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J. Electromagn. Eng. Sci > Volume 24(5); 2024 > Article
Sung: Novel Multi-Band SIW Antenna with Filtering Function

Abstract

In this paper, the basic structure of a quad-band substrate-integrated waveguide (SIW) antenna with filtering characteristics is proposed. The proposed antenna consists of a four-slot SIW and a single-probe feed. Despite having a single feed, it was able to independently control the four resonant frequencies. However, since a basic quad-band SIW antenna has three radiation null frequencies, the left skirt characteristics of f1 and the right characteristics of f4 were not good. To improve this, four pairs of vias and a pair of U-slots were introduced to add two radiation null frequencies to the lower part of f1 and the upper part of f4, respectively. This time, the parameters of the basic structure did not change, except for those of the added structure. The fabricated antenna operated at 3.9, 4.03, 4.2, and 4.3 GHz, and exhibited a radiation null below −15.6 dBi at 3.6, 3.95, 4.09, 4.27, and 5.07 GHz.

Introduction

With the rapid development of wireless mobile communication, a method that simultaneously supports various services on a single device is preferred [1]. However, devices are becoming slimmer over time. In the early days, many antennas had to be used to match a large number of service bands. Owing to its relatively easy design, dual-band antennas reduced the number of antennas to half. Nonetheless, among microwave components, the antenna is one of the most difficult to integrate and usually has a large size, due to which it often encounters issues related to the unavailability of space. The multi-band antenna is one of the ways in which this problem can be solved [2, 3].
Table 1 summarizes the characteristics of triple- and quad-band antennas based on their substrate-integrated waveguide (SIW) structure. Since the SIW structure is easy to manufacture using a thin dielectric substrate, a substantial amount of research has been conducted on it. Among SIWs, half-mode SIW (HMSIW) and quarter-mode SIW (QMSIW) cavity resonators have been found to be suitable for multi-band antenna applications, since they have the ability to reduce antenna size by at least 50% [4, 5].
Furthermore, triple- or quad-band antennas using the SIW structure have been proposed for self-triplexing [68] and self-quadruplexing applications [911]. Generally, a quadruplexer system combines a single-feed quad-band antenna and a quadruplexer. In contrast, self-quadruplexing antennas consist of four feed structures. An advantage of this structure is that it does not require a multiplexer, considering that different frequency bands use different ports [9].
A multi-band SIW antenna divides a square, rectangular, or—as in the case of [9]—a circular patch into independent regions by properly arranging vias.
Notably, the antenna size in [8] and [11] was large because they used an SIW structure instead of HMSIW or QMSIW. In contrast, since the eighth-mode SIW cavity was used in [6], the antenna size was very small. In HMSIW and QMSIW cavity resonators, the boundary between two adjacent resonators is composed of via array. As a result, it exhibits a short impedance, which weakens the E-field at the boundary. This structure suppresses the E-field from passing through the adjacent antenna. Accordingly, a stable isolation of approximately 20 dB can be easily obtained without extra effort [1214]. However, isolation of 60 dB or more between ports is generally required by communication systems [15]. Therefore, it is difficult to establish that these antennas do not need a multiplexer since another filter is required after each port to satisfy the high isolation characteristics.
Only a few studies presenting multi-band SIW antennas with a single feed have been published so far [16, 17]. However, the triple-band antenna proposed in [16] displays different radiation patterns at three resonant frequencies. Meanwhile, the antenna proposed in [17] exhibits dual-band characteristics and therefore is not suitable as a multi-band antenna.
The structure proposed in this paper is a quadruple-band antenna based on a single feed, while that considered in [1820] is a quadruple-band antenna based on four different feeds. In other words, there is a difference between these studies with regard to the implementation method of the quadruple band antenna. Furthermore, since the structures considered in [1820] do not have filtering functions, the proposed antenna is considered functionally superior.
In this study, a quad-band antenna is constructed by applying four slots to a SIW structure, which is then surrounded by three side vias. While a multi-band SIW antenna generally uses a multi-port, the antenna proposed in this study uses a single-feed structure. The proposed structure has three remarkable features. First, despite its single-feed structure, the four resonant frequencies of the antenna can be independently controlled, which is uncommon. Second, the difference between the two phenomena considered in this study (resonant frequency and radiation null frequency) is analyzed through electric field distribution. Third, a pair of vias is added next to each slot to improve the left-skirt characteristics of f1, and a pair of U-slots is applied to improve the right skirt characteristics of f4. The aim of the proposed filtenna, or filtering antenna, is to achieve superior out-of-band gain suppression and flat in-band gain so that the need for using the band-pass filter at the RF front end can be removed.

