Triangular Ring Antennas for Dual-Frequency Dual-Polarization or Circular-Polarization Operations

In this letter, triangular ring (TRG) antennas are proposed for dual-frequency dual-polarization (DFDP) and circular-polarization (CP) operations. By simultaneously exciting the two orthogonal modes of the TRG, the DFDP or CP performance is achieved. By varying the vertex angle of the TRG, the frequency ratio (FR) of the two orthogonal modes could be continuously tunable for the DFDP operation. By moving the FR close to 1, a CP antenna element is developed for the sequential-rotation array. Due to the geometry advantage of the TRG, the array could be designed to have a low sidelobe level (SLL). Measured results show that an overlapped bandwidth (of return loss, axial ratio, and gain) of 10% and an SLL of below
$-$
20 dB are achieved. The fabricated 120
$^{\circ}$
TRG (for the DFDP operation) and the 75
$ ^{\circ}$
TRG (for the CP operation) are 28.2% and 65.5% the size of the conventional equilateral triangular patch antenna, respectively.