The spinning-dipole technique applied to the measurement of waveguide modes

The paper describes an instrument for the measurement of field distribution in a multi-mode waveguide, and shows how the information obtained in this way can be analysed to yield the relative amplitudes and phases of the various wave modes present. The technique of measurement is based on the use of a minute dipole supported by a nylon thread and spinning about an axis perpendicular to its length, so that the electric field incident upon it is scattered with a frequency corresponding to twice the speed of rotation. Arrangements are made to amplify and record this scattered field while the dipole is moved from one point to another over the cross-section of the guide, thus enabling relative values of the field strength, its direction and phase to be obtained. The instrument was employed to examine the mode purity of rectangular-to-circular H01 mode waveguide transducers and of the output field distributions from various waveguide tapers and bends terminated in a matched load. By making comparison with the results obtained by other methods, it was concluded that the spinning-dipole technique gave an accurate assessment of the mode content of the fields in the waveguides investigated.