Time Domain Characteristics of Broadband Antipodal Fermi Antenna and Its Application to Through-wall Imaging

1. INTRODUCTION There is a great demand for broadband antennas used for many applications such as the ultra-wideband (UWB) communications, the EMI measurements and the wideband radars. Considering the transmission and reception of pulse signals, not only broadband characteristics of input impedance and radiation pattern but also the distortionless condition are required. Among many types of broadband antennas such as the bow-tie antenna, the log-periodic dipole array antenna, the spiral antenna, the TEM horn antenna and the double ridged horn antenna, the tapered slot antenna (TSA) is well known as traveling wave operation, low weight, thin structure , easy to fabricate, well suited for microwave integrated circuits (MICs) and is expected for the applications described above. Recently, Sugawara et al. have proposed a TSA called " Fermi antenna " [1] having a profile defined by the Fermi-Dirac function as well as the corrugation on the side of the substrate. In our previous paper [2], the antipodal Fermi antenna (APFA) with the combination of antipodal feeding section [3] and Fermi-Dirac taper section has been proposed and high gain and the lower cross polarization level are obtained in a broadband frequency range. The FDTD analysis was also performed showing that the VSWR and the radiation patterns were almost agree with the measured data. In this paper, time domain characteristics of the APFA designed in [2] are investigated. 2-element H-plane APFA array is also developed as a quasi-monostatic pulse radar for the application of through-wall imaging. 2. GEOMETRY Fig. 1 shows the geometry of the APFA. The Fermi-Dirac taper is determined by f (x) = a/(1+e −b(x−c)) [4]-[6] where a denotes the asymptotic value of the width of the taper for x → ∞ and c denotes the x coordinate of the inflection point of the Fermi-Dirac function. Because of the relation of f 0 (c) = ab/4, b is related to the gradient at the inflection point c. Also there is a relation of f (c) = a/2 and W = 2a when b(L − c) À 1. Dimensions of the APFA are shown in Fig. 1. 3. EXPERIMENTS The vector network analyzer 8722ET (Agilent Technologies) is used for frequency-domain measurement. The time-domain data were obtained by the frequency-domain data in the frequency range from 2GHz to 18GHz (Bandwidth=16GHz) which yield a time resolution of ∆t=0.0625ns. The transmission coefficient S 21 between two APFAs separated with a distance d in the opposite …