A new perturbation method for measuring microwave fields in free space

A perturbation method for measuring free-space microwave fields is described. A short thin metal rod forms the perturbing element. In order to avoid confusing the reflection from this rod with unwanted reflection the rod is arranged to spin about an axis perpendicular to its length. The perturbation of the field at the source is therefore modulated at a characteristic frequency, and by using an a.c. amplifier, steady reflections can be eliminated. The apparatus described operates at a wavelength of about 3.2 cm. Experimental results are given which verify the theory for linearly polarized waves. Extension of the method to elliptically polarized waves is possible in principle, but inherent ambiguities may be troublesome in practice.