Time-Domain Circularly Polarized Antennas

The transient properties of short-pulsed elliptically/ circularly polarized antennas, namely, the transmitting and receiving effective heights, are introduced. Due to the pulsed temporal dependence of the radiation pattern, an instantaneous axial ratio is introduced as a quality measure of the polarization performance. For these antennas, it is shown that the trace of the tip of the electric-field polarization vector in time depends on the pulse-width (bandwidth) regime of the excitation signal. Only for quasi-monochromatic or narrow-band excitations is a ldquopurerdquo elliptical/circular trace achieved (i.e., axial ratio approaching unity for circular polarization). For short-pulsed excitations, a deformation of the elliptical/circular polarization vector trace is obtained due to a Hilbert-transform dependence in the antenna´s effective heights and the finite duration pulsed-envelope. A remedy for that deformation, for circular polarization, is obtained by using an array of sequentially rotated circularly polarized antenna elements, which extends the temporal duration for which an instantaneous axial ratio lower than some prescribed value is obtained beyond that of a single antenna.