A Single Radiator-Based Circularly Polarized Antenna for Indoor Wireless Communication Applications

This paper introduces a novel technique for inducing circular polarization in a single radiator through the implementation of a sequentially rotated feeding network. Analogous to the operational principles of sequentially rotated antennas employing multiple radiators, the creation of circular polarization (CP) with a solitary radiator becomes achievable through the distinctive phase and angular arrangement facilitated by the feeding network. This innovative approach not only results in a substantial reduction in complexity but also contributes to an overall reduction in antenna size, all while upholding commendable CP performance in terms of both axial ratio (AR) bandwidth and beamwidth.


I. INTRODUCTION
T HE rapid evolution of wireless communication systems necessitates circularly polarized (CP) antennas that facilitate communication between transmitters and receivers with less stringent requirements regarding polarization orientation.In comparison to linearly polarized antennas, CP antennas exhibit lower sensitivity to multipath fading and interference [1], [2].Antennas with wide axial ratio (AR) bandwidth, impedance bandwidth, directional radiation patterns, and high gain are crucial for maintaining overall system performance.
CP antennas can be implemented in single-layer or multilayer configurations, with the former being popular due to its compact size.Achieving CP characteristics in a single-layer configuration involves employing a single patch radiator with truncated corners, multiple feeds featuring proper phase offsets, and circular-shaped geometries.While enhancing AR performance is feasible with multiple radiators arranged in an array configuration, this approach falls short of achieving a satisfactory AR bandwidth, particularly when designed with thin dielectric substrates [3].Furthermore, the impedance bandwidth is limited due to the high-quality factor (Q) nature of thin substrates [4], resulting in limited gain and less directive radiation patterns.
In contrast, multi-layer structures have demonstrated substantial performance improvements.A comprehensive study [5] indicated a significant enhancement in AR bandwidth from 0.8% to 4.6%.Alternative approaches, such as incorporating circular parasitic patches as directors [6], have also proven effective for improving AR bandwidth.
The challenge of achieving an even wider AR bandwidth, exceeding 10%, using a single radiator has been addressed in [7].The proposed technique involves sequentially rotating individual radiators with a unique phase offset, c 2024 The Authors.This work is licensed under a Creative Commons Attribution 4.0 License.
For more information, see https://creativecommons.org/licenses/by/4.0/Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.leading to a substantial improvement in AR bandwidth.Additionally, the AR beamwidth is enhanced due to the cancellation of cross-polarization fields and reinforcement of co-polarization fields attributed to the assigned phase offset [7].
The fundamental mechanism for CP generation using linearly polarized elements involves two mutually orthogonal radiation field components with a 90-degree offset at the feeds, as illustrated in Fig. 1(a) [7].The drawback of this configuration is poor AR beamwidth due to extra spatial phase delays caused by offset in the broadside direction.To address this issue, two more elements are added, as depicted in Fig. 1(b), resulting in improved AR bandwidth and beamwidth [7].The sequentially-rotated radiator approach, while offering significant improvements, suffers from increased size due to the number of radiators.The primary objective of the proposed antenna is to realize circular polarization using a single radiator in an aperturecoupled configuration.Four slots are utilized that are placed perpendicularly to each other, and feed through 90 • phase shifted signals, which couple the EM energy to the top conducting patch.As observed in Fig. 2(a), it is depicted that two perpendicularly polarized fields generated from the two orthogonally placed linearly polarized elements and provided with the necessary phase shifts will lead to the generation of circular polarization.As discussed previously, using only two elements will lead to extra spatial phase delays, leading to poor AR due to higher cross-pol levels.So, in this case, 2, more elements are utilized for good cross-pol cancellation, leading to a better axial ratio.In the proposed antenna, the slots are arranged diagonally, and the electric field due to a single slot is illustrated in Fig. 2(b).Fig. 2(c) shows the final configuration using 4 slots placed diagonally in an orthogonal manner.The sequential feeding network featuring a progressive 90 • phase difference is used to excite the slots and eventually coupling of EM energy to the top patch.Specifically, the ports feeding the four coupling slots have phases of 0 • , 90 • , 180 • , and 270 • with nearly equal amplitudes.This paper proposes a technique to achieve the same CP characteristics with a single radiator.The configuration involves the physical rotation of coupling slots sequentially with the same phase offset, enhancing the AR bandwidth and beamwidth similar to the sequentiallyrotated radiators configuration [7].The key advantage of this proposed configuration is its compact size, as only one radiator is involved in CP generation.To validate the technique, an antenna for radio-frequency identification (RFID) reader applications is proposed.RFID systems necessitate antennas with excellent CP performance to establish communication links between readers and tags in a complex environment.Based on the proposed sequentially-rotated feeding architecture, an aperture-coupled patch antenna was realized, covering an impedance bandwidth of S11 less

