Lorentz reciprocity and the apparent phase centre of ultra-wideband antennas

The transmission and reception properties of an antenna can be expressed using two characteristic transfer functions that describe the directional and dispersive behaviour. From these functions an apparent phase centre may be determined based on either the transfer function for transmission or reception. Using the Lorentz reciprocity principle, which expresses a symmetry of Maxwell's equations, a simple relation between the two transfer functions can be established. Based on this relation, it is shown that the phase centres for transmission and reception are equal. Furthermore, a definition for a delay domain phase centre is proposed.