A Filtering Antenna with Slots and Stacked Patch Based on SISL for 5G Communications

Abstract

A filtering antenna based on the Substrate Integrated Suspended Line (SISL) platform applied for the n78 band of 5G is presented in this paper. The antenna has a segmented feed line, a rectangular driven patch etched with a double I-slot, and a squared stacked patch with grooves at the edges of both sides. The etched slots and the stacked patch introduce two new resonance frequencies increasing the bandwidth. Furthermore, the etched slots excite a deep radiation null in the low-frequency band, and the stacked patch coupled with the driven patch produces two deep radiation nulls in the high-frequency band. Three radiation nulls enable high selectivity of the antenna. The filtering antenna works at 3.2–3.89 GHz, which can be applied to the 5G (n78, 3.3–3.8 GHz) frequency band. The peak gain in the band can reach 9.21 dBi, and the out-of-band suppression levels are higher than 18.47 dB.

1. Introduction

Wireless communication is in the era of the coexistence of 4G and 5G, and electromagnetic waves of different frequency bands are easy to interfere with one another. The filtering antenna is able to integrate the antenna and the filter, which can filter the interference signal while receiving and transmitting the electromagnetic wave signal and realize the miniaturization of the 5G communication system.

The traditional approach to design involves creating the filter and antenna separately and then connecting them in series to select the desired frequency band and filter out unwanted signals [1,2,3,4,5]. The filtering circuit or network can also be introduced into the antenna for collaborative design [6,7,8,9]. In [4], the filtering antenna is loaded with a first-order microstrip filter using an open-loop resonator and a third-order hairpin bandpass filter in the RF path to realize the filtering function of the antenna. However, these design methods often result in issues such as increased size, complex structure, added losses, and impedance mismatches in communication systems.

A method of shaping the microstrip patch antenna is proposed to improve these shortcomings. Common methods include etching slots [10,11,12,13,14,15], shorting pins [14,15,16,17,18], parasitic structure or stacked patches [15,17,19,20,21], shorting loops [22], etc. This design method does not require any filter or filtering circuit, which not only simplifies the circuit and solves the problem of insertion loss caused by introducing a filter, but also reduces the size of the antenna and facilitates the miniaturization of the system. For instance, in [11], a method of etching four slots on a single patch was employed to achieve multiple operating modes in the design of a filtering antenna. However, this design method uses a relatively large number of slots, which can easily disturb the current and field distribution on the patch, greatly affecting the resonance mode and increasing the design difficulty. In [14], the joint action of slots, strips, and shorting pins makes the antenna generate a filtering response, but it is relatively sensitive. In [20], the antenna is loaded with two layers of stacked patches of different sizes, and a radiation null appears at each edge of the frequency band to achieve filtering. However, this inevitably enlarges the profile and volume of the antenna. In [21], the antenna uses two coupling slots for feeding, forming multiple coupling paths. However, the slot-coupling may introduce certain radiation and scattering losses, which may increase the antenna’s loss. Therefore, the gain of the antenna is only 4.8 dBi. In [22], the radiation suppression level of the lower higher stopband is ameliorated by introducing a shorting loop and a pair of stubs. However, its complexity and profile are greatly increased. In the above filtering antenna design, some methods will increase the size and complexity of the antenna. Etched slots may have little effect on the patch. A combination of methods is usually used to achieve the required bandwidth and filtering response. In practical designs, not all antenna designs incorporating these structures can achieve filtering characteristics. For example, in [23], the antenna is loaded with stacked patches, etched slots, and pins, but its purpose is to enhance the pattern and obtain multiple operating bands. In [24], the designed antenna uses a crescent-shaped substrate, a rotated L-shaped monopole, and a DGS, but it does not obtain filtering characteristics.

It is worth mentioning that the most commonly used method for analyzing the filtering principle of a filtering antenna is to analyze the field distribution at the radiation nulls. For example, in [16,20,24], the filtering characteristics are explained by analyzing the cancellation of radiation through the electric field or current distribution at each radiation null.

