Wide E-Plane Beamwidth Microstrip Patch Antenna Using H-Shaped Gap-Coupling with Three Parasitic Patches for the K-Band

. This article presents a new wide E-plane beamwidth rectangular microstrip patch antenna adopting H-shaped gap-coupling with three parasitic patches for the K-band vehicle parking monitoring system, while keeping single substrate. Both side and upper parasitic elements serve as λ and λ /2 resonators and are excited primarily by magnetic and electric couplings, respectively. The synthesis of radiation patterns from main and auxiliary elements results in the E-plane wide-beamwidth property. The proposed gap-coupling antenna provided the peak gain of 4.82dBi, E-plane half-power beamwidth (HPBW) of 143.4 ° , and co-to-cross-pol ratios of 22.7 and 20.2dB within the HPBW for the E-and H-planes, respectively. The presented geometry showed excellent ﬁ gure of merits compared with others for the K-band.


Introduction
A wide beamwidth in antennas is beneficial for maximizing the area of coverage in a point-to-multipoint wireless communication system [1], a wide scanning range of phased array systems [2], global positioning satellite (GPS) applications [3], and smart multistandard wireless devices like communication-enabled smart vehicles [4]. For the vehicle parking monitoring system using a radar sensor with wideangle object detection, the antenna should satisfy the requirement of wide beamwidth. In addition to that, the horizontal polarization is preferred for automotive radar target detection since the horizontally polarized backscattering coefficients are smaller than the vertically polarized backscattering coefficients for dry and wet asphalts at large incidence angles of 25°to 80°a t 24 GHz [5,6]. Various approaches have been studied to achieve the wide-beamwidth characteristics. The first technique adds an extra resonator [7] or parasitic elements [1,6,[8][9][10].
This article proposes a new rectangular microstrip patch antenna using H-shaped gap-coupling with three parasitic radiators to achieve the wide E-plane beamwidth property with horizontal polarization at the K-band of 24-24.25 GHz, which is allocated for the radar application. Side and upper parasitic elements work as radiators resonating at λ and λ/2, respectively, producing the wide-beamwidth characteristic. The proposed antenna geometry with a planar single substrate contributes to low profile, easy fabrication, and low cost.

Antenna Design
The single microstrip patch antenna as the main patch is used for easy integration, being fed by a conducting strip directly connected to the edge of the patch. To widen the E-plane beamwidth, additional patches are required. Two side parasitic patches, of which lengths are twice the length of the main patch, are implemented, and the upper halves of both side patches contribute to the wide E-plane beamwidth. For wider beamwidth, the upper parasitic patch between both side parasitic patches and above the main path is inserted. Consequently, the H-shaped gap-coupled patch antenna is proposed. The geometry of the proposed H-shaped gapcoupled patch antenna, including both side and the upper parasitic patches, was designed on the microwave substrate RO4350B with a thickness of 10 mil and a relative dielectric constant ε r of 3.48 using commercial Electromagnetic Simulation Software (CST Microwave Studio). Figure 1 shows the simplified concept of the H-shaped gap-coupled antenna for the wide E-plane beamwidth. The synthesis of the beam patterns of the type A and B antennas gives rise to the wide E-plane beamwidth characteristic of the proposed antenna. (Type A). Each side parasitic square patch with approximate one wavelength is implemented on the same layer of the main square patch. The parasitic elements serve as λ resonated radiators, and the type A antenna includes the main and both side patches, as depicted in Figure 1. The parasitic element of the antenna is primarily magnetically coupled with the feed patch. Figures 2(a) and 2(b) display the radiation patterns and their synthesis produced by four magnetic currents, and Figures 2(c)-2(e) show the E-plane radiation pattern, surface current, and Efield distributions of the type A antenna at 24.1 GHz in simulation, respectively. The lower half of both λ resonated radiators has the same surface current direction as the main patch, and the upper half has the opposite direction. The principle of widening E-plane beamwidth can be explained by the synthesis of the radiation patterns generated by four magnetic currents, J m,m1 , J m,m2 , J m,p1 , and J m,p2 , in the main and parasitic patches. The magnetic currents J m,m1 , J m,m2 , J m,p1 , and J m,p2 are placed with the distance of 0.24λ o on y -axis, and the magnitude of J m,p1,2 is approximately 0.45 times that of J m,m . The magnetic currents J m,p1 and J m,p2 are placed with the distance of 0.385λ o on x-axis. Only considering the peak antenna gain caused by J m,p1 and J m,p2 , the magnitude of the equivalent magnetic current in the middle of both magnetic currents is corresponding to approximately 0.9 times that of J m,m from array factor calculation. The wide-beamwidth property is obtained by the synthesis of the Δ beam pattern generated by J m,m1 and J m,p1,2 with the

