Research Article  Open Access
HaiNa Song, ZhaoHui Chen, JianFeng Li, "A Triband Patch Antenna with Monopolelike and Patchlike Radiation Patterns for Multifunctional Wireless Systems", International Journal of Antennas and Propagation, vol. 2020, Article ID 3268729, 12 pages, 2020. https://doi.org/10.1155/2020/3268729
A Triband Patch Antenna with Monopolelike and Patchlike Radiation Patterns for Multifunctional Wireless Systems
Abstract
A triband patch antenna with monopolelike and patchlike radiation patterns for multifunctional wireless systems is proposed. The antenna consists of a single square radiation patch with an annular slot, a ground plane, and a toploaded metal sheet. The toploaded metal sheet is shorted to the ground plane for producing a zerothorder resonant (ZOR) mode, which has an omnidirectional radiation pattern at the lowest operation band, and its performance is robust to the location of the probe feed. With the annular slot and the offcenter probe feed, a dualresonant TM_{01} mode is excited, yielding unidirectional radiation patterns for the two upper operation bands. The ZOR and the dualresonant TM_{01} modes can be independently controlled, and a triband antenna prototype with a square patch of 24 mm is fabricated and tested. The first bandwidth is 2.5–2.7 GHz with omnidirectional radiation pattern, the second and the third bandwidths with unidirectional radiation are 3.3–3.9 GHz and 4.8–6.1 GHz, and the realized gains over the three bands are about 2.6, 6.5, and 7.5 dBi, respectively.
1. Introduction
With merits of 360° full coverage, low profile, low cost, and easy fabrication, monopolar patch antennas are widely used in portable devices, such as the mobile phone, laptop, and GPS. Centerfed circular patch antennas can excite TM_{0m} modes with omnidirectional radiation patterns in the azimuth plane [1], but suffering narrow bandwidth. Many techniques have been proposed to solve the problem, including diskloaded [2] and loaded annular ring [3], but the first nonzero mode of these antenna is TM_{02} (1.0 λ) rather than the eigenmode TM_{01} mode (0.5 λ), resulting in large antenna sizes. Great efforts have been devoted to the study of TM_{01} mode, the most common method is centerfed circular patch shorted with via holes [4, 5], and monopolar patch antennas with TM_{01} mode were presented [6]. By utilizing shorting vias, annular ring, and stacked patch, monopolar radiation patterns were obtained over a broadband [7] or two operation bands [8].
Moreover, antennas with monopolelike and patchlike radiation patterns over different bands are desired by multifunctional wireless systems, such as vehicular communications. WMAN (Wireless Metropolitan Area Network) requires omnidirectional signal coverage for vehicular communication system accessing to Internet, and cartocar communication needs unidirectional signal coverage for safe driving control in vehicular communication systems. To realize the dualsense radiation patterns, artificial magnetic conductor and shorted vias were employed [9], and a small patch was stacked on a larger patch with vias [10].
Most of dualsense radiation pattern antennas are circular shaped and centerfed. But triangular, rectangular, and square antennas are also needed in practical applications. Monopolar patch antennas in equilateral triangular shape [11] and in squarering shape [12] were presented, while a conicalpattern antenna consisting of four symmetrical square radiating slots and artificial magnetic conductors was studied [13]. Four monopoles shorted to the ground and one square patch loaded with a slot were utilized by the antenna to obtain dualband and dualsense radiation patterns [14]. However, the reported antennas suffer from complicated structure and large size.
Resonant frequency of zerothorder resonant (ZOR) antenna depends on the antenna reactance instead of physical size [15], and that is useful to reduce antenna size. Taking advantage of the mushroom structure, omnidirectional ZOR antennas were proposed [16, 17], and a mushroom patch was inserted in a rectangular patch to generate the ZOR and TM_{010} modes at the same frequency [18]. Utilizing a toploaded small metal strip, a dualband circular patch antenna with omnidirectional ZOR mode for the lower band and unidirectional TM_{01} mode for the upper band was studied in [19].
