SHORTED CIRCULAR MICROSTRIP ANTENNAS FOR 50 Ω MICROSTRIP LINE FEED WITH VERY LOW CROSS POLARIZATION

A circular microstrip antenna (CMSA) cannot be fed directly with 50Ω microstrip (MS) line, without an inset or an impedance transformer, as all around the periphery the impedance is equally high. The feeding technique such as inset fed or quarter wave transformer makes the antenna geometry asymmetrical. In this paper, a technique using single shorting post and a pair of shorting posts is proposed to match the peripheral impedance of the CMSA with that of the 50Ω-MS-line feed for operation around 2.45GHz. The shorting posts perturb the current distribution on the patch, altering the input impedance at the periphery. By selecting proper shorting posts positions a wide range of impedance has been adjusted without altering the patch geometry. Due to symmetric arrangement of the double shorting posts, the proposed antenna configuration has a very low cross-polarization ratio of better than −65 dB at the broadside direction for 2.45GHz. The simulated results of the directly fed 50Ω-MS-line CMSAs are experimentally validated with good agreement.