диафрагмированные волноводные фильтры / 10c4d153-84ff-40c5-892d-9c63668ee227

Abstract — A novel compact waveguide resonator in planar form with EBG is presented in this paper. Dielectricfilled rectangular waveguide is fitted with periodical arrays of metal plates at its top and bottom surfaces. Periodic resonators are designed and simulated with a resonant frequency of 2.4 GHz. Proposed structures are compact and perform a higher Q factor than their conventional waveguide planar counterparts. This is achieved by alternation of electromagnetic properties of the waveguide. A novel compact EBG-substrate bandpass filter in planar form demonstrates feasibility of the proposed resonators.

I. INTRODUCTION

Modern tendencies of research and development in telecommunications today are led by customers demand for mobile, robust and convenient information services at any place any time. As a result, at microwaves, suitable for mass production, low-cost passive devices are widely required in order to build the essential components of microwave communication systems. The printed-circuit- board (PCB) technology has been widely applied for microwave integrated circuits (MIC) due to the simplicity to fabricate the structures that operate at microwave frequencies.

High quality resonators are essential for the most of microwave circuits and systems. At microwave frequencies, the quality factor (Q factor) of metal transmission line resonant circuits is known to be proportional to their volume. Consequently, rectangular waveguide resonators are employed in order to achieve higher Q. However, the difficulty of integration and high cost of conventional rectangular waveguides makes it improper to utilize them for modern applications. The classical rectangular waveguide theory, as a result, is extensively employed in order to build the novel planar passive circuits [1]-[3], viable to meet the actual requirements.

The planar rectangular waveguide, which is filled by a dielectric material of higher permittivity constant, is known to operate at lower frequency. Such structures are also further reduced in size [4]. The losses in planar structures, such as conductor losses, dielectric losses and radiation losses, can be overcome using the planar-to- waveguide transitions. Tapers provide the modal energy conversion from waveguide, supporting mode TE10, to the microstrip, and vice versa. The side walls of the rectangular waveguide can be realized within the substrate, either as an array of metallized posts, metallized grooves or paste side walls. Hence, the ground

plane becomes one metallic wall, while the tapered microstrip line provides the other metallic wall. The result of the design is the complete planar MIC structure that exhibits a good resonant performance at microwave frequencies.

New approach for designing planar rectangular waveguide [7] can be successfully applied in order to improve performance and reduce physical dimensions of planar passive circuits. The concept of electromagnetic bandgap (EBG) substrates implies that the dielectric substrate of the waveguide is patterned with twodimensional lattices of metallic plates at the top and bottom surfaces. The slow-wave effect, which is formed by these high-impedance surfaces, allows the circuit to operate at lower frequencies.

II. DESIGN AND ANALYSIS OF WAVEGUIDE

RESONATORS IN PLANAR FORM

Fig. 1 shows different configurations of Ȝ/2 rectangular waveguide resonators in planar form. The waveguide structure is designed as a MIC dielectric substrate, which integrates the guiding part, microstrip and taper in one complete module. Permittivity of a dielectric material İr = 2.2, thickness h = 1.52 mm, and the substrate used is Rogers RT/Duroid 5880. The finite-element method (FEM) is used for analysis of the proposed structures.

A. Rectangular Waveguide Resonators in Planar Form

Analysis of a resonator, as shown in Fig. 1(a), is conveniently based on the common rectangular waveguide theory. It is a planar structure that integrates resonator, microstrip and taper, preserves the guided wave properties of a conventional rectangular waveguide with the equivalent width a. The dielectric waveguide resonator [5], which resonant frequency f0 = 2.4 GHz, defines width a = 55 mm as an equivalent width for a 1.52 mm-thick dielectric substrate, while length of the waveguide lwg = 60 mm. It contains the resonant part, which is formed by two discontinuities realized as metallic via-holes transversely inserted into the substrate. Diameter of the two vias d = 1.4 mm. Walls on both sides of the waveguide connect the top surface to the ground plane and are constructed by transversely-cut metallized grooves, which are further filled with copper. Mode conversion provides interface of the rectangular waveguide with other planar circuits at input and output ports of the resonator, and is performed by tapers. The latter are also integrated on the substrate.

Fig. 2 plots the simulated transmission coefficient of the proposed resonator (solid line). It has been designed on a 1.524-mm-thick substrate, exhibiting resonance at 2.4 GHz. This structure can be fabricated using the PCB process [6], and is suitable for mass-production, and compact in size.

B. EBG-Substrate Resonators

Layout of an electromagnetic-bandgap-substrate resonator is illustrated in Fig. 1(b). Its design is based on the rectangular waveguide resonator structure, as shown in Fig. 1(a). Furthermore, EBG substrate is realized by placing the two-dimensional lattice of patterned metallic plates at the top and bottom surfaces of the waveguide. These periodic arrays of plates, constructed by the PCB technology, are connected to the ground by a set of metallic protrusions, transversely inserted through the substrate. This new conducting surface alters electromagnetic properties of the waveguide and allows a complete bandgap for the surface waves of either TE or TM polarization.

