The Ka-band, 26 - 36 GHz, was chosen for the availability of low-noise HEMT amplifiers[6,7], for optimum atmospheric transmission and stability, and for low contamination from foreground emission. Dust emission is negligible at wavelengths exceeding several mm. Nonthermal emission from radio point sources, however, can be considerable. These sources will be identified and monitored using an external facility, such as the Australian array (ACTA).
A cut away view of the receiver is shown in Figure 5. The corrugated horn, lens, and shroud are discussed in the section 3. The thermal break is made by a thin wall stainles s steel waveguide between the feed horn and the polarizer. The vacuum seal is made at the interface between the stainless waveguide and the throat section of the horn.
The receivers are very similar to those used to measure the Sunyaev Zel'dovich effect at 1-cm on the OVRO and BIMA arrays. Based on the performance of these receivers with four stage InP HEMT amplifiers, we expect receiver noise temperatures of 12 K to 16 K, including the contribution from the warm lens.
The receivers are sensitive to only one polarization. To reduce the sensitivity to linearly polarized foreground emission, such as synchrotron emission, we utilize 1/4 wave polarizer to transform form the linear polarization of the amplifier to circular polarization.
The polarizer also serves to reduce cross talk between array elements. By adding a rotating 1/2 wave polarizer between the 1/4 wave polarizer and the feed, we modulate the phase of any noise wave emitted from one receiver and received by another, for example the out-going noise wave from the first stage amplifier. By modulating all array elements synchronously we reduce the amount of cross talk without affecting the phase of astronomical signals.
The front plate of the receiver is used to attach the receivers to the telescope. The relative pointing of the array elements is controlled to within ten arcseconds by ensuring the feed patterns are orthogonal to this precision ground front plate.