Bandwidth Enhancement on Half-Mode Substrate Integrated Waveguide Antenna Using Cavity-Backed Triangular Slot

This paper proposes bandwidth enhancement of a cavity-backed slot antenna using a triangular slot on a half-mode substrate integrated waveguide structure antenna. The bandwidth enhancement was achieved by combining the ﬁxed TE 101 and the downward shifting TE 102 modes, resulting in hybrid modes. The design evolution of the slot antenna from a half nonresonating rectangular slot to a triangular slot antenna increased the fractional bandwidth. The simulation result showed that fractional bandwidth increased from 6.27% to 9.1%. It was conﬁrmed by measurement that the fractional bandwidth of 9.87% was achieved which reﬂects a 350MHz bandwidth with center frequency at 3.84GHz. The measured gain at center frequency was 4.2dBi. It is shown that the radiation characteristics obtained from both measurement and simulation results are in very good agreement.


Introduction
Cavity-backed slot (CBS) antennas using the substrate integrated waveguide (SIW) technology provide easy integration in the planar form and enable low-cost fabrication for various applications. To meet the requirements of various applications, researchers have proposed miniaturization, bandwidth enhancement, circular polarization, gain improvement, dual band, and self-diplexing. e miniaturization of the SIW-CBS antenna was proposed in [1][2][3][4]. e miniaturization of the SIW-CBS antenna can vertically cut the magnetic field of the full-mode SIW (FMSIW) into two parts of the half-mode substrate integrated waveguide (HMSIW) structure, as shown in [1][2][3][4]. By using the HMSIW structure, 50% miniaturization of the antenna was obtained; however, all the fractional bandwidths in [1][2][3][4] were below 7%.
Bandwidth enhancements for the SIW-CBS antenna have been proposed in [5][6][7][8]. Yun et al. [5] proposed substrate removal under the slot radiator, which successfully enhanced the impedance bandwidth although the bandwidth increased only up to 2.16% of the fractional bandwidth. Researchers have also implemented a hybrid mode or a multiresonance mode to enhance the impedance bandwidth, as shown in [6][7][8]. A shorting via above the slot creates double resonance, which was implemented to enhance the bandwidth [6]. A 3.35% increase in the fractional bandwidth was achieved using this technique; however, tuning the shorting via location was not an easy task. Luo et al. [7] and Mukherjee et al. [8] presented hybrid resonance using the half cavity of two adjacent resonances between TE 101 and TE 102 while TE 101 was kept as the first mode. However, the fractional bandwidths are still narrow.
In this paper, bandwidth enhancement is proposed using a triangular slot on the HMSIW structure. e triangular slot was etched on the ground layer. e position, length, and width of the triangular slot will be varied to generate the first mode TE 101 and the second mode TE 102 closer to each other.
eir appropriate combination will result in impedance matching for a hybrid resonance by keeping the first mode TE 101 fixed and the second mode TE 102 shifting downward frequency. is provides a significant advantage because the mixing of HMSIW structure together with the triangular slot could effectively enhance the bandwidth to reduce antenna dimensions. Furthermore, the proposed antenna could be used for the fifth-generation telecommunications where its application needs broad bandwidth characteristic and compact design antenna.

Antenna Design
Bandwidth enhancement for a cavity backed by a triangular slot antenna in the HMSIW structure is proposed using a geometric structure, as shown in Figure 1. e triangular slot was etched on the ground layer that acted as a nonresonating slot. e top layer acted as a metallic reflector. e triangular slot excited the first mode TE 101 at 3.8 GHz and the second mode TE 102 at 3.98 GHz.
eir impedance matched the frequency range as a hybrid resonance. e geometry evolution to achieve the proposed antenna geometry shown in Figure 1 is depicted in Figures 2(a)-2(c). In the first step, the FMSIW cavity structure with rectangular slot on the ground plane was used, as shown in Figure 2(a). e resonant frequency for the TE mnp mode was calculated using the following equation [9]: where for the first mode TE 101, m � 1, n � 0, and p � 1; c is the velocity of light in free space; h is the substrate thickness; and µ r and e r are the relative permeability and relative permittivity of the substrate, respectively. In fact, only the TE m0p modes can resonate in the rectangular SIW and TE m0 can be propagated in the SIW waveguide because the transverse magnetic (TM) modes are not supported in the SIW technology owing to the noncontinuous walls [10][11][12]. e effective length (L eff ) and effective width (W eff ) of the rectangular cavity are determined as follows [13]: where L c and W c are the length and width, respectively, of a rectangular cavity; D is the diameter of the embedded metallic cylinders; S is the distance between the two adjacent center points of the embedded metallic cylinders, and λ 0 is the free space wavelength. e conditions S/D ≤ 2 and λ 0 / D ≥ 10 should be considered to minimize the leakage energy. e antenna design uses Rogers RT/Duroid 5880 with dielectric relative permittivity ε r � 2.2, loss tangent δ � 0.0009, and substrate thickness h � 1.575 mm. e singleresonant FMSIW structure has one drawback: it has a narrow bandwidth. e second step of antenna evolution involved miniaturizing and bandwidth enhancement of the SIW-CBS antenna. e miniaturization was achieved by cutting the magnetic wall vertically and making it an HMSIW-CBS antenna, as shown in Figure 2(b). Bandwidth enhancement was achieved using hybrid modes between TE 101 and TE 102 . e hybrid modes were achieved by dividing the HMSIW patch into a quarter patch of TE 101 and TE 102 . ese two modes could be excited by varying the slot length and slot position of the nonresonating slot by a length more than a quarter wavelength. Finally, the bandwidth of the HMSIW-CBS antenna could be enhanced by flaring the rectangular nonresonating slot into a triangular slot, as shown in Figure 2(c).

