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To solve the difficulty of cascading arbitrary antenna elements in a compact single-layer structure for achieving a high-gain omnidirectional-like circularly polarized (OCP) array antenna, a new design methodology based on complementary source radiation is introduced in this letter. By utilizing the inherent quadrature-phase difference of the electric and magnetic fields in a resonant cavity, the parallel magnetic current and electric current with a ${\text{90}}^\circ$ phase difference are realized in the open cavity and two symmetrical arms of the proposed antenna, respectively. The radiation intensity of electric current source is enhanced by reshaping the structure with cutting corners and introducing slots. Thanks to the simple and symmetrical structure of the element, the proposed methodology effectively addresses the cascaded difficulty of existing OCP antennas, making it possible to achieve the required gain of the OCP array antenna by appropriately cascading elements. To verify the design methodology, a seven-element center-fed array antenna is designed and fabricated in a single-layer dielectric substrate. The OCP array antenna achieves a maximum realized gain of 6.5 dBic and its volume is just $4.12\lambda _{0} \times 0.58\lambda _{0} \times 0.015\lambda _{0}= 0.035\lambda _{0}^{3}$.

A low-profile dual-polarized (DP) omnidirectional metasurface antenna is proposed in this letter. By conducting characteristic mode analysis on a metasurface, one mode with a vertically polarized (VP) omnidirectional radiation pattern and another one with a horizontally polarized (HP) counterpart are first found. To allow for an easy excitation, a method that can independently manipulate both modes is then developed. After optimization, the VP modal current concentrates at the center of the metasurface, whereas the HP modal current mitigates to the edges of the metasurface. As a result, the two modes can be excited simultaneously with a simple feeding strategy, namely, a central probe is used to excite the VP mode, and four slots under the four edges of the metasurface are used to excite the HP mode. To verify the design, a prototype is simulated, fabricated, and measured. The results show that the proposed antenna achieves a DP omnidirectional radiation with a wide bandwidth of 39.4% and high port isolation of 32 dB.