SIW Directional Coupler with Improved Isolation for X-Band Applications

Abstract

This paper presents the design of a high-isolation directional coupler for X-band applications, utilizing substrate-integrated waveguide (SIW) technology. The coupler features a simple structure, compact size, and ease of integration with other planar circuits. Typically, the S-parameters of a directional coupler are determined by the dimensions of the SIW and the aperture (or hole) of the coupler. In this study, we introduce additional via lines to modify the SIW and the coupler aperture, aiming to achieve high isolation. First, two via lines are embedded in the center, converging into two central vias that form the coupler’s aperture. The power ratio within the coupler is controlled by adjusting the width of the aperture and the overall width of the SIW. Specifically, the width of the SIW at the aperture position is affected by adding vias on the two outer sides of the SIW. By incorporating these vias, we can effectively manage the power distribution across the four ports while ensuring sufficient isolation among them. The proposed design achieves an insertion loss of 3.3 dB, a coupling factor of 6 dB, and an isolation factor of 28.6 dB at 10 GHz. The experimental results demonstrate that the coupler maintains $S_{41}$ less than −20 dB over a 30% fractional bandwidth, ranging from 8.6 GHz to 11.6 GHz.

1. Introduction

A directional coupler is a passive microwave component used for power division. This device has four ports. In the coupler, the power entering port 1 is transmitted to port 2 (the through port) and to port 3 (the coupled port), while port 4 (the isolated port) does not receive any power [1]. Traditionally, the main power from input port 1 goes through port 2, while the remaining power is coupled into port 3. Under ideal conditions, port 4 will not receive any power from the other ports, ensuring absolute isolation at port 4. Due to these features, directional couplers are commonly used in applications such as radar, satellite communication, point-to-point radio, and antenna beam-forming networks [2,3].

Traditionally, directional couplers are implemented in rectangular waveguides because they provide low insertion loss and high-quality factors. However, rectangular waveguides can be costly, bulky, and challenging to integrate with printed structures. To address these issues, a new technique called substrate-integrated waveguide (SIW) has been introduced. SIW combines aspects of microstrip and waveguide technologies.

An SIW consists of two metal layers with a substrate sandwiched in between and metallic vias connecting the two ground planes of the substrate. It offers several significant advantages, including low loss, high Q-factor, cost-effectiveness, and ease of integration with other planar devices [4,5]. Recently, many circuits have been designed using the SIW structure, including filters, couplers, power dividers, and antennas [6,7,8,9,10,11,12,13,14,15,16,17,18,19,20].

To enhance the ability to integrate directional couplers with other circuits on the same board, several studies have proposed designs for directional couplers implemented in substrate-integrated waveguide (SIW) structures [2,3,6,9,10,11]. References [2,9,10,11] specifically describe a design using a single-aperture (hole) structure. In this configuration, coupling is achieved through two narrow apertures located on the common broadside wall of two adjacent SIWs. The power distribution among the four ports of the coupler can be adjusted by varying the width of the apertures. This design is characterized by its simple configuration and compact size.

In [12,13,14,15], couplers with multiple apertures are introduced. These designs offer a wider and flatter bandwidth; however, the increased number of apertures results in a larger overall coupler size. Meanwhile, in [16,17], the authors propose a method for adjusting the width of the SIW to control the S-parameters of the coupler. This approach can improve the performance of the S-parameters significantly.

Previous studies have shown that it is possible to design directional couplers using substrate-integrated waveguides (SIWs). However, achieving the desired directivity for these couplers, which should range from 20 dB to 40 dB, has been challenging. In this paper, we propose integrating metallic vias to enhance the isolation performance of the coupler. These vias help to separate the electromagnetic fields of adjacent SIWs and enable control over the S-parameters of the coupler by allowing adjustments to the width of the SIW. By controlling the width of the SIW, we can reduce the transmitted wave from port 1 to port 4. Both simulation and experimental results demonstrate that the proposed coupler can achieve a fractional bandwidth of 30% with an isolation factor greater than −20 dB.

