диафрагмированные волноводные фильтры / 8fcbbd86-da25-4a24-b9dc-1a88b0f2c8c7

ABSTRACT—A novel substrate integrated E-plane waveguide is proposed. It is used to implement E-plane type of waveguide circuits on printed circuit boards which cannot be accomplished by the conventional substrate integrated waveguide. The new waveguide structure has two copper strips at the middle layer of the substrate. The strips follow the directions of the plated through holes that emulate the vertical wall of a conventional rectangular waveguide. Each strip provides a path for the longitudinal current along the vertical waveguide wall. Simulation is carried out to demonstrate that the proposed waveguide is able to guide horizontally polarized electromagnetic wave. As an example, an E-plane waveguide filter is designed based on the new waveguide structure.

I.INTRODUCTION

With the development of wireless systems, waveguide components have been widely used in various microwave and millimeter-wave communication systems due to the advantages such as high quality factor (Q-factor) and high power capability [1], [2], [3], [4]. However, the bulky size and not being suitable for high density integration make it hard to achieve low-cost fabrication and production [5], [6], [7].

The idea of substrate-integrated waveguide (SIW) technology offers a promising solution on the above mentioned problem [8], [9], [10], [11], [12]. This integrated waveguidelike structure offers a platform with high performance and lowcost. Most significantly, it allows the integration of waveguide circuits with microstrip line circuits on the same circuit board. It not only preserve the advantage of classical rectangular waveguides with high quality factor, but also has excellent power handling capability. Many components including passive circuits, active circuits and antennas have been proposed based on the substrate integrated waveguide technique since then. An SIW naturally utilizes the copper plating of a circuit board as the top and bottom walls of a waveguide and uses a series of plated through holes as the vertical walls. This synthesized waveguide is successfully applied to design many H-plane type of waveguide circuits where the electric field is in a direction normal to the circuit board. However, it is not suitable to implement E-plane type of waveguide circuit, where the electrical field is parallel to the circuit board. It is because the synthesized vertical wall only allows current to flow along the through hole, but not in the longitudinal direction which is required by the E-plane type of circuit.

In this paper, the concept of substrate integrated E-plane waveguide (SIEW) is presented. It introduces a metal strip at the middle of all through holes to provide a path for the

Fig. 1. Geometry of the substrate integrated E-plane waveguide

longitudinal current along the vertical walls. The proposed SIEW allows the propagation of wave with electric field parallel to the circuit board. An SIEW is designed to operate at 12-13GHz. An E-plane SIEW filter is also presented. As a complement structure to SIW, the SIEW provides more design possibilities for researchers who are working on substrateintegrated circuits. The rest of the paper is organized as follows. Section II presents the waveguide geometry. Section III discusses simulation results. An E-plane filter as an application of the SIEW is proposed in Section IV. Section V is the conclusion.

II.WAVEGUIDE GEOMETRY

The configuration of a substrate-integrated E-plane waveguide is shown in Fig. 1. The waveguide is built by binding two pieces of circuit boards together. The boards are made of the same material and with the same thickness of h. The top board has copper plating on top surface only. The bottom board has two copper strips on its top surface and copper plating on the bottom surface. Two rows of plated through holes are drilled to connect the top and bottom plating. The holes are closely aligned with a spacing p and are along the two copper strips at the middle of the waveguide. Those holes also penetrate the copper strips. The longitudinal cross-sectional view at the middle of the waveguide is illustrated in Fig. 2. Herein, holes with diameter d=1mm and spacing p=1.5mm are used. The plated through holes and the middle copper strips altogether function as the vertical metallic walls of a rectangular waveguide to support currents both longitudinally and vertically. Printed circuit board with thickness of 2.5mm,

Fig. 2. The longitudinal cross-sectional view at the middle of the waveguide

dielectric constant of 10.7 and loss tangent of tanδ=0.0023 is used in this study. The total thickness of the proposed waveguide is 5mm. It is considered as the long dimension of the waveguide since the distance between two arrays of holes is 3.66mm.Thus, the fundamental mode will be similar to the TE10 mode of a rectangular waveguide, with electric field parallel to the circuit board. Consequently, the E-plane is parallel to the circuit board. The distance between the two middle strips is 2.46mm and the width of each strip is 3mm.