Single Antenna Design

Fig. 1 shows two SIW antennas. In Model 1, a short boundary condition is set on three sides, with the upper side of the patch set as an open boundary condition by removing the vias. Model 2 shows a short boundary condition by applying vias to all four sides, representing a conventional SIW antenna. To compare the antenna characteristics in detail, all parameters were kept identical.
Fig. 2 presents a comparison of the simulated reflection coefficients of the two antennas presented in Fig. 1. The antenna parameters for the simulation were set as follows: L = 35.2 mm, W = 40.8 mm, df = 19.2 mm, p = 1 mm, and r = 0.3 mm. Rogers 5880 (ɛr = 2.2 and h = 1.57 mm) was used as the substrate for the antenna design. As is well known, a conventional SIW antenna exhibits a single resonance at 3.66 GHz, and an open-ended SIW antenna exhibits dual resonance (2.74 GHz and 3.77 GHz).
To analyze the resonance mode, the E-field distribution at the resonance frequency was examined. The maximum and minimum values in the E-field distribution were set equally, with a brighter E-field representing a stronger E-field, and a darker E-field indicating a weaker E-field. The E-field distribution at 3.77 GHz, as shown in Fig. 2, confirms the existence of an E-field on the open wall, although its amplitude is very weak. As a result, it can be considered the same as the fundamental mode of a conventional SIW antenna. In contrast, in the case of the open-ended SIW antenna, another resonance is formed at 2.74 GHz. The electric field distribution implies that this resonance was formed by radiation from the upper side of the patch. Notably, this characteristic is similar to that of an open waveguide antenna.
Fig. 3 shows a comparison of the simulated radiation patterns of the two antennas on the yz-plane. The radiation pattern of Model 1 at f2 coincides with that of the conventional SIW antenna. In the case of the radiation pattern at f1, a weak null occurs at θ = 0° because, as seen in the E-field distribution, strong E-fields form on the upper edge of the patch and the slot, which cancel each other out at θ = 0°. Therefore, considering the impedance matching and radiation pattern of Model 1 at f1, it is not used as the operating frequency in this study. However, a radiation null was formed between 2.74 GHz and 3.77 GHz because of this resonance. Therefore, it will be used with regard to the left skirt characteristics of f1 of the quad-band antenna.

Quad-Band Antenna Design

Fig. 4 depicts the quad-band SIW antenna with the filtering function proposed in this study. In the proposed antenna, each slot forms an independent resonance. The length of slot #i is set to Li and the width is set to Wi, with the distance between the feed point and the slot determining the resonant frequency. Accordingly, slot #1—the farthest from the feed point—forms the lowest f1, while slot #4, located closest to the feed point, forms the highest f4. The distance between slot #i and the feed point is set to di. To simplify the analysis without increasing the design parameters, the spacing between the slots is set to 2.8 mm. Initially, the length Li and width Wi of the four slots are set to be the same, although Li is set slightly differently through a parameter study.
Fig. 5 shows the simulated reflection coefficient when the slot length is changed by 1 mm. The results indicate that L1 forms f1, L2 forms f2, L3 forms f3, and L4 forms f4, with the resonant frequency independently controlled by adjusting each length. Notably, in Fig. 5(c), when L3 is 25.7 mm, f4 changes. This is different from the fact that the resonant frequency can be adjusted independently because L3 decreases, f3 increases, which pushes f4 up. Furthermore, since the proposed quad-band antenna is designed to use the same resonance structure, its structure is very simple compared to other quad-band antennas. In addition, the size of the proposed quad-band antenna is not large because the spacing between slots is very small (2.8 mm).
Fig. 6 depicts the simulated E-field distributions formed on the yz-plane. Simulations were performed at the four resonant frequencies of the proposed antenna.
The left and right sides of the slots exhibit a phase difference of 180°. On the left side of the slot, a vertical E-field forms in the upward direction. Meanwhile, the right side of the slot exhibits the formation of a vertical E-field in the downward direction. As a result, an E-field is formed in the horizontal direction at the top of the slot, as shown in Fig. 6. Drawing on this E-field distribution, the broadside radiation pattern at θ = 0° can be obtained.
Figs. 79 display the simulated vector E-field and surface current distributions at radiation null frequencies. The formation principle of the radiation null is as follows. As observed in the E-field distribution, the in-phase E-fields formed in two slots are canceled out at the midpoint, due to which the radiation pattern has a null characteristic at θ = 0°. For example, at the second radiation null frequency, the null was formed by slots #2 and #3. As mentioned above, the second radiation null frequency formed between the resonant frequencies formed by slots #2 and #3. Accordingly, as shown in Fig. 8(a), the radiation occurs in slots #2 and #3. Furthermore, Fig. 8(b) shows that the E-field from the substrate is formed vertically upward near the two slots. As a result, the E-field formed at the upper part of the midpoint of the two slots is directed downward, indicating that the radiation pattern is null at θ = 0°.
Fig. 10 shows the simulated and measured reflection coefficients and gains. The simulation and measurement results are observed to be largely consistent. With regard to the measured results, the proposed quad-band antenna exhibits resonance characteristics at 3.745, 4.023, 4.197, and 4.323 GHz. Since the proposed antenna exhibits a broadside radiation pattern at resonant frequencies, the gain was valued at φ = θ = 0°. The antenna gain is 1.6, 2.2, 3.6, and 4.3 dBi at the four resonant frequencies, and radiation nulls of −15 dB or more are observed at 3.81, 4.06, and 4.27 GHz. Fig. 11 shows the measured and simulated radiation patterns at the four resonant frequencies.