The effect of electromagnetic bandgap structures for rectangular waveguides has been investigated, and various kinds of patterns have been reported [7]-[9]. Due to high surface impedance, which they exhibit within the stopband, they are usually referred to as high-impedance surfaces. The surface impedance of EBG substrates is frequency dependent, and can be described using equivalent parallel resonant LC-circuit. In the frequency range, where surface impedance is high, the magnetic field is small, and the surface therefore behaves as a magnetic conductor.

Incorporating EBG substrates for a planar rectangular waveguide we can achieve the new type of resonators. These circuits, characterized by the slow wave effect, can operate at lower frequencies. The cutoff frequency of a rectangular waveguide, commonly defined as

in this case is not limited only by its inverse proportion to the dielectric constant İr, but also depends on the new propagation properties of the EBG-substrate waveguide. This results in significant reduction of physical dimensions of such resonators.

Proposed structure is realized as a multilayer circuit board, with three EBG substrate layers. The top and bottom layers are made on an EBG substrate, as described in part A of this section, but the top surface contains a lattice of square plates, connected to the ground by a set of metal protrusions.

Frequency (GHz)

Fig. 2. Simulated transmission coefficient of a rectangular waveguide resonator in planar form and EBG-substrate resonator.

CHARACTERIZATION OF RESONATORS BY Q FACTOR

Frequency (GHz)

Fig. 3. Simulated transmission coefficient of a rectangular waveguide EBG-substrate bandpass filter.

The square metal plates are 6.5-mm wide, and their period is 6.9 mm, while the copper protrusions are 0.5 mm in diameter. Another layer is a dielectric substrate, incorporated between the two layers. All three substrate layers are 0.508-mm-thick Rogers RT/Duroid 5880, designed with equal thickness (h1 = h2 = h3) in order to improve simplicity of fabrication process. Dielectric constants of the substrates İr1 = İr2 = İr3 = 2.2. As shown in Fig. 1(b), discontinuities of the resonator are formed by the two via-holes, 1.4 mm in diameter each, that are drilled through the dielectric.

The resonator has been designed on a three-layer EBG substrate, bringing the total thickness of the structure (h = 1.524 mm) to be the same as of the rectangular waveguide planar resonator.

Simulated transmission coefficient of the EBGsubstrate resonator is shown on Fig. 2 with a dashed line (with EBG).

C. Characterization of Proposed Structures

Table I and Table II present comparison results of the rectangular waveguide resonator without use of EBG and its EBG-substrate counterpart. It shows that incorporation of an EBG substrate for a rectangular waveguide resonator, due to new electromagnetic properties of the structure, results in significant size reduction. The cutoff frequency of a proposed EBGsubstrate resonator is lower than that of the rectangular waveguide planar resonator, and allows using different equivalent width of a rectangular waveguide for the design.

The Q factor comparison underlines advantage of the resonator that incorporates an EBG-substrate. A significant improvement is observed for loaded and unloaded values.

III. APPLICATION OF EBG-SUBSTRATE RESONATORS

In order to illustrate feasibility of the proposed structures an EBG-substrate bandpass filter in planar form was designed and simulated. The equivalent width of a conventional rectangular waveguide for the filter is

a = 7.112 mm, which corresponds to WR-28 (WG-22). As shown in Fig. 3, simulated passband of the filter lies in a frequency range of a rectangular waveguide with equivalent width a, which corresponds to WR-51 (WG-19). This illustrates feasibility to break the limit of size reduction, imposed by a conventional rectangular waveguide theory, and make use of the slow-wave effect of EBG substrates.

IV. CONCLUSION

Novel low cost, compact resonator structures have been designed and simulated. A conventional rectangular waveguide resonator in planar form has been reconfigured, incorporating the EBG substrates in the design. Altered electromagnetic properties of such substrates have been shown to give an advantage for further improvement of resonator structures. Feasibility of the approach has been illustrated for the novel compact EBG-substrate waveguide bandpass filter in planar form.

REFERENCES

[1]C.-Y. Chang, W.-C. Hsu, "Novel planar, square-shaped, dielectric-waveguide, single-, and dual-mode filters,"

IEEE Trans. Microwave Theory and Tech., vol. 50,

pp. 2527-2525, November 2002.

[2]D. Deslandes and K. Wu, "Integrated microstrip and rectangular waveguide in planar form," IEEE Microwave and Wireless Comp. Lett., vol.11,no.2,pp.68-70, Feb.2001.

[3]Y. Cassivi, L. Perregrini, K. Wu and G. Conciauro, "Low cost and high-Q-millimeter-wave resonator using substrate integrated waveguide technique," 2002 European Conference Digest, vol. 2, pp. 737-740.

[4]A. Shelkovnikov, D. Budimir, "Novel waveguide E-plane bandpass filters in planar form," 2003 Asia-Pacific Microwave Conference Digest, Seoul, Korea, Nov. 2003.

[5]A. Shelkovnikov, D. Budimir, "Rectangular waveguide resonators in planar form for filter applications," in IEEE Antennas and Propagation Society Symp. Digest, vol. 2, Monterey, USA, June 2004, pp. 2167-2170.