Using the method of moments, we analyze three types of series-fed array antennas for circular polarization. First, we present a reference antenna's radiation characteristics with symmetrical dual loops fed by balanced parallel lines. Next, we modify the symmetrical dual loops into asymmetrical ones to enhance a circularly polarized (CP) wave bandwidth. It is found that the modified antenna has a bandwidth of 35% for a 3 dB gain-drop criterion, where the axial ratio is less than 3 dB. Finally, we use unbalanced parallel lines for feeding the asymmetrical dual loops to simplify the feed system. It is revealed that the antenna shows a CP wave bandwidth of 19%, which is twice as wide as that of the reference antenna.
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