Novel antenna array configuration using miniature double box branch-line couplers for wideband circularly polarised applications

Abstract

This article presents a wideband circularly polarised (CP) antenna array using a miniaturised double box branch-line coupler. This was achieved by creating a slow wave structure by introducing open-circuit stubs on the inside arms of the branch-line coupler. The 2 × 2 antenna array is made of four CP slot radiating elements excited through 1 × 4 feeding network. Moreover, sequentially rotation method is used to increase the axial-ratio (AR) bandwidth of the array. The CP slot radiating element is fed by coplanar waveguide. The feedline of the horn-shaped radiator includes a circular matching stub. Ground connected asymmetric T-shaped stub, a pair of grounded L-shaped square strips, and single strip line are shown to induce CP and enhance the antenna's 3 dB impedance bandwidth and AR performance. The antenna constituting the array has impedance bandwidth of 2.45 GHz from 4.8 to 7.25 GHz that corresponds to a fractional bandwidth of 40%, and its 3 dB AR extends between 4.8 and 6.25 GHz. Compared to a conventional double-box branch-line coupler the size of the proposed coupler is reduced by 35% by increasing the capacitance of the coupler's arms. This also is shown to increase the impedance bandwidth of the coupler. A 2 × 2 array was fabricated, and its performance verified. Measured results confirm the proposed antenna array exhibits CP covering the bands of WiMAX IEEE 802.16, C-band and ITU-R F386.9. The measured 3 dB AR bandwidth of the antenna array extend from 3 to 7.3 GHz and its impedance bandwidth extends from 3.2 to 9.0 GHz. The antenna array radiates with a peak gain of 11.5 dBic.