A Transient UWB Antenna Array Used with Complex Impedance Surfaces

The conception of a novel Ultra-Wideband (UWB) antenna array, designed especially for transient radar applications through the frequency band (300？MHz–3？GHz), is proposed in this paper. For these applications, the elementary antenna must be compact and nondispersive, and the array must be able to steer in two dimensions. The geometry of the elementary antenna and its radiation characteristics are presented. The array beam steering is analyzed and a technique making the increase of the transient front-to-back ratio possible is described. 1. Introduction In the recent years, investigations directed to the realization of Ultra-Wideband (UWB) systems that radiate transient waveforms and exhibit operating bandwidths of over one decade are made intensively in many countries. Such systems radiate impulses with rise times around 100？ps and duration of few nanoseconds. Such radiated waveforms make them interesting for many applications such as transient radar cross section (RCS) measurements [1] or UWB synthetic aperture radar (SAR) systems [2]. The main interest of such systems is that the radar resolution is proportional with the radiated bandwidth. Contrary to narrowband systems, a second advantage is the capacity of identifying stealth targets. This paper describes a novel antenna array appearing in the N generators/N antennas configuration, because it presents the advantage of increasing the radiation power on one hand and offering the agility to the array on the other hand. Moreover, this architecture permits a working continuity even if an elementary source is defective. With the frequency spectrum [300？MHz–3？GHz], this kind of system permits to detect hidden objects (wall, foliage, or ground penetration, etc.). A major difficulty of limiting the synchronization of the radiation source jitters yet exists. A solution which consists in using impulse optoelectronic devices permits to bypass this difficulty, with the generation of ultra short electrical waveforms with a small jitter (2？ps typically) [3]. Section 2 presents the elementary antenna and its radiation characteristics. Then, the array beam steering is analyzed in Section 3, and a technique involving a diminution of the back radiations and the amount of radiations in the transverse plane is presented in Section 4. Indeed, the increase of the transient front-to-back ratio is essential to limit fratricidal effects. 2. The Elementary Antenna The elementary antenna is inspired by a travelling wave antenna, the Valentine antenna [4], because it has good linearity properties. Moreover, the antenna