Research Article  Open Access
Wen Huang, Jia Li, Ping Li, Xi Guo, "Compact Microwave Components with Harmonic Suppression Based on Artificial Transmission Lines", International Journal of Antennas and Propagation, vol. 2019, Article ID 4923964, 16 pages, 2019. https://doi.org/10.1155/2019/4923964
Compact Microwave Components with Harmonic Suppression Based on Artificial Transmission Lines
Abstract
In this paper, compact microwave components, including a Wilkinson power divider and a 3 dB branchline coupler based on artificial transmission lines (ATLs) with harmonic suppression, are presented. A section ATL is consisted of microstrip stepped impedance transmission lines and a microstrip interdigital capacitor. To achieve a compact size, the stepped impedance transmission lines are folded into a rightangled triangle shape. For the ATL, the interdigital capacitor is used to suppress harmonics. By employing two sections of 70.7 Ω ATLs with a rightangled triangle shape to replace conventional transmission lines, the proposed power divider working at 0.9 GHz achieves a size miniaturization with the 58.8% area of a conventional case. In addition, the power divider has good harmonic suppression performance. In the design of a branchline coupler, two pairs of ATLs with 50 Ω and 35.4 Ω are utilized. For 50 Ω ATLs, the ATLs are designed to a rightangled triangle shape. Meanwhile, to obtain a more compact size, these 35.4 Ω ATLs are modified to an isosceles trapezoid shape. The proposed branchline coupler operating at 0.9 GHz accounts for merely 33.4% of a coupler adopting conventional microstrip transmission lines. Moreover, the harmonics of a branchline coupler are suppressed effectively as well. Finally, measured results of the proposed Wilkinson power divider and branchline coupler display good performance and agree with their simulated results well.
1. Introduction
The rapid development of the modern communication system makes a high requirement for size miniaturization of microwave components [1–3]. At present, size miniaturization methods of microwave components include loading lumped elements [4], using right/lefthanded transmission lines [5], employing a defected ground structure (DGS) [6], utilizing a microstrip electromagnetic bandgap (EBG) [7], and applying slowwave transmission lines with inductive and capacitive loading [8]. In these miniaturization methods, using artificial transmission lines (ATLs) is a good solution due to a flexible design and easy fabrication on a printed circuit broad. Besides, harmonic suppression can improve the signaltonoise ratio of the communication system, which is also a hot research spot for microwave components. To achieve harmonic suppression, these methods are proved to be effective, such as using an ultrawideband bandstop filter [9], an electromagnetic bandgap [10], open stubs [11], a defected ground structure [12], and inductively loaded slowwave transmission lines [13].
The Wilkinson power divider, which works as a divider and combiner of power, is extensively used in a feed network for an antenna [14]. The conventional Wilkinson power divider consists of two sections of 70.7 Ω quarterwavelength transmission lines and an insolation resistor. Therefore, the size of the Wilkinson power divider is mainly determined by the size of quarterwavelength transmission lines at operating frequency, which results in a large occupied area, especially when operating at low frequency. Besides, the conventional Wilkinson power divider realized by microstrip lines cannot suppress harmonics. Therefore, some miniaturization methods of Wilkinson power dividers are reported, such as using short circuited halfwavelength and quarterwavelength resonators [15], loading coupled resonator topology [16], utilizing topology consisted of halfwavelength resonators and quarterwavelength resonators [17], and using open dualtransmission line stub and Ltype artificial lowpass transmission line structures [18]. These power dividers mentioned above not only have an obvious reduced size but also have a good harmonic suppression performance. In [15], harmonic suppression is realized by using lowpass filters. In [16], a nettype resonator is designed to obtain rejection of harmonic responses. In [17], the mixed electric and magnetic coupling and crosscoupling between three quarterwavelength resonators generate three transmission zeroes to suppress harmonics. In [18], open dualtransmission line stub, Ltype artificial lowpass transmission lines, open stub, and extension line modules are designed to suppress harmonics. These power dividers mentioned above can achieve a compact size and good harmonic suppression, but 15 dB bandwidths of these cases are narrow, which are, respectively, only 2.4%, 4.5%, 6.5%, and 21.1%.