II. ANTENNA GEOMETRY
The design of the antenna with circular polarization relies on exciting orthogonal modes with a 90 • phase shift and equal magnitude.In this section, we introduce the geometry of an aperture-coupled multilayer circularly polarized antenna fed by a single sequentially-rotated feed.The proposed multilayer antenna configuration using a single radiator is depicted in Fig. 3.The antenna comprises two main components: a bottom printed circuit board (PCB) and a single radiator separated by an air gap.The bottom PCB features a sequentially rotated feeding network with the desired phase offset on the bottom layer and rotated slots inclined at an angle of 45 • on the top side of the board.The PCB used is the FR-4 substrate with a height, relative permittivity, and loss tangent of 0.8 mm, 4.34, and 0.025, respectively.Fig. 3 displays the layered view, top patch, and side view of the antenna.
Fig. 3(a) illustrates that the antenna consists of four layers, including one substrate and three metal layers.The antenna is excited by the sequentially rotated feeding network, coupling electromagnetic radiation to the top square radiator through sequentially rotated inclined slots etched on the ground plane.Such an aperture-coupled patch configuration generally exhibits a reasonable impedance bandwidth, as well as directive radiation patterns.The enhanced bandwidth of an aperture-coupled patch setup stems from increased design flexibility afforded by tuning stub length and coupling aperture size.These elements allow precise adjustment to counteract impedance shifts and optimize resonance, yielding a double-tuning effect.Additionally, stacked patch configurations provide further tuning capabilities, boosting bandwidth and performance.Furthermore, aperture-coupled designs enable precise control over radiation characteristics, yielding directive radiation patterns.By manipulating the dimensions of the aperture, patch, and coupling mechanism, designers can tailor patterns for specific needs like beam steering or shaping.
The top conducting patch is depicted in Fig. 3(b), positioned 15 mm away from the ground plane, as visible in Fig. 3(c).A parametric study to investigate the effect of H p on the antenna performance is carried out.Fig. 4 (a-c) shows the results of such analysis in terms of S11, AR, and gain versus frequency.As observed, the spacing H p does not have much effect on the S11 and gain performance yet AR is more dependent on H p .Based on the simulation results, the value of H p as 15 mm is selected for practical realization.
The four slots etched on the ground plane are depicted in Fig. 5(a), inclined at an angle of ±45 • and sequentially rotated by 90 • relative to the adjacent ones.The hourglassshaped slot is chosen for its contribution to maximum coupling with minimal back radiation.The hourglass-shaped slot in an aperture-coupled patch antenna optimizes coupling while minimizing back radiation through several key mechanisms.Its unique form efficiently channels electromagnetic energy from the feed network to the radiating patch.The narrowing towards the center concentrates electromagnetic fields, enhancing coupling efficiency.Moreover, this shape enables precise control over field distribution within the antenna, minimizing back radiation and maximizing forward radiation.By adjusting slot dimensions, impedance matching between the feed network and patch is improved, minimizing reflection losses and maximizing power transfer efficiency [8].The sequential feeding network is illustrated in Fig. 5(b), featuring a progressive 90 • phase difference.Specifically, the ports feeding the four coupling slots have phases of 0 • , 90 • , 180 • , and 270 • with nearly equal amplitudes.The length of the feed line is optimized to cancel its reactance with respect to the slot aperture.The dimensions of the antenna are provided in Table 1.
The E-field distribution is analyzed across four different frequency bands to determine the polarization type-whether left-handed or right-handed.As depicted in Figure 6, it is evident that the E-field propagates in a clockwise direction, indicating left-handed circular polarization.It is worth noting that by inverting the feed network (feed phases given as 0 • , −90 • , −180 • , −270 • ) while keeping the rest of the antenna configuration unchanged, right-hand circular polarization (RHCP) can be easily realized.Therefore, the proposed sequentially rotated circularly polarized antenna offers a convenient method for generating both LHCP and RHCP, making it highly promising for wireless communication applications.

