Primary Antenna Elements

Each of DASI's 13 primary antenna elements consists of a 20-cm diameter, wide flare-angle corrugated horn; to make the array maximally compact, the receiver dewars were designed to fit entirely within the footprint of the horns. Unlike mini-Cassegrain elements, horns provide unobstructed apertures, with correspondingly lower sidelobe response and better cross-talk characteristics. To further reduce cross-talk from correlated amplifier noise, the receivers are equipped with front-end isolators, and each antenna element is in turn surrounded by a corrugated shroud.

Each element is preceded by a high density Polyethylene lens, permitting a very compact horn design, while at the same time flattening the aperture distribution, resulting in a high aperture efficiency -- 84%, compared to 69% for an unlensed horn. At polar temperatures, the lens contributes no more than $2.5~{\rm K}$ to the system temperature.

The measured beam pattern, which determines the field of view of the interferometer, has a FWHM of $3\fdg 4$, with first sidelobes below $-20~{\rm dB}$, in excellent agreement with the predicted pattern.

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