With space at a premium, there is strong interest to develop a single ultra wideband (UWB) conformal phased array aperture capable of supporting communications, electronic warfare and radar functions. However, typical wideband designs transform into narrowband or multiband apertures when placed over a ground plane. Therefore, it is not surprising that considerable attention has been devoted to electromagnetic bandgap (EBG) surfaces to mitigate the ground plane's destructive interference. However, EBGs and other periodic ground planes are narrowband and not suited for wideband applications. As a result, developing low-cost planar phased array apertures, which are concurrently broadband and low-profile over a ground plane, remains a challenge.
The array design presented herein is based on the infinite current sheet array (CSA) concept and uses tightly coupled dipole elements for wideband conformal operation. An important aspect of tightly coupled dipole arrays (TCDAs) is the capacitive coupling that enables the following: (1) allows field propagation to neighboring elements, (2) reduces dipole resonant frequency, (3) cancels ground plane inductance, yielding a low-profile, ultra wideband phased array aperture without using lossy materials or EBGs on the ground plane. The latter, is of course, critical for retaining the aperture's wideband behavior under conformal installations.
This dissertation focuses on the realization of wideband phased array apertures using tightly coupled dipole arrays. A methodology for designing planar apertures is presented including: element selection, material loading, and unbalanced to balanced conversion for wideband feeding. Multiple solutions and practical design examples are presented to increase bandwidth, reduce height, avoid common mode excitation and retain low-cost planar PCB manufacturability. Using one of these designs, a 64 element low-profile X-band array prototype is fabricated and measured. The conformal array is capable of scanning up to 70 degrees and 60 degrees in the E- and H-planes, respectively. The active VSWR is less than 2 from 8 to 12.5 GHz (1.6:1) and the array height is only lambda/7 at the lowest frequency of operation. A unique feature of the proposed array is its planar layered PCB construction. Specifically, a single microwave laminate is used for the aperture while another supports all associated baluns and matching networks. Good agreement between simulations and measurements confirm the proposed concepts.