A Circular Polarization Alterable Square MPA with Wide Frequency Tuning Biswajit Dwivedy Department of Electronics and Communication Engg. National Institute of Technology Rourkela, India Email: [email protected] Santanu Kumar Behera Department of Electronics and Communication Engg. National Institute of Technology Rourkela, India Email: [email protected] Abstract—A wide-range frequency tunable (2.01 GHz- 2.41 GHz) square patch antenna with simultaneous CP alteration ability is designed and its performance is analyzed. Frequency tuning is achieved by varying the capacitance of four varactors connected between the central portion and four additional segments of the main patch. Circular polarization at different bands are achieved by using a wideband 90 ◦ phase shifter and a DPDT switch is used to vary the sense of circular polarization rotation. The antenna shows very good axial ratio value at all frequencies with a maximum gain of 2.2 dBc. Keywords—frequency reconfigurable; circular polarization; var- actor; axial ratio I. I NTRODUCTION In RF communication major challenges which are becoming worse day by day are spectrum congestion problem arising due to uninterrupted increase in modern features along with users and signal quality degradation by multipath propagation based fading. The earlier problem is handled by frequency tunable antennas using MEMS, p-i-n diodes, RF switches and varac- tors etc [1]. To withstand the later problem, polarization agile antennas are considered, as they can provide better weather penetration and enhance information content by their two sets of co-polar and cross-polar data [2]-[3]. So intense importance is given on development of such antennas which are less complex, compact, efficient and with ability to reconfigure both frequency and polarization to mitigate above discussed problems [4]-[5]. In this article, design of a microstrip line fed square patch antenna having both frequency and circular polarization agile characteristic within 2.01 GHz - 2.41 GHz is discussed and its performance is analyzed. II. DESIGN AND FUNCTIONAL DESCRIPTION A. Design Procedure And Frequency Reconfigurability The design of the proposed antenna starts with a simple quarter wave length transformer matched square patch antenna designed on 62 mil FR-4 substrate at the centre frequency of 2.4 GHz following standard design formulae [6]. The antenna radiates in basic TM 10 fundamental linear polarized mode. To achieve frequency tuning, an annular square shape portion with some additional lengths on four sides are carefully removed TABLE I. DIMENSIONAL DETAILS OF THE ANTENNA Para. Ls Lz Wz Lx W f g L (λ/4) W (λ/4) Val.(mm) 60 11.4 1 25 3 1 11.3 0.9 Fig. 1. Layout of the proposed antenna from the main patch for connection of four varactor diodes. Specific varactor (SMV-2025, Skywork solutions) is selected for tuning purpose as it has low series resistance (0.8 Ω) and high range of capacitance variation (1.15 pF-8.81 pF). Due to the simultaneous variation of capacitances of four varactors overall electrical length of the antenna is varied causing change in resonant frequency of the antenna. All dimensional details of the antenna layout shown in Fig. 1 are presented in Table. I. From Fig. 2, it is revealed that the resonant frequency of the antenna varies between 2.41 GHz to 2.01 GHz for variation of the capacitances from 1.15 pF to 8.81 pF. B. CP Alteration with Frequency Tuning To achieve circular polarization the main patch is fed orthogonally with a phase difference of +90 ◦ for LHCP or - 90 ◦ for RHCP respectively. Current distribution of the antenna at 2.16 GHz which is shown in Fig. 3 clearly illustrates RHCP for a phase difference of -90 ◦ . The circular polarization performance of the antenna for different values of capacitances Fig. 2. S-parameters of the antenna for different values of varactor capacitance