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A design method is described for reconfigurable circularly-polarized (CP) microstrip array antennas with conical-beam radiation. The proposed antenna structure consists of four L-shaped patch antennas arranged in a square-ring formation. Each patch antenna is shorted to the ground plane through conducting walls. With a top-loaded monopole feed, two orthogonal resonant modes, loop mode and monopole mode, can be excited simultaneously. The effects of the shorting walls on the resonant frequencies of the two modes are investigated, and it is found that the resonant frequency (fmono) of the monopole mode has a more obvious variation than that (floop) of the loop mode when the width or the number of the shorting walls is changed. In addition, if the two modes are properly coupled, the microstrip array can generate a CP radiation; meanwhile, the polarization sense is left-hand CP if floop > fmono but is right-hand CP if floop <; fmono. A reconfigurable antenna prototype is fabricated, and the experimental results show that the prototype not only has the characteristic of omnidirectional radiation but also possesses the ability of switching between left-hand and right-hand CP. Simulated results carried out by HFSS are also provided and they agree well with the measured results.
Date of Publication: Aug. 2010