Directional dependence of the minimum phase property of antenna transfer functions

It has been shown that a number of directional antennas exhibit minimum phase transfer functions on their principal axes. Here we quantify the dependence of this property on direction for a particular broadband antenna. We present measured data for the transfer function of a broadband double-ridged horn on axis and off-axis in the E- and H-planes. It is seen that the transfer function is clearly minimum phase on axis, but degenerates gradually and systematically as the observation direction is moved off the principal axis. The degradation is much smaller and much more gradual for off-axis directions in the E-plane than in the H-plane. It is postulated that one source of this degradation is interference between the primary radiation from the horn and fields diffracted by the front edges of the sides of the horn.