With the rapid development of wireless communications,antennas have also been developing towards miniaturization,broadband and multi-frequency.Recently,periodic structures have attracted more and more attention,and have been applied in antenna design.This dissertation investigates circularly polarized microstrip antennas using periodic structures.It is organized as follows:Firstly,a compact dual-band circularly polarized(CP)microstrip antenna over reactive impedance surface(RIS)is proposed.Stacked patch radiators are fed separately and two independent feed ports are for signals with two different frequencies,which increase the isolation of two frequency bands.The overall volume of the dual-band CP antenna is 0.235 × 0.235 × 0.042λ03,where λ0 denotes the free-space wavelength at 1.765 GHz.When port 1 is excited and port 2 is terminated by a 50-Ω load,the measured 10-dB return loss bandwidth is 5.9%and 3-dB axial ratio(AR)bandwidth is 0.9%for the L-band.When port 2 is excited and port 1 is terminated by a 50-Ω load,the measured 10-dB return loss bandwidth is 7.3%and 3-dB AR bandwidth is 1.1%for the S-band.The antenna gain at boresight is 1.9 dBic for the L-band,and 1.9 dBic for the S-band.Next,a new artificial magnetic conductor(AMC)unit structure which is a square patch loaded by two-sided edge rectangular slots is proposed.Using this AMC structure,a compact broadband CP microstrip antenna is designed,fabricated,and measured.The center frequency of the antenna is 3.0 GHz,and the ove,rall volume of the broadband CP antenna is 0.40×0.40×0.048λ03,where λ0 denotes the free-space wavelength at 3.0 GHz.The measured 10-dB return loss bandwidth is 41.3%and 3-dB AR bandwidth is 14.0%.The antenna gain at boresight is 5.3 dBic,and the gain fluctuation is less than 1 dB in the whole 3-dB AR bandwidth.Finally,a new miniaturized AMC unit structure which is a square patch loaded by two inner rectangular slots is proposed.Using this miniaturized AMC structure,a compact broadband CP microstrip antenna is designed,fabricated,and measured.The center frequency of the antenna is 3.2 GHz,and the overall volume of the broadband CP antenna is 0.43 x 0.43 x 0.051 λ3,whereλ0 denotes the free-space wavelength at 3.2 GHz.The measured 10-dB return loss bandwidth is 25.9%and 3-dB AR bandwidth is 14.0%.The antenna gain at boresight is 5.1 dBic.However,its gain fluctuation is more than 2 dB in the whole 3-dB AR bandwidth.To cope with this problem,an improved miniaturized AMC unit structure is reproposed by reducing the size of two inner rectangle slots which are loaded on the square patch structure.Using this improved miniaturized AMC structure,an improved compact broadband CP microstrip antenna is designed,fabricated,and measured.The center frequency of the antenna is 2.65 GHz,and the overall volume of the broadband CP antenna is 0.44×0.44×0.042λ0-3,where λ0 denotes the free-space wavelength at 2.65 GHz.The measured 10-dB return loss bandwidth is 27.2%and 3-dB AR bandwidth is 11.3%.The antenna gain at boresight is 5.0 dBic,and the gain fluctuation is less than 1 dB in the whole 3-dB AR bandwidth. |