диафрагмированные волноводные фильтры / 1d30b4d0-99b4-4661-9233-6ce1a89cfc55

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International Journal of Electronics

2012, 1–12, iFirst

A novel H-plane filter using double-layer substrate integrated waveguide with defected ground structures

Hassan Aghayaria*, Nader Komjanib and Nima Molaei Garmjanib

aDepartment of Communication Engineering, Saeb Institute of Higher Education, Abhar, Iran; bDepartment of Electrical Engineering, Iran University of Science & Technology (IUST), Tehran, Iran

(Received 30 August 2011; final version received 1 August 2012)

The novel double layer substrate integrated waveguide (SIW) technology is used for realisation of conventional H-plane filter, which is manufactured in waveguide. This proposed filter is totally realised in double layer dielectric substrate with metallic vias and fabricated using a standard printed circuit board (PCB) process. In previous studies, prototypes of E-plane and H-plane filter were designed and fabricated in standard waveguides. The H-plane type of those two has the same frequency response as that of the E-plane type, while its cross section is one-quarter. Similarly, the SIW H-plane filter, which is presented in this article, has the same dispersion characteristics as that of waveguide filter while its dimensions are very shorter. Moreover, by using a sandwich model of double layer SIW, the interleaved metal vane is fabricated between two substrates easily. We can also improve the frequency response of the SIW H-plane filter using defected ground structure (DGS). Therefore, in DGS SIW H-plane filter, which is presented, the return loss and insertion loss in passband are less than conventional H-plane filter. The improvement of the spurious response is the other trait of DGS SIW H-plane filter.

Keywords: substrate integrated waveguide (SIW); defected ground structure (DGS)

1. Introduction

In the last several years, waveguide filters have been widely used in millimetre wave and microwave circuits with their salient features such as low insertion loss, high-quality factor and high-power transmission. Nevertheless, it is difficult to be integrated in microwave and millimetre wave integrated circuits because of their bulky size, rigid and non planar structure. The H-plane filter is the conventional type of waveguide filters, which are fabricated with interleaved metal vane within a rectangular waveguide (Kim and Lee 2005). Upkeep of this metal vane in the middle of waveguide is the important compositional problem.

Recently, a novel planar structure technique called substrate integrate waveguide (SIW) is introduced as it shares many of the advantages with printed circuits such as small size, low cost and more easily integration with the microwave and millimetre wave integrated circuits. Many passive and active components based on SIW technique such as

*Corresponding author. Email: n.molaei@gmail.com

ISSN 0020–7217 print/ISSN 1362–3060 online2012 Taylor & Francis http://dx.doi.org/10.1080/00207217.2012.727101 http://www.tandfonline.com

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Coupler (Djerafi and Wu 2007), Phase Shifter (Cheng, Hong, and Wu 2007), Power Divider (Yang and Fathy 2007), Balun (Zhang and Wu 2007), Diplexer (Tang, Hong, Chen, Luo, and Ke-Wu 2007) and Wide Band Filters (Zhang, Xu, Yu, and Dong 2007) have been reported recently.

In this article, we represent a novel design of double-layer SIW based on a sandwich model for realisation of S-band partial H-plane filter in the substrate. Using of this model, the interleaved metal vane in the middle of waveguide filter is fabricated between two substrates more easily and the dimensions of the SIW H-plane filter are very shorter than waveguide filter.

In addition, the linear imperfections in the defected ground structure (DGS) form are used for reduction of the return loss and insertion loss of SIW filter in the frequency response. Therefore, in DGS SIW H-plane filter, the return loss and insertion loss in passband is less than conventional H-plane filter and the improvement of the spurious response in DGS SIW H-plane filter will be achieved.

Moreover, a transition circuit between SIW and stripline has been designed for excitation of the filter. A good agreement between measured and simulated results for the frequency response of proposed DGS SIW H-plane filter will be presented.

2. Partial H-plane waveguide filter

A partial H-plane waveguide filter consists of alternative resonators with evanescent waveguide segment. Evanescent waveguide segments are performed by inserting H-plane septa between the metal vane and side wall of waveguide (position of d and a in Figure 1 (Kim and Lee 2005)).

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In Figure 1, Ji,iþ1 is admittance inverter value between ith and (i þ 1)th resonator, wj is the length of jth evanescent waveguide segment and rk is the length of kth the resonator.