A branchline coupler is another important microwave component used in a modern communication system. However, the conventional 3 dB branchline coupler is also confronted with the problem of a large occupied area brought in by two pairs of quarterwavelength transmission lines with 50 Ω and 35.4 Ω. In order to reduce the size, some solutions are reported, such as loading a πequivalent artificial transmission line [19], using asymmetrical structures [20], utilizing highimpedance transmission lines and interdigitated shunt capacitors [21], applying artificial transmission line combined with meandered lines and resonators [22], and employing transmission lines loaded resonators [23]. However, only the couplers in [21–23] can realize harmonic suppression. In [21], the secondorder harmonic is suppressed to 20 dB by using interdigitated shunt capacitors. In [22], shunt resonators generate suppression for highfrequency harmonics, but a secondorder harmonic and thirdorder harmonic cannot be suppressed. In [23], harmonics are suppressed due to the transmission zeros of resonators. In addition, these cases in [19, 21] are realized with size reduction, but 15 dB bandwidths are only 8.9% and 7.2%. These methods are difficult to achieve a reduced size, good bandwidth, and harmonic suppression at the same time.
In this paper, a method of using ATLs for size miniaturization and harmonic suppression employed on the Wilkinson power divider and 3 dB branchline coupler is presented. These ATLs consist of stepped impedance transmission lines and interdigital capacitors. In the design of ATLs, these stepped impedance lines are folded into a rightangled triangle shape or isosceles trapezoid shape for a more compact size. Then, the interdigital capacitor plays a function of harmonic suppression. Finally, the proposed Wilkinson power divider and branchline coupler have similar bandwidths of 58.2% and 17% compared with conventional components, while a compact size and harmonic suppression are realized at the same time.
2. Theoretical Analysis of an ATL
2.1. Design Concepts
As shown in Figure 1(a), a section of the 70.7 Ω ATL with a rightangled triangle shape is composed of five sections of microstrip lowimpedance transmission lines and six sections of microstrip highimpedance transmission lines. Additionally, there are two sections of uniform transmission lines connected to ports. The lowimpedance lines and highimpedance lines are cascaded alternately and then folded into a rightangled triangle shape for a compact size. Meanwhile, an interdigital capacitor is placed in the middle of an ATL to suppress harmonics. The equivalent circuit of the 70.7 Ω ATL is given by Figure 1(b). Each uniform transmission line connected to a port can be equivalent to a series inductor and a shunt capacitor to ground. Correspondingly, the highimpedance lines can be equivalent to series inductors L_{as2}, L_{as3}, and L_{as4}, and these lowimpedance lines work as equivalent shunt capacitors C_{as2}, C_{as3}, and C_{as4} to ground. The interdigital capacitor is placed between several highimpedance lines and between lowimpedance lines, so it can be equivalent to several series capacitors in parallel and parasitic capacitors to ground. For the sake of simplicity, as marked in Figure 1(a), the microstrip interdigital capacitor is divided into two parts as interdigital A1 and interdigital A2 and then be equivalent to series capacitors C_{ap1} and C_{ap2}. Besides, the shunt capacitors C_{a11}, C_{a12}, C_{a21}, and C_{a22} represent parasitic capacitors of the interdigital capacitor.
