N. W. Halverson, J. E. Carlstrom, M. Dragovan, W. L. Holzapfel, and J. Kovac
Lensed corrugated horns were chosen as the antenna elements for DASI because of their compactness and low crosstalk characteristics. Each horn has a 20 cm diameter, as large as possible within the constraint of minimum baseline length, in order to achieve maximum sensitivity and to increase the u,v plane coverage for efficient mosaicing. The full width half power (FWHP) angle of the main beam is 3.4°. Corrugated horns have the advantage of an unobstructed and tapered aperture amplitude distribution with correspondingly low sidelobes, low spillover, and minimal crosstalk between elements. A diffraction limited compact horn is achieved by combining a wide-angle (30° semi-flare angle) corrugated horn with a High Density Polyethylene (HDPE) lens to collimate the phase front at the aperture. Figure 3 depicts an exploded view of the horn/lens assembly.
The lens is meniscus with a refracting hyperbolic front surface, and a non-refracting spherical rear surface to match the spherical wavefront of the wide-angle horn. This particular lens type was chosen to increase the aperture efficiency by redistributing power towards the outside of the lens. The theoretical aperture efficiency for the DASI lensed horn is 84%, compared to 69% for an unlensed diffraction limited corrugated horn. The lens material HDPE was chosen for its desirable index of refraction, low loss tangent, and machinability. With a loss tangent tand @ 170 mrad, a lens at ambient (polar) temperature contributes 2.5 K to the system noise temperature. The lens has grooves ~ l/4 deep that act as an anti-reflection coating.
Special consideration must be given in the design of the throat section, where the mode conversion and transition to the wide flare angle occur, not to excite spurious higher order modes which would degrade the beam. A throat section similar to that described in Refs. 4,5 is employed, with an extremely narrow initial groove width (0.043 cm) gradually tapering in width and depth to reduce return loss. This type of design is particularly critical when combined with a wide-angle flare section to improve the impedance match to the rapidly increasing inner diameter of the horn. The measured return loss is < -27 dB across the band without the lens, and < -20 dB with lens. The predicted and measured beam pattern of the lensed horn is shown in Figure 4.