Characteristic Modes Approach to Design Compact Superdirective Array With Enhanced Bandwidth

Abstract

Superdirective arrays have become the key technique in enhancing the directivity of electrically small antennas. By forming an array of closely spaced unit elements, a high directivity can be achieved. Despite the high directivity, these arrays suffer from narrow bandwidth and low efficiency. In this paper, we present an approach to enhance the bandwidth of superdirective arrays using the network characteristic modes. The key step is to use an internally loaded compact wideband unit element. The loads inside the unit element are used to manipulate the characteristic modes in order to optimize its impedance bandwidth. A two-element endfire array is then formed. The modal coefficients of the characteristic fields are then optimized at multiple frequency points in order to radiate a maximal directivity in the endfire direction. The best current excitation that can maintain a constant directivity as a function of frequency is then deduced. Finally, a feeding network that can provide the desired excitation at the array ports is designed and integrated on the array system, where the 1 dB directivity bandwidth is about 23% with $D_{max}=7.1$ dBi. The antenna impedance bandwidth is equal to 15%.