Small-size printed CPW-fed antenna for ultra-wideband communications D. Abed, H. Kimouche and B. Atrouz A novel printed monopole antenna fed by a coplanar waveguide (CPW) is proposed in view of ultra-wideband applications. Its impedance bandwidth defined by a voltage standing-wave ratio (VSWR , 2) is from 3 to 11.3 GHz with a fractional bandwidth of 116%. A parametric study of the proposed antenna was carried out in order to optimise the main parameters. Details of the proposed antenna design and measured results are presented and discussed. Introduction: Ultra-wideband (UWB) technology was approved by the Federal Communications Commission (FCC) in February 2002 [1]. According to the FCC regulations, the frequency band from 3.1 to 10.6 GHz can be used for short-range and high speed wireless com- munications systems. In those systems the UWB antenna has become a key component, which presents the most exciting of challenges. 35 3 4.45 0.3 0.3 R 1 = 9.25 R 2 = 8 3 35 1.58 FR4 R g = 20 e r = 4.32 50 W SMA connector x y z Fig. 1 Geometry of proposed UWB antenna Fig. 2 Photograph of fabricated antenna prototype Recently, various planar broadband antennas have been studied and reported for UWB communications. The majority of these antennas have regular shaped configurations, such as circular, elliptical, triangular, modified triangular, rectangular, and modified rectangular [2–6], which have a microstrip-line feed and a partial ground plane on the opposite side of the printed circuit board (PCB). Similarly, a coplanar waveguide (CPW)-fed antenna has been reported for ultra-wideband operations. Owing to its single-layer substrate, wide bandwidth and a small total size, the CPW-fed antenna is a good candidate for UWB systems. In [7] a modified rectangular shape monopole antenna CPW-fed was reported. This antenna configuration yielded large bandwidth, from 2.8 to 10.6 GHz, for a voltage standing-wave ratio (VSWR) , 2. Also, a printed CPW-fed taper arc slot antenna was reported to yield ultra-wide bandwidth performance of over 9.69 GHz (from 2.89 to 12.58 GHz) for VSWR , 2 with improved omnidirectional radiation pattern [8]. 2 3 4 5 6 7 8 9 10 11 12 VSWR frequency, GHz 0 1 2 3 4 5 simulated VSWR for h =0 mm simulated VSWR for h =0.15 mm simulated VSWR for h =0.3 mm measured VSWR for h =0.3 mm Fig. 3 Measured and simulated VSWR against frequency with different feedgap values 270° 240° 210° 180° 0 0 –10 –20 –30 –40 –10 –20 –30 –40 150° 120° 90° 60° 30° 300° 330° 0° 270° 240° 210° 180° 150° 120° 90° 60° 30° 300° 330° 0° 0 0 –10 –20 –30 –40 –10 –20 –30 –40 a b azimuthal pattern Fig. 4 Far-field radiation pattern for proposed antenna at two frequencies a At 3.7 GHz b At 5.7 GHz In this Letter, a new small-size CPW-fed modified-elliptical antenna is proposed for ultra-wideband communications. The theoretical study of this configuration was carried out using the method of moments (MoM). Antenna structure and design: The configuration of the proposed UWB antenna is shown in Fig. 1, where an FR4 substrate with relative permit- tivity of 4.32 and thickness of 1.58 mm is used. It can be seen from the Figure that the antenna radiator consists of a modified-elliptical shape. The antenna is fed by a 50 V CPW transmission line with a signal strip of width 3 mm and a gap distance of 0.3 mm between the signal strip and the coplanar ground plane. Two finite ground planes with the same size defined by R g ¼ 20 mm are situated symmetrically on each side of the CPW line. Experimental results: A prototype of the proposed UWB monopole antenna with the optimal geometrical parameters was fabricated as shown in Fig. 2. The impedance bandwidth was measured using an Agilent 8719ES vector network analyser (VNA). Measured and simu- lated VSWR are shown in Fig. 3. The measured bandwidth defined by VSWR , 2 of the proposed antenna with a feed gap of 0.3 mm is ELECTRONICS LETTERS 14th August 2008 Vol. 44 No. 17