In these equations, Yg is the waving admittance of partial H-plane waveguide.

The partial H-plane filter is a one type of direct-coupled resonator filter. Thus, evanescent waveguide segment can be depicted by an admittance inverter circuit as shown in Figure 2.

The susceptability values of Ba and Bb in Figure 2 are functions of the length of evanescent waveguide segment. The normalised inverter value of admittance inverter circuit and negative electrical length, ’ are given by (Matthaei, Yong, and Jones 1980) as:

The design of the filter, which is based on numerical techniques, is performed in Kim and Lee (2005) by the following four steps. The first step is the determination of a unit cell to extract S-parameters. The unit cell is composed of H-plane waveguide on both sides and the evanescent waveguide segment in the centre.

Using commercial EM simulators such as HFSS or CST studio, a unit cell with varying lengths of H-plane septum for evanescent waveguide segment was simulated and its S-parameters in the centre frequency of the filter were extracted. We assumed that only the dominant mode propagates in the unit cell structure. The second step is the conversion of the extracted S-parameters into ABCD matrices. The extracted S-parameters must follow S11 ¼ S22, S12 ¼ S21 and jS11j2þjS21j2¼ 1 because the unit cell is a symmetrical and reciprocal structure. In the third step, the ABCD matrix is achieved. Later, the susceptibility values of Ba and Bb is given by ABCD matrices of the evanescent waveguide for corresponding septum length.

The length of the evanescent waveguide segment of filter is achieved in the last step using (2) and the normalised inverter values for equal-ripple bandpass filter as a function

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of the septum length (Matthei et al. 1980). The negative electrical length ’j is determined by (3) for each length of the evanescent waveguide segment and the resonator length rk is given by:

where 0 is the propagation constant of H-plane waveguide in the centre frequency of the filter.

The said steps were used for designing an S-band H-plane waveguide filter and these designed filter sizes were shown in Table 1. This filter was manufactured in standard S- band waveguide and a good agreement between its simulated and measured results was presented (Kim and lee 2005).

We simulated this filter in the Ansoft HFSS11 and its simulated results were shown in Figure 3.

3. Substrate integrated waveguide (SIW)

The substrate integrated waveguide is a new type of planar structure that is based on a low-cost printed circuit board (PCB) process such as microstrip, stripline or coplanar waveguides.

The SIW is manufactured in a substrate with arrays of metallic vias to realise the side walls of waveguide, and its transitions to planar structures such as the microstrip, stripline or coplanar waveguide are designed and integrated on the same substrate.

Table 1. Designed H-plane waveguide filter sizes (unit: mm).

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SIW components can be integrated together without any transition circuit. A transition circuit is used for integrating the SIW component and other planar structures. The performance factors of the SIW component, such as insertion loss, quality factor and power transmission, are better than those of microstrip stripline or coplanar waveguide.

However, the analysis of via structure of the SIW component is much more complicated compared with the conventional waveguide devices. By applying a generalised BI-RAM method, the propagation characteristics of the SIW component can be achieved. It was assumed that a TE10-like mode in the SIW has dispersion characteristics akin to the TE10 mode of a dielectric filled rectangular waveguide with an equivalent width. This equivalent width was named as the effective width of the SIW (see Figure 3). Other important parameters of SIW are the diameter of vias (D) and the distance between them (b).

The SIW parameters of Figure 3 can be approximated in the habit together as follows (Matthei et al. 1980; Hong, Cui and Wu 2003):

D2

W ¼ arwg þ 0:95b ð4Þ

The practical conditions in the design of SIW as ðb DÞ 0:2 and ðb DÞ=D 0:5 should be happened.

If (b D) is greater than 0.2 , the energy leakage between consecutive vias will be great. If the diameter of via holes (D) is over the range, the reflection loss of the SIW filter will be great. Since the field distribution in the SIW component is similar to a conventional rectangular waveguide, it is possible to use the same formula to compute the cut-off frequency of the SIW by:

Thus, the air-filled waveguide with width a is equal to the dielectric filled waveguide

4. SIW H-plane filter sandwich model

The SIW H-plane Filter is made of two dielectric substrates with metallic vias. The sketch of H-plane metal vane is created in upper surface of one substrate and lower surface of another substrate is wasted. The diameter of metallic vias and distance between them are chosen as explained in the previous section. More details of this plane are shown in Figure 4.

