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.