(a)
(b)
For the equivalent circuit of the 70.7 Ω ATL, the total inductance of equivalent series inductors can be represented by L_{t1}, and the total capacitance of equivalent shunt capacitors can be presented by C_{t1}. And they can be given by
In the design of the 3 dB branchline coupler, 50 Ω and 35.4 Ω ATL are designed. For a 50 Ω ATL, the layout and its equivalent circuit are given by Figure 2. It has similar layout and equivalent circuit as a 70.7 Ω ATL. The total inductance L_{t2} of equivalent series inductors and total capacitance C_{t2} of equivalent shunt capacitors are calculated by
(a)
(b)
The layout and equivalent circuit of the 35.4 Ω ATL are shown in Figure 3. The 35.4 Ω ATLs are modified to an isosceles trapezoid shape to realize a more compact size for the branchline coupler. Referring to its layout, the 35.4 Ω ATL is comprised of three sections of lowimpedance transmission lines and four sections of highimpedance transmission lines. The two transmission lines connected to ports can be equivalent to series inductor L_{cs1} and shunt capacitor C_{cs1} to ground. In order to analyze the interdigital capacitor more accurately, the interdigital capacitor is divided into two parts. As marked in Figure 3(a), interdigital capacitor C1 and interdigital capacitor C2 can be equivalent to series capacitors C_{cp1} and C_{cp2} and parasitic capacitors C_{c11}, C_{c12}, C_{c21}, and C_{c22} to ground. The total series inductance L_{t3} and total shunt capacitance C_{t3} are given by
(a)
(b)
According to the uniform transmission line theory, characteristic impedance () and phase propagation constant () of per unit length are determined by and , where and are the inductance and capacitance per unit length along the transmission line and represents the operating angular frequency. For ununiform transmission lines such as ATLs, when the physical length of the ATL is less than oneeighth of a guided wavelength, the ununiform transmission lines can approximate to uniform transmission lines [24]. Then, for the ATL, and , where and are the total series inductance and shunt capacitance and is the physical length of a unit ATL. The characteristic impedance and electrical length of a unit of an ATL can be calculated as
From formulas (4) and (5), we can find, for a unit ATL with the same length, if and increase proportionally, that would remain unchanged, while the electrical length would increase. In turn, with a given characteristic impedance and electrical length of a unit of an ATL, when and are increased proportionally, characteristic impedance would be unchanged, while phase propagation constant of per unit length would be increased, so the required physical length is significantly reduced.
For stepped impedance transmission lines, using the lines with high characteristic impedance is equivalent to loading more inductance per unit length, while employing the lines with low characteristic impedance is equivalent to loading more capacitance per unit length than conventional microstrip transmission lines. In the design of an ATL, stepped impedance transmission lines folded into a rightangled triangle shape would add more capacitance and inductance for per physical length, so the required physical length can be further reduced. In a practical design, the 35.4 Ω ATL is modified to an isosceles trapezoid shape to realize a more compact size of a branchline coupler.
Owing to the impedance invariance of half wavelengths, conventional transmission lines including stepped impedance transmission lines have many harmonic passbands at multiples of central frequency. In order to suppress harmonics effectively, a microstrip interdigital capacitor is used in the ATL, which is equivalent to several series capacitors. These series capacitors are shunted with inductors, forming parallelresonant circuits, as shown in Figures 1(b), 2(b), and 3(b). According to the stopband characteristic of parallelresonant circuits at resonant frequency, these highorder harmonics at multiples of central frequency will be suppressed effectively by this way. Besides, the performance of harmonic suppression can be changed by changing the number and size of an interdigital capacitor’s fingers. Finally, in order to realize a more compact structure of the proposed components using ATLs, each quarterwavelength line is replaced by one unit ATL with 90°, though the optimum number of ATL units is proved to be two in [8, 13].
Based on the above analysis, taking the ATL with the required 70.7 Ω characteristic impedance and 90° electric length as an illustration, the design procedure of ATL can be described as follows: Step 1.Based on the layout of an ATL just like that in Figure 1(a), create a rough model of an ATL in the software of IE3D. Obviously, the physical dimensions of each section in the model as shown in Figure 4 are arbitrary.Step 2.The ATL is simulated by software of IE3D. The value of characteristic impedance can be obtained by simulation directly, and electric length can be got by simulated Ang().Step 3.Compare the simulated characteristic impedance and electric length with the required goals. When required goals are not achieved, then, according to equations (4) and (5), decide the value of and should be increased or reduced.
