A Broadband Circularly Polarized Endfire Loop Antenna for Millimeter-Wave Applications

In this letter, a broadband circularly polarized (CP) endfire loop antenna is proposed for millimeter-wave (mmW) applications. The loop consists of two arms constructed by printed strips with straight edge connections implemented by metalized via-holes, and is fed by substrate integrated waveguide differentially. The quasi-traveling wave currents flowing along strips and via-holes, which generate horizontal and vertical polarizations, respectively. By using the offset strip, comparable amplitudes of two polarizations are realized to achieve a good CP radiation. Furthermore, a dielectric rod is applied for higher gain. The experimental prototype reported that the impedance bandwidth of 28.3% from 23.75 to 31.4 GHz, the 3 dB axial-ratio bandwidth of 25% from 24.25 to 31 GHz, and the gain no less than 9 dBic for endfire radiation. In addition, the whole antenna is curved on one substrate with a low profile of 0.14 lambda(0) (lambda(0) is the free-space wavelength at 27 GHz). Thus, the antenna is attractive for fifth-generation (5G) applications in mmW integrated circuits.