Results and Discussion
3.1. Simulation Study. As depicted in Figure 2, the radiation characteristics of each step evolution are displayed, and simulation study using Ansys HFSS software is considered. Figure 3 shows comparison results of the reflection coefficient for the antenna evolution shown in Figure 2. e fractional bandwidth of FMSIW-CBS is only 1.81% (70 MHz) for 3.86 GHz of center frequency. Furthermore, the fractional bandwidth of HMSIW increases to 6.27% (240 MHz). is HMSIW structure prevents the shifting of the resonant frequency to a lower frequency because the electric field radiates across the dielectric aperture by the first mode TE 101 . e modification of nonresonating slot into a triangular slot will cause more impedance bandwidth improvement up to 350 MHz, which has 9.1% for fractional bandwidth. It means that the proposed antenna fractional bandwidth increases five times higher than the FMSIW-CBS antenna.
e bandwidth enhancement for a cavity backed by a triangular slot antenna in the HMSIW structure can be explained from the real impedance simulation result, as displayed in Figure 4. It is shown that the first mode TE 101 and the second mode TE 102 are close to each other. e hybrid resonances between TE 101 and TE 102 will enhance the impedance bandwidth. Hybrid resonance occurs without changing the first resonant mode TE 101 . e fractional bandwidth was improved to 9.1%. In the real impedance plot, it is shown that the operating band has a wideband as compared with other operating bands.
In addition, the simulated H-field vector distribution corresponds to a combination between the first mode TE 101 and the second mode TE 102 , as shown in Figure 5. e simulated H-field vector distributions are illustrated with the same scale of 0-100 A/m. e H-field vector distribution is separated by the triangular slot into a lower quarter cavity part and an upper quarter cavity part. Figure 5(a) shows the simulated H-field vector distribution at 3.8 GHz; the field distributions in both parts are out of phase. e dominant field distribution of the H-field vector is located in the lower cavity. Figure 5(b) shows the simulated H-field vector distribution at 3.98 GHz. e field distributions in both parts are in the phase indicated by the amount and magnitude of the H-field vector distribution in the upper cavity. e upper quarter cavity itself is more dominant than the lower cavity. e combination of the weak first mode TE 101 and the strong second mode TE 102 results in hybrid-mode resonance at 3.8 GHz, as shown in Figures 5(a) and 6(a). Although the fields distributed in the two cavities are out of phase, their great difference in magnitude can make the triangular slot radiate effectively. Figures 5(b) and 6(b) show that the hybrid-mode resonances at 3.98 GHz are the combination of a strong first mode TE 101 and a weak second mode TE 102 . e magnitudes of the field vector distributions are in phase. As a result, the triangular slot can radiate electromagnetic waves. is investigation follows the methodology given in [7].