(a)
(b)
(c)
(d)
(e)
In tuning of characteristic impedance , when the simulated characteristic impedance is lower than 70.7 Ω, according to equation (4), we can increase total inductance or reduce total capacitance . When the simulated characteristic impedance Z_{1} is higher than 70.7 Ω, we can reduce total inductance or increase total capacitance .
In tuning of electric length , when electric length is smaller than 90°, according to equation (5), we can increase total inductance or total capacitance . When simulated electric length is larger than 90°, we can reduce total inductance or total capacitance .
According to equation (1), changing the total inductance L_{t1} or total capacitance C_{t1} can be realized by changing the inductance and capacitance of each related section. For example, extending line length or reducing line width of these highimpedance lines can increase inductance of L_{as2}, L_{as3}, and L_{as4}, and reducing the area of lowimpedance lines can reduce capacitance of C_{as2}, C_{as3}, and C_{as4} as shown in Figure 1(b).
So tuning physical dimensions of each section can change total inductance L_{t1} and total capacitance C_{t1}, to obtain the designed 70.7 Ω and 90°. Step 4.Tune fingers of interdigital capacitors to adjust the performance of harmonic suppression, including frequency of transmission zero and range of stopband.Step 5.To obtain equivalent inductance and capacitance of ATL to verify our design, with designated 70.7 Ω characteristic impedance and 90° electric length, we use formulas (4) and (5) to obtain L_{t1} and C_{t1}.Step 6.Each section as shown in Figure 4 is simulated by the simulation software of IE3D and converted to , , or equivalent circuit. And then parameter extraction of equivalent elements is based on impedance or admittance matrices.
2.2. Parameter Extraction
The process of parameter extraction is similar for the ATL of 70.7 Ω, 50 Ω, and 35.4 Ω, so we take the 70.7 Ω ATL as an example to analyze. In order to analyze the equivalent circuit of the 70.7 Ω ATL easily, the ATL is divided into several sections, as shown in Figure 1. Each section and their equivalent circuit are given in Figure 4. Obviously, a more accurate equivalent circuit can be achieved by dividing the ATL into more sections when analyzing the ATL. To extract the equivalent element values of each section, fullwave simulation software IE3D is applied to calculate impedance or admittance matrices from the corresponding equivalent circuits of each section.
The admittance matrix of the equivalent circuit for section A1 in Figure 4(a) is expressed as
From the formula above, the equivalent value of elements can be obtained by
Due to similar type circuit, the equivalent value of elements in Figures 4(b) and 4(c) can be readily calculated based on the admittance matrix by the same way.
For section A4 in Figure 4(d), its impedance matrix can be given by
Then, the value of equivalent elements can be calculated as
For interdigital capacitor () as shown in Figure 4(e), its admittance matrix can be expressed by
Then, the equivalent capacitance value can be calculated by
All values of equivalent elements as marked in Figures 1(b), 2(b), and 3(b) are given in Table 1.

The final optimized size of ATLs used in the proposed Wilkinson power divider and branchline coupler are listed in Table 2 as marked in Figures 1(a), 2(a), and 3(a). The physical lengths of the 70.7 Ω, 50 Ω, and 35.4 Ω ATLs are 0.098λ_{g}, 0.104λ_{g}, and 0.124λ_{g}, respectively.

2.3. Harmonic Suppression
For the proposed ATLs, the interdigital capacitors can play a role of harmonic suppression. For the interdigital capacitors, the performance of harmonic suppression is mainly determined by the number, length, width of finger, and gap width between fingers [24]. In order to realize the compact size and easy fabrication, the gap width between fingers is chosen to be 0.2 mm. Then, other factors affecting harmonic suppression would be discussed.
Taking the 70.7 Ω ATL mentioned above as an example, the interdigital capacitors have 19 fingers and the length and width as marked in Figure 1(a). The 70.7 Ω ATLs with different fingers’ numbers, lengths, and widths of interdigital capacitors are simulated.
