Broadband Planar End-Fire Circularly Polarized Antenna for K-Band

A K-band planar end-fire circularly polarized (CP) complementary antenna with broadband capabilities is proposed in is paper. A multi-mode substrate integrated waveguide (SIW) cavity, with one side open for aperture radiation, is utilized as a multi-mode resonant magnetic dipole to provide x-axis polarization component. A printed electric dipole, which provides y-axis polarization component, is connected to the aperture by a phase extension line that introduces a 90° phase difference between the two orthogonal polarization components. This configuration enables CP performance. Simulation results indicate that the proposed antenna achieves a bandwidth of 11.94% ($18.73-21.11 \mathrm{GHz}$) with $\left|S_{11}\right|{\lt}-10 \mathrm{~dB}$. In addition, the proposed antenna exhibits an axial ration (AR) below 3 dB throughout the working bandwidth and demonstrates a stable inband gain variation between 4.24 dBi and 4.38 dBi. Therefore, this antenna meets the fundamental requirements for K-band $\mathbf{C P}$ antennas.