A novel CPW-fed broadband circularly polarized (CP) slot antenna provides enhanced bandwidth. Its ground plane is a stepped stairs shape where CP is introduced. To enhance the bandwidth, a meander line branch is embedded in the side perpendicular to the feed line introducing a wideband orthogonal mode. The measured –10 dB impedance bandwidth is 103.2 percent (from 2.12 to 6.64 GHz) and the ARBW is 72.0 percent (from 2.82 to 5.99 GHz), which agrees well with simulations.
CP antennas can receive arbitrarily polarized electromagnetic waves, and electromagnetic waves radiated by CP antennas can be received by antennas of any polarization.1 Further, the CP antenna can suppress interference from inclement weather and alleviate the effects of multipath reflections.2
The way to generate a circularly polarized wave is to excite two orthogonal modes with an equal amplitude and a phase difference of 90 degrees.3-5 Various types of CP antennas have been proposed with broad impedance and axial ratio (AR) bandwidths. In addition, the demand for antenna miniaturization in modern communication systems has increased recently.4
Most CP antenna designs are asymmetric5 and fed by L-shaped feed lines.6 To produce CP, improve performance and widen the bandwidth, many methods are employed, such as the introduction of slots7,8 and the embedding of various types of stubs.9-11 However, achieving performance in all areas, simultaneously, is challenging. For example, an antenna with a novel chifre-shaped feed line demonstrates a wide bandwidth of 72 percent and a low profile, but with an insufficient CP characteristic.12
Metamaterials with unique physical characteristics have been used in CP antenna design. For example, Marouf and Ziani13 demonstrated a 3×3 array of circular patches with the circular patch antenna at the center of the array fed by a modified CPW slot. Dual CP radiation was achieved, but bandwidth was no greater than 20 percent.
Weng et al.14 described a dual-band, CP slot antenna with broadband characteristics in both AR and impedance, while Hoang et al. demonstrated ARBW improvement of a CP printed monopole antenna using a lumped capacitor.15
A high gain and broadband planar CP antenna was designed by Guthi and Damera with a single-layer substrate.16-18 To realize a broadband antenna initially, a circular patch antenna was fed by a slot with a coplanar feed line in the ground plane. A metasurface was created with a 3×3 array of circle patches with the circular patch antenna at the center of the array. Moreover, the slot was also modified into an L-shape to generate circular polarization. In addition, stubs were introduced in the slot to improve the impedance bandwidth. Other technologies, such as artificial magnetic conductor reflectors,19 asymmetrical dipoles with via holes20 and multilayer structures21 have also been used to widen the antenna bandwidth.
The above designs are large and non-conformal, and their bandwidths are relatively narrow. A monopole antenna with an asymmetrical ground was described by Bao and Ammann.22 Its impedance bandwidth was 96.5 percent with an ARBW of 63.3 percent, while it was more compact than most CP antennas, its peak gain was just 3.5 dBic, which is extremely low relative to its size.
The CPW slot-feed has been shown to have potential for wideband CP antennas,23-27 but most are linearly polarized. In this work, a CPW-fed slot antenna with an asymmetrical ground is described. With an asymmetrical ground, a wide ARBW is achieved, then a meander line is etched on the ground to further improve its bandwidth and enhance CP performance.
ANTENNA CONFIGURATION
The antenna geometry shown schematically in Figure 1 is based on the traditional design of a CPW-fed slot antenna. Its dimensions, determined through parametric analysis, are listed in Table 1. A rectangular metallic section of the ground plane on the left of the feed line is removed to produce a CP wave. Further, a meander line protruding off the ground plane is etched to the left of the feed line to improve CP performance. The dielectric substrate of the single-layer antenna is an FR4 composite with a dielectric constant of 4.4. The antenna is fed from a coaxial line with a standard SMA connector.
Figure 1 Antenna structure.
The design evolution is shown in Figure 2. It starts with a conventional symmetrical CPW-fed slot antenna (see Figure 2a). The slot is then enlarged on one side to produce an asymmetrical surface current, introducing CP (see Figure 2b). To further enhance the CP bandwidth and improve impedance, an embedded slender meander line is positioned perpendicular to the feed line (see Figure 2c) introducing wideband orthogonal modes.
Figure 2 Design evolution: Antenna 1 (a), Antenna 2 (b) and Antenna 3 (c).
Figure 3 Simulated impedance bandwidth and ARBW of Antenna 1 (a), Antenna 2 (b) and Antenna 3 (c).
Figure 4 Surface current distribution of Antenna 1 (a) and Antenna 2 (b) at 5 GHz.
The simulated –10 dB impedance bandwidth and ARBW are shown in Figure 3. Compared with previous structures, which have achieved a broad band or dual-band circular polarization by loading with complex structures (such as spiral or branched microstrip), this antenna is designed based on a traditional CPW-fed slot antenna, so it has the advantages of a low profile and ease of fabrication.
To illustrate the effect of introducing the CP mode, the current distributions of Antennas 1 and 2 at 5 GHz are simulated (see Figure 4). The current distribution vectors of Antenna 1 are mainly vertical and more dominant at the base of the feed, indicating that Antenna 1 radiates only linearly polarized waves (see Figure 4a). With the ground plane shape of Antenna 2, horizontal currents are enhanced in the top branch of the ground plane. Two currents distributed perpendicularly are obtained, introducing the capability for CP. To further improve CP as well as impedance bandwidth, a meander line is placed perpendicular to the feed, introducing wideband orthogonal modes.
To demonstrate the CP mode introduced by Antenna 3, the time-varying current on the metal surface at phases of 0, 90, 180 and 270 degrees at 5 GHz is shown in Figure 5. The dominant surface current flows counterclockwise. Consequently, it radiates a right-hand CP (RHCP) wave in the +z direction. The dominance of surface current distribution on the feed line, the edge of the step-stairs ground and the meander line underscores the contribution of the meander line on CP radiation. To achieve the left-hand CP (LHCP), the meander line and step-stairs ground are simply positioned on the opposite side of the feed.
Figure 5 Time-varying current on the antenna conductive surface at 5 GHz: 0 (a), 90 (b), 180 (c) and 270 (d) degrees.
Figure 6 Simulated |S11| and AR with different Lf.
Figure 7 Simulated |S11| and AR with different b.