Based on the SISL structure, a variety of filtering antennas that can be applied to different 5G frequency bands are proposed [25,26,27,28,29,30]. For example [26], the antenna uses low-cost FR4 material, which has the advantages of low cost and high gain. The antenna uses differential feeding, which can improve the performance of the antenna, but the use of two feeding ports makes the antenna design more complicated and requires high matching requirements. The SISL structure has the advantages of low loss, small size, lightweight, high integration, and self- packaged. The use of SISL structure can make the design of antenna structure more flexible and diverse. At the same time, low-cost dielectric materials can be used to reduce the antenna cost.

In this paper, a high gain, high selectivity, and coverage of n78 band SISL filtering antenna with etched slots and a stacked patch for 5G systems is proposed. The antenna uses the SISL structure as the design platform. The filtering performance is achieved through loading a stacked patch and two symmetrical I-slots on the edge of the rectangular driven patch. The I-slots can generate a radiation null at the low-frequency edge of the frequency band, and the stacked patch can engender two radiation nulls on the high-frequency side. The suppression level is higher than 18.47 dB.

2. Antenna Devise and Operating Principle

2.1. Configuration

The layout configuration of the filtering antenna is depicted in Figure 1. The proposed antenna uses the SISL structure consisting of five layers of the substrate (substrate1-5) and ten layers of metal (G1–G10) as the design platform. All five layers of substrate are made of FR4 epoxy materials with meagre cost, the thickness of each layer is h₁ of 0.6 mm, h₂ of 2 mm, h₃ of 0.127 mm. The hollow parts of substrate2 and substrate4 are two air cavities, where most of the electric field is distributed, resulting in very low dielectric loss. The core circuit is designed on the dielectric substrate 3, and the dielectric substrate 1 and 5 are utilized as cover plates. The metallized vias surround each layer of substrates. The rivet holes are arranged at the outermost part of the substrates to facilitate the assembly of the physical model.

Figure 2 exhibits the specific configuration and parameters of the antenna on the G6 layer, G2 layer, and G5 layer. As depicted in Figure 2a, a rectangular driven patch is placed on the G5 layer, and a pair of symmetrical longitudinal I-slot (slot1) and a transverse I-slot (slot3) are etched on the driven patch. The square stacked patch with strip groove (slot2) etched on the edge is placed on the G2 layer, as evidenced in Figure 2b. In Figure 2c, a segmented feed line is placed on the G6 layer, and the metallized via (via1) through the substrate3 connects the driven patch with the feed line.

2.2. Antenna Analysis

Figure 3 expresses the design steps of the proposed antenna structure. The simulated S11 and realized gain of the antenna in each step are depicted in Figure 4.

Initially, only the traditional rectangular driven patch is placed on the G5 layer, which is Ant1 in Figure 3a. It is manifest from Figure 4 that Ant1 generates a resonant frequency f₂ at 3.68 GHz, and there is no radiation null. For the purpose of increasing the bandwidth and achieving the filtering response, additional resonances and radiation nulls should be led into. Therefore, the rectangular driven patch is deformed, and a couple of slots (slot1) are symmetrically etched on the conventional rectangular driven patch, as shown in the Ant2 in Figure 3b. It can be seen from Figure 4 that the S11 and realized gain of Ant 2 have changed significantly, and a new resonance frequency point f₁ is generated at 3.3 GHz, but the matching of this point is poor. At this time, f₂ is not easy to be observed because of serious mismatch. A radiation null n₁ is produced at 2.92 GHz.

Then, based on Ant2, the grooved stacked patch on the G2 layer is loaded, which is called Ant3, as shown in Figure 3c. Ant3 generates the third resonant frequency f₃ at 3.9 GHz, and the matching of f₁ and f₂ is improved. Moreover, two new radiation nulls, n₂ and n₃, are generated at 4.34 GHz and 4.78 GHz. The bandwidth of the antenna band has been initially broadened, and the antenna frequency selection feature has been initially realized, but the matching of f₃ is relatively poor, the frequency at the radiation null n₁ moves to the low frequency. The out-of-band suppression level in the low-frequency band has been slightly reduced. The out-of-band suppression level in the high-frequency band is relatively high, but the roll-off rate needs to be improved.

Finally, an I-slot (slot3) is etched on the driven patch for optimal matching to get the proposed antenna, as exhibited in Figure 3d. In Figure 4, the matching of f₃ is significantly improved. Moreover, n₂ move to the lower frequency direction, resulting in high selectivity. The out-of-band suppression level in the low-frequency band has decreased somewhat. The out-of-band suppression level in the high-frequency band is relatively high, but the roll-off rate has been somewhat improved. The simulation impedance bandwidth of the proposed antenna is 18% and peak gain is 8.97 dBi.