III. RESULTS AND DISCUSSION
The correlation between the results (simulated and experimental) for the proposed sequentially rotated CP antenna is established by simulating the antenna in CST software, followed by fabricating the antenna using an in-house MITS PCB prototyping machine.The multilayer fabricated CP antenna, as illustrated in Fig. 7, occupies a total size of 150 × 150 × 15.8 mm 3 (0.42λ • × 0.42λ • × 0.04λ • ) at 845 MHz.The antenna's multilayers are stacked together using Teflon rods.The final antenna prototype is soldered with a 50-SMA connector at the input port present on the bottom layer for measurement.The simulated and measured values of the reflection coefficient and axial ratio are depicted in Fig. 8 (a-b) which are obtained using a Keysight N5227B vector network analyzer.The simulated and measured impedance bandwidth (|S11| ≤ −10 dB) is (13.25%)845-965 and (13.15%) 845-964, respectively.The simulated antenna illustrates a 3 dB AR BW of (7.80%) 862-932 MHz, while the 3 dB AR BW for the fabricated antenna is (7.45%) 865-932 MHz, showing a strong correlation.
The radiation pattern along xz and yz planes at a center frequency of 915 MHz is simulated and measured.It is illustrated in Fig. 9 (a-b) that the proposed CP antenna provides wide HPBW, a steady directional pattern, and a front-to-back ratio of 8 dB over the band of interest.A slight discrepancy is observed which could be due to the feeding cables in the chamber during measurement and fabrication tolerances.Nevertheless, the simulated and measured levels of RHCP polarization are below −15 dB which indicates a decent circular polarization.The measured values of halfpower beam-widths (HPBW) are 70 • and 79 • and 3 dB AR beam-widths are 62 • and 59 • along xz and yz planes, respectively with a boresight axial ratio of 1.69 as illustrated in Fig. 9(c).
Similarly, the radiation pattern along xz and yz at 902 MHz and 928 MHz is measured as shown in Fig. 10 (a) which illustrates that the patterns are directional with a reasonable front-to-back ratio.The HPBW and the 3dB AR Beam-width are shown in Fig. 10(b).The measured and simulated gains are plotted in Fig. 11, where a gain value of more than 4.8 dBic is observed across the band of interest.The measured boresight gain at 915 MHz is 7.44 dBic.Overall, a reasonable agreement between simulation and   To test the CP performance of the proposed antennas, a linearly-polarized tag (ALN 9540) was presented along the boresight direction as depicted in Fig. 12(a).The maximum readable distance was then recorded with the tag rotated at different angles with respect to the line-of-sight direction from the antenna as shown in Fig. 12(b-c).It is evident that at an Effective Isotropic Radiated Power (EIRP) of 2.1W, the maximum readable distance is 2.7 meters, while at an EIRP of 4W, the maximum readable distance is 4 meters.The EIRP is calculated referring to equation (1).EIRP = Reader output power + antenna gain −Cable Loss (1) The EIRP of 2W (outdoor) and 4W (indoor) is chosen for testing, as that is the permissible limit suggested by the Federal Communications Commission (FCC).The results also demonstrate the good circular polarization (CP) characteristics of the proposed antenna.
In Table 3, comparisons of the proposed antenna are conducted with other state-of-the-art circularly polarized (CP) RFID antennas.In [9], [10], RFID antennas with a low profile are proposed; however, both antennas suffer from low impedance bandwidth (IBW), axial ratio (AR), and gain values, which are less than 3%, 1.64%, and 4 dBic, respectively.A low-profile 2-element RFID antenna with wide IBW (16.4%) is proposed in [11], but it has a low AR bandwidth (4.4%) and gain (4.5 dBic).In [12], a high-gain (18.8 dBic) antenna with an IBW of 10.6% is illustrated, but the antenna has a very large profile (220 × 220 × 11 mm 3 ) with a low AR bandwidth (6%).A single-fed antenna with a profile of 150 × 150 × 25 mm 3 , having satisfactory IBW and AR bandwidth, is proposed in [13]; however, the gain and overall volume of the antenna may not be suitable for many applications.In [14], an antenna with decent IBW and AR bandwidth and a low profile is proposed, although the antenna displays a very low gain of only around 2.2 dBic.An antenna with high IBW (36%) and AR bandwidth (18.6%) with a profile of 95 × 100 × 13.6 mm 3 is depicted in [15]; however, this antenna also suffers from low gain (3.1 dBic).In [16], an antenna with a size of 150 × 150 × 21 mm 3 is illustrated, achieving a very wide IBW (30.2%) and AR bandwidth (24.2%); however, the overall volume and gain (5.6 dBic) limit its use for specific RFID applications.As observed, although the antennas in [17], [18] offer wider IBW and AR BW, the overall size is large in terms of the operating wavelength.Moreover, DRA's generally require high precision in terms of manufacturing and positioning.Out of the RFID antennas discussed, only [10] is analyzed for both front-to-back ratio and AR beam width.
The proposed CP antenna features left-hand circularlypolarized (LHCP) radiation characteristics with a gain of 7.8 dBic, a front-to-back (F/B) ratio of 8 dB, AR beamwidth of more than 105 • , IBW of (13.15%) 845-964 MHz, and an AR bandwidth (AR<3 dB) of (7.45%) 865-932 MHz.The read range of the reader antenna is 2.7 meters and 4 meters at an effective isotropic radiated power (EIRP) of 2.1 W and 4 W, respectively.The use of sequential rotational feed with inclined slot geometry helps in achieving enhanced AR beam width and bandwidth with only a single radiator at the top.

