  This work proposes a small pattern and polarization diversity multi-sector
annular antenna with electrical size and profile of ${ka=1.2}$ and
${0.018\lambda}$, respectively. The antenna is planar and comprises annular
sectors that are fed using different ports to enable digital beamforming
techniques, with efficiency and gain of up to 78% and 4.62 dBi, respectively.
The cavity mode analysis is used to describe the design concept and the antenna
diversity. The proposed method can produce different polarization states (e.g.
linearly and circularly polarized patterns), and pattern diversity
characteristics covering the elevation plane. Owing to its small electrical
size, low-profile and diversity properties, the solution shows good promise to
enable advanced radio applications like wireless physical layer security in
many emerging and size-constrained Internet of Things (IoT) devices.
