Conditions for Direction-Independent Distortion in UWB Antennas

Many measures for the assessment of ultrawideband (UWB) antenna performance have previously been presented. Nevertheless, it remains to be succinctly stated exactly what constitutes an ideal UWB antenna. Here, one particular criterion, direction-independent distortion, and the conditions required to satisfy it are presented. We show that the realized gain (i.e., gain including mismatch) must be a separable function of angle and frequency, and that the phase pattern must satisfy a narrow set of restrictions. Within certain restrictions, the property of direction-independent distortion is independent of input waveform. We show that the class of antennas known as "maximum directivity" antennas is one type of antenna that satisfies these requirements. Three numerical examples are given, including an unloaded linear dipole, a resistively loaded linear dipole, and a resistively loaded tapered dipole. The examples show that the unloaded linear dipole does not at all satisfy the requirements for direction-independent distortion while both loaded dipoles do, with the loaded tapered dipole satisfying the requirements exceedingly well