A License Plate Recognition and Speed Detection System - Index of

Fig. 31 Assembled Oscillator
The microwave oscillator circuit performs two functions. First, it produces a 2.6 GHz
signal which is broadcast via a quarter-wave dipole patch antenna. Second, it uses a quarter wave
antenna mounted in a resonant cavity to receive the reflected signal from the object whose speed
we want to measure. Two one pound coffee cans (with the ends cut out of one of them) are
soldered together to form the resonant cavity. A stiff piece of wire is cut so that it extends
approximately 1.1 inches into the can. It is mounted 1⅞ inches from the closed end of the can.
This forms a ¼ wave dipole antenna housed in a resonant cavity. The metal walls of the coffee
cans reflect the incoming radiation in a way that allows only those signals with wavelengths that
are multiples of the can diameter to undergo constructive reinforcement. This concentrates and
intensifies the radiation prior to entering the detector circuitry. The resonant cavity also helps
narrow the angle from which the antenna can receive a signal, which improves the pointing
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