When the fingers’ number of interdigital capacitors changes, while their length and width remain unchanged, the simulated are shown in Figure 5(a). It can be observed that when the fingers’ number is reduced, the frequency of transmission zero in stopband becomes higher. In addition, compared to the ATL without interdigital capacitors, the ATL with interdigital capacitors has obvious harmonic suppression. The influence of fingers’ length on harmonic suppression is given in Figure 5(b). When the number of fingers equals to 19 and fingers’ width , it can be seen that the frequency of transmission zero becomes higher as fingers’ length is shortened. Figure 5(c) reveals that the frequency of transmission zero varies as fingers’ width changes when the interdigital capacitors have 7 fingers with 5.2 mm length. When fingers’ width increases, the frequency of transmission zero would be lower slightly.
(a)
(b)
(c)
The result of such simulations is because the reduction of fingers’ number, length, and width would reduce the capacitance of C_{ap1} and C_{ap2}. The reduced capacitance of C_{ap1} and C_{ap2} would make the resonant frequency of the parallel resonant circuit higher, which is composed of C_{ap1}, C_{ap2}, and L_{as4}. And the resonant frequency is the frequency of transmission zero in a stopband, so the harmonic suppression is affected.
2.4. Propagation Characteristics
All of these proposed ATLs are designed to operate at 0.9 GHz and with 90° phase delay. The layout of ATLs are simulated by the electromagnetic simulation solver of IE3D, while their corresponding equivalent lumped circuits as given in Figures 1(b), 2(b) and 3(b) are simulated by software of ADS. The comparisons of simulated parameters between the layout and equivalent lumped circuit are given in Figure 6. Additionally, when the 70.7 Ω, 50 Ω and 35.4 Ω ATLs are simulated, the reference impedances of the ports are set to 70.7 Ω, 50 Ω, and 35.4 Ω, respectively.
(a)
(b)
(c)
As shown in Figure 6(a), for the 70.7 Ω ATL, the simulated parameters show that it has a 152.2% bandwidth of 0.9 GHz, from 0 GHz to 1.37 GHz. At 0.9 GHz, the return loss is 45.63 dB while insertion loss is less than 0.1 dB. Moreover, harmonic suppression performance is also shown in Figure 6(a). For the secondorder harmonic frequency at 1.8 GHz, is 10.66 dB, while for the thirdorder harmonic frequency at 2.7 GHz, is 17.71 dB. For these harmonic frequencies, there are no passbands.
For 50 Ω ATL as shown in Figure 6(b), the layout simulation displays a 170% bandwidth of 0.9 GHz, from 0 GHz to 1.53 GHz. At 0.9 GHz, the ATL achieves 39.56 dB return loss and insertion loss less than 0.1 dB. Moreover, its harmonic suppression performance is verified by 12.5 dB harmonic suppression at the thirdorder harmonic frequency at 2.7 GHz.
For the 35.4 Ω ATL as exhibited in Figure 6(c), it has a 161.1% bandwidth of 0.9 GHz, from 0 GHz to 1.45 GHz. When operating at 0.9 GHz, 52.7 dB return loss and less than 0.1 dB insertion loss are realized. For the 35.4 Ω ATL, the thirdorder harmonic is suppressed to 26.63 dB, which displays good harmonic suppression performance.
The simulated characteristic impedance and phase delay are exhibited in Figures 7 and 8. For the 70.7 Ω ATL, at 0.9 GHz, the real part of characteristic impedance is 70.7 Ω and the imaginary part is 0.42 Ω, while the simulated results of the equivalent circuit are 70.93 Ω and 2.08 Ω. For the 50 Ω ATL, the 50.0 Ω real part and 0.56 Ω imaginary part of characteristic impedance can be obtained, while the results of equivalent circuit simulation are 50.5 Ω and 0.1 Ω. For the 35.4Ω ATL, the real part of characteristic impedance is 35.4 Ω and the imaginary part is 0.24 Ω, while the corresponding results of the equivalent circuit are 35.85 Ω and 0.15 Ω.
