Omnidirectional Dual-Band Dual Circularly Polarized Microstrip Antenna Using ${\rm TM}_{01}$ and ${\rm TM}_{02}$ Modes

An omnidirectional dual-band dual circularly polarized (CP) antenna with wide beam radiation patterns using TM
₀₁
and TM
₀₂
modes is investigated. The proposed antenna is composed of a circular patch with eight curved slots and a disk-loaded coaxial probe. The horizontal and vertical polarizations are obtained by the currents flowing on the patch and on the coaxial probe, respectively. Omnidirectional CP fields can be achieved at both resonant modes when the two orthogonally polarizations are equal in amplitude and different in phase by 90°. With a low profile of 0.017 λ
₀
, the proposed antenna exhibits a wide beamwidth of 100° in the elevation plane at both modes. The measured axial ratios (ARs) at the two resonant modes in the azimuth plane are 1.71 and 2.82 dB, respectively. The measured left-hand CP (LHCP) and right-hand CP (RHCP) gains in the azimuth plane are 0.1 dBic at the lower center frequency and -1.1 dBic at the upper center frequency, respectively. These measured results are congruent with the simulated data. In addition, the methods to broaden the bandwidth of the proposed antenna are given and discussed in details.