Broadband Circularly Polarized Square-Ring-Loaded Slot Antenna With Flat Gains

A novel broadband circularly polarized (CP) squarering-loaded slot antenna with flat gains is proposed in this letter. The antenna is fed by an L-shaped microstrip line with tapered section for impedance matching. A square ring is also loaded, and a new resonant frequency with the minimum axial ratio (AR) is produced, broadening the CP bandwidth of the original slot-coupled microstrip-fed antenna. The antenna is successfully designed and fabricated. Measured results show that its 10-dB return-loss bandwidth is 3.94 GHz (4.56-8.5 GHz with the center frequency 6.53 GHz) and that its 3-dB AR bandwidth can reach 3.7 GHz (4.75-8.45 GHz with the center frequency 6.6 GHz). The antenna occupies an overall volume of 0.8λ
₀
× 0.8λ
₀
× 0.13λ
₀
with respect to 6 GHz. Its peak gain is 4.4 dBic, and the gain variation within 4.75-8.2 GHz band is less than 1 dBic, which means a good flat gain feature in the broad frequency band. The measured results agree well with the simulated ones.