Quad-Band Antenna with Improved Filtering Performance

The quad-band antenna presented in Fig. 4 has four slots, each independently forming its own resonance, and three radiation nulls between the four resonant frequencies, indicating its filtering characteristics. However, the skirt characteristics on the left side of f1 and on the right side of f4 were found to be unfavorable. To improve these characteristics, four pairs of vias and a pair of U-slots were added to the antenna presented in Fig. 4, as shown in Fig. 12. Fig. 13 shows the fabricated quad-band antenna with improved filtering property.
A pair of vias was added to each side of the four slots by s1. As a result, a radiation null formed below f1, thus improving the left skirt characteristics of f1. Furthermore, a pair of U-slots was implemented between slot #4 and the feed point. This led to the formation of a radiation null on the upper side of f4, in turn improving the characteristics of the right skirt of f4. To ensure an accurate comparison, all other parameters were kept identical.
To verify the principle of the two newly formed radiation nulls, the reflection coefficients of three antennas (the antenna without the four pairs of vias, the antenna without a pair of U-slots, and the antenna shown in Fig. 12) were compared, as shown in Fig. 14. For accurate comparison, all parameters, except for the newly added four pairs of vias and the pair of U-slots, were kept the same: La = 4.4 mm, Lb = 14.4 mm, Lc = 7 mm, s1 = 0.4 mm, s2 = 3.6 mm, and s3 = 6 mm. The results presented in Fig. 14 confirm that all three antennas maintained the four resonance characteristics, and the matching characteristics did not change significantly. Fig. 14(b) compares the simulated gains of the three antennas. All three antennas exhibit three radiation nulls (fnull,2, fnull,3, fnull,4), with fnull,2, fnull,3, and fnull,4 referring to the three frequencies of the radiation null presented in Figs. 79. Notably, fnull,1 refers to the frequency of the radiation null formed by the vias located next to the slot while fnull,5 is the frequency of the radiation null formed by the U-slots.
As shown in Fig. 2, the removal of the via array on top of the patch made it an open wall. As a result, radiation was observed in this part, attaining a frequency of 2.74 GHz. For the same reason, the antenna without the four pairs of vias formed a weak resonance at 2.87 GHz. With the addition of each pair of via to the slot, the resonant frequency increased from 2.87 GHz to 3.56 GHz. Accordingly, the fnull,1 increased from 3.03 GHz to 3.62 GHz. This points to an improvement in the skirt characteristics on the left side of f1, as shown in Fig. 14(b). Furthermore, due to this phenomenon (the resonant frequency increasing from 2.87 GHz to 3.57 GHz as a result of the open wall), the resonant frequency of f1 increased from 3.6 GHz to 3.8 GHz.
In the final antenna, weak resonance was observed at 5.14 GHz. The antenna without vias exhibited a weak resonance at 5.04 GHz caused by the radiation formed in the U-slot. As shown in Fig. 14(a), in the case of the antenna without the U-slot, the resonance disappears near 5 GHz. This resulted in the final antenna and the antenna without vias forming radiation nulls at 5.1 GHz and 4.9 GHz, respectively, indicating an improvement in the right skirt characteristic of f4. Furthermore, as the total length of the U-slot (La + Lb+ Lc) increased, the resonant frequency of the 5 GHz band decreased, and the skirt characteristics improved further. Therefore, the radiation null frequency is closely related to the length of the U-slot [21], as expressed by the following equation:
fnull,4=C2Ltotalɛeff,
where Ltotal = La + Lb + Lc.

Conclusion

This study proposes a single-feed quad-band antenna. The proposed structure has three remarkable features. First, despite its single-feed structure, the antenna's four resonant frequencies could be independently controlled. Second, six slots were added to a deca-band antenna, keeping the other parameters unchanged, except for an increase in the length of the patch due to the additional slot. The proposed antenna showed good matching characteristics, even without undergoing a tuning process. Third, by adding a pair of vias next to each slot and a pair of U-slots, improvements were observed in the left skirt characteristics of f1 and right skirt characteristics of f4, respectively.