IV. CONCLUSION
In this paper, a technique for circular polarization (CP) generation using a single radiator has been proposed.
Leveraging the operational mechanisms of conventional sequentially-rotated radiator antennas, the implementation of a sequentially-rotated feeding network enabled similar CP performance while maintaining size compactness.The technique was validated using a commercial radio-frequency identification (RFID) system at the 915 MHz band, where the antenna exhibited satisfactory CP performance.The proposed technique is a generic approach suitable for any type of feeding excitation, including coupling through slots or pins.

FIGURE 1 .
FIGURE 1.The mechanism for CP generation: (a) using two mutually orthogonal elements with proper phase offset, (b) using two pairs of (a) with 90• phase offset for the improvement of AR bandwidth and beam width.

FIGURE 2 .
FIGURE 2. The illustration of the mechanism generating CP performance using a single radiator with a sequentially rotated feeding network: (a) the basic building blocks showing the composition of two mutually-orthogonal pairs of E-field components; (b) simulation verification of the aperture-coupled square patch radiator to generate the required E-field components and (c) final configuration of the proposed antenna featuring single radiator.

FIGURE 4 .
FIGURE 4. The effect of Hp on (a) S11, (b) AR, and (c) gain versus frequency.

FIGURE 6 .
FIGURE 6. E-field distribution at (a) 880 MHz (b) 900 MHz (c) 920 MHz, and (d) 940 MHz validating the generation of CP through a single square patch radiator.

FIGURE 7 .
FIGURE 7. Fabricated antenna prototype: (a) top view, (b) top side of bottom layer PCB, (c) bottom side of bottom layer PCB, (d) side view.

FIGURE 8 .
FIGURE 8. Simulated and measured results of (a) reflection coefficient and (b) axial ratio.

FIGURE 9 .
FIGURE 9. Simulated and measured radiation patterns of the sequentially rotated CP antenna at 915 MHz (a) XZ plane (b) YZ plane (c) axial ratio beamwidth.

FIGURE 10 .
FIGURE 10. Results along XZ-and YZ-planes at 902 MHz (left) and 928 MHz (right) in terms of (a) measured 2D radiation patterns and (b) axial ratio beamwidth.

FIGURE 11 .
FIGURE 11.Simulated and measured the gain of the proposed antenna.

FIGURE 12 .
FIGURE 12. RFID antenna read distance measurement: (a) The test setup for tag-reading measurement.The fabricated antenna prototypes (ANT1 and ANT2) are integrated with the ALR-9800 reader.The linearly polarized tag is rotated at different orientations for maximum readable distance measurement.(b) distance for EIRP = 2.1 W; (c) distance for EIRP = 4W.