Main and Upper Patches (Type B).
Additional radiation is required in the main lobe direction of the positive angle in the elevation plane to widen the beamwidth. Therefore, an H-shaped gap-coupled patch antenna is introduced that includes the type A antenna and an additional upper parasitic square patch element. The upper parasitic patch with a length of approximately a half wavelength operates as a λ/2 resonated radiator like the main one. The parasitic element of the antenna is dominantly electrically coupled with the feed patch. Figures 3(a) and 3(b) illustrate the simulated E-plane radiation pattern and surface current of the type B antenna, respectively. The type B antenna has a peak gain of 7.01 dBi, E-plane HPBW of 58.3°, and main lobe direction of 31°in the elevation plane at 24.1 GHz. The type B antenna has the main lobe direction of a positive angle but a high side-lobe level in the elevation plane. The sidelobe of the antenna is caused by surface currents with approximately 180°out of phase, operating at TM 02 mode in terms of phase. Due to the side-lobe, the H-shaped gap-coupled antenna geometry-comprising the main, upper, and both side patches-has no effect on the enhancement of the E-plane beamwidth. Therefore, the side-lobe of the type B antenna must be removed for the wide-beamwidth characteristic of the entire antenna with H-shaped gap-coupling.
For the phase delay (except for phase reversal) of the upper parasitic patch compared with the fed patch and the wide E-plane beamwidth property of the proposed Hshaped gap-coupled antenna employing type A and B antennas, the proposed H-shaped gap-coupled antenna has been optimized by CST Microwave Studio with the parameters of the length of the upper patch and the gap between the upper and main patches. Consequently, the length of the upper parasitic patch is reduced by 0.1 mm, corresponding to 0.014λ g , changing the electrical length compared with the driven patch. Figures 3(c)-3(e) illustrate the simulated E-plane radiation pattern and distributions of surface current and E-field of the revised type B antenna. Note that the side-lobe of the previous type B antenna is eliminated. In simulation, it has the peak gain of 7 dBi, E-plane HPBW of 71.9°, and main lobe direction of 19°in the elevation plane at 24.1 GHz.

Proposed H-Shaped Gap-Coupled Patch Antenna.
The design procedure of the proposed H-shaped gap-coupled antenna is as follows: Firstly, a conventional microstrip square patch antenna was designed for the K-band of 24-24.25 GHz. The designed sole main microstrip patch antenna has the peak gain of 6.41 dBi and HPBW of 78.2°a nd 75.9°in the E-and H-planes, respectively. Secondly, both side parasitic square patches, working as λ resonated radiators, were added to the main patch with coupling gaps between the main and side parasitic patches (type A antenna). The spacings between the fed and side patches were optimized for the wide E-plane beamwidth property, as shown in Figure 4. Finally, the upper parasitic square patch, operating as a λ/2 resonated radiator, was added to     International Journal of RF and Microwave Computer-Aided Engineering the type A antenna with a coupling gap between the main and upper patches and coupling gaps between the upper and side patches (H-shaped gap-coupled antenna). The proposed H-shaped gap-coupled antenna employing the type A and B antennas together has gap-couplings of "driven patchboth side patches," "driven patch-upper patch," and "upper patch-both side patches." The spacings between the fed and upper patches between the fed/upper and side patches were optimized for the wider E-plane beamwidth property compared with the type A antenna, as illustrated in Figure 5. Figure 6 shows the designed H-shaped gapcoupled antenna for the wide E-plane beamwidth characteristic. The dimensions of the H-shaped gap-coupled patch antenna are tabulated in Table 2. Figure 7 illustrates the simulated E-plane radiation pattern and distributions of surface current and E-field of the proposed wide-beamwidth antenna, which are the superimposed results of the type A and revised type B antennas as depicted in Figures 2(d) and 2(e) and 3(d) and 3(e). The proposed H-shaped gapcoupled antenna provides the peak gain of 5.21 dBi and HPBW of 127.4°and 94°in the E-and H-planes at 24.1 GHz in simulation. The lower half of both λ resonated radiators (side patches) has the same surface current direction as the main patch, and the upper half has the opposite direction. Moreover, λ/2 resonated radiator (upper patch) has phase delay compared with the main patch. The designed planar microstrip antenna can be analyzed using cavity model [1,39,40]. As shown in Figures 7(c) and 7(d), considering the E-field intensity, the cavity model of the proposed antenna is depicted with slots S1-S5. When φ = 0, the radiation pattern in yz-plane can be derived as follows [41]: where k is the wave number in free space, l d is the length of the side patch, l g is the gap between the main and upper patches, β is the difference in phase excitation compared with the slot 2 (β 2 = π, Figures 2 and 3), θ and φ are the polar and azimuth angles in the coordinated system, E 0 is the maximum amplitude of the electric field, and h and W are the thickness of the substrate and the width of the patch, respectively. The volume of the proposed H-shaped gap-coupled antenna, including the ground plane, becomes 1:24λ o × 1:30λ o × 0:0204λ o (λ o : free-space wavelength). The size reduction can be expected by replacing each patch with a microstrip magnetic dipole antenna consisting of a patch and a metal wall.  10 Spacing between the fed and side patches S 1 S1 = 0.05 mm S1 = 0.10 mm S1 = 0.15 mm S1 = 0.20 mm S1 = 0.25 mm