Acknowledgments

This work was supported by a Kyonggi University Research Grant (2022).

Fig. 1
SIW antenna geometry: (a) Model 1 and (b) Model 2.
jees-2024-5-r-244f1.jpg
Fig. 2
Reflection coefficient and E-field distribution of the two SIW antennas.
jees-2024-5-r-244f2.jpg
Fig. 3
Radiation pattern of the two antennas on the yz-plane.
jees-2024-5-r-244f3.jpg
Fig. 4
Quad-band SIW antenna with filtering function.
jees-2024-5-r-244f4.jpg
Fig. 5
Simulated results: (a) L1, (b) L2, (c) L3, and (d) L4.
jees-2024-5-r-244f5.jpg
Fig. 6
Simulated vectors of E-field on the yz-plane at (a) f1, (b) f2, (c) f3, and (d) f4.
jees-2024-5-r-244f6.jpg
Fig. 7
Simulated vectors of (a) surface current on the xy-plane and (b) E-field on the yz-plane at the first radiation null.
jees-2024-5-r-244f7.jpg
Fig. 8
Simulated vectors of (a) surface current on the xy-plane and (b) E-field on the yz-plane at the second radiation null.
jees-2024-5-r-244f8.jpg
Fig. 9
Simulated vectors of (a) surface current on the xy-plane and (b) E-field on the yz-plane at the third radiation null.
jees-2024-5-r-244f9.jpg
Fig. 10
Simulated and measured results.
jees-2024-5-r-244f10.jpg
Fig. 11
Simulated and measured radiation patterns: (a, b) E-plane and H-plane at f1, (c, d) E-plane and H-plane at f2, (e, f) E-plane and H-plane at f3, and (g, h) E-plane and H-plane at f4.
jees-2024-5-r-244f11.jpg
Fig. 12
Proposed quad-band antenna with improved filtering property.
jees-2024-5-r-244f12.jpg
Fig. 13
Fabricated quad-band antenna with improved filtering property.
jees-2024-5-r-244f13.jpg
Fig. 14
Measured and simulated antenna performance: (a) reflection coefficient and (b) gain.
jees-2024-5-r-244f14.jpg
Table 1
Comparison of the proposed antenna performance with that of existing references
Study Freq. (GHz) Isolation (dB) Size Independent control Filtering function
Priya and Dwari [6] 4.95 / 5.3 / 5.9 20.5 0.08λ02
Kumar and Raghavan [7] 6.53 / 7.65 / 9.1 18.1 0.42λ02
Chaturvedi et al. [8] 4.2 / 5.2 / 5.8 23 1.2λ0 × 0.9λ0
Iqbal et al. [9] 3.5 / 4.9 / 5.4 / 5.8 23 0.32 λ02
Priya et al. [10] 8.2 / 8.8 / 9.7 / 11 24.7 0.9λ0 × 0.9λ0
Kumar et al. [11] 8.8 / 10.4 / 11.4 / 12.3 26 1.17λ0 × 1.17λ0
Guan et al. [16] 4.4 / 7.8 / 9.5 Single-feed 0.59λ0 × 0.59λ0
Niu et al. [17] 3.27 / 3.76 Single-feed N/A
Kumar [18] 3.5 / 5.2 / 5.5 / 5.8 23.6 0.45λ0 × 0.45λ0
Chaturvedi et al. [19] 5.2 / 5.5 / 5.8 / 6.2 / 6.8 20 0.9λ0 × 0.7λ0
Chaturvedi and Kumar [20] 2.45 / 3.5 33 0.48λ0 × 0.4λ0
This work 3.9 / 4.03 / 4.2 / 4.3 Single-feed 0.45λ0 × 0.45λ0

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Biography

jees-2024-5-r-244i1.jpg
Youngje Sung, https://orcid.org/0000-0002-9310-7450 was born in Incheon, Korea, in 1975. He received his B.S., M.S., and Ph.D. degrees from Korea University, Seoul, Korea, in 2000, 2002, and 2005, respectively. From 2005 to 2008, he was a senior engineer working in the Antenna R&D Laboratory, Mobile Phone Division, Samsung Electronics, Korea. In 2008, he joined the Department of Electronic Engineering, Kyonggi University, Suwon, Korea, where he is currently a professor. His research interests include reconfigurable antennas, cellphone antennas, wideband slot antennas, multifunction devices, compact circular polarized antennas, and compact dual-mode filters. Prof. Sung serves as a reviewer for IEEE Transactions on Microwave Theory and Techniques, IEEE Transactions on Antennas and Propagation, IEEE Microwave and Wireless Components Letters, IEEE Antennas and Wireless Propagation Letters, Progress in Electromagnetic Research, IET Electronics Letters, and IET Microwaves, Antennas and Propagation.

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