1 Introduction
In recent years, circularly polarization (CP) antennas have been attracted extensive attention in wireless systems such as navigation, radar, satellite and radio-frequency identification (RFID) because of its flexibility in the orientation angle between transmitter and receiver, its better mobility and weather penetration, and less multipath reflection [
1‐
3]. Circular polarization can generally be achieved by exciting two orthogonal modes with equal amplitude but 90° phase difference.
Planar printed antenna has the advantages of low cost, easily manufacture and low profile, which is often used in the design of CP antenna [
4]. CPW feed has small dispersion and low radiation loss, and its working frequency can reach millimeter wave band. In the CPW fed microstrip antenna, the active devices can be integrated much easier, with series and shunt connection on one side of the substrate, avoiding the via-connection in the substrate [
5].
In order to achieve both wide impedance bandwidth and wide axial ratio (AR) bandwidth with single antenna, wideband circularly polarized antenna was proposed by trimming the feed structure and the loading slots. J. W. Wu et al
. proposed an asymmetric feeding monopole rectangular radiation patch in 2010, with embedded slits and stubs on the deformed ground to achieve 31.6% AR bandwidth and 102.5% impedance bandwidth [
6]. Zhang et al
. proposed a CPW fed planar printed monopole antenna with broadband CP characteristics in 2013. The proposed antenna consists of a rectangular patch and an improved ground plane. The AR and the impedance bandwidth are enhanced by adding vertical stub and cutting horizontal slot [
7]. It shows that the antenna can achieve wide impedance bandwidth and wide AR bandwidth at the same time, with the 3-dB AR bandwidth reaching 44.9% and the 10 dB impedance bandwidth reaching 76.9%. The broadband CP monopole antennas in [
8‐
10] mainly utilized complex patch structures or embedded slots & stubs ground planes. Ref. [
11] reported the coupling effect between a monopole antenna and a short sleeve for the excitation of both the monopole and the traveling wave resonant modes. However, the corresponding 3-dB AR bandwidth is only 5%.
It is worth noting that special slot antennas fed by CPW structures, such as the staircase-shaped slot adopted in [
12] and the regular-hexagonal structure adopted in [
13], also exhibit the broadband CP performance. Ref. [
14] proposed a CPW-fed 3D MIMO ground-radiating cubic antenna (CA) for biotelemetry, which achieves CP radiation in two bands of 2.45 GHz and 5.8 GHz. However, with the large size of the antenna, it is difficult to conform it to the other communication devices. The CP antenna reported in Ref [
15] is composed of three asymmetrical rectangular slots. The axial ratio bandwidth can reach 2.3 GHz. The 3-dB axial ratio bandwidth overlaps with the 10-dB impedance bandwidth very well. Its working frequency spectrum can cover the bands of RFID, WLAN and WiMAX. Ref. [
16] proposed an ultra-wideband (UWB) CP antenna with continuous phase feeding. Its 3-dB AR bandwidth reaches 133.3%, but the feeding network is very complex with the large antenna size. Ref. [
17] performed CPW feeding for the first time on a quasi-magnetic dipole (PQMD), in which the planar quasi-magnetic dipole as well as the printed electric dipole work together to achieve the circular polarization performance with the extreme narrow working bandwidth. Ref. [
18] uses dual port feedings to achieve double circular polarization modes for the port isolation, while the antenna proposed in this paper only needs one port to achieve right/left circular polarization in the ± z axis direction at the same time. Ref. [
19] adopts coplanar waveguide feed to excite quadrilateral-shaped monopole antenna, which shows excellent CP performance with small size. An array element in antenna is proposed in the paper. Ref. [
20] proposed a monopole antenna fed by a square coplanar waveguide surrounding the antenna. The impedance bandwidth is 12 GHz, but the AR bandwidth is relatively narrow. In addition, wavelet analysis can be used to design fractal geometry. When fractal structure is used for antenna design, a tortuous and complex current path can be constructed. The current distribution on the antenna has a certain level and self-similarity, so it can produce multi-frequency radiation characteristics. When multiple frequencies on the antenna are close to each other, broadband operation can be realized [
21‐
28]. However, the design of fractal circularly polarized antenna is relatively difficult and can’t be used practically until now. Above all, it is still a challenge for the optimization of the monopole antenna with both wide AR bandwidth and wide impedance bandwidth.
In the paper, a novel CPW-fed quarter-circular monopole antenna with asymmetric ground plane and bent L-shaped stub line loading is designed to achieve wide impedance matching and broadband CP operating bandwidth. The CP radiation is implemented on the quarter-circle microstrip patch monopole using an asymmetric ground plane. To expand the impedance bandwidth and the AR bandwidth, a bent L-shaped stub line is assembled on the left of the monopole, and a circular patch is coupled above the monopole. The measurements show that the impedance bandwidth is 76.9%, and the 3-dB AR bandwidth is 39.8%. At the same time, the size of the antenna is relatively small, with the low profile and the simple feeding structure.
4 Conclusions
Circularly polarized antennas play a vital role in wireless communication systems and are widely used in devices such as radar, satellites, televisions, and drones. The CPW single-feed technology proposed in the paper, can achieve broadband circular polarization by the exciting quarter-circular radiating monopole patches, the folded microstrip stub lines, and the coupled circular patches in the structure. The single-fed circularly polarized antenna is simple in structure, with low-cost, which can be used in mobile equipment terminals.
The designed CPW single-feed UWB circularly polarized antenna has 10-dB impedance bandwidth of 76.9% and 3-dB axial ratio bandwidth of 39.8%. In the operating band, the measured gain is greater than 3-dBi and the antenna can generate right/left hand circularly polarized waves in the direction of ± z axis. It has wide application prospects due to its low profile, easy confomity with other communication devices and wider axial ratio bandwidth.
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