Comparison of UWB Antennas Considering Pattern Variation With Frequency

The radiated waveform shape of a good UWB antenna should be the same in all directions of interest. This property is termed “waveform omni-directionality”. Such antennas can be characterized by a single normalized transfer function or impulse response, making system optimization, equivalent circuit modeling, propagation analysis and pulse shaping filter design much easier. Waveform omni-directionality is directly related to the antenna pattern stability with respect to frequency. It may not be feasible to realize UWB antennas with absolutely stable patterns; however, a quantitative figure for pattern stability enables one to compare UWB radiation from various antennas in a range of directions of interest. In this paper we use a newly proposed Pattern Stability Factor (PSF) to compare several probe-fed monopole UWB antennas and microstrip-fed printed planar monopole UWB antennas. The PSF is also used to determine the pattern stability bandwidth of those antennas. The PSF is defined in the frequency domain because UWB regulations (e.g. on bandwidth, mask level etc.) are specified in this domain but it also represents waveform fidelity and omni-directionality in the time domain. It is important to note that the PSF is independent of the gain of the antenna. For a good PSF, a flat gain response (versus frequency) is not necessary but the gain frequency response curves should be similar (apart for a constant) in all directions of interest. A pattern stable antenna fulfills this condition because, although its absolute gain pattern may change with frequency, the fractional change is the same in all directions of interest.