horn. This is realised by providing a groove discontinuity at the input of the horn. The depth and width of the groove discontinuity in the presence of corrugations and cascaded input waveguides were optimised using CHAMP software to obtain symmetrical as well as the amplitude taper. In the presence of a groove, the computed amplitudes of TEII, TMll and TE12 modes at the input of the corrugated horn are 0.84, 0.48 and 0.20, respectively, at 36.5 GHz. The multifrequency corrugated feed employing a groove discontinuity at the input of the corrugations is shown in Fig. 1.Results and discussion: The measured co-polar radiation patterns with a groove discontinuity at 18.7 and 23.8GHz closely match the computed patterns as shown in Figs. 2 and 3, respectively. The radiation patterns with and without a groove were found to be identical at 18.7 and 23.8 GHz and therefore patterns without a groove are not included in Figs. 2 and 3. The measured radiation patterns with and without a groove at 36.5GHz are shown in Fig. 4. As shown in this Figure the measured patterns without a groove are very narrow and patterns with a groove are wider. Although the measured patterns with a groove at 36.5 GHz do not closely match the computed patterns these patterns clearly exhibit the beam broadening effect due to the groove. The mismatch can be attributed to the horn and power coupling waveguides not being fabricated as a single piece. The worst case measured crosspolar patterns in the diagonal plane are shown in Figs. 2 -4. Measured return loss at all the frequencies is shown in Fig. 5. 1°F -40 -5 O E n I n i i i I i i I i I* I 18.50 18.60 18.70 18.80 18.90 23.50 23.65 23.80 23.95 24.10 36.00 36.25 36.50 36.75 37.00 frequency, GHz 1901/51 Fig. 5 Measured return loss at 18.7, 23.8 and 36.5 GHz frequency bands 18.7GHz -0- 36.5GHz-I- 23.8GHz Conclusion: The technique to generate TMll and TE12 modes using a groove at the input of corrugations is very effective for microwave radiometer feeds for prime focal parabolic reflectors for multifrequency operation.

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