III.WAVEGUIDE CHARACTERISTICS AND PERFORMANCE

The simulation is carried out using HFSS software. The SIEW is excited using horizontally polarized wave with electric field parallel to the circuit board. The electric field distribution in the transverse cross section at the middle of the waveguide is plotted in Fig. 3. As can be seen, the excited mode inside the SIEW is similar to the TE10 mode of a conventional rectangular waveguide. The electric field distribution in the longitudinal cross section is shown in Fig. 4, which demonstrates that the electromagnetic energy is confined and guided inside the proposed structure. Fig. 5 shows the current distribution on the middle strips. It shows that most energy on the strips is close to the through holes and the SIEW has little leakage. The simulated S-parameters from 12-13GHz are shown in Fig. 6. Good impedance matching with return loss at around 40dB and insertion loss close to 0dB can be observed.

Considering the electrical connectivity of the plated through holes and the middle strips cannot be always guaranteed during fabrication, the effects of such disconnectivity is investigated. Fig. 7 shows two different configurations of the SIEW, one with the hole plating connected to the middle strips and one with those disconnected. The simulated S- parameters are plotted and compared in Fig. 8. It can be seen that the disconnectivity of the hole plating and the middle strips has little effect on the waveguide performance. It is because the plated through holes only support vertical current

Fig. 3. Electric field distribution in the transverse cross section.

Fig. 4. Electric field distribution inside the SIEW.

Fig. 5. Surface current on the middle strips

Fig. 7. SIEW with middle strips and plated through holes (a) connected and

(b) disconnected

and the middle strips only support longitudinal current. Those two types of currents are orthogonal to each other and neither relies on the electrical connectivity between the hole plating and the middle strips.

Fig. 8. Effects of the disconnectivity of the middle strips and the plated through holes

As discussed, the major geometrical difference between SIW and SIEW is the introducing to middle strips in SIEW. To confirm the significant role of the middle strips on the propagation of horizontally polarized wave, a conventional SIW without the middle strips is simulated. All the dimensions are the same as that in Fig. 2 except that the two middle strips are removed. The same as for the SIEW case, the SIW is excited by a horizontally polarized electric field. The simulated electric field distribution is illustrated in Fig. 9. It can be seen that the electromagnetic energy is radiated in all directions instead of guided by the SIW. So, the SIW is unable to guide the propagation of the wave without the help of the two middle strips. Particularly, the strong electric field observed in the gaps between the neighboring plated through holes indicates that those gaps act as slot antennas to radiate electromagnetic energy. The resulting S-parameters are compared to that of the SIEW in Fig. 10, which also shows that the SIW is very lossy for the horizontally polarized wave due to leakage between the plated through holes.

Fig. 11. Longitudinal cross-sectional view of the SIEW filter

Fig. 12. S-parameters of the SIEW filter

IV. INDUCTIVE SEPTA FILTER DESIGN

An inductive septa E-plane bandpass filter is designed based on the SIEW structure. The filter geometry is shown in Fig. 11. Five pieces of copper septa are placed at the same layer of the middle strips. All of them connect the two middle strips. The simulated S-parameters are plotted in Fig. 12, where the return loss is better than 20dB in the pass-band of 12.4- 12.7GHz and the insertion loss is at around 3dB in the passband. The loss is mainly due to dielectric loss of the substrate.

V.CONCLUSIONS

The concept of substrate integrated E-plane waveguide is proposed. The new waveguide structure is built by stacking two printed circuit boards with plated through holes penetrating both boards and two copper strips sandwiched between the two boards. It supports the propagation of electromagnetic wave with horizontally polarized electric field, which cannot be achieved by using the conventional substrate integrated waveguide structure. An E-plane waveguide is designed for a frequency band from 12 to 13GHz. Simulation shows good return loss at 40dB and insertion loss close to 0dB. As an application example, an E-plane bandpass filter is designed based on the proposed waveguide structure. It can be expected that other E-plane type of waveguide circuits can be implemented on printed circuit board using this new synthesized E-plane waveguide structure.

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