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Experimental characterization of UWB antennas can in many cases be performed using traditional frequency-domain antenna measurements, combined with careful application of well-known integral-transform and linear time-invariant (LTI) system techniques. In this paper, UWB antenna descriptors, including impulse response, energy gain, correlated energy gain, and correlation coefficient, are determined from frequency-domain measurements of transfer functions. Results are presented for two representative UWB antennas: a double-ridged (DR) horn, and a 1200 biconical antenna (Figure 1). The fundamental quantity is taken as the antenna transfer function, which is measured using a modified three-antenna method. One critical step is to extract the free-space component of the port-to-port transfer function, which, in tumn, requires careful attention to definition and measurement of the distance. After the transfer function is obtained, standard linear system techniques can be freely applied. The transfer-function measurement technique involves redundant information due to (a) the use of the threeantenna method with identical antennas, and (b) the use of complex data from a vector network analyzer. This redundancy can be exploited to check consistency and repeatability, by comparing the measured phase with Hilbert-transformed data, as well as by comparing the three estimates with each other. The extracted linear phase is a highly sensitive indicator of antenna displacement, and defines a unique distance for any given pair of antennas in a given orientation. In the three-antenna case with identical antennas, the result is a unique reference plane for the antenna that is not tied to a particular frequency.

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The time delay pattern for a UWB antenna is considered. Many different definitions of time delay are possible. Possibilities include (a) time delay for maximum correlation with a template function, either in terms radiated field or port-to-port transfer function, (b) time delays extracted from the linear phase part of a transfer function, and (c) time delays based on timedomain waveform features.Each of these three general definitions may be computed by multiple means. The delays can be absolute or differential, that is, taken relative to the respective delay in a reference direction. It is shown that some of definitions are equal under certain very restrictive conditions. Example data is given for a broadband (1-18 GHz) double ridged horn antenna.Index Terms-antenna measurements, antenna radiation patterns, time delay pattern, ultra-wideband antennas.

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