In this paper, a compact triband square patch antenna with monopolelike and patchlike patterns is presented. Compared with the circular patch antenna [19], the proposed square patch antenna has a more compact size and better impedance matching bands to covering more communication bandwidth, so the proposed square patch antenna is more suitable to apply in multifunctional wireless systems. For the proposed square patch antenna, the method for the square radiation patch to excite monopolelike ZOR mode is introducing a rectangular metal sheet, and its performance is robust to the probe feed position. By etching a small annular slot into the square radiation patch, its original TM_{01} mode is split into two TM_{01} modes with controllable resonant frequencies, and they work at the two upper operation bands with unidirectional radiation pattern. Based on the equivalent circuit as well as the distributions of electric field and current, the principles of the antenna are discussed. These concepts are then verified by a triband antenna prototype; it exhibits omnidirectional radiation in 2.5–2.7 GHz and unidirectional radiations in both 3.3–3.9 GHz and 4.8–6.1 GHz. The radiation efficiencies are larger than 77%, 84%, and 86% for the three operation bands, respectively.
2. Antenna Design
Figure 1 illustrates the configuration of the proposed antenna; the antenna consists of a square patch with a small annular slot, a ground plane, and a rectangular metal sheet. With an air gap of size H between them, FR4 substrates 1 and 2 have relative permittivity of 4.4, a dielectric loss tangent of 0.002, and a thickness of H_{s} = 1.6 mm. The square ground plane with side length of L_{g} is printed on the bottom layer of FR4 substrate 1. The square radiation patch with side length of L_{p} is printed on the bottom layer of FR4 substrate 2 and is fed by an offcenter coaxial probe. F_{y} is the offset distance of the probe feed from the center of the square radiation patch along the yaxis. Taking the probe feed as center, a small annular slot with an outer radius r_{1} and an inner radius r_{2} is etched in the square radiation patch for producing a dualresonant TM_{01} mode with unidirectional radiation pattern.
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A rectangular metal sheet printed on the top layer of FR4 substrate 2 is shorted to the ground plane through two conductive vias to excite a monopolelike ZOR mode working at the lowest operation band. The dimension of the rectangular metal sheet is , and the two conductive vias are arranged symmetrically with a distance of . There are two clearance circles etched on the radiation patch and are placed around the two conductive vias, respectively, and the radiation patch and the conductive vias are separated.
3. Analysis of Working Mechanism
In our design, numerical analysis of the antenna is conducted in the commercial electromagnetic software HFSS in Version 2018, which solves the electric field and magnetic field by using finite element method (FEM). HFSS adopts adaptive mesh generation technology; the solution type and the convergence error are chosen as Driven Modal and 0.01, respectively, in the simulation of the proposed design.
3.1. Generation of the DualResonant TM01 Mode
To reveal the operating mechanism of the proposed antenna, a square patch antenna named Design 1 is first investigated. Design 1 has the same dimensions of the proposed antenna shown in Figure 1 but without the metal sheet and the two conductive vias. The resonant frequency f_{0} of TM_{01} mode is decided by [20, 21]where c is the velocity of electromagnetic waves in the free space, ε_{eq} denotes the equivalent permittivity of the substrate with an air gap, ε_{re} = 4.4 is relative permittivity of the substrate, and H_{s} = 1.6 mm and H = 4 mm represent the thickness of the substrate and the air between the two substrates. Based on (1), the value of f_{0} is 4.8 GHz when the square radiation patch side length L_{p} = 24 mm, and that is agreed well with the simulated result for the case of F_{y} = 0 mm, as shown in Figure 2. Coupling existing in the annular slot can split the original TM_{01} mode (at about 4.8 GHz) into two TM_{01} modes. But at the central area of the square radiation patch, the input impedance of the antenna is high due to the strong electric field and the weak magnetic field. Therefore, the impedance matching is poor when F_{y} = 0 mm, and only one TM_{01} mode appears.