Figure 8 indicates that the 70.7 Ω ATL has 90.0° phase delay at 0.9 GHz while 90.13° for the equivalent circuit. The 50 Ω ATL has 90.0° phase delay at 0.9 GHz while 91.52° for the equivalent circuit. The 35.4 Ω ATL has 90.1° phase delay and 91.15° for the equivalent circuit.
From the analysis above, these proposed ATLs not only have the similar performance like conventional transmission lines within the bandwidth but also have effective suppression ability for the harmonics. For the ATLs, the simulated parameters of the layout agree well with that of the lumped equivalent circuit at low frequency. However, at high frequency, there is a deviation of insertion losses, due to the equivalent circuit of the interdigital capacitor. In addition, the precision of the equivalent circuit would be improved by dividing the interdigital capacitor into more numbers of equivalent series capacitors. The simulation comparisons between the layout and lumped model indicate the accuracy of extracting parameters of the equivalent circuit.
3. Circuit Design and Measurements
The proposed Wilkinson power divider and branchline coupler are designed to operate at 0.9 GHz. Simulations are performed by a fullwave EM simulation software of IE3D. The proposed Wilkinson power divider and branchline coupler are fabricated on a substrate of F4B with 1 mm thickness and relative permittivity and loss tangent . Measurements are carried out on an Agilent 8510C network analyzer.
3.1. Wilkinson Power Divider
The layout of proposed Wilkinson power divider is given by Figure 9(a). The proposed Wilkinson power divider consists of two sections of 70.7 Ω ATLs employed to replace these conventional quarterwavelength transmission lines. And a chip resistor of 100 Ω (type: 0805) is used in the design. The photograph of the fabricated Wilkinson power divider is shown in Figure 9(b). The proposed Wilkinson power divider occupies a size of , that is , where is the guided wavelength on the substrate at 0.9 GHz. Comparing to a power divider implemented by conventional microstrip lines, the fabricated power divider only occupies a 58.8% size.
(a)
(b)
Figure 10 shows the simulated and measured , , and of the proposed Wilkinson power divider. The simulated and measured parameters indicate that the measured results have a good agreement with simulated results. For simulation, the central frequency is 0.9 GHz while measurements exhibit the central frequency of 0.91 GHz. There exists a frequency deviation of 0.01 GHz for central frequency due to fabrication tolerance and measurement devices. Then, the fabricated Wilkinson power divider for 0.91GHz has a bandwidth of 58.2% for less than 15 dB, from 0.61 GHz to 1.14 GHz, and the return loss at 0.91 GHz is 32.12 dB. At 0.91GHz, measured is 3.13 dB and is 3.26 dB. In addition, as shown in Figure 10, the fabricated Wilkinson power divider has 12.5 dB suppression for the secondorder harmonic of 1.82 GHz and 24.7 dB suppression for the thirdorder harmonic of 2.73 GHz.
Figure 11 shows isolation and phase difference (Ang()Ang()). Measured results display at 0.91 GHz that the power divider has a 32.8 dB isolation, which reveals good isolation performance between output port 2 and port 3. Besides, there is only a 0.23° phase difference between two output port 2 and port 3 at 0.91 GHz.
Figure 12 gives the comparison of parameters between the proposed Wilkinson power divider and conventional case operating at 0.9 GHz, which obviously indicates that the harmonic is effectively suppressed in the stopband in the proposed structure. Meanwhile, a similar bandwidth performance as that of the conventional case is achieved when size miniaturization is realized.
Finally, the performance comparisons of this proposed power divider (this work) with several previous designs are listed in Table 3. It indicates the proposed Wilkinson power divider in this work has similar electrical performance as the conventional case and good harmonic suppression but with a small size. Especially, the relative bandwidth is similar to the conventional one.
 
: guided wavelength at central frequency. 
3.2. BranchLine Coupler
In the design of the proposed 3 dB branchline coupler as shown in Figure 13(a), two sections of 50 Ω ATLs and two sections of 35.4 Ω ATLs are employed to replace these corresponding conventional quarterwavelength transmission lines. The two sections of 50 Ω ATLs are designed to be with a rightangled triangle shape, while the two sections of 35.4 Ω ATLs are modified to an isosceles trapezoid shape in order to realize a more compact size. All of these ATLs are combined into a compact rectangle shape. The photograph of the fabricated 3 dB branchline coupler is given in Figure 13(b). The size of the fabricated branchline coupler is , that is , where is the guided wavelength on the substrate at 0.9 GHz. The size of the proposed branchline coupler is effectively reduced to a 33.4% area of a conventional one at 0.9 GHz.
(a)
(b)
Figure 14 shows the simulated and measured , , and of the proposed coupler. The parameters reveal a good agreement between simulated and measured results. As observed in Figure 14, the central frequency is 0.9 GHz at simulation while the central frequency is 0.91 GHz at measurement. A frequency deviation of 0.01 GHz exists between simulation and measurement because of fabrication tolerance and measurement devices. For measured results, at a central frequency of 0.91 GHz, it has a 17% bandwidth with less than 15 dB, from 0.84 GHz to 0.99 GHz. At 0.91 GHz, the return loss is 29.35 dB, which shows its good impedance match with ports. And at 0.91 GHz, and are 3.2 dB and 3.15 dB, respectively. In Figure 15, and phase difference between the two output ports are given. At 0.91 GHz, measured is 26.3 dB. The phase difference (Ang()Ang()) is 90.6°, while the bandwidth is 34 MHz, from 881 MHz to 915 MHz.
In addition, at the secondorder harmonic frequency of 1.82 GHz, is 1.26 dB, is 12.6 dB, and is 11.1 dB. At the thirdorder harmonic frequency of 2.73 GHz, is 0.34 dB, is 29.5 dB, is 33.9 dB. It is demonstrated that at the secondorder and thirdorder harmonic frequencies, there are no passbands. So harmonic suppression performance is verified. As a comparison, parameters of the conventional branchline coupler are given in Figure 16. By observation, the bandwidth of the proposed branchcoupler is comparable to that of the conventional one, while these harmonic passbands at harmonic frequencies are effectively suppressed.
Table 4 shows the performance comparisons of the proposed branchline coupler (this work) and several previous designs. The performance comparisons reveal that the proposed branchline coupler has similar bandwidth, insertion loss, and isolation with other designs, but better harmonic suppression with a miniaturized size.
 
: guided wavelength at central frequency. 
4. Conclusion
In this paper, a compact Wilkinson power divider and a compact 3 dB branchline coupler with harmonic suppression performance based on artificial transmission lines are proposed. These ATLs consist of stepped impedance transmission lines and interdigital capacitors. In order to reduce the size, these ATLs are folded into a rightangled triangle shape, and especially, the 35.4 Ω ATLs employed in the branchline coupler are modified to an isosceles trapezoid shape. Then, the feasibility of the proposed power divider and branchline coupler design are demonstrated, and the simulated and the measured results are in good agreement. The proposed Wilkinson power divider realizes the 58.8% occupied area of the conventional case and good harmonic suppression performance. The occupied area of the proposed branchline coupler is reduced to 33.4% of the conventional case. At the same time, good harmonic suppression performance is demonstrated. Both proposed components have similar bandwidth as the conventional design but with compact size and good harmonic suppression. The proposed Wilkinson power divider and branchline coupler can be extensively used in the modern communication system.
Data Availability
All data included in this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that there are no conflicts of interest.
Acknowledgments
This work was supported by the Chongqing Research Program of Basic Research and Frontier Technology (cstc2017jcyjAX0128), the Chongqing Postgraduate Innovative Research Project (CYS18243), and the National Natural Science Foundation of China (61401054).
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Copyright © 2019 Wen Huang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.