There are three general type of nontracking interferometers. The Mill's cross and similar "cross" instruments combine the outputs of two antenna that produce narrow fan beams in orthogonal planes. The product of the fan beams is a narrow pencil beam, aligned with the intersection of the fan beams. The area of spectral sensitivity is equivalent to that of a rectangular aperture of dimensions equal to half the length of the two arms in the corresponding directions. A taper must be applied at the ends since the strong sidelobes which result from the sharp boundary of spectral sensitivity are undesirable.
Interferometer, Mill's Cross, Two-Element Interferometer