In addition, the impedance plot of a given frequency band is given. The impedance plot for the given frequency band is shown in Figure 5. In the frequency band of 3.24–3.88 GHz, the real part of the antenna impedance is around 50 Ω and the imaginary part is around 0, achieving impedance matching. However, at other frequencies, there is impedance mismatch, revealing the antenna’s filtering response.

2.2.1. Three Resonance Frequencies

Figure 6b–d presents the surface current distribution and current path diagram at three resonance frequencies of the proposed antenna to visually compare the change of resonance mode in the design steps.

Since the proposed antenna is developed from the traditional rectangular microstrip antenna, the operating mode of the radiation patch of Ant1 at resonance frequency f₂ is TM₀₁ mode, as expressed in Figure 6a. As exhibited in Figure 6b, the current at the resonance frequency f₁ is mainly distributed around slot1, so mode 1 results from slot1 on the driven patch. The initial mode TM₀₁ of the driven patch at resonance frequency f₂ is plotted in Figure 6c. The resonance frequency f₃ is generated after loading the stacked patch. According to Figure 6d, its operating mode2 at resonance frequency indicates that the driven patch and the stacked patch are coupled together to generate resonance frequency f₃.

2.2.2. Three Radiation Nulls

In Figure 7, the surface current distributions at n₁, n₂, and n₃ are presented to explain the cause of radiation nulls. Figure 7a indicates that the current on the driven patch is primarily concentrated around slot1, with equal current amplitude on both sides of slot1 but in opposite directions. This weakens the radiation effect on the driven patch and generates radiation null n₁. After loading the stacked patch, radiation nulls n₂ and n₃ are generated. Figure 7b shows that at n₂, the surface current direction of the driven patch and the stacked patch is opposite, resulting in the maximum suppression of radiation energy on the driven patch and the generation of radiation null n₂. Similarly, in Figure 7c, a robust current distribution occurs in the center of the driven patch and the stacked patch at n₃, with opposite current directions in these two parts. This cancels out the radiation energy and produces radiation null n₃. In conclusion, the electromagnetic radiation at the corresponding frequency is canceled, resulting in radiation nulls. The gain decreases rapidly on both sides of the frequency band, achieving a good roll-off rate and realizing the filtering characteristics of the antenna.

2.2.3. Analysis of Critical Parameters

The critical parameters of the driven patch, stacked patch and slots are analyzed one by one. Figure 8a shows that the decrease of the length ls₁ of slot1 makes f₁ shift to a higher frequency direction, which verifies that f₁ is generated by slot1. At the same time, because the length change of slot1 will have some influence on the current distribution of the driven patch, it will affect the matching of f₂. Figure 8b plots the influence of the width lp₁ of the driven patch on the S11. The change of lp₁ has the most significant influence on the matching of the resonance frequency f₂, the matching of f₂ worsens as the increase of lp₁. Figure 8c can prove that f₃ is affected by the width wp₂ of the stacked patch. With the increase of wp₂, f₃ shifts to the lower frequency direction, and the matching becomes poor. It has some influence on the resonance frequency f₂.

Figure 9 shows the effects of some parameters on radiation nulls. The radiation null n₁ moves to a lower frequency direction with the growth of ls₁, as illustrated in Figure 9a. The radiation null n₂ moves to a lower frequency direction with increasing of wp₂ and a higher frequency direction with increasing of ls₂, as demonstrated in Figure 9b,c. The changes of wp₂ and ls₂ also affect the out-of-band suppression level at high frequencies. Figure 9d displays the effect of the length ws₃ of slot3 on the radiation null. Both n₁ and n₂ shift to a lower frequency direction as ws₃ increases, confirming the effect of adding slot3 on n₁ and n₂ in Figure 4b. Ls₂ and ws₃ affects the current distribution at 4.84 GHz, resulting in the movement of n₃, as shown in Figure 9c,d. The proposed antenna is simulated and optimized by ANSYS software, and the optimized parameters are shown in

Table 1. Optimized antenna dimension. (Unit: mm).