In order to design a SIW H-plane Filter, the dielectric constant of the substrate and its height were assumed equal to 3.55 and 0:03200, respectively. Moreover, the diameter of the metallic vias (D) and the distance between them (b) were chosen 0.8 mm and 1.5 mm, respectively. The SIW width was also obtained as 12.63 mm via the formulation of Section 3. Therefore, the dimensions of the designed filter are obtained as shown in the Table 2.

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Figure 4. Filled waveguide to substrate mapping and SIW parameters.

Table 2. Dimensions of traditional H-plane and SIW H-plane filter (unit: mm).

Figure 5. Sandwich model of two substrate and metal vane between them.

According to Table 2, it is recognised that the width, height and length of the filter were reduced noticeably.

The SIW H-plane filter which is presented in Figure 5 was simulated by wave port excitation in HFSS and its results are shown in Figure 6.

According to the simulated results of SIW H-plane filter (Figure 6), the insertion loss and the return loss were increased to 1.17 dB and 9.83 dB, respectively.

5. Response improvement by DGS and excitation mechanism

In this section, the linear defects in DGS form are used for reduction of the return loss and insertion loss in frequency response of the SIW H-plane filter. Radiation of slotted from linear defects in the ground of SIW filter is the very important point. Hence, the surface current density in the top and bottom layers of H-plane filter was plotted in Figure 7 (Cassivi et al. 2002).

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Figure 6. Simulation results of SIW H-plane filter with wave port excitation.

*

Figure 7. JS in the top and bottom layer of H-plane filter.

Figure 8. Configuration of linear defects in the top and bottom of DGS SIW H-plane filter.

The configuration of linear defects shown in Figure 8 is used and their location and

dimension were optimised for response improvement of DGS SIW H-plane filter. This

*

configuration was selected to the linear defects and JS was not secant. The parameters of the defects which were shown in Figure 8 are seen in Table 3.

The DGS SIW H-plane filter, presented in Figure 7, was simulated by wave port excitation in HFSS and its results are shown in Figure 9.

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Figure 9. Simulation result of DGS SIW H-plane filter with wave port excitation.

Figure 10. SIW to microstrip transition and its parameters.

Figure 11. Side view of the DGS SIW H-plane filter with stripline excitation.

For the excitation of SIW component, a few structures were purposed (Karmakar, Roy, and Balbin 2000). Typical form of them is made of the tapered microstrip line between the SIW section and 50 microstrip lines. This plane is observed in Figure 10.

Here, the excitation mechanism of DGS SIW H-plane filter is made of a tapered strip line between metal vane, which is located in the middle of sandwich model and 50 stripline. The stripline width on SIW section (q) is obtained from the input impedance of SIW. The length of transition (p) is optimised by the software to reach the minimum of the reflection coefficients. These parameters are as follows:

q ¼ 5:8 mm, w ¼ 0:83 mm, p ¼ 10:5 mm

The final plane of designed DGS SIW H-plane filter is shown in Figure 11.

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Figure 12. Top and bottom view of the designed DGS SIW H-plane filter with strip line excitation.

Figure 13. Simulation result of DGS SIW H-plane filter with strip line excitation.

We simulated the final plane of the DGS SIW H-plane filter with strip line excitation presented in Figure 12 and its results are shown in Figure 13.

The simulation showed the return loss and the insertion loss better than 21.5 dB and 1.3 dB, respectively. Therefore, a good agreement between simulated results of conventional H-plane waveguide filter and presented DGS SIW H-plane filter is realised.

The spurious responses in the stop band of SIW and DGS SIW filters have been simulated and are shown in Figure 14. The simulations have shown that the stop band response of the DGS SIW filter is better than the SIW filter.

6. Fabrication and measurement

The prototype of DGS SIW H-plane filter is fabricated on the double-layer substrate using a traditional PCB process in which the minimum radius of the metallic via is 0.4 mm. Using this fabricating process, metallic vias in the SIW filter can be located at the design position with a high accuracy of 0.02 mm and SIW-stripline transition can be realised with an error of 0.05 mm. The used substrate for this fabrication is Rogers RO/4003 with the following parameters:

"r ¼ 3:55, h ¼ 0:032000, tan ¼ 0:002