Cartesian multipole mechanism governing wideband circular polarization in single-fed elliptical dielectric resonator antennas

The physical origin of wideband circular polarization (CP) in a single-fed elliptical dielectric resonator antenna (DRA) is investigated through complementary resonator-based Cartesian multipole and complete-antenna vector-spherical-harmonic (VSH) analyses. Rather than treating CP radiation as a mere empirical consequence of geometric tuning, this work relates the axial ratio (AR) bandwidth to the frequency-dependent amplitude and phase balance of the two orthogonal broadside fields and their corresponding multipolar radiation channels. A top cross-slot and two off-centered metallic cylinders are introduced not merely as structural perturbations, but as controlled degrees of freedom to redistribute dielectric polarization currents and thereby tailor the relative amplitudes and phase difference between broadside far-field components across the operating band. The multipolar results show that the l = 1 channels remain dominant throughout the operating band, while the increased l = 2 participation provides additional amplitude–phase correction toward the upper band. This physical model is validated experimentally on a fabricated prototype with normalized dimensions of 0.45λc × 0.47λc × 0.22λc. Measured results demonstrate a 3-dB AR bandwidth of 36.77% from 5.46 to 7.92 GHz, a 10-dB impedance bandwidth of 55.24% from 4.52 to 7.97 GHz, and a peak gain of 4.76 dBic. These results demonstrate that the combined Cartesian multipole and VSH analyses provide a physics-based diagnostic and design-guidance framework for wideband circularly polarized DRA design beyond empirical optimization.