Propagation of Higher-Order Waveguide Modes from the OMT into the Far Field

The design of a circularly symmetric Q-band (30- 58 GHz) feed horn with a turnstile orthomode transducer (OMT) is analyzed using electromagnetic simulation to quantify the activation of higher order modes in the waveguide that connects these two components of the feed assembly. The effect of these higher order modes on the far-field beam pattern is demonstrated, as propagated through MeerKAT offset-Gregorian reflector optics. This investigation follows from a study that found that a dipole OMT with asymmetric balun design unexpectedly activated higher order waveguide modes in the L-band (856-1712 MHz) which propagate into the far field for the same reflector geometry. In that case a fundamental mode analysis proved to be ineffective at explaining the measured far-field beam patterns in the upper half of the operating range. There is a high degree of symmetry in the Q-band design studied in this paper, and this work determines that the higher order mode activation is approximately -20 dB lower than reported for the L-band dipole OMT design. The effect of introducing deliberate asymmetry into the turnstile OMT design is also analyzed to assess how a small manufacturing error could change the outlook. The proposed analysis method is computationally attractive for evaluating the effect of several adjustments to the design of the OMT on the far-field patterns.