Measuring a microwave field close to a conductor

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Abstract
The modulated scatterer technique can be used to measure a microwave field with very little disturbance. If the transmitting antenna is itself used to receive the scattered waves (monostatic configuration), the method is unaffected by the presence of conductors in the measurement region. However, when the scattering probe is very close to a conductor an additional effect, not allowed for in the simple theory, arises from waves that have been scattered more than once. A general theory is given for this proximity effect; an important result is that the measured positions of the zeros of the field are not affected whatever the shape of the conductor. An explicit expression is derived for the special case when the surface of the conductor is a plane, and an experiment to show up the proximity effect with this geometry has verified the applicability of the theory. It is concluded that the error in amplitude made by using the results from the present probe, uncorrected for proximity, is less than 5% for distances from the plane conductor greater than lambda /6, and less than 1% for distances greater than 2 lambda . The phase error is less than 5 degrees for distances greater than lambda /4 and less than 1 degrees for distances greater than 1.2 lambda .

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