A Triband Patch Antenna with Monopole-like and Patch-like Radiation Patterns for Multifunctional Wireless Systems

A triband patch antenna with monopole-like and patch-like radiation patterns for multifunctional wireless systems is proposed. -e antenna consists of a single square radiation patch with an annular slot, a ground plane, and a top-loadedmetal sheet.-e toploaded metal sheet is shorted to the ground plane for producing a zeroth-order 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 off-center probe feed, a dual-resonant TM01 mode is excited, yielding unidirectional radiation patterns for the two upper operation bands.-e ZOR and the dual-resonant TM01 modes can be independently controlled, and a triband antenna prototype with a square patch of 24mm is fabricated and tested. -e first bandwidth is 2.5–2.7GHz with omnidirectional radiation pattern, the second and the third bandwidths with unidirectional radiation are 3.3–3.9GHz and 4.8–6.1GHz, and the realized gains over the three bands are about 2.6, 6.5, and 7.5 dBi, respectively.


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. Center-fed 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 disk-loaded [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 center-fed 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 [11] and in square-ring shape [12] were presented, while a conical-pattern 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 dual-band and dual-sense radiation patterns [14]. However, the reported antennas suffer from complicated structure and large size.
Resonant frequency of zeroth-order 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 top-loaded small metal strip, a dual-band 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 monopole-like and patch-like 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 monopole-like 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.
ese 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. e radiation efficiencies are larger than 77%, 84%, and 86% for the three operation bands, respectively. 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. e square ground plane with side length of L g is printed on the bottom layer of FR4 substrate 1. e square radiation patch with side length of L p is printed on the bottom layer of FR4 substrate 2 and is fed by an off-center coaxial probe. F y is the offset distance of the probe feed from the center of the square radiation patch along the y-axis. 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.

Antenna Design
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 monopole-like ZOR mode working at the lowest operation band. e dimension of the rectangular metal sheet is M l × M w , and the two conductive vias are arranged symmetrically with a distance of V d . ere 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.

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.

Generation of the Dual-Resonant 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. e 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. erefore, 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. erefore, a dual-resonant 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. e 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. e off-centered 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. at 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  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.

Generation of Monopole-like Mode.
By utilizing a rectangular metal sheet, which is shorted to the ground plane by two conductive vias, a monopole-like 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 top-loaded rectangular metal sheet and the radiation square patch, so the probe feed can be located offcenter without affecting the monopole-like 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. e 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. e 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. e 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. e 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. e 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 . en 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  R rad . 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. 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 V d between the two conductive vias, leading to a smaller antenna reactance, as shown in Figure 9(a). e 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 V d 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 top-load rectangular metal sheet, but it has ignorable effect on the dual-resonant TM 01 mode. 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. e 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 monopole-like radiation patterns, as presented in Figures 12(a) and 12(b).
is is further proved that the performance of the ZOR mode is robust to the location change of the probe feed. e 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.

Results and Discussion
e proposed triband patch antenna with a profile of about 0.06λ 0 at 2.5 GHz and dual-sense radiation patterns is fabricated, and its photographs are shown in Figure 13. e 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.
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 monopole-like radiation pattern. e 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 eta � −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. e simulated Co-pol pattern agrees well with the measured one, and the discrepancy for simulated and measured Cross-pol levels is due to the limit of the anechoic chamber. e 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. e 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. Table 1 compares the proposed triband patch antenna with the other four reported antennas having omnidirectional and unidirectional radiation patterns. e comparison International Journal of Antennas and Propagation 9 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. is 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. e 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, monopole-like, and patch-like radiation patterns is more suitable to be applied in the multifunctional communication system with limited space.

Conclusions
A compact triband antenna with a single square patch, lowprofile, monopole-like, and patch-like 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 off-center feed probe, a ZOR mode with monopole-like radiation pattern at the lowest operation has been obtained, and a dual-resonant TM 01 mode has been excited, yielding unidirectional radiation pattern for the middle and the highest operation bands. e triband antenna has been fabricated and tested, and the measured and simulated results are in a good agreement. e measured realized gain and the radiation efficiency in 2.5-2.7 GHz are 2.0-3.2 dBi and 77-80.5%, respectively. e measured realized gain and the radiation efficiency in 3.3-3.9 GHz are 5.5-6.8 dBi and 84-88%, respectively. e 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
e numerical and experimental data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.