Received: 16 February 2017 DOI: 10.1002/mop.30738 Compact multiband antenna fed with wideband coupled line impedance transformer for improvement of impedance matching Sajeed Mulla | Shraddha S. Deshpande Department of Electronics Engineering, Walchand College of Engineering, Sangli, Maharashtra, India Correspondence Sajeed Sirajuddin Mulla, Department of Electronics Engineering, Walchand College of Engineering, Sangli, Maharashtra, India. Email: sajeed.mulla@walchandsangli.ac.in Abstract The design of the Compact Multiband Antenna (CMA) fed with Wideband Coupled Line Impedance transformer (CMA-WCLIT) over a wide frequency range is proposed. The wide band coupled line impedance transformer is used for improvement of impedance matching. The performance of the CMA-WCLIT is demonstrated by comparing its input impedances with antenna impedances of the conventional CMA. Calculated input impedances in HFSS, for respective bands are Z in1 5 51 – j0.33 X,Z in2 5 48 – j4.5 X, Z in3 5 49 – j2.4 X. Comparing to CMA, the return loss (RL) of CMA- WCLIT for GSM and WIFI bands are reduced by 6dB and 19dB respectively, however there is no further reduction of RL for UMTS-FDD band. A prototype is simulated (HFSS), manufactured and tested, the simulated results validates the procedure of proposed design. The proposed antenna can be operated for GSM-900,UMTS-FDD, and WiFi. KEYWORDS antennas, coupled transmission line, directional coupler, impedance matching, microstrip line, mobile antennas, multiband antennas 1 | INTRODUCTION The great demands in designing the Compact Multiband Antenna (CMA) and Broadband Antenna are due to evolution of mobile communication. The mobile phones are required to operate over multiple frequency bands such as GSM, PCS, UMTS, and WLAN bands. A variety of antenna configurations have been suggested by various researchers, such as fractal antennas, and printed Planner Inverted F Antenna. 1,2 These antennas have good performance, 1,3 whereas impedance matching for desired frequency increases the difficulty in antenna design and manufacturing. 1 The planner CMA for the mobile handset is reported by Xu Jing et al., 3 but different impedance matching networks used for each antenna branch leads to the complicated antenna struc- ture. In this article, CMA fed with Wide band Coupled Line Impedance Transformer (CMA-WCLIT) is presented. This antenna is simple to design using dipole (or monopole) antenna elements of uniform width and single coupled line as the impedance transformer. The purpose of using coupled line impedance transformer is to avoid separate impedance matching network for each antenna, that can helps to provide independent frequency bands with controlled Return Loss (RL) and radiation patterns. 2 | IMPEDANCE MATCHING NETWORKS USED IN PLANNER MICROSTRIP ANTENNA A multiband resonant antenna has several dipoles cut for dif- ferent resonant frequencies of interest connected together at common feed and fed with single transmission line. 1 In the multiband antenna system, the antenna impedance is reactive and it is desirable to place antenna tuner between feed and antenna for impedance matching. 3 Hence, antenna tuner cir- cuit to resonate each dipole of the multiband antenna for desired frequency is necessary. Presently, different matching networks are being used for tuning the microstrip antenna at the resonant frequency. These are as follow. The impedance transforming properties of k/4 transmis- sion line can be used for matching the antenna impedance to the characteristic impedance (Z 0 ) of the microstrip line as described in Ref. [4]. The input impedance of k/4 line termi- nated with real load Z L is Z in 5 z 2 0 Z L . Rearranging the equation gives impedance of Quarter Wave Transformer (QWT) for which the load impedance Z L can be transformed into input impedance Z in , the equation is Z 0 5 ffiffiffiffiffiffiffiffiffiffi Z in Z L p . It requires the real load and operated over narrow band frequency 4 so net- work can be connected to each antenna element for imped- ance matching, which is not practical. The limitation of narrow bandwidth (BW) in QWT can be overcome by using tapered line section 4,5 called as Tapered Quater wave Transformer (TQWT). The TQWT operated as the broadband TQWT. The change of impedance MULLA AND DESHPANDE | 2341