The paper is organized as follows: Section 2 presents the theoretical considerations of SIWs and SIW directional couplers. Section 3 details the design concept of the proposed X-band directional coupler. The experimental results of the two configurations are discussed in Section 4. Section 5 provides a comparison and discussion of the findings. Finally, Section 6 concludes the paper.

2. Theory of SIW and a SIW Directional Coupler

2.1. Substrate-Integrated Waveguide

The structure of the substrate-integrated waveguide (SIW) transmission line is depicted in Figure 1a [4]. The SIW transmission line comprises a top metal layer, a ground layer, and a middle substrate layer. There are two rows of metallic vias along the line that facilitate the transmission of signals within them. The SIW line supports ${TE}_{10}$ mode exclusively at the operating frequency indicated in Figure 1b.

The essential design parameters of a substrate-integrated waveguide (SIW) transmission line are shown in Figure 2. In the figure, w and l represent the width and length of the SIW, respectively. The parameters for the vias are d and p, which denote the diameter of the via and the distance between two adjacent vias. It is important for the values of d and p to satisfy Equations (1)–(3) in order to minimize loss.

$$d<{\displaystyle \frac{{\lambda }_g}{5}}$$

(1)

$$p<2d$$

(2)

where ${\lambda }_g$ is the guiding wavelength in SIW.

$${\lambda }_g={\displaystyle \frac{2\pi }{\sqrt{\frac{{\left(2\pi f_c\right)}^2{\epsilon }_r}{c^2}-{\left(\frac{\pi }{w}\right)}^2}}}$$

(3)

where c, $f_c$ are the speed of light and frequency, respectively.

2.2. SIW Directional Coupler

A basic structure of a four-port directional coupler is illustrated in Figure 3. This coupler is used to divide and distribute power. An incident signal enters through Input Port 1, where most of its power is transmitted to Transmitted Port 2. Power is also coupled to Coupled Port 3 from the input signal at Port 1. Finally, Port 4 is an isolated port, which does not receive any power from the input.

The fundamental parameters of the directional coupler are listed below:

Coupling:

$$C\left(dB\right)=10\log \left({\displaystyle \frac{P_1}{P_3}}\right)=-20\log \left(S_{31}\right)$$

(4)

Insertion loss:

$$IL\left(dB\right)=10\log \left({\displaystyle \frac{P_1}{P_2}}\right)=-20\log \left(S_{21}\right)$$

(5)

Isolation:

$$I\left(dB\right)=10\log \left({\displaystyle \frac{P_1}{P_4}}\right)=-20\log \left(S_{41}\right)$$

(6)

Directivity

$$D\left(dB\right)=-20\log \left(S_{31}/S_{41}\right)=C-I$$

(7)

The simple SIW directional coupler features a single-aperture design [2]. Figure 4 illustrates the single-aperture SIW coupler. In this type of coupler, the critical parameter is the width of the aperture $W_a$, as it influences the power distribution between the output ports. Tappers are used to connect transmission lines to the SIW. By incorporating tappers, the bandwidth of the circuit increases. The key parameters of the tapper include the width of the tapper $W_t$, the width of the 50-ohm transmission line $W_0$, and the length of the tapper $L_t$.

3. Design of a X-Band Directional Coupler

In this section, the design of a basic X-band coupler is performed first. Then, the proposed high-directional coupler is designed.

3.1. A Basic X-Band Directional Coupler

A basic X-band directional coupler was designed using a single-aperture structure, as illustrated in Figure 4. The substrate material selected for this design was Roger 4350. The main characteristics of the coupler are as follows: $S_{21}$ should be in the range of −3 to −2 dB, $S_{31}$ should be in the range of −6 to −5 dB, and both $S_{11}$ and $S_{41}$ should be minimized as much as possible. The designed parameters of the coupler are detailed in

Table 1. The parameters of the single-aperture X-band coupler.

