диафрагмированные волноводные фильтры / 4f2b9d9c-7742-43b0-a32f-2ac7dadcbc5e

Abstract—A partial -plane filter, which can be compared with a conventional -plane filter, is proposed as a new type of compact direct-coupled resonator filter. The partial -plane filter has the same frequency response as that of the -plane filter while its cross section is one quarter. Measured results are in good agreement with computed results.

Index Terms—Direct-coupled resonator filter, -plane filter, folded waveguide.

I. INTRODUCTION

PREVIOUSLY, a large number of filters made by hollow metal waveguide were developed as microwave and mil- limeter-wave filters. However, they have the disadvantage that their volumes are relatively bulky at the low frequency. In this letter, a partial -plane filter is proposed as a new type of compact direct-coupled resonator filter, which is implemented by a partial -plane waveguide [1]. A partial -plane waveguide, suggested by the concept of folded waveguide [2], [3], has the same dispersion characteristics for the first two dominant modes as those of a rectangular waveguide while its cross section is

one-quarter cross section.

A partial -plane filter can be compared with an -plane filter [4], [5] since the one has the inserted metal septa in -plane while the other has the inserted metal septa in -plane. Both filters also have inductive coupled structures. It will be shown that a partial -plane filter has the same frequency response as that of a conventional -plane filter while its cross section is one-quarter. In addition, the proposed filter has advantages of low cost and mass-producible property even though it has higher insertion loss than -plane filter due to its compactness. A partial -plane filter has been fabricated using coaxial to partial -plane waveguide transition in the H-band. Good agreement between measured and computed results will

be presented.

II. THEORY AND DESIGN

A partial -plane waveguide has a shape of interleaved -plane metal vane within a rectangular waveguide. Fig. 1 shows the schematic of the partial -plane waveguide and electric field distributions of its first two dominant modes. The detailed analytical expression and the mode definition

Manuscript received September 9, 2004; revised November 16, 2004. This work was supported by University IT Research Center Project.

The authors are with the Department of Radio Science and Communication Engineering, Hongik University, Seoul 121-791, Korea. (e-mail: jeonglee@hongik.ac.kr).

Digital Object Identifier 10.1109/LMWC.2005.847708

Fig. 1. Schematic and E-field distributions of partial H-plane waveguide: (a) structure and (b) dominant (upper) and second (below) modes.

Fig. 2. Dispersion characteristics of partial H-plane waveguide (a: 23.8 mm, b: 12 mm, d: 20.2 mm, and metal vane thickness: 0.1 mm) and rectangular waveguide (width: 47.55 mm and height: 22.15 mm) in H-band.

are described in [1]. The dispersion characteristics compared with rectangular waveguide in the H-band are shown in Fig. 2. Dispersion characteristics of the first two dominant modes (TE modes) are the same as those of the rectangular waveguide. Note that the cross section of the partial -plane waveguide is one-quarter as that of the rectangular waveguide.

The structure of the proposed partial -plane filter made by a partial -plane waveguide is illustrated in Fig. 3. It consists of resonators alternating with evanescent waveguide sections. Evanescent waveguide sections are implemented by inserting -plane septa between the positions of and of Fig. 1. The partial -plane filter is a direct-coupled resonator filter like an -plane filter. Thus, evanescent waveguide sections can be represented by an admittance inverter circuit as shown in Fig. 4. The susceptance values of and are functions of the length of

Fig. 5. Susceptance values for the length of evanescent waveguide.

evanescent waveguide. Normalized inverter value and negative electrical length are given by [6]

(1)

(2)

where is a wave admittance of a partial -plane waveguide. Filter design based on numerical simulation is carried out by the following four steps. First, we determine a unit cell to extract

-parameters. The unit cell consists of partial -plane waveguide on both sides and evanescent waveguide in the center of unit cell. Using commercial EM simulators HFSS and/or CST studio we simulate a unit cell with varying a length of -plane septum for evanescent waveguide and extract the -parameters for the center frequency of the filter. It is assumed that only dominant mode propagates in the unit cell. The second step is to convert extracted -parameters for each septum length into matrices. Since a unit cell is a symmetrical and reciprocal structure, extracted -parameters must follow , , and . In the third step, we obtain matrices of evanescent waveguide for each septum length using the relationship between -parameters and matrix. Subsequently, matrices of evanescent waveguide for corresponding septum length give the susceptance values of and . Fig. 5 shows the susceptance values against length of evanescent waveguide and those of an -plane filter obtained by the same method. In the last step, the lengths of evanescent waveguide sections of filter ( ) are obtained using (1) and the normalized inverter values for equal-ripple bandpass filter [6] as a function of septum length. The negative electrical lengths for each are defined as (2) and resonator lengths are given by

(3)

Fig. 7. Fabricated partial H-plane filter. H-plane metal vane with H-plane septa (bottom figure) is inserted in partial H-plane filter (upper figure).

Fig. 8. Simulated and measured responses of partial H-plane filter.

where is a propagation constant of partial -plane waveguide for the center frequency of filter.

III. FABRICATION AND MEASUREMENT

Using the previously described method, we designed the partial -plane filter and -plane filter in the -band. The specifications for both filters are: 5-GHz center frequency, 0.01-dB pass-band ripple, and 5% relative bandwidth. The inserted metal vane thickness of both filters was 0.1 mm. Fig. 6 shows the simulated pass-band and spurious responses of both filters. Frequency responses of both filters are almost the same. The designed filter sizes are compared in Table I. The overall length of a partial -plane filter is almost the same as that of an -plane filter while its cross section is one quarter. To compare the insertion loss due to conduction loss, the conductivity of 5.8 10 is

assumed. The calculated insertion losses of partial -plane and

-plane filters are 0.06 dB and 0.032 dB, respectively. Thus, it is found that insertion loss of partial -plane filter is lager than that of the -plane filter because of its compactness.

To verify our approach, the partial -plane filter has been designed along with coaxial to partial -plane waveguide transition. As shown in Fig. 1(b), the -field of dominant mode is concentrate on end of the -plane metal vane. So, we introduce a coaxial transition structure. A coaxial probe from the narrow sidewall is inserted at the rectangular intaglio in the

-plane metal vane. It is located at about a quarter wavelength long distance from the end metal wall of the filter. A coaxial to partial -plane waveguide transition is made of commercially available SMA connector. The photograph of fabricated partial -plane waveguide is shown in Fig. 7. Frequency responses obtained by simulation and measurement of partial -plane filter

are shown in Fig. 8, showing a good agreement.

IV. CONCLUSION

As a new type of compact direct-coupled resonator filter, a partial -plane filter has been presented. The proposed partial -plane waveguide filter has been implemented and compared with the conventional -plane filter. Both filters have the same frequency characteristics, while the partial -plane filter has one quarter cross section of the conventional -plane filter. For the fabrication, a coaxial to partial -plane waveguide transition is introduced. A partial -plane filter has been fabricated in -band and the measured results are in good agreement with the computed results.

REFERENCES

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