## 1. Introduction

[2] Considerable interest currently exists in the development of antenna technologies for future radiotelescopes such as the Square Kilometre Array (SKA) [*Hall*, 2004]. The SKA requires large collecting area for high sensitivity. In addition, high survey speed is desired, particularly over the frequency range from around 0.7 to 1.8 GHz. One approach currently being pursed is the use of reflector systems with focal-plane arrays (FPAs), thus forming antenna elements with substantial effective area and substantial field of view. A contiguous, fully-sampled field of view is highly desirable [*Fisher and Bradley*, 2000], and one approach to the wideband problem is based on FPAs of tapered-slot or Vivaldi elements [*Ivashina et al.*, 2004; *Veidt et al.*, 2007].

[3] There exist other concepts for wideband arrays [*Lee et al.*, 2006a] that may be considered for FPA applications. Of particular interest are planar and low-profile arrays that may have advantages in terms of cost or ease of integration with low-noise receivers. Such arrays may also be easier to model than Vivaldi arrays, and provide another point of view that may assist in understanding the limitations of dense arrays in such applications. One possibility is the capacitor-coupled dipole concept developed by *Munk* [2003]. Another is the connected-array concept, in either the dipole form proposed by *Hansen* [2004] or the slot form developed by *Lee et al.* [2006b]. As discussed by *Lee et al.* [2006a], connected arrays offer wide bandwidth by approximating Wheeler's continuous current sheet model. However the planar connected-array structures developed so far are single-polarization antennas, whereas dual polarization is required in the FPA applications of current interest.

[4] In this paper, we analyze the efficiency of a planar dual-polarized connected array in relation to wideband FPA applications. The array consists of conducting patches in a checkerboard form, which can be viewed as connected dipoles or connected slots. Alone in free space a large array of such patches is self-complementary and therefore well matched to frequency-independent loads connected between the patch corners [*Mushiake*, 1992]. Previous work on similar structures includes analysis of TEM horn arrays [*McGrath and Baum*, 1999], groundplane effects [*Dardenne and Craeye*, 2003] and bandwidth-enhancement by dielectric loading [*Gustafsson*, 2006]. Here we extend this work by analyzing a structure that includes transmission lines used to divert the array current to loading circuits conveniently located at the groundplane [*Hay et al.*, 2007]. The power transferred into the loads from focal-region and plane wave fields is analyzed and simple forms of loading are compared, including optimum single-ended loading and optimum differential-mode loading with short- and open-circuit common-mode terminations. With differential-mode loading, common-mode currents on the transmission lines are shown to reduce efficiency at certain frequencies. We analyze the distributions of the corresponding currents on the array and the results give some insight into the relationship between the resonances and the array geometry.