Simple models of the antenna power patterns as a function of the sky angle in units of half the primary beam FWHM (θ_fwhm). In the 3 cases shown, the illumination is Gaussian with an edge taper of 12.5 dB but 3 different ratios of the secondary-to-primary diameters (i.e. f_b, the antenna blockage factors) are considered (see e.g. Goldsmith 1998, Chap. 6). The middle and bottom panels respectively model ALMA and PdBI antennas. The red lines define the minimum angular sizes for which the antenna power pattern is less than a given fraction.