Measurement Results
To measure the radiation pattern and return loss, the widebeamwidth antenna, designed using microstrip line feed with a planar single-substrate structure, is required for an SMA connector. When the tip of the SMA connector is aligned with the signal microstrip line, the body of the SMA connector affects the beam pattern of the antenna due to a relatively short distance from the radiator to the body. To reduce the influence of the body, the proposed wide-beamwidth antenna was designed and fabricated, con-sidering the probe feed to the signal microstrip line, as depicted in Figure 8(a). As a result, the size of the ground plane was enlarged. The radiation pattern of the introduced antenna was measured in anechoic chamber. Figure 9 shows the measurement set-up using compact antenna test range for the radiation pattern.
The simulated and measured return losses, radiation patterns, gains, and efficiencies are illustrated in Figures 8(b)    The simulated gains and E-plane HPBWs are 5.56 dBi and 137.9°, respectively, at the center frequency of 24.1 GHz. The measured gain and E-plane HPBW were 4.82 dBi and 143.4°, respectively, at a center frequency of 24.1 GHz. The maximum detection range for a radar system can be determined as follows [42]: where P T , G T , G R , λ, σ, and S min are the transmit power, transmit antenna gain, receive antenna gain, operating

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International Journal of RF and Microwave Computer-Aided Engineering wavelength, radar cross-section value, and receiver sensitivity, respectively. When a radar system requires higher antenna gain, an array adopting the proposed antenna can be used, while maintaining the wide E-plane beamwidth property.
The simulated and measured antenna efficiencies were 79.1% and 75.3%, respectively, at a center frequency of 24.1 GHz. The measured results agree reasonably well with the simulated results. The discrepancies in simulated and measured results for gains and HPBWs would be mainly attributed to manufacturing tolerances and dielectric loss at the K-band. The fabricated antenna, which uses the SMA connector, has a wider E-plane HPBW compared with the proposed antenna described in Section 2.3 with the ideal port, due to the larger size of its ground plane.
The ratio of the maximum copolarization to crosspolarization radiation levels (the co-to-cross-pol ratio) is used to evaluate the polarization purity. Within the beamwidth of interest, the co-to-cross-pol ratio can be calculated from [43] Co − to − cross − pol ratio = maximum copolarization radiation level maximum cross − polarization radiation level : The measured co-to-cross-pol ratios in the E-and Hplanes were 19.8 and 17.3 dB over the entire angular range and 22.7 and 20.2 dB within the HPBW, respectively. The measured co-to-cross-pol ratios are acceptable, considering the cross-polarization of the conventional microstrip antenna of approximately 20 dB in the H-plane due to the effect of transverse current [44]. Table 3 shows a performance comparison of the previous reported K-band wide-beamwidth antennas in terms of the total volume, including ground plane size, HPBW, FBW, gain, front-to-back ratio (FTBR), and figures-ofmerit (FOMs). Several FOMs are defined by HPBW and gain, related to the antenna's total volume with the ground plane size [25]. FOM 1 , FOM 2 , and FOM 3 are defined as   Figure 9: Measurement set-up using compact antenna test range.

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International Journal of RF and Microwave Computer-Aided Engineering  13 International Journal of RF and Microwave Computer-Aided Engineering keeping a planar single substrate, which results in easy fabrication and low cost compared with other antenna structures requiring multilayer fabrication process.

Conclusion
This article proposed and fabricated a new wide E-plane beamwidth patch antenna that introduces H-shaped gapcoupling with three parasitic elements for the K-band of 24-24.25 GHz. The side and upper parasitic elements operate as λ and λ/2 resonators gap-coupled by the driven rectangular patch. The wide E-plane beamwidth property was achieved by synthesizing the radiation patterns of the total radiators. The proposed antenna with a planar single-substrate structure contributes to low profile, easy fabrication, and low cost. The measured co-to-cross-pol ratios for the E-and H-planes are reasonable. The designed antenna illustrates excellent FOMs compared with the other wide-beamwidth antennas for the K-band. Furthermore, the size reduction can be anticipated by replacing each patch with a microstrip magnetic dipole antenna including a patch and a metal wall.

Data Availability
Data are available upon request.

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