Connected array antennas : analysis and design

Many of today's satellite communication and radar systems necessitate phased array antennas that are capable of wideband/multi-band operation and good polarization purity over a wide scan volume. However, the antenna solutions typically used for wideband wide-scan applications trade-off matching performance against polarization purity. For this reason, in recent years, a new approach has arisen for the design of broadband arrays, aiming at reducing cross polarization. This antenna solution consists of arrays of long dipoles or slots periodically fed, and are referred to as connected arrays of slot or dipoles. Connected array antennas represent one of the most promising concepts in the field of very wideband arrays, for being able to achieve both broad band and low cross polarization. The wideband performance is due to the fact that the connections between neighboring elements allow currents to remain nearly constant with frequency. Another attractive feature of connected arrays is their capability to achieve good polarization purity, in virtue of the planarity of the radiators. Besides the advantageous physical properties, connected arrays are based on simple geometries that lead to the derivation of analytical solutions for the antenna parameters. Closed-form expressions based on a spectral Green's function representation are derived for the input impedance, the current distribution over the array and the radiation patterns. Important advantages result from this representation with respect to numerical solutions: above all, the reduction of computational costs and the gain in physical insight on the wave phenomena. A convenient circuit representation of the array unit cell is derived. The circuit describes rigorously and analytically the transition between free-space radiation and guiding transmission line. Contrarily to standard Thévenin circuit for receiving antennas, this representation can be used to evaluate the power scattered by the antenna. The results have been applied to the analysis of the scattering and absorption of a real connected-dipole prototype array backed by a frequency selective ground plane. Good agreement was achieved between measurements and results from the equivalent network. A novel measurement technique based on passive RCS measurements in the main planes was used to characterize the active matching of the radiating part of the antenna in transmission. Finiteness effects can be particularly severe in connected arrays, due to electrical connection and the high mutual coupling between the elements. As a consequence, the overall behavior of a finite wideband array can be sensibly different with respect to infinite array analysis. Thus, it is crucial to include edge effects already in the preliminary assessment of the array performance. An efficient numerical procedure is derived for the characterization of the edge effects. The method requires only one unknown per elementary cell, independently from the cell geometrical parameters. This is possible thanks to the use of an appropriate connected array Green's function in the integral equation. This procedure is of general applicability and can be used for arrays with and without backing reflectors and for arbitrary scan angle. An alternative analytical representation is also derived to provide physical insight on the nature of the edge-waves. The analytical approximation of the spatial current distribution on the finite array is derived, for the specific case of a connected array of dipoles operating in free space, and scanning only in the E-plane. The key step is to represent the total current as sum of the infinite array contribution plus edge-born waves. The final analytical expression is given in terms of Fresnel functions and allows qualitative considerations on the nature of the electric current distribution, in terms of spreading and attenuation. The analytical expressions represent a powerful tool that can be used both for modeling and design. A connected array of dipoles with 40% bandwidth, when scanning in elevation to 45o, has been designed. Practical designs require the implementation of ad hoc feed structures that avoid common-mode currents to propagate on the feed lines. This problem has been addressed and feed structures that perform common-mode rejection have been designed. Measurements form a prototype demonstrator were presented for validation and showed good performance.