Broadband Dual Circularly Polarized C- Band Antennas

Abstract— Designing and fabricating an antenna with compact size, improved bandwidth, gain and directivity has been an important issue especially for the satellite based applications. In addition to this multipath interference has to be managed. To satisfy these criteria, microstrip fed broadband monopole antenna of different structures (U-Shaped slot monopole antenna, modified asymmetric hexagonal shaped monopole) having dual circularly polarization is designed in this paper for C-Band applications. Once CP is generated, then new challenge for generating wide impedance bandwidth, AR bandwidth, high gain and directivity should be handled. These antennas are designed and simulated using HFSS software of version 17. The designed antennas are compared based on its performance. The simulated results are verified with fabricated antennas and tested with Vector Network Analyzer (VNA) Agilent E5071C. Keyword Monopole, Dual Circularly Polarized, HFSS (High Frequency Structure Simulator), C-Band, Axial Ratio (AR) Bandwidth.


III. ANTENNA DESIGN
The specifications and procedures for designing modified asymmetric hexagonal shaped monopole antenna, and U-shaped slot monopole antenna are discussed.

A. Design Specifications
In order to design and simulate antenna using software like HFSS, various specifications like frequency of operation for the antenna, type of feed and its location etc should be considered. In addition to this, type, speed and memory capacity processor are also playing a major role. For asymmetric hexagonal shape monopole antenna, FR4 substrate of 1mm thickness with 32mm × 32 mm dimension [1] and for its modified work FR4 substrate of 1mm thickness with 32mm × 28 mm dimension is considered. In case of U-Shaped slot monopole antenna, 1mm thickness of FR4 substrate with 48mm × 41 mm dimension is considered. The centre frequency for these antenna structures is 6 GHz and loss tangent value is about 0.02.

B. Design Analysis and Procedures for Modified Asymmetric Hexagonal Shaped Monopole Antenna
Generally monopole antenna is linearly-polarized. By modifying the structure, it can generate circular polarization. Various complex methods have been used to produce CP from monopole. In [1], the antenna covers portion of L-Band (1.4-2) GHz, S-band (2-4) GHz, C-Band (4-8) GHz and smaller portion of X-Band (8-9) GHz.The frequency of operation is from 1.4 to 9.0533 GHz. So there may be interference for the C-Band applications. In addition to this gain, directivity has to be improved.
Two orthogonal tapered microstrip feed lines with modified ground plane, LSSs and ILSSs is shown in Fig.1. This modified antenna covers only the range of C-Band (4-8) GHz. It is achieved by modifying the design of L-Shaped strips and Inverted L-Shaped Strips along the edges of the radiating monopole. Ground plane structure is modified by changing its position and dimension. The circular polarization bands produced due to LSSs and ILSSs get overlapped to produce wideband operation and wide Axial Ratio bandwidth.   Extension length ( L  ) and actual length can be expressed as The width and length of the ground plane calculation is given as, These are all some of the formulas involved in calculating the parameters of the modified asymmetric hexagonal shaped monopole antenna.

C. Design Analysis and Procedures for U-shaped slot monopole antenna
The antenna is designed using two microstrip-fed ports, a U-shaped slot, and a frame structure ground is shown in Fig.2. In this antenna design, U-shaped slot and asymmetric feeding structure together produces broadband circular polarized wave. In order to excite U-shaped slot this CP antenna make use of microstrip fed rectangular radiator. After the generation of CP, new challenges for generating wide AR bandwidth, high gain and impedance bandwidth will appear. In order to expand the impedance bandwidth, the metal patch of size ( Various parameters values for designing modified asymmetric hexagonal shaped monopole antenna and Ushaped slot monopole antenna are shown in Table I and II.

