Comparison of calculated beam patterns of the CBI1 and CBI2 antennas across the observing band of 26–36 GHz. Top: far-out beam patterns – (black) 1.4-m antenna at 26 GHz, (blue) 1.4-m at 31 GHz, (green) 1.4-m at 36 GHz and (red) 0.9-m antenna at 31 GHz (no shield can). Note that the main spillover lobe at 100° is lower in all cases for the larger antenna and is negligible at the top of the observing band. Bottom: main lobe of the calculated beam patterns. Solid lines indicate the 1.4-m antenna. Dashed lines indicate the 0.9-m antenna. In both cases, the colour scheme is green, 36 GHz; red, 31 GHz and black, 26 GHz.