Multiport broadband matching and its application in the design of feed networks for array antennas

Abstract

The design of 2N-port networks that allow good power flow between N isolated resistors and a general, passive, N-port network is discussed. A new design procedure is derived by first solving a multiport matching problem exactly for a particular class of load network. Then a strategy to extend this design to general load networks is presented. A congruence transformation that approximately diagonalizes the impedance matrix of a given multiport over a band of frequencies is found. It is shown how this congruence transformation can be practically implemented as a network at microwave frequencies. This 2N port transforming network is placed in front of the given multiport, so that the impedance looking into the transforming network consists of N approximately isolated impedances. Then N two port matching networks are designed to be placed between the isolated resistors and the input to the transforming network. These matching networks together with the transforming network constitute a 2N port lossless coupling network with the desired performance. A specific multiport network model for array antennas is developed. A new expression for the power dissipated by a load with a Toeplitz immittance matrix, excited by a progressive phase signal, is derived. It is shown how the design of coupling networks at a particular frequency, and for a particular excitation pattern, is straightforward, but the performance can be unsatisfactory at other frequencies or with other excitation patterns. Results of several coupling network design techniques as applied to a feed network for a four element, linear array of slots at 3 GHz are presented.