Circularly Polarized Antenna Array With Suppressed Sidelobes for Electronic Toll Collection

In this letter, a 4×4 circularly polarized antenna array with suppressed sidelobes for electronic toll collection applications is proposed. The antenna array operates at 5.8 GHz and is designed based on the weighted constrained method of the maximum power transmission efficiency (WCMMPTE). Different constraints to control the polarization and sidelobe levels are simultaneously introduced to realize the desired polarizations and sidelobe levels. While the optimized distribution of excitations for the 4×4 antenna array is first obtained analytically by the WCMMPTE, a few elements with negligible exciting amplitudes are then removed to reduce the array size to yield a 12-element array together with a feeding network which are fabricated and tested. The measured results indicate that the fabricated array has a left-hand circular polarization gain of 15.3 dBi with sidelobe levels below –30 dB and half-power beamwidths of 30° in both
ϕ
= 0° and
ϕ
= 90° planes, which meet the requirements of ETC system. Moreover, a 3 dB axial ratio bandwidth of 720 MHz and a low axial ratio of 0.3 dB at 5.8 GHz have been achieved, which demonstrate a good circular polarization performance.