A Compact Wideband Circularly Polarized Microstrip Slot Antenna with Parasitic Elements

A compact wideband circularly polarized (CP) microstrip slot antenna (MSA) with parasitic elements is designed in this letter. The CP MSA comprises a square-loop sequential-phase (SP) feeding configuration, four rotated rectangular patches, and four L-shaped slots embedded in the ground plane. The square-loop SP feeding structure comprises a square loop and an arc-shaped strip, which could provide a 270° phase difference. Four rotated rectangular patches are placed at the edge of the square-loop feeding configuration using a capacitively tightly coupled feeding method to stimulate the CP resonant mode. After selecting these elements and tuning proper dimensions, the broad operating bandwidths of 4.38–5.25 GHz (18%) for |S11| <–10 dB and 4.65–5.31 GHz (13.2%) for AR <3 dB could be realized. Hence, the designed CP MSA has a potential application value in wireless communication.


Introduction
Because of the obvious merits, antenna arrays have been massively used in wireless communication systems. Compared with single-antenna units, antenna arrays have excellent characteristics in terms of wide bandwidth, high gain, and stable radiation patterns [1,2]. Conversely, circular polarization (CP) antennas have obtained considerable attention in satellite and mobile communication [3,4] because of their outstanding features of overcoming polarization misalignment and reducing multipath interference with respect to linear-polarization antennas. us, CP antenna arrays (CPAAs) have naturally been significantly studied because of their combination of the advantages of the two structures.
us far, several effective methods have been proposed for designing the CPAAs [5][6][7][8][9][10]. To compare and analyze these structures, the feeder network is essential in antenna array design. For example, two T-junction power dividers are joined as a sequential phase (SP) shifting feeding network in [5], two phase-shifted swaps and a 1 : 8 power divider are combined as stripline feeding network in [6], and Wilkinson power combiners and delay lines are used as a beamforming network in [7]. Among these feeding strategies, the SP feeding network is one of the potential candidates in the design of antenna arrays, which could excite a stable and SP difference to the port of the feedline. When the SP feeding network is introduced into the design of 2 × 2 CPAAs, the impedance bandwidth (IBW) and axial ratio (AR) bandwidth of CPAAs could be improved simultaneously. e SP feeding network could be categorized as follows: a series-parallel feeding structure [11,12], aperturecoupled structure [9], coplanar waveguide-fed structure [13], and a square-loop rotated feed-network structure [14][15][16][17][18][19][20][21][22].
Additionally, based on the SP feed-network, the stacked technology is effective in CPAAs [21][22][23][24][25]. Round [21] and rectangular [22] patches as parasitic elements are stacked above the SP rotated feeding structure, which realize 6% and 28.1% of CP bandwidth, respectively. Although the stacked structure enhances the CP bandwidth, the profile of CPAAs is too high to meet the requirements of system integration.
is study develops a new scheme of the CP microstrip slot antenna (MSA) with 2 × 2 array and low-profile characteristics. e proposed CP MSA comprises a square-loop SP feed-network, four rotated rectangular patches, and four L-shaped slots embedded in the ground plane. e squareloop SP feed-network is composed of a square loop and an arc-shaped strip, which could provide a 270°phase difference. Four rotated rectangular patches are placed at the edge of the square-loop feeding configuration using a capacitively tightly coupled feeding method to stimulate a CP resonant mode. After selecting these elements and tuning proper dimensions, broad operating bandwidths of 4.38-5.25 GHz (18%) for |S 11 | < −10 dB and 4.65-5.31 GHz (13.2%) for AR < 3 dB could be realized. Figure 1 shows the entire structure and detailed geometry of the proposed CP MSA with SP feeding characteristics.

