operation of this principle in everyday life, albeit unconsciously. For instance, its effects are shown whenever a vehicle passes whilst sounding its horn. As the vehicle approaches, the sound from its horn is perceived to be at a higher pitch than when it is receding.

As applied to traffic radar, the Doppler effect means that a radio frequency signal emitted from a stationary radar and reflected from a moving target will experience a change in frequency of the reflected wave directly proportional to the speed of the target, relative to the radar transmitter.

In whichever direction the target is moving, the change in frequency will be the same for the same relative speeds. The difference between an approaching and a receding target being that an approaching target will give a reflected frequency higher than the transmitted frequency, a receding target will give a reflected frequency lower than the transmitted frequency.

The Speedar transmits at 24.10GHz. The transmitted microwaves strike a moving target vehicle and some are reflected back. If the vehicle is approaching, more waves, ie. a higher frequency, will be reflected back to the radar; if the vehicle is receding, less waves, ie. a lower frequency, will be reflected back to the radar. The difference in frequency between the transmitted signal and that received back after reflection is known as the Doppler shift. This frequency shift falls within the audio band for the transmitter frequencies and vehicle speeds for which the equipment is used.

The circuitry in the Speedar compares the frequency of the transmitted and reflected signals in a homodyne mixer and from the Doppler audio difference frequency that is detected, the circuitry calculates and displays the speed and direction of the moving target.

The Doppler frequency is calculated according to the following equation.





Cos U

= the doppler frequency in Hz

= the transmitter frequency in Hz

= the speed of the target in metres/second

= the angle of travel of the target relative to the radar