A larger F_{y} means a larger distance between the probe feed and the center of the square patch. With the probe feed moving away from the center, the balance between the electric and magnetic fields can be enhanced to improve the impedance matching gradually, and TM_{01} mode at about f_{0} = 4.8 GHz is splitted into two TM_{01} modes. Therefore, a dualresonant TM_{01} mode with controllable resonant frequencies of f_{1} and f_{2} is produced, due to the capacitance existing in the annular slot. When the annular slot is close to the central part of the square patch, where antenna input impedance is high, the effect of the annular slot on the resonant modes will become weak. In other words, with a larger F_{y}, the annular slot has a stronger capacity of splitting the original TM_{01} mode into two TM_{01} modes, so the ratio of f_{1}/f_{2} becomes larger, as shown in Figure 2.
Keeping r_{2} = 2.0 mm, a smaller r_{1} represents a smaller annular slot width (r_{1} − r_{2}), leading to a stronger coupling of the annular slot, thereby both f_{1} and f_{2} shift towards the lower frequencies, as shown in Figure 3. Observing from Figures 2 and 3, we can find that the trends of f_{1} and f_{2} are the same with varying r_{1} but are opposite when F_{y} is changed, so f_{1} and f_{2} can be effectively controlled by tuning r_{1} and F_{y}, and r_{1} and F_{y} are chosen as 3.0 and 8.0 mm for the desired operation bands in the paper.
The current distributions and the simulated 3D radiation patterns of Design 1 at f_{2} = 3.75 GHz and f_{1} = 5.35 GHz are shown in Figures 4 and 5, respectively. The offcentered feed can modify the current distribution of the square patch, so the currents have the same direction in general and only one maximum, as shown in Figure 4. That suggests that the two resonant modes of Design 1 at 3.75 and 5.35 GHz are TM_{01} mode, based on the cavity model of patch antenna [22]. For the two TM_{01} modes, the only one theoretical maximum of radiation at the broadside direction is obtained in some degree, as shown in Figure 5. The original TM_{01} mode of the square patch is at about f_{0} = 4.8 GHz and (f_{0} − f_{2}) > (f_{1} − f_{0}). As a result, the uniformity of current distribution at f_{2} is worse than that of at f_{1}, as shown in Figure 4; the direction of the radiation pattern at f_{2} deviates from the broadside direction in some degree, but the TM_{01} mode at f_{1} still has a maximum of radiation in the broadside direction, as shown in Figure 5.
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3.2. Generation of Monopolelike Mode
By utilizing a rectangular metal sheet, which is shorted to the ground plane by two conductive vias, a monopolelike ZOR mode for the square radiation patch is achieved, and that is the most outstanding contribution of the proposed design. Experimental results show that the performance of the ZOR mode is robust to the location change of the probe feed, and that is different from the reported omnidirectional antennas [16, 17], of which probe feeds must be fixed at the center of the circular radiation patch. In our design, there are the two clearance circles around the two conductive vias, which isolates the toploaded rectangular metal sheet and the radiation square patch, so the probe feed can be located offcenter without affecting the monopolelike radiation pattern.
For illustrating the excitation principle of the ZOR, an equivalent circuit of the proposed triband square patch antenna is derived and is shown in Figure 6. L_{f} models the inductance of the feed probe, L_{P} describes the inductive effect associated to the radial currents flowing on the square radiation patch, and C_{R} is the capacitance of the annular slot. The capacitive coupling between the square patch part inside the annular slot and the ground plane is expressed by C_{P1}; C_{P2} is the capacitive coupling between the part of the square patch outer the annular slot and the ground, while C_{M} is the capacitive coupling between the rectangular metal sheet and the square radiation patch. The inductances of the conductive vias and the rectangular metal sheet are modeled by L_{v} and L_{M}, respectively, while R_{P} represents the antenna radiation resistance. By applying the rectangular metal sheet and the two conductive vias, two parallel branches both having L_{v} and L_{M}/2 are introduced and can be replaced by one branch with an inductance of (L_{v} + L_{M}/2)/2. The branch having C_{M} and (L_{v} + L_{M}/2)/2 is parallel to the branch with C_{p2}, ZOR mode is only related to the shunt components [18], and thus the proposed square patch antenna can support a ZOR mode at around 2.6 GHz.