W	L	d	p	${\mathit{W}}_{\mathit{a}}$	${\mathit{W}}_0$	${\mathit{W}}_{\mathit{t}}$	${\mathit{L}}_{\mathit{t}}$
24 mm	28.3 mm	0.5 mm	1 mm	13.9 mm	1.15 mm	2.6 mm	5 mm

.

The simulated S-parameters of the conventional coupler are illustrated in Figure 5, with the results obtained from the CST simulator. As shown in the figure, the insertion loss $S_{21}$ of the designed coupler is approximately −3 dB, while the $S_{31}$ is about −5 dB, within the frequency range of 8.5 GHz to 11.5 GHz. Within this band, the $S_{11}$ is smaller than −10 dB, and the isolation $S_{41}$ is less than −15 dB. At a frequency of 10 GHz, the S-parameters ${S_{11},S}_{21},S_{31},S_{41}$ are measured at −20 dB, −2.2 dB, −4.6 dB, and −19 dB, respectively.

Figure 6 illustrates the simulated electric field (E-field) of the X band coupler at a frequency of 10 GHz, demonstrating the coupler’s capabilities.

3.2. A High-Isolation X-Band Direction Coupler

Based on the structure of the basic directional coupler illustrated in Figure 4, it can be inferred that the coupler’s S-parameters are primarily determined by the width of the aperture $W_a$. Achieving the desired $S_{21}$ and $S_{31}$ while minimizing $S_{11}$ and $S_{41}$ within a specific frequency band by solely adjusting $W_a$ is a challenging task. Therefore, in this design, we propose adding via lines to modify the width of the substrate-integrated waveguide (SIW). This adjustment will help ensure that the S-parameters of the coupler meet the necessary requirements across the circuit’s bandwidth. The main objective of this design is to maintain a high directivity factor.

The electric field distribution of the basic coupler shown in Figure 6 indicates that power from the input port (port 1) leaks into the isolated port (port 4). To minimize this leakage, we propose adjusting the width of the SIW to modify the reflex waves. Two via lines are positioned near the outer edges of the SIW at the aperture location to implement the idea. This adjustment helps control the power transfer ratio among the ports. Figure 7 illustrates the structure of the proposed directional coupler. From the figure, we can see that the width of the SIW is determined by the length of the added line at the outer vias ($L_a$) and the distance from the added line to the outer line ($X_a$). To enhance the control over the S-parameters of the coupler, we incorporated two additional via lines in the center of the SIW, which create the coupler’s aperture. By modifying the two parameters, Wa and Ya, we can effectively manage the S-parameters of the coupler.

The design methodology follows these steps:

• Begin by designing a basic directional coupler, as illustrated in Figure 4. The design method is detailed in [2]. In this step, select $W_a$ to meet the required $S_{21}$ and $S_{31}$ values, while keeping $S_{11}$ and $S_{41}$ as small as possible.
• Introduce additional vias and adjust the design parameters $X_a, L_a, Y_a$ by analyzing the S-parameters of the coupler, with the goal of minimizing $S_{41}$ in the X-band.

Figure 8 illustrates the process of adjusting $X_a$ to assess the impact of added vias on the performance of the coupler. This control procedure can be evaluated through the electric field distribution. Figure 8a–d show the electric field distribution of the coupler at 10 GHz for the cases where $X_a=1.0$, $X_a=0.8$, $X_a=0.5$, and $X_a=0.3$. The scattering parameter $S_{41}$ of the coupler at 10 GHz for these values is −29.2 dB, −31.8 dB, −33 dB, and −32 dB, respectively.

From the electric field distribution, it is evident that adding the via lines reduces the transmitted wave from port 1 to port 4. Additionally, the signal at the isolated port results from the reflected signal from port 3 to port 4. Incorporating the added via lines results in smaller amounts of reflected power in Figure 8b–d compared to Figure 8a.

Figure 8e demonstrates the effect of controlling the width of the substrate-integrated waveguide (SIW) across the frequency band. It shows that for $X_a=1.0$ and 0.8, better isolation factors are achieved at higher frequencies. However, in the X-band, $X_a=0.5$ and 0.3 exhibit better isolation factors. Specifically, in the X-band, $X_a=0.5$ is the optimal choice for achieving the highest directivity.

