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A shared-radiator dual-band dual-sense circularly-polarized (CP) microstrip antenna with duplexing and filtering functions is presented. For each band, there is a coupling network, which consists of a hairpin resonator, a straight slot, and a U-slot, and thus two paths with an inherent 90 • or-90 • phase difference are available for the input signal to couple to the patch, left-handed CP (LHCP) characteristic and right-handed CP (RHCP) characteristic are respectively realized at different bands for the proposed antenna with a single patch. Such a dual-band dual-sense CP patch antenna is highly integrated and outperforms other traditional CP antennas state-of-the-art. Moreover, the duplexing and filtering functions are seamlessly integrated into the dual-band patch antenna with a low-profile (0.044λ o), and application of such an integrated device in wireless systems can potentially reduce the volume, complexity, and cost of RF front-ends. Based on resonator-based topologies, the principles of achieving the dual-sense CP, duplexing, and filtering functions are illustrated. As a proof-of-concept, an antenna working at C-band is prototyped and tested. Good agreements between the measured and simulated results are achieved, showing an LHCP radiation characteristic for the low-band operation and a RHCP radiation characteristic for the high-band operation, respectively. The antenna also exhibits very good frequency selectivity and out-of-band rejection performance. At the low-band, the impedance and 3-dB axial-ratio (AR) bandwidths are 4.7% and 1.1%. At the high-band, the impedance and AR bandwidths are about 7.3% and 3.2%. The isolation between the two channels/bands is over 24 dB. INDEX TERMS Antenna, patch antenna, dual-band, dual-polarized, circularly-polarized, filtering antenna, duplexing antenna.

A compact single-radiator microstrip patch antenna with highly integrated duplexing and filtering functions is proposed for dual-band dual-polarized (DBDP) base-station applications, and the center frequency ratio of the upper and the lower operation band is just 1.14. The single radiation patch with four high isolation channels is oriented along the 45 • diagonal of the ground plane to achieve the ±45 • dual linearly polarizations (LP) at each of the two operation bands. For each polarization, the dual-band full-duplex characteristic is obtained by conceiving the strong mutual coupling between the combined resonator and the radiation path, and the mutual coupling then couples to two groups of resonator-based channels with different working frequencies. Therefore, high channel isolation is achieved, without resorting extra decoupling structures. Moreover, the mutual coupling of each resonator-based channel can help to improve its corresponding operation band and produce the third-order filtering feature, leading to enhancement of the full-duplex characteristic for the proposed antenna. Such a single-patch four-port DBDP antenna could significantly reduce the complexity and volume of the RF frontend of the base station. The removal of the bandpass filters and duplexers in the design could also reduce the potential cost of the RF frontend. The measured results agree well with the simulations, showing an excellent performance in terms of bandwidths, channel isolations, polarization isolations, radiation patterns, and gains. Base-station, dual-band, duplexing, dual-polarization, filtering, microstrip antenna. INDEX TERMS

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A compact low-profile dual-band antenna with a single circular radiation patch is presented. The circular radiation patch is top-loaded by a metal strip, leading to a zeroth-order resonant (ZOR) mode with monopole-like radiation pattern at the lower operation band, and the ZOR mode is not affected by the position change of the probe feed. Thereby an off-center probe feed can be used to excite a TM 01 mode with patch-like radiation pattern at the upper operation band, and its performance remains good stability when the size of the metal strip is varying. The equivalent circuit, electric field and current distributions of the proposed antenna are studied to investigate the principles of the ZOR and the TM 01 modes, and all the results show that the two operation bands can be effectively and independently controlled. As a proof-ofconcept, a circular patch antenna prototype with low profile of about 0.023 λ 0 at 2.35 GHz is fabricated and tested. Good agreements between the measured and simulated results are achieved, showing monopole-like and patch-like radiation patterns within 2.28-2.40 GHz and 3.30-3.60 GHz, respectively, and the average realized gains are about 2.5 dBi and 8.5 dBi for the two operation bands. INDEX TERMS Antenna, patch antenna, monopolar patch antenna, dual-band, zeroth-order mode, TM 01 mode, omnidirectional radiation, broadside radiation.