Mode Conversion in Circular Waveguides

In order to obtain optimum illumination at the aperture of a dualmode conical horn,1 as in the open cassegrain antenna, 2 it is necessary to control the relative amplitudes and phases of the TEn and TM U modes. The mode combination is established (considering the antenna as a transmitter) by exciting the dominant mode (TEn) in the circular waveguide that feeds the horn, and converting a part of that signal to propagate in the TMn mode by introducing suitable transmission line discontinuities in the vicinity of the waveguide to cone transition (Fig. 1). The general requirement that a discontinuity be suitable for mode conversion as described above is that it perturb the incident transverse electric field in such a way as to produce a component of electric field in the direction of propagation. This may be done by introducing a conducting surface transverse to the direction of propagation, in which case the coupling arises directly from the fact that the component of electric field tangent to the surface must vanish. To be generally useful, the mode conversion configuration must provide controlled mode conversion over a useful band of frequencies without. introducing significant reflection of the input TEn mode. It is further required that the converter discontinuity be circularly symmetric, both to prevent excitation of unwanted modes and to permit operation of the device for signals of arbitrary polarization. Three basic schemes are possible, the iris, the groove, and the step, each of which is shown in Fig.