When comparing the electric field distributions of the two couplers shown in Figure 6 and Figure 8, it is evident that the coupler in Figure 8 exhibits better isolation than the one in Figure 6. The coupler in Figure 8 includes additional vias in the outer lines of the substrate-integrated waveguide (SIW), which distinguishes it from the coupler in Figure 6. The frequency band in which the basic coupler from Figure 5 maintains a performance of $S_{41}$ below −15 dB is approximately 3 GHz, whereas the coupler in Figure 8d has a band of nearly 4 GHz. These results suggest that the additional vias enhance the isolation of the coupler.

After optimizing the design parameters of the proposed coupler to achieve high isolation, the parameters of the X-band coupler are presented in

Table 2. The parameters of the high-isolation X-band coupler.

${\mathit{Y}}_{\mathit{a}}$	${\mathit{L}}_{\mathit{a}}$	${\mathit{X}}_{\mathit{a}}$
0.6 mm	5 mm	0.5 mm

. In this circuit, the diameter of the center via is 0.4 mm, and the distance between two adjacent vias is 0.8 mm.

The simulated results of the proposed coupler are presented in Figure 9. The results indicate that in the frequency range of 8.5 GHz to 11.5 GHz, the insertion loss $S_{21}$ of the proposed coupler is approximately −2 dB, the $S_{31}$ is nearly −5 dB, while $S_{11}$ and $S_{41}$ are smaller than −15 dB. At the center frequency of 10 GHz, the measured values of ${S_{11},S}_{21},S_{31},S_{41}$ are −32 dB, −2.0 dB, −4.7 dB, and −33 dB, respectively. Additionally, the electric field distribution of the proposed coupler can be seen in Figure 10.

The figure demonstrates that the proposed coupler exhibits a high level of isolation. Figure 11 compares $S_{41}$ measured results of the basic coupler with that of the proposed coupler. In the X-band, the proposed coupler achieves better performance, with higher isolation than the basic coupler. Both couplers provide the same level of −5 dB $S_{31}$; however, the directivity of the proposed coupler is superior to that of the basic coupler.

4. Experimental Results

Figure 12 showcases photographs of the two X-band couplers. Image (a) displays a basic one-aperture coupler, while image (b) features the proposed coupler. Both couplers are compact, measuring 51.7 mm × 25.9 mm. The couplers are designed using Rogers 4350 substrate, with a substrate height of 0.5 mm, ${\epsilon }_r=3.48$, and 1 oz of copper cladding.

Figure 13 presents the measured results for the basic coupler. In the frequency band ranging from 8.5 GHz to 11.5 GHz, $S_{21}$ is nearly −3 dB, while $S_{31}$ is approximately −6 dB. Additionally, $S_{11}$ is less than −10 dB, and $S_{41}$ is below −15 dB. At the center frequency of 10 GHz, the measured values ${{\mathrm{S}}_{11},\mathrm{S}}_{21},{\mathrm{S}}_{31},{\mathrm{S}}_{41}$ are −13.7 dB for the return loss, −3.0 dB for the insertion coefficient, −6.2 dB for the coupling loss, and −21 dB for isolation, respectively.

Figure 14 presents the measurement results of the proposed coupler. As shown in the figure, the proposed circuit exhibits good characteristics for a directional coupler. In the frequency band from 8.5 GHz to 11.5 GHz, $S_{21}$ is nearly −3 dB, while the coupling level $S_{31}$ is approximately −6 dB. Additionally, the isolation level $S_{11}$ is less than −10 dB, and $S_{41}$ is less than −20 dB. At a center frequency of 10 GHz, the measured values of ${S_{11}, S}_{21}, S_{31}, S_{41}$ are −20.2 dB, −3.3 dB, −6.0 dB, and −28.6 dB, respectively.

