The Experimental Characterization of UWB Antennas via Frequency-Domain Measurements

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 120deg 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 turn, 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 three-antenna 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.