Compact Single-/Dual-Polarized Filtering Dielectric Resonator Antennas

A compact single-polarized filtering dielectric resonator (DR) antenna (DRA) with high selectivity is investigated. The DRA is fed by hybrid microstrip line/conformal strip, excited in its fundamental TE_1δ1^y mode. Owing to different loading effects of the microstrip stub and conformal strip, the resonance frequency of TE_1δ1^y mode excited by the two feed lines is slightly different. Such stepping resonances yield a wide bandwidth of 21.9% and a very flat gain of 5.1 dBi. The hybrid-feeding scheme also establishes a cross-coupled structure in the DRA, which introduces two radiation nulls right near the band edges. A compact wideband filtering DRA (FDRA) with quasi-elliptic bandpass response is, therefore, obtained without requiring any specific filtering circuit. This single-polarized design is also modified to realize a dual-polarized FDRA by adding another orthogonal port with the same feeding scheme. To reduce mutual coupling between the two ports, the microstrip stubs are folded to L shape, and four additional metal posts are inserted into the DR. As a result, the isolation is improved by 14 dB, from 7.2 to 21.2 dB.

I. Introduction

In mobile communications, a transmitted electromagnetic wave will generally reach a receiver through different paths, leading to fluctuations in the amplitude and phase of received signals. To tackle the multipath fading problem and settle polarization mismatch issue, dual-polarized antennas are often required in end user or base station. Patch antennas [1], magnetoelectric dipole antennas [2], traveling-wave antennas [3], and dielectric resonator antennas (DRAs) [4] that with dual polarization have been extensively investigated. On the other hand, it is increasingly popular to integrate the bandpass filter and antenna into a single module for reducing the system size [5]–[7] as well as insertion loss [8], [9]. Various filtering antennas that featuring filter-like frequency response for both the reflection coefficient and antenna gain have been proposed in recent years, including filtering patch antennas [10]–[12], slot antennas [13]–[15], and dipole/ monopole antennas [16]–[21]. However, thus far, most of the existing designs are limited to single polarization, and there are very few dual-polarized filtering antennas [6], [9]. The dual-polarized filtering patch antenna in [6] was designed based on the filter synthesis approach. A second-order stub-loaded resonator filter was used as the feeding network, whereas the patch antenna was taken as the last-stage resonator of the filter. This antenna working at 5 GHz had an impedance bandwidth of 10% and an average gain of about 5 dBi. Differently, the dual-polarized patch antenna in [9] was realized using antenna fusion approach. Small modifications were introduced into the radiator and feeding circuit of the antenna to control its input impedance, providing a filter-like frequency response [22]. More specifically, two orthogonal H-shaped microstrip feeding lines were coupled to the driven patch to realize dual polarization. The H-shaped line also provided a sharp roll-off rate at the lower band edge, whereas the stacked patch offered a radiation null at the upper stopband. Quasi-elliptic bandpass response was consequently achieved for both polarizations. Due to the stacked patch, the antenna could provide a wider impedance bandwidth of 13% and a higher gain of about 9 dBi.