Contents I. Introduction
With the rapid development of modern wireless communication systems, circularly polarized (CP) antennas have been paid more attentions because of their advantage on reducing multipath interferences. CP antennas have been widely used in many systems, such as radio frequency identification devices (RFIDs) and wireless local area networks. Traditional microstrip CP antennas have very narrow bandwidth. A communication system, which works at several frequency bands, needs more than two microstrip CP antennas. To realize the simultaneous operation of multiple communication systems in the same module, broadband CP antenna has become a hot research topic. There exist some basic types to achieve broadband CP antennas, such as microstrip slot antenna [1], substrate integrated waveguide (SIW) slot antenna [2], loop antenna [3], monopole antenna [4], [5], slot-monopole antenna [6], [7], and dipole antenna [8]. A planar square slot CP antenna is proposed for RFID reader with an arc-shaped strip [1]. The operating band can reach about 142 MHz and the 3-dB axial ratio bandwidth (ARBW) is about 166 MHz. A wideband CP antenna element and antenna array are achieved by positioning a slot-coupled rotated strip above a slot cut onto the broad wall of an SIW for 60-GHz wireless personal area networks applications [2] with low-temperature cofired ceramic technology. The ARBW is about 10.7% (60.2–67 GHz). A CP antenna array using shorted loop fed by a four-port structure with sequential phase of 0°, 90°, 180°, and 270° is presented in [3]. The ARBW is improved from 2% of single CP patch to 7% of the CP array. A monopole antenna with asymmetric feed and a chifre-shaped structure is designed for broadband CP radiation [4]. By adjusting the location of the monopole microstrip feed, a CP antenna with 72% impedance bandwidth (IBW) and 41.6% 3-dB ARBW is achieved. Another antenna uses a C-shaped monopole and an open loop to realize a wider CP bandwidth, the ARBW is extended to 65.2% [5]. A combination of a slot and a monopole antenna is used to produce CP wave with 30% ARBW in [6]. Another slot-monopole structure antenna applies a triangular monopole and a slot etched on the ground plane to produce CP wave with 46.8% ARBW [7]. A new broadband rotational symmetry CP antenna is designed based on dipole antenna, which ARBW is 96.6% [8]. The CP antenna with dual polarization can meet the requirement of polarization diversity in many different communication systems. Many articles have made study of dual-CP antennas [9]–[14]. A patch antenna with two cross slots, which is fed by two L-strips, produces dual-CP radiation [9]. It exhibits an IBW and ARBW of about 10% and 4.8%, and the isolation is better than 15 dB. A square slot antenna loaded with a T-shaped strip and two microstrip T-junctions is produced for dual-CP radiation [10]. The ARBW is 19.6% and the isolation is better than 20 dB. A dual-CP antenna fed by two fed ports connected with a circular split-ring microstrip line underneath the ground plane is designed for RFID reader [11]. It provides IBW of 18% and ARBW of 11%, and the isolation within the RFID operating band is better than 25 dB. Another RFID reader uses a branch-line coupler with H-shaped ground slots to excite dual-CP radiation with wideband isolation [12]. The IBW is 29.1%, and ARBW is 18.7%. A broadband dual-CP patch antenna is fed by four cross slots [13]. The IBW, 10-dB isolation bandwidth, and 3-dB ARBW are 24%, 19%, and 16%, respectively. Two asymmetric T-shaped feed lines and inverted-L grounded strips with three straight strips are used to achieve dual-CP radiation [14]. The ARBW can reach up to 60%. A new monopole structure is designed for dual-CP in [15]. The ARBW is expanded to 80.7%.