Parametric Analysis.
A parametric study of the proposed antenna, as depicted in Figure 1, was conducted to investigate the effects of the position Sp, the width Wt, and the length Lt of the triangular slot on the impedance bandwidth parameter. Figure 7 shows the simulation results for the variations in the reflection coefficient with changes in the position Sp of the triangular slot. As mentioned before, the two hybrid modes depend on the quarter cavity parts separated by the triangular slot position Sp. Two hybrid modes would be achieved between TE 101 and TE 102 if the combination between them is in phase or out of phase; this would depend on the position of the triangular slot. If the triangular slot position Sp moves to the bottom of the cavity, the first resonant frequency will shift to the higher frequency and the second resonant frequency will disappear. Critical investigations on the H-field vector will appear on both sides of the quarter cavity. en, the hybrid modes will disappear and become only a single resonance. is means that the impedance bandwidth will be narrow, and the reflection coefficient will improve. e single resonance shifts into the middle of the impedance bandwidth ranges. e length Lt of the triangular slot was designed as more than a quarter wavelength resonant length of the SIW-CBS conventional antenna. e impedance bandwidth and the improvement in the reflection coefficient also depended on the width Wt and the length Lt of the triangular slot, as shown in Figure 8. Increasing the width of the triangular slot as shown in Figure 8(a) will cause the first and the second mode resonances to shift to higher frequencies. In addition, the reflection coefficient values will be high and the impedance bandwidth will decrease. e triangular slot width Wt of our proposed antenna was chosen to obtain the maximum bandwidth that could be achieved if the slope angle of the triangular slot was 10° [14]. Figure 8(b) reveals that if the triangular slot length increased, the reflection coefficient would be smaller than before. It also causes the impedance bandwidth to become wider than before. e optimum reflection coefficient was chosen to be 25 mm for the triangular slot of length Lt.

Measurement Results.
To confirm the simulation studies, the proposed antenna was then fabricated and measured. e fabrication results are exhibited in Figure 9. A triangular slot was etched on the ground layer, and a quarter transmission line was used to connect the current path. All embedded metallic cylinders connected the top layer and the ground layer. e reflection coefficient of the proposed antenna was validated by the measurement. Figure 10 shows a comparison of the simulation and the measurement of the reflection coefficient. e reflection coefficient for the    ere was a slight discrepancy between the measurement and simulation results, which was caused by the parasitic effect of the antenna that is influenced by the soldering process in the connector, but overall it can be tolerated.  International Journal of Antennas and Propagation e gain simulation and measurement are shown in Figure 10. e gain measurements for the H-plane were 3.89 dBi and 2.89 dBi at 3.72 GHz and 3.98 GHz, respectively. e gain measurement for the E-plane was 4.2 dBi at both frequencies.
e differentiation between simulation and measurement gain occurs because of the measurement condition such as antenna tilt. e simulation and measurement gain that are achieved using the HMSIW structure are not higher than the FMSIW-CBS single resonance because of the HMSIW topology itself. Figure 11 shows the simulation and measurement radiation pattern performance for the antenna design. e measured frequency used 3.72 GHz and 3.98 GHz, which are the antenna-working frequencies. e measured copolarization and cross-polarization far-field radiation pattern used a horn antenna as the reference antenna. e measurements of the normalized copolarization antenna agreed well for the H-plane (phi � 0°) and E-plane (phi � 90°).
e measured copolarization has a unidirectional antenna.
is occurs because the electric field of the antenna design has radiation on the patch and the ground plane, even when the difference between them was approximately 10 dB. e E-plane was approximately 90°for the half-power beamwidth antenna, whereas the H-plane was approximately 120°for the halfpower beamwidth. e cross-polarization level measurements were −3 dB in the E-plane and −8 dB in the H-plane.

International Journal of Antennas and Propagation
High cross-polarization levels occurred because of linear polarization impurities. Finally, Table 1 shows a comparison with previous results related to the miniaturization and bandwidth enhancement of the CBS antenna. e HMSIW structure was an effective method for miniaturization. Approximately 50% miniaturization of the antenna design was achieved using the HMSIW structure without decreasing the fractional bandwidth, and the triangular slot on the ground layer enhances the fractional bandwidth of the CBS antenna. Table 1 reveals that this study has achieved bandwidth enhancement up to 9.87% from the conventional SIW-CBSA with miniaturization dimension compared with other studies. Reference [8] reported bandwidth enhancement similar to that in this study, but the dimension is still large because of the use of FMSIW-CBSA. Gain simulation Gain measurement S11 simulation S11 measurement

Conclusions
In this study, the bandwidth enhancement of a HMSIW-CBS using a triangular slot structure antenna has been studied for simulation and measurement. It shows that bandwidth enhancement was influenced by the position, length, and width of the triangular slot. e appropriate combination of position, length, and width of the slot can generate the hybrid mode between the TE 101 and TE 102 modes, which depends on the phases of TE 101 and TE 102 modes. With the proposed method, an increase in fractional bandwidth of up to 9.87% from the conventional SIW-CBS was achieved. It is also shown that the fractional bandwidth increases up to 5.45 times than the conventional SIW-CBS.

Data Availability
e data used in this study are available upon request to the corresponding author.

Conflicts of Interest
e authors declare no conflicts of interest.