Antenna Mechanism.
To reveal the operated mechanism of the proposed CP MSA, Figure 2(a) shows four evolutionary models (Ant. I to Ant. IV). e models depict a stepby-step design process, and for comparison, Figures 2(b) and 2(c) show the corresponding |S 11 | and AR values. Note that the square-loop SP structure could excite the fundamental mode (TM 11 ), and the corresponding resonance-frequency point (f 11 ) can be tuned by using the average circumference of the square loop, and the formula is as follows [17]: where ε r represents the dielectric constant of the substrate, c represents the speed of light, and D2 and D3 represent the square-loop side-lengths. When the length of the arc-shaped strip is a quarter of the waveguide wavelength, the 90°phase difference could be obtained, which is a necessary condition for exciting CP resonant modes. Instead of using the traditional placement method that places four rectangular/ square patches at the edge of the square loop [16,17,22], we chose to place four rotated rectangular patches at the corners of the square loop (Ant. I) through proximity coupling. As shown in Figure 2, Ant. I with four rotated rectangular patches has two AR resonant points, with AR values being less than 3 dB. is is because the square-loop SP structure could provide a stable continuous phase (0°, 90°, 180°, and 270°). When four L-shaped strips are separately connected with the square-loop, Ant. I has poor return-loss and AR values. To improve the CP bandwidth of Ant. II, the Lshaped strips and rectangular patches are introduced into Ant. III. e 3 dB ARBW and −10 dB IBW of Ant. III could be improved. However, the CP performance of Ant. III is not as good as that of Ant. I, meaning that the four L-shaped strips do not increase the bandwidth. Furthermore, when the L-shaped strip as a parasitic slot is etched into the ground plane, the ARBW and IBW of Ant. IV could be increased. Regarding performance improvement, the introduction of L-shaped slots could generate a new current flow path and extend the previous current path, which deviates the AR resonance points. Finally, the proposed antenna could realize a wide −10 dB IBW of 19.8% (5.64, 5.08-6.2 GHz) and a 3 dB ARBW of 17.5% (5.5, 5.02-5.98 GHz). Furthermore, Table 1 presents a comparison with previously published CP antennas to demonstrate our design's superior performance. From the table, although the CPBW and peak gain of the proposed antenna is not as good as some referenced antennas in Table 1, the proposed antenna features a new design and low profile, which is the greatest contribution of the proposed antenna.   is a necessary condition for exciting CP, the surface current is mainly distributed on the rectangular patch, with a small amount of distribution on edge of the L-shaped slot, whether at low frequency (4.7 GHz) or high frequency (5.2 GHz). Note that the surface currents on the rectangular patch and L-shaped slot with different phases (0°and 90°) are orthogonal. e total surface current of the proposed CP MSA is the vector sum of the surface currents on the edge of L-shaped slots and rectangular patches. As shown in Figure 3, the total surface currents could rotate counterclockwise with varying phases (0°and 90°) at AR resonance points (4.7 and 5.2 GHz). us, the proposed CP MSA radiates right-hand CP waves in the +z direction. Additionally, according to the above analysis, we can conclude that the etched L-shaped slots introduce a new current flow path and make the ground plane participate in the radiation. Furthermore, the CP bandwidth of the proposed CP MSA can be improved.

Experimental Results
In this section, a simulated model of the proposed CP MSA was printed and tested to verify the rationality of the design. International Journal of Antennas and Propagation measured gains are smaller than the simulated gains at the entire frequency band due to the measurement errors. e simulated and measured peak gains are 10.2 and 9.1 dBi, and the simulated radiation efficiencies are greater than 90% within the CPBW. Additionally, Figure 5 depicts the normalized radiation patterns at 4.7, 5.0, and 5.2 GHz. e figure shows the simulated and measured left-hand CP waves are less than the right-hand CP parts by at least 20 dB in the yozand xoz-plane, which reveals that the proposed CP MSA could radiate right-hand CP waves. Because of the error in manufacturing and measured environment, minor differences between the tested and simulated values could be found.

Conclusion
In this study, a compact wideband CP MSA with parasitic elements is designed. e proposed CP MSA consists of a square-loop SP feed network, four rotated rectangular patches, and four L-shaped slots embedded in the ground plane. e square-loop SP feed network is composed of a square loop and an arc-shaped strip, which could provide a 270°phase difference. Four rotated rectangular patches are placed at the edge of the square-loop feeding configuration using a capacitively tightly coupled feeding method to stimulate a CP resonant. e bandwidth of CP MSA could be improved by combining CP modes produced by parasitic elements. After selecting these elements and tuning proper dimensions, the broad operating bandwidths of 4.38-5.25 GHz (18%) for |S 11 | < −10 dB and 4.65-5.31 GHz (13.2%) for AR < 3 dB could be realized. Hence, the proposed CP MSA has a potential application value in wireless communication.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.