International Symposium on Antennas and Propagation
Electromagnetic characterisation of direct machined corrugated horn for the ALMA band 10 receiver
M. Candotti, Y. Uzawa, Y. Fujii, S. Shitov, K. Kaneko,
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The Atacama Large Millimeter/submillimeter Array (ALMA) is a major new facility for world astronomy. ALMA will be comprised of a giant array of 12-m submillimetre quality antennas, with baselines of several kilometres. The ALMA project is an international collaboration between Europe, Japan and North America in cooperation with the Republic of Chile, . Band 10 is the highest band of the ALMA interferometer ranging from 787 to 950GHz. The cryogenic receiver, capable of orthogonal polarisation signal detection, is a heterodyne radiometer which employs Superconductor-Insulator-Superconductor (SIS) mixer for direct conversion of the RF signal to the 4-12GHz IF band. The orthogonal signal polarisation detection is achieved by means of a suspended wire grid, which allows simultaneous illumination of the secondary reflector of the orthogonally polarised receiver. The SIS mixer is fed by a corrugated horn coupled to the 12-m antenna by means of a couple of off-axis ellipsoidal mirrors, which design guarantees frequency independent illumination of the sub-reflector. At such high frequencies, fabrication of corrugated horns is in general obtained by electroformation process, since the corrugation depths reach dimensions as small as ~10-1mm. In this paper the experimental electromagnetic characterisation of corrugated horns obtained from direct machining procedure is presented. A number of two horns made of copper and gold plated have been fabricated and their electromagnetic beam pattern measured in the near-field zone using a custom made amplitude and phase near-field beam pattern measurement system. Far-field beam patterns are subsequently calculated by Fourier transformation and compared with software far-field simulations of the corrugated horn obtained with commercial mode matching software .