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A unique coplanar-waveguide (CPW)-fed circularly polarised square slot antenna with enhanced impedance bandwidth (IBW) is presented. The antenna structure includes a pair of rectangular-shaped notches located at two opposite corners of the slot for achieving a significantly enhanced IBW of 12.06 GHz (2.76-14.82 GHz), and a pair of reverse L-shaped ground arms in the slot for realising circularly polarised radiation with 1.86 GHz (4.27-6.13 GHz) bandwidth. This proposed technique has the advantages of covering the whole of the ultra-wideband spectrum (3.1-10.6 GHz), an average gain of 3 dBi and a reduced antenna size of 25 × 25 × 0.8 mm 3 .

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Among the other features, multiple beam antennas have the advantage of reduced multi-path fading and co-channel interference. Because of the outstanding features of the Butler matrices, they have been proposed extensively in recent years as feed networks of the antenna array applications such as broadband and circular polarisation. In this study, a beam steering array antenna composed of a broadband circularly polarised square slot antenna and a novel, Butler matrix feed network designed with a proposed broadband branch-line coupler is introduced. The results show that a compact and its improvements are discussed. In this study, a broadband double box coupler with impedance bandwidth over 5-7.4 GHz frequency and the phase error less than 3°is employed. Also the measured impedance bandwidth of the proposed beam steering array antenna is 39% (from 4.7 to 7 GHz). The minimum 3 dB axial ratio bandwidth between ports, support 4.55-6.7 GHz frequency range. The measured peak gain of the proposed array antenna is 10.1 dBic that could scan solid angle ∼25 steradian. Fig. 12 Measured gain and AR against frequency for ports 1-4