Parameter	Value	Parameter	Value	Parameter	Value
Wt	77	lp1	35.6	ls3	1.5
Lt	78.8	wp1	41	ws3	11
Wa	65	lp2	32.5	lf1	6
La	66.3	wp2	32.5	wf1	5.8
La1	63	ls1	17.8	lf2	21
d1	14.7	ws1	5	wf2	3.1
d2	20.125	ls2	0.8	lf3	5
ds1	11	ws2	17	wf3	2.1
h1	0.6	h2	2	h3	0.127

.

3. Antenna Measurement and Discussion

The proposed filtering antenna is fabricated and measured. Figure 10 exhibits the pictures of the manufactured filtering antenna. The simulation results and measurement results of the proposed antenna are illustrated in Figure 11. It can be concluded that the measurement impedance bandwidth is 19.5% (3.2–3.89 GHz), which is wider than the simulation result of 18% (3.24–3.88 GHz), and the impedance matching is also improved compared with the simulation result. The maximum measured gain at 3.25 GHz is 9.21 dBi. Three measured radiation nulls are located at 2.94 GHz, 4.1 GHz, and 4.74 GHz, respectively. The out-of-band suppression levels of 18.47 dB and 25.22 dB are reached in upper and lower frequency bands and have high roll-off rates, resulting in higher frequency selectivity. Figure 11b shows that the maximum efficiency of the antenna in the frequency band can reach 93.5%. The errors between measurement and simulation results may come from assembly errors and measurement environment. The simulated and measured normalized radiation patterns in the yoz and xoz plane of the proposed antenna at 3.3 GHz, 3.58 GHz, and 3.84 GHz are shown in Figure 12. The proposed filtering antenna has good radiation characteristics in the working frequency band. The measurement results show better than normal consistency with the simulation results. Because the simulated cross-polarization is small, it is not displayed in the radiation patterns of yoz plane.

The comparisons with other works are exhibited in

Table 2. Comparisons with other filtering antenna.

Ref.	Bandwidth (GHz)	Peak Gain (dBi)	Rejection Level (dB)	Size (λ03)	Material
[11]	3.34–3.66 (9.14%)	7.6	12.7	0.457 × 0.457 × 0.043	F4B
[16]	2.3–2.9 (23%)	8.9	20	0.8 × 0.8 × 0.06	None
[17]	2.705–2.847 (5.1%)	6.3	None	0.76 × 0.44 × 0.015	Taconic TLY-5
[20]	2.15–2.5 (15.2%)	5.2	13	0.66 × 0.62 × 0.062	RO4003C
[21]	4.45–5.52 (21.5%)	4.8	15.9	0.4 × 0.4 × 0.09	F4BM
[23]	1.575 (3.5%) & 3.4–3.6 (5.71%)	3.15 & 4.3	None	0.13 × 0.13 × 0.067	ARLON 1000
[24]	4.8–5.99 (22.05%)	2.5	None	0.55 × 0.55 × 0.014	FR4
[26]	4.28–5.34 (22.04%)	10.4	14.5	0.78 × 0.8 × 0.08	FR4
This	3.2–3.89 (19.5%)	9.21	18.47	0.93 × 0.91 × 0.06	FR4

. The bandwidth of the proposed antenna (19.5%) can be kept at a high level compared to [11,17,20,23]. The gain (9.21 dBi) is higher than [11,16,17,20,21,22,23,24]. The minimum out-of-band rejection level is 18.47 dB, which is better than most antennas except for [26]. The processing cost of the antenna is lower than that of other reference antennas by using FR4 material. However, the antenna proposed in this work has a relatively larger size.

4. Conclusions

This paper introduces an SISL-based 5G filtering patch antenna with wide bandwidth, high gain, and high selectivity. Instead of using a filter or filtering circuit, the antenna utilizes the design method of etching slots and stacked patch to achieve the desired filtering characteristics. The introduction of two new modes results in a filtering response with three radiation nulls, leading to bandwidth enhancement and high selectivity. The antenna has the properties of low cost, excellent radiation performance, and frequency selectivity, making it suitable for application in 5G communication systems. In the future, methods such as loading branches, pins, or further improving patch shapes will be considered in the design of antenna improvements to enhance the suppression level in the low-frequency band, promote antenna miniaturization, reduce the volume of 5G communication systems, improve system performance, and further reduce manufacturing costs.