Synthesis of Ultra-Wideband Array Antennas
Acquisition of ultra-wideband signals by means of array antennas requires essentially frequency-independent radiation characteristics over the entire bandwidth of the signal in order to avoid distortions. Factors contributing to bandwidth limitation of arrays include array factor, radiation characteristics of the array element, and inter-element mutual coupling. Strictly speaking, distortion-free transmission or reception of ultra-wideband signals can be maintained if the magnitude of the radiated field of the array remains constant while its phase varies linearly with frequency over the bandwidth of interest. The existing wideband-array synthesis methods do not account for all factors affecting the array bandwidth and are often limited to considering the array factor and not the total field of the array in the synthesis process.
The goal of this study is to present an ultra-wideband array synthesis technique taking into account all frequency-dependent properties, including array total pattern, phase of the total radiated field, element field, element input impedance, and inter-element mutual coupling. The proposed array synthesis technique is based on the utilization of frequency-adaptive element excitations in conjunction with expressing the total radiated field of the array as a complex Fourier series. Using the proposed method, element excitation currents required for achieving a desired radiation pattern, while compensating for frequency variations of the element radiation characteristics and the inter-element mutual coupling, are calculated.
An important consideration in the proposed ultra-wideband array design is that the "phase bandwidth", defined as the frequency range over which the phase of the total radiated field of the array varies linearly with frequency, is taken into account as a design requirement in the synthesis process. Design examples of linear arrays with desired radiation patterns that are expected to remain unchanged over the bandwidth of interest are presented and simulated. Two example arrays, one with a wire dipole as its element and another using an elliptically-shaped disc dipole as the element are studied. Simulation results for far-field patterns, magnitude and phase characteristics, and other performance criteria such as side-lobe level and scanning range are presented.
Synthesis of two-dimensional planar arrays is carried out by employing the formulations developed for linear arrays but generalized to accommodate the geometry of planar rectangular arrays. As example designs, planar arrays with wire dipoles and elliptical-shaped disc dipoles are studied. The simulation results indicate that synthesis of ultra-wideband arrays can be accomplished successfully using the technique presented in this work. The proposed technique is robust and comprehensive, nonetheless it is understood that the achieved performance of a synthesized array and how closely the desired performance is met also depends on some of the choices the array designer makes and other constraints, such as number of elements, type of element, size, and ultimately cost.