When comparing the experimental results of the two couplers, it can be concluded that the proposed coupler has similar performance to the basic coupler. Furthermore, the proposed coupler exhibits better directivity than that of the basic coupler.

Figure 15 displays the $S_{41}$ measurement results for both couplers. The figure shows that the proposed coupler demonstrates a smaller $S_{41}$ measurement in the X-band compared to the basic coupler. As a result, the isolation factor and directional factor of the proposed coupler are significantly better than those of the basic coupler.

5. Comparison and Discussion

The performance of the proposed circuit is compared with several X-band couplers, as shown in

Table 3. Comparison table.

		This Work	Ref. [9]	Ref. [3]	Ref. [12]
Simulation results	$S_{11}$$@ f_c$	−32 dB @ 10 GHz <−15 dB @ 33% BW	−11 dB @ 10 GHz	−15 dB	<−15 dB @ 35% BW
	$S_{21}$$@ f_c$	−2.0 dB @ 10 GHz	about −2 dB @ 10 GHz	−3.52 dB	about −4 dB
	$S_{31}$$@ f_c$	−4.7 dB @ 10 GHz	about −3 dB @ 10 GHz	−6.57 dB	about −4 dB
	$S_{41}$$@ f_c$	−33 dB@ 10 GHz	−10 dB @ 10 GHz	−26.4 dB	about −20 dB
	Bandwidth	(8.48–11.78) GHz	(8.5–10.5) GHz	(8.5–11) GHz	(7.5–11) GHz
Experimental results	$S_{11}$$@ f_c$	−20.2 dB @ 10 GHz <−10 dB @ 30% BW	about −9 dB @ 10 GHz	-	<−14 dB @ 32.3% BW
	$S_{21}$$@ f_c$	−3.3 dB @ 10 GHz	about −3 dB @ 10 GHz	-	about −5 dB
	$S_{31}$$@ f_c$	−6.0 dB @ 10 GHz	about −6 dB @ 10 GHz	-	about −5 dB
	$S_{41}$$@ f_c$	−28.6 dB @ 10 GHz <−20 dB @ (8.61–11.65) GHz	about −14 dB @ 10 GHz	-	<−14 dB @ (7.56–10.49) GHz
	Bandwidth	(8.6–11.6) GHz	(8.5–10.5) GHz	-	(7.5–11) GHz
Material		RO4350	RO4350		RT Duroid 5880

. The table indicates that the proposed coupler exhibits favorable parameters in both simulation and experimental results. Additionally, measurements show that the proposed coupler achieves a level $S_{41}$ of −20 dB over a frequency range from 8.6 GHz to 11.6 GHz, resulting in a fractional bandwidth of 30%. Therefore, it can be concluded that the proposed coupler is suitable for wideband and high-isolation applications.

6. Conclusions

This paper presents the design of a wideband, high-isolation X-band directional coupler utilizing substrate-integrated waveguide (SIW) technology. The S-parameters of the directional coupler are influenced by the dimensions of the SIW and the aperture size of the coupler. To control the width of the SIW at the aperture position, we introduce vias on the two outer sides of the SIW. The configuration of the coupler’s aperture is achieved by embedding two via lines in the center of the SIW. By adjusting the parameters of these additional vias, we can modify the S-parameters of the coupler to achieve high isolation. As a result, the proposed coupler demonstrates high isolation across a wide frequency band. The simulation results obtained using CST show that the coupler achieves parameters of ${S_{11},S}_{21},S_{31},S_{41}$ at 10 GHz of −32 dB, −2.0 dB, −4.7 dB, and −33 dB, respectively. Experimental results for the circuit also demonstrate commendable performance, with values of ${S_{11},S}_{21},S_{31},S_{41}$ of −20.2 dB, −3.3 dB, −6.0 dB, and −28.6 dB, respectively. Over a 30% fractional bandwidth from 8.6 GHz to 11.6 GHz, the performance of the coupler remains below −20 dB of $S_{41}$. These results indicate that the proposed coupler has potential for use in wideband and high-isolation applications.