Dual-Frequency Decoupling Networks for Compact Antenna Arrays

Decoupling networks can alleviate the effects of mutual coupling in antenna arrays. Conventional decoupling networks can provide decoupled and matched ports at a single frequency. This paper describes dual-frequency decoupling which is achieved by using a network of series or parallel resonant circuits instead of single reactive elements. 1. Introduction The adverse effects of mutual coupling on the performance of multiport antennas are well known [1]. The effects can be countered by using a decoupling network which provides an additional signal path to effectively cancel the external coupling between array elements to yield decoupled ports [2–4]. In its simplest form, the decoupling network consists of reactive elements connected between neighbouring array elements, but this approach only applies when the mutual admittances between elements are all purely imaginary [2]. The design of decoupling networks for arrays with arbitrary complex mutual admittances has been described [3–5]. In [6], closed-form design equations for the decoupling network elements of symmetrical 2-element and 3-element arrays were presented. This concept was extended to the decoupling of larger, circulant symmetric arrays through repeated decoupling of the eigenmodes [7]. However, these methods are only applicable to the decoupling of arrays over a small bandwidth at a single frequency. This paper describes dual-frequency decoupling of arrays. The procedure is based on the methods described in [6, 7], but where each reactive element in the single-frequency decoupling network is replaced with either a series or parallel combination of an inductor and a capacitor in order to achieve simultaneous decoupling and matching at two frequencies. 2. Theory The procedures described in [6] or [7] may be employed to design a network at two distinct frequencies, and . Subsequently, the relations provided in [8] can be used to design L-section impedance matching networks which match the decoupled port impedances to the system impedance at and . Refer, for example, to the decoupling and matching networks for a 2-element array, shown in Figure 1. ？ and are the elements of the decoupling network at frequency , while and are the corresponding values at frequency . The element values are obtained in closed form from [6]. Ports 1′ and 2′ will thus be decoupled at both frequencies, but the port impedances and are not matched to . The matching networks shown in Figure 1 are for the case where and . and and are the matching network elements at frequency , whereas and are the elements at frequency .