The design of a dual frequency corrugated horn, working at S- and X-band simultaneously, is presented. The horn was designed to feed the 20 m radio telescope at the Onsala Space Observatory (a classical Cassegrain), via two feed reflectors. The feed system is intended for geodetic measurements using very long baseline interferometry (VLBI). Large corrugated horns are mainly designed by using software based on mode matching. The moment method (MM) for bodies of revolution (BOR) has successfully been used to design small corrugated horns. The design of large horn antennas using MM for BOR is difficult because of the large memory requirements. Internal resonances may also occur and give incorrect results. These resonances are difficult to distinguish from actual performance problems. However, the paper shows that it is possible to identify the internal resonances by scanning the frequency and thereby it is possible to design even large horns, using the MM for BOR.
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