In this article, as low-profile fish-shaped monopole antenna is proposed for circular polarization radiation. The modal characteristics of the proposed antenna were analyzed and manipulated using characteristic mode analysis technique.Four characteristic modes with different resonant frequencies and current distributions were chosen as operating modes. In addition, a suitable position excited the desired modes, which achieve broadband circular polarization performance and dual-band performance. Based on the above analysis, a prototype of the proposed antenna was fabricated with dimensions of 60 mm  60 mm  1.5 mm to validate simulated results. Measured results show that the 3-dB axial ratio bandwidth (ARBW) of the proposed antenna is 40.7% (4.9-7.4 GHz) and the antenna broadside peak gain is 4.2 dBic within usable bandwidth. In addition, the measured impedance bandwidths of the proposed antenna are 12.2% (2.3-2.6 GHz) and 53.7% (4.5-7.8 GHz). Compared to other designs, the proposed circularly polarized antenna is a promising design operating in WiMAX band and industrial, scientific, and medical bands.broadband, characteristic mode analysis, circularly polarized antenna, multiband
| INTRODUCTIONIn wireless communication systems, the main challenges that researchers designing antennas face are multipath interference, multipath fading, polarization mismatch, and the "Faraday rotation" effect. 1,2 Circularly polarized antennas (CPAs) can meet these challenges better than linearly polarized antennas, especially in the fields of GPS, radar, satellite system, and microwave sensing.Generally, to achieve circular polarization (CP) radiation, two equal-amplitude and orthogonal mode currents are obtained using either a single-feed technique or a multifeed technique. 3 The multifeed technique can achieve satisfactory antenna performance by power dividers or hybrid couplers or both with a 90 phase shift, but it causes additional complex design, large size, high loss, and low radiation efficiency. Compared to the multifeed technique, the single-feed technique can achieve the CP radiation by perturbation segments, parasitic elements, and slots. But then the single-feed technique may have limitations of narrow impedance bandwidth and axial ratio bandwidth (ARBW). Recently, researchers have proposed many designs for CPA, 4-14 such as a C-shaped antenna with triangular stubs and slits which obtains broadband CP performance. 4 Similarly, a combination of a Cshaped structure and a I-shaped structure is used to obtain a D-shaped dual-band CP antenna 5 ; an asymmetric E-shaped antenna achieves the CP radiation, and its 3-dB ARBW is Abbreviations: ANA, anti-nuclear antibodies; APC, antigen-presenting cells; IRF, interferon regulatory factor.
