EVLA Memo 137 Performance Tests of the EVLA K, Ka, and Q-Band Receivers

Efficiency observations performed in January and February 2009 during clear dry weather with the EVLA receivers mounted on antenna #24 show excellent performance at K, Ka, and Q bands. System temperatures are well below project requirements, with mid-band values of 37, 43, and 58 K at K, Ka, and Q bands, respectively. Antenna efficiencies decline smoothly with increasing frequency, from a high of ∼ 55% at 18 GHz to ∼ 28% at 49 GHz, and are well fitted by a Ruze law, with a zero-frequency efficiency of 61% and surface roughness of 0.42 mm. Antenna spillover temperatures at the zenith rise slowly with band, with values near 14, 16 and 18K at K, Ka, and Q bands. Spillover appears to be nearly constant at all three bands between 60 and 20 degrees elevation – this range is thus the best for determining atmospheric opacity through tip curves. All bands show a 1 – 2 degree rise in spillover at the zenith, presumably due to ground spillover seen at all lines of sight around the antenna reflector. Spillover at low elevations has a sharply band-dependent behavior: At K-band, a ∼ 4K excess is seen at all frequencies, while at Ka-band, there is very little change in spillover, even down to 8 degrees elevation. At Q-band, the behavior is notably different, with a ∼ 4K excess observed at the low-frequency end, and a ∼ 6K deficit at the high frequency end. There were no peculiar changes in detected power like those noted in earlier tests which were performed while in the D-configuration. We conclude that these changes were caused by reflected ground radiation off adjacent antennas, and not by some inherent instability in the receivers.