The effects of the square patch side length L_{p} and rectangular metal sheet width M_{w} on the input impedance and S_{11} of the proposed antenna are studied in Figures 7(a) and 7(b), respectively. For the lowest resonant mode at about 2.6 GHz, the resistance tends to the maximum value of about 50 Ω when the reactance of the input impedance goes to zero, which proves that the lowest resonant mode is a parallel resonant mode. In Figure 7(a), we also can see that the two TM_{01} modes both are series resonant modes. The parallel resonant mode at about 2.6 GHz is also a ZOR mode, which is related to the shunt components; its resonant frequency is not affected by the varying L_{p} in theory, since its resonant frequency depends on antenna reactance rather than antenna physical dimensions. However, a smaller L_{p} brings to a smaller effective area of capacitor plate between the square patch and the ground plane and gives rise to a smaller capacitance C_{p2}. Then the resonant frequency of the ZOR mode moves to the higher frequency, but this tendency can be held up by increasing the value of C_{M} and that can be deduced from Figure 6. A larger M_{w} results in a larger C_{M}. When L_{P} is decreased from 26 to 23 mm, M_{w} is increased from 4.8 to 8.8 mm correspondingly; resonant frequency of the ZOR mode is preserved at around 2.6 GHz, as shown in Figures 7(a) and 7(b). However, despite the fact that a larger M_{w} is chosen, the resonant frequencies of the two TM_{01} modes move towards the higher frequencies with the decreasing of L_{P}, as shown in Figure 7.
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Moreover, the increase of the rectangular metal sheet length M_{L} also leads to an increase of C_{M}; the ZOR mode resonant frequency shifts to the lower frequency, as shown in Figure 8, but the two TM_{01} modes are almost not affected by M_{L}. L_{M} is decreased with the decrease in the distance between the two conductive vias, leading to a smaller antenna reactance, as shown in Figure 9(a). The resonant frequency of the ZOR mode, which mainly depends on the antenna reactance, moves to the lower frequencies, as shown in Figure 9(b). However, the effect of the varying on the resonant frequencies of the two TM_{01} modes is slight. Observing from Figures 7–9, we can draw a conclusion that the resonant mode at about 2.6 GHz is ZOR mode and can be controlled by the dimensions of the topload rectangular metal sheet, but it has ignorable effect on the dualresonant TM_{01} mode.
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Figure 10 shows the variation of S_{11} of the proposed antenna with respect to different F_{y}; its effects on the two TM_{01} modes are the same as that shown in Figure 2, but for the ZOR mode, the effect can be neglected. In Figure 11, the 3D electric fields and the equivalent magnetic currents of the proposed triband square antenna are sketched for the cases of F_{y} = 0 mm and F_{y} = 8.0 mm. The electric field distributions for both the two cases are similar. Omitting the slight disturbances of the two conductive vias and the probe feed, the electric fields under the square patch and the rectangular metal sheet vary in phase and are uniformly vertical to the square patch for the two cases. Based on the cavity model of the patch antenna, the equivalent magnetic loop currents along square patch peripheries can be produced and give rise to perfect monopolelike radiation patterns, as presented in Figures 12(a) and 12(b). This is further proved that the performance of the ZOR mode is robust to the location change of the probe feed. The radiation patterns from the two TM_{01} modes at 3.6 and 5.3 GHz are shown in Figures 12(b) and 12(c), and their performances are similar as that in Figures 5(a) and 5(b), respectively, showing once again that the effect of the rectangular metal sheet on the two TM_{01} modes is very slight.
