An Omnidirectional Antenna for Sparse Circular-Ring Phased Array Antenna

In this letter, a high gain omnidirectional antenna with low gain ripple and a uniformly distributed sparse circular-ring phased array antenna (SCPAA) with a prime number of elements are proposed. The omnidirectional antenna, consisting of 12 head-to-end connected elements, exhibits a cosecant-squared radiation pattern in the elevation direction and an omnidirectional radiation pattern in the azimuth direction. The proposed SCPAA with 17 elements arranged on a 4 m diameter circular-ring exhibits a minimum sidelobe level of -7.65 dB and good 360° scanning performance. The measured VSWRs of the omnidirectional antenna elements are less than 2.0 over the 1.02–1.10 GHz band. In the azimuth direction, the minimum gain reaches 6.29 dBi at 1.06 GHz. The measured gain ripples of No.2 and No.11 antenna elements are ±0.52 dB and ±0.15 dB, respectively. In the elevation direction, the cosecant squared beam covers an airspace of 58° with a low sidelobe level of -22.3 dB. Compared with conventional mechanically scanned secondary surveillance radar (SSR) antennas, the proposed SCPAA has lower development and maintenance costs and enables higher data rates.