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PROC. 29th INTERNATIONAL CONFERENCE ON MICROELECTRONICS (MIEL 2014), BELGRADE, SERBIA, 12-14 MAY, 2014

Bandstop Waveguide Filters with Two or Three Rejection Bands

S. Stefanovski, M. Potrebić, D. Tošić, and Z. Cvetković

Abstract - This paper presents a novel design of bandstop waveguide filters with two or three rejection bands, using quarter-wave resonators. Bandstop waveguide filters are designed using printed-circuit inserts realized with single-mode and multi-mode resonators. The filter response is analyzed depending on the parameters of the resonators and their positions on the printed-circuit insert. Various models of dual-band and triple-band filters are developed by employing different combinations of the resonators, as examples of the proposed multi-band filter design.

I. INTRODUCTION

Bandstop filters are considered essential components of telecommunication systems, intended to suppress undesired signals. Particularly, waveguide filters represent sustainable solution for satellite and radar systems, since they are developed to meet strict requirements regarding high power and small losses.

It is known that one of the methods for waveguide filter design is the insertion of discontinuities of various shapes and positions in the waveguide [1]. For example, split ring resonators, realized as printed-circuit inserts in the rectangular waveguide, are widely implemented for bandstop filter design, either as single-mode [2] or multi-mode [3]-[6]. These days, multi-band filters are of particular interest for consideration.

In this paper we present a novel design of bandstop waveguide filters using quarter-wave resonators. Design starts from the implementation of single-mode resonators in order to obtain single-band filter. Further, by employing various positions of these resonators, dual-band and triple-band waveguide filters are developed. It is important to emphasize that there is possibility to independently tune each of the resonators, so each frequency band can be independently controlled. Considered single-mode and multi-mode resonators are implemented as printed-circuit inserts in the transverse planes of the WR90 standard rectangular waveguide, in order to obtain H-plane bandstop waveguide filters operating in X-band. For the filter design

and analysis, WIPL-D software [7] is used to perform full-wave simulations.

The objective of our research is to propose a novel design of multi-band bandstop waveguide filters using quarter-wave resonators and to demonstrate its application by means of various models with different positions of these resonators. In this manner, the implementation of the presented method can be verified.

II. SINGLE-BAND BANDSTOP FILTER USING

QUARTER-WAVE RESONATOR Filter design starts from the model using single-mode

quarter-wave resonator, with resonant frequency of 11 GHz. The three-dimensional (3D) electromagnetic (EM) model of the filter using one quarter-wave resonator, attached to the top waveguide wall, is shown in Fig. 1. The parameters of the resonator are tuned by means of a series of simulations in WIPL-D software, in order to meet the requirement regarding the resonant frequency. According to Fig. 1, for the resonator with f0 = 11 GHz, the dimensions are as follows: d1 = 1.55 mm, d2 = 3.1 mm, c = 0.2 mm, p = 0.45 mm, lstr = 0.5 mm.

p

1d2d

ca

b strl

Fig. 1. 3D EM model of the single-band bandstop filter using quarter-wave resonator.

For the filter design, the WR90 standard rectangular

waveguide of width a = 22.86 mm and height b = 10.16 mm is used. It is assumed that the dominant mode of propagation is the transverse electric TE10 mode. The waveguide filter is excited by quarter-wave probes. The printed-circuit inserts are realized on RT/Duroid 5880 microstrip board (εr = 2.2, thickness h = 0.8 mm) and are inserted in the transverse cross-section of the described rectangular waveguide. The WIPL-D model of the considered waveguide filter is shown in Fig. 2.

The filter response is analyzed for various values of the resonator length and width, while the other dimensions remain the same, and the obtained results are given in

S. Stefanovski is Ph.D. student at the School of Electrical Engineering, University of Belgrade, Bulevar kralja Aleksandra 73, 11120 Belