Compact Microwave Components with Harmonic Suppression Based on Artificial Transmission Lines

In this paper, compact microwave components, including a Wilkinson power divider and a 3 dB branch-line 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 right-angled triangle shape. For the ATL, the interdigital capacitor is used to suppress harmonics. By employing two sections of 70.7Ω ATLs with a right-angled triangle shape to replace conventional transmission lines, the proposed power divider working at 0.9GHz 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 branch-line coupler, two pairs of ATLs with 50Ω and 35.4Ω are utilized. For 50Ω ATLs, the ATLs are designed to a right-angled triangle shape. Meanwhile, to obtain a more compact size, these 35.4Ω ATLs are modified to an isosceles trapezoid shape. The proposed branch-line coupler operating at 0.9 GHz accounts for merely 33.4% of a coupler adopting conventional microstrip transmission lines. Moreover, the harmonics of a branch-line coupler are suppressed effectively as well. Finally, measured results of the proposed Wilkinson power divider and branch-line coupler display good performance and agree with their simulated results well.


Introduction
The rapid development of the modern communication system makes a high requirement for size miniaturization of microwave components [1][2][3].At present, size miniaturization methods of microwave components include loading lumped elements [4], using right/left-handed transmission lines [5], employing a defected ground structure (DGS) [6], utilizing a microstrip electromagnetic bandgap (EBG) [7], and applying slow-wave 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 signal-to-noise 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 ultra-wideband band-stop filter [9], an electromagnetic bandgap [10], open stubs [11], a defected ground structure [12], and inductively loaded slow-wave 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 quarter-wavelength transmission lines and an insolation resistor.Therefore, the size of the Wilkinson power divider is mainly determined by the size of quarter-wavelength 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 half-wavelength and quarter-wavelength resonators [15], loading coupled resonator topology [16], utilizing topology consisted of half-wavelength resonators and quarter-wavelength resonators [17], and using open dualtransmission line stub and L-type 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 net-type resonator is designed to obtain rejection of harmonic responses.In [17], the mixed electric and magnetic coupling and cross-coupling between three quarterwavelength resonators generate three transmission zeroes to suppress harmonics.In [18], open dual-transmission line stub, L-type 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 branch-line coupler is another important microwave component used in a modern communication system.However, the conventional 3 dB branch-line coupler is also confronted with the problem of a large occupied area brought in by two pairs of quarter-wavelength transmission lines with 50 Ω and 35.4 Ω.In order to reduce the size, some solutions are reported, such as loading a π-equivalent artificial transmission line [19], using asymmetrical T-structures [20], utilizing high-impedance 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][22][23] can realize harmonic suppression.In [21], the second-order harmonic is suppressed to 20 dB by using interdigitated shunt capacitors.In [22], shunt LC resonators generate suppression for high-frequency harmonics, but a second-order harmonic and third-order 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 branch-line 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 right-angled 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 branch-line 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.

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 right-angled triangle shape is composed of five sections of microstrip low-impedance transmission lines and six sections of microstrip highimpedance transmission lines.Additionally, there are two sections of uniform transmission lines connected to ports.The low-impedance lines and high-impedance lines are cascaded alternately and then folded into a right-angled 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 L as1 and a shunt capacitor C as1 to ground.Correspondingly, the high-impedance lines can be equivalent to series inductors L as2 , L as3 , and L as4 , and these low-impedance lines work as equivalent shunt capacitors C as2 , C as3 , and C as4 to ground.The interdigital capacitor is placed between several high-impedance 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.
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 L t1 = 2L as1 + 2L as2 + 2L as3 + 2L as4 , In the design of the 3 dB branch-line 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 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 branch-line 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

Port2 Port1
According to the uniform transmission line theory, characteristic impedance Z i (i = 1,2,3) and phase propagation constant β i (i = 1,2,3) of per unit length are determined by Z i = L i /C i and β i = ω L i ⋅ C i , where L i and C i are the inductance and capacitance per unit length along the transmission line and ω represents the operating angular frequency.For ununiform transmission lines such as ATLs, when the physical length of the ATL is less than one-eighth of a guided wavelength, the ununiform transmission lines can approximate to uniform transmission lines [24].Then, for the ATL, L ti = L i l i and C ti = C i l i , where L ti and C ti are the total series inductance and shunt capacitance and l i is the physical length of a unit ATL.The characteristic impedance Z i and electrical length θ i 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 L ti and C ti increase proportionally, that Z i would remain unchanged, while the electrical length θ i would increase.In turn, with a given characteristic impedance Z i and electrical length θ i of a unit of an ATL, when L i and C i are increased proportionally, characteristic impedance Z i would be unchanged, while phase propagation constant β i of per unit length would be increased, so the required physical length l i 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 right-angled 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 pass-bands 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 LC parallel-resonant circuits, as shown in Figures 1(b), 2(b), and 3(b).According to the stopband characteristic of LC parallel-resonant circuits at resonant frequency, these high-order 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 quarter-wavelength 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 Z 1 can be obtained by simulation directly, and electric length θ 1 can be got by simulated Ang(S 21 ).
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 L t1 and C t1 should be increased or reduced.
In tuning of characteristic impedance Z 1 , when the simulated characteristic impedance Z 1 is lower than 70.7 Ω, according to equation (4), we can increase total inductance L t1 or reduce total capacitance C t1 .When the simulated characteristic impedance Z 1 is higher than 70.7 Ω, we can reduce total inductance L t1 or increase total capacitance C t1 .
In tuning of electric length θ 1 , when electric length θ 1 is smaller than 90 °, according to equation ( 5), we can increase total inductance L t1 or total capacitance C t1 .When simulated electric length θ 1 is larger than 90 °, we can reduce total inductance L t1 or total capacitance C t1 .
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 low-impedance 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 4 International Journal of Antennas and Propagation 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 Aj (j = 1, 2) 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 branch-line 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 L a6 = 5 2 mm and width W a6 = 0 2 mm 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 L a6 = 5 2 mm and width W a6 = 0 2 mm remain unchanged, the simulated S 21 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 W a6 = 0 2 mm, 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 W a6 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.
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.

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 As shown in Figure 6(a), for the 70.7 Ω ATL, the simulated S-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   6(a).For the second-order harmonic frequency at 1.8 GHz, S 21 is -10.66 dB, while for the third-order harmonic frequency at 2.7 GHz, S 21 is -17.71 dB.For these harmonic frequencies, there are no pass-bands.
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 third-order 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    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 70.93Ω and 2.08 Ω.For the 50 Ω ATL, the 50.0Ω 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 35.4ΩATL, the real part of characteristic impedance is 35.4Ω and the imaginary part is 0.24 Ω, while the corresponding results of the equivalent circuit are 35.85Ω 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 S-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.

Circuit Design and Measurements
The proposed Wilkinson power divider and branch-line coupler are designed to operate at 0.9 GHz.Simulations are performed by a full-wave EM simulation software of IE3D.The

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International Journal of Antennas and Propagation proposed Wilkinson power divider and branch-line coupler are fabricated on a substrate of F4B with 1 mm thickness and relative permittivity ε r = 2 65 and loss tangent δ = 0 005.Measurements are carried out on an Agilent 8510C network analyzer.

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 quarter-wavelength 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 29 9 mm × 34 7 mm, that is 0 13λ g × 0 16λ g , where λ g is the guided wavelength on the substrate at 0.9 GHz.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 S 11 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 S 21 is -3.13 dB and S 31 is -3.26 dB.In addition, as shown in Figure 10, the fabricated Wilkinson power divider has 12.5 dB suppression for the second-order harmonic of 1.82 GHz and 24.7 dB suppression for the third-order harmonic of 2.73 GHz.
Figure 11 shows isolation and phase difference (Ang(S 21 )-Ang(S 31 )).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 S-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.12 International Journal of Antennas and Propagation mm, that is 0 2λ g × 0 15λ g , where λ g is the guided wavelength on the substrate at 0.9 GHz.The size of the proposed branch-line coupler is effectively reduced to a 33.4% area of a conventional one at 0.9 GHz. Figure 14 shows the simulated and measured S 11 , S 21 , and S 31 of the proposed coupler.The S-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 S 11 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, S 21 and S 31 are -3.2 dB and -3.15 dB, respectively.In Figure 15, S 41 and phase difference between the two output ports are given.At 0.91 GHz, measured S 41 is -26.3 dB.The phase difference (Ang(S 21 )-Ang(S 31 )) is 90.6 °, while the 90 °± 1 °bandwidth is 34 MHz, from 881 MHz to 915 MHz.
In addition, at the second-order harmonic frequency of 1.82 GHz, S 11 is -1.26 dB, S 21 is -12.6 dB, and S 31 is -11.1 dB.At the third-order harmonic frequency of 2.73 GHz, S 11 is -0.34 dB, S 21 is -29.5 dB, S 31 is -33.9 dB.It is demonstrated that at the second-order and third-order harmonic frequencies, there are no pass-bands.So harmonic suppression performance is verified.As a comparison, S -parameters of the conventional branch-line coupler are given in Figure 16.By observation, the bandwidth of the proposed branch-coupler is comparable to that of the conventional one, while these harmonic pass-bands at harmonic frequencies are effectively suppressed.4 shows the performance comparisons of the proposed branch-line coupler (this work) and several previous designs.The performance comparisons reveal that the proposed branch-line coupler has similar bandwidth, insertion loss, and isolation with other designs, but better harmonic suppression with a miniaturized size.

Figure 1 :
Figure 1: A section of 70.7 Ω ATL with a right-angled triangle shape: (a) layout and (b) equivalent circuit.

Figure 2 :
Figure 2: A section of 50 Ω ATL with a right-angled triangle shape: (a) layout and (b) equivalent circuit.

Figure 3 :
Figure 3: A section of 35.4 Ω ATL with an isosceles trapezoid shape: (a) layout and (b) equivalent circuit.
), 2(b) and 3(b) are simulated by software of ADS.The comparisons of simulated S-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.

Figure 8 :
Figure 8: Simulated phase delay of the proposed ATLs.

Figure 14 :
Figure 14: Simulated and measured S 11 , S 21 , and S 31 of the proposed branch-line coupler.

Figure 15 :
Figure 15: Simulated and measured S 41 and phase difference of the proposed branch-line coupler.

Figure 16 :
Figure 16: Simulated S-parameters of the conventional branch-line coupler.

Table 1 :
Detailed element parameters of ATLs.

Table 3 :
Performance comparisons of power dividers.
λ g : guided wavelength at central frequency.

Table 4 :
Performance comparisons of branch-line couplers.
14g : guided wavelength at central frequency.14InternationalJournal of Antennas and