Suppressing Zenith Directional Radiation Using Collinear Array for STAR Antennas

In this paper, a method using a collinear array to suppress the zenith directional radiation for simultaneous transmit and receive (STAR) antennas is proposed. At first, the principle of pattern multiplication is introduced to analyze the operation mechanism of the antenna. Then, two collinear array examples, based on mode orthogonality and near-field cancellation STAR antenna units, spaced at half a wavelength apart, are introduced for verification. Results show that this collinear array configuration successfully reduces radiation in the undesired zenith direction of the sequentially rotated array (SRA) mode, achieving good omnidirectional Co-horizontally polarized (HP) and Co-vertically polarized (VP) radiation. Moreover, it substantially improves the realized gain of the STAR antenna in the azimuthal plane without adversely affecting its reflection coefficients and isolation.