IV. RESULTS AND DISCUSSIONS
The S-Parameter graph for the modified asymmetric hexagonal shaped monopole antenna is shown in Fig.3. It represents amount of reflected power with respect to ports.  The Fig.6 shows that the simulated results agree well measured results. The quality of circular polarization of an antenna can be explained through AR bandwidth graph. Based on the AR < 3dB, circular polarization is typically defined. The Fig.7 represents that the 3-dB Axial Ratio bandwidth graph covers the range from 4.84 GHz to 7.46 GHz. Hence it can be used for limited applications like WIFI, WIMAX, WLAN, some weather radars, cordless phone etc under dual CP concept.  The surface current distribution of modified asymmetric hexagonal shaped monopole antenna at φ=0 ̊ and φ=90 ̊ is shown in the Fig. 9. The gain plot for modified asymmetric hexagonal shaped monopole antenna is shown in Fig.10. In this peak gain of 5.7570 dB is achieved at 6 GHz. From the Fig. 11, it is shown that the peak directivity of 5.9740 dB is achieved at 6 GHz. The radiation pattern for modified asymmetric hexagonal shaped monopole antenna with respect to elevation plane and azimuthal plane are shown in Fig 12( The co polarization and cross polarization graph for modified asymmetric hexagonal shaped monopole antenna is shown in Fig. 13. On plotting the component phi of the electric field (rEphi) for phi=0, it represents the co polar component, however, on plotting the rEtheta on the same plane it represents cross polar component. A purely polarized antenna will have low cross polarized radiation. By using this modified antenna structure, interference get completely eliminated for C-Band applications with improved gain,directivity when compared with previous antenna design [1]. But 3-dB Axial Ratio Bandwidth has to be improved in order to support more applications of C-Band under dual circular polarization with improved gain, directivity and less design complexity.
So U-shaped slot monopole antenna with dual CP mode is designed.The picture of the fabricated U-Shaped Slot Monopole Antenna is shown in Fig. 14 and it is tested with Agilent E5071C.The Fig.15 shows S-Parameter graph of U-Shaped slot monopole antenna which covers only C-Band (4-8) GHz. Hence interference due to other bands gets eliminated. The Simulated results are verified with measured results using fabricated antenna and tested with Agilent E5071C. The Fig.16 shows the testing setup. The Fig.17 shows combined graph of measured and simulated S-Parameter. The simulated results agree well measured results  Fig. 18, it is shown that VSWR value < 2 from 3.92 to 8.03. Hence retransmission is less.  The Fig.21 shows that the U-shaped slot antenna is having peak gain of 7.2831 dB at 4 GHz.  The simulated far field radiation pattern of U-Shaped slot antenna with respect to elevation plane and azimuthal plane is shown in the Fig.23. It represents the direction of the strength of the radio waves. The co polarization and cross polarization graph for U-shaped slot monopole antenna is shown in Fig. 24. On plotting field (rEphi) for phi=0, it represents the co polar component, however, on plotting the rEtheta on the same plane it represents cross polar component.  This U-shaped slot monopole antenna is having the advantage that broadband CP characteristics can be easily achieved due to its simplicity in its structure with good gain, directivity and support almost all applications of C-Band under dual circular polarization due to its 3 dB AR bandwidth improvement. Table III shows the comparision between modified asymmetric hexagonal shaped monopole antenna and U-Shaped slot monopole antenna. V. CONCLUSION The Dual CP based broadband monopole antennas have been designed and analyzed with the results obtained for C-Band applications. The results show that the frequency of operation from 4GHz to 8GHz is achieved for both the antennas. The U-Shaped slot monopole antenna is having good gain, directivity, AR bandwidth and less complexity when compared with modified asymmetric hexagonal shaped monopole antenna. This antenna design can be useful for applications like Wi-Fi (5.15-5.85)GHz, WIMAX (5.15-5.85)GHz, WLAN (5GHz, 5.2GHz, 5.8GHz), cordless phone, weather radars, INSAT (4.5-4.8)GHz, Terrestrial microwave applications (4-6) GHz ,Military applications (7.25-7.3,7.3-7.75)GHz, RSA(7.75-7.85) GHz and other C-Band wireless system.