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4. Results and Discussion
The proposed triband patch antenna with a profile of about 0.06λ_{0} at 2.5 GHz and dualsense radiation patterns is fabricated, and its photographs are shown in Figure 13. The simulated and measured S_{11} shown in Figure 14 are in a good agreement and both are less than −10 dB in 2.5–2.7 GHz, 3.3–3.9 GHz, and 4.8–6.1 GHz.
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It is clear from Figure 15(a) that both the measured and simulated radiation patterns in xy and xz planes at 2.6 GHz show a monopolelike radiation pattern. The radiation patterns in yz and xz planes at 3.6 and 5.3 GHz are shown in Figures 15(b) and 15(c), respectively. It is observed that the antenna has the maximum radiation at the direction of Phi = 90^{o} and Theta = −30^{o} for 3.6 GHz, while the maximum radiation is in the broadside direction at 5.3 GHz, and that is because the uniformity of current distribution at f_{2} is worse than that of at f_{1}, as shown in Figure 4. The simulated Copol pattern agrees well with the measured one, and the discrepancy for simulated and measured Crosspol levels is due to the limit of the anechoic chamber. The results show that the proposed antenna has different radiation patterns for the three operation bands of 2.5–2.7 GHz, 3.3–3.9 GHz, and 4.8–6.1 GHz. Figure 16 shows the simulated and measured gains for the three operation bands are 2.0–3.2 dBi, 5.5–7.0 dBi, and 6.2–8.1 GHz, respectively. The measured efficiency is also given in Figure 16, and the efficiencies are higher than 77%, 84%, and 86% for the lower, middle, and highest operation bands, respectively.
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Table 1 compares the proposed triband patch antenna with the other four reported antennas having omnidirectional and unidirectional radiation patterns. The comparison focuses on the number of antenna radiator, operation bands, size of antenna radiators in terms of their wavelengths at the lowest operating frequencies, antenna profiles in terms of wavelengths at the lowest operating frequencies, and the performance of radiation patterns. This comparison shows that just the proposed antenna, the antenna [14], and the antenna [19] employ a single one radiator to achieve the omnidirectional radiation pattern and the unidirectional radiation pattern simultaneously. However, the sizes of radiator for the two antennas [14, 19] are larger than that of the proposed antenna. The antenna [14] has a lower antenna profile compared with the proposed antenna, but only the proposed antenna has three operation bands. Comparing with the other antennas, we can say that the proposed antenna with a single square patch, compact size, lowprofile, triband, monopolelike, and patchlike radiation patterns is more suitable to be applied in the multifunctional communication system with limited space.

5. Conclusions
A compact triband antenna with a single square patch, lowprofile, monopolelike, and patchlike radiation patterns has been presented. By using a rectangular metal sheet shorted to the ground plane, a small annular slot etched on the square radiation patch, and an offcenter feed probe, a ZOR mode with monopolelike radiation pattern at the lowest operation has been obtained, and a dualresonant TM_{01} mode has been excited, yielding unidirectional radiation pattern for the middle and the highest operation bands. The triband antenna has been fabricated and tested, and the measured and simulated results are in a good agreement. The measured realized gain and the radiation efficiency in 2.5–2.7 GHz are 2.0–3.2 dBi and 77–80.5%, respectively. The measured realized gain and the radiation efficiency in 3.3–3.9 GHz are 5.5–6.8 dBi and 84–88%, respectively. The measured realized gain and the radiation efficiency are, respectively, 6.2–8.1 dBi and 86–88.5% in 4.8–6.1 GHz.
Data Availability
The numerical and experimental data used to support the findings of this study are included within the article.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
Acknowledgments
This work was supported by the National Science Foundation for Young Scientists of China under Grant 61901126. The authors would like to thank Professor BinJie Hu for his invaluable assistance in the preparation of this manuscript.
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Copyright
Copyright © 2020 HaiNa Song et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.