14 Circularly Polarized Slotted / Slit - Microstrip Patch Antennas

In modern wireless communication systems, small circularly polarized microstrip antennas with good performance are desirable mainly at low microwave frequencies. The radio frequency identification (RFID) systems in the ultra-high frequency (UHF) band have gained much interest in many services [Finkenzeller2004]. The reader antenna is one of the important components in the RFID reader system and has been designed with circularly polarized (CP) radiation. The RFID system consists of the reader and the tag. Normally, the UHF tag antennas are linearly polarized. The circularly polarized microstrip antennas (CPMAs) can reduce the loss caused by the multipath effects between a reader and the tag antenna. The circularly polarized reader antenna is useful in particular when tag antenna system is in rotating motion or when its respective orientation cannot be ensured. Total frequency span of 840 960 MHz is used over worldwide for UHF RFID systems. The UHF RFID system operates at the bands of 902 928 MHz for North-South America, 865 867 MHz for Europe and 840 955 MHz for Asia-Pacific region. The compact CPMA design to cover total UHF RFID frequency span for handheld RFID applications is very difficult. Overall sizes of the broadband CPMAs to cover woldwide RFID applications are bulky [Chen2009, Chung2007]. However these broadband antennas are not suitable for handheld or portable reader applications. Design of the compact CPMA is attractive for handheld/ portable device applications. Small size of the CPMA can be achieved at the cost of limited gain and narrow 3-dB Axial ratio (AR) bandwidth/ 10-dB return loss impedance bandwidth. A CPMA with a low-profile, small-size, and light weight is required in a portable/ handheld RFID reader. However the compact CP reader antenna should be covered at least one country frequency span used for RFID UHF band [Nasimuddin2010, Nasimuddin2009]. A typical technique for producing CP is to excite two orthogonal linearly polarized modes with a 90o phase difference of the patch antenna. The single-feed CP annular-ring, square and circular microstrip patch antennas with perturbation elements have reported in [Chen1999, Chen2001, Row2004, Huang1998, Hyun2008, Row 2005, Sharma 1983]. Using perturbation or strips or slots on a radiating patch two orthogonal modes can be generated at around resonance frequency with 90o phase-shift for CP radiation requirements. The single-feed circularly polarized microstrip antennas [Haneishi1988, Wong2002] are generally compact when compared with the dual-feed CPMAs [Tragonski1993]. Single-feed CPMA is simple, compact structure, easy manufacture, and low-cost. However, the single-feed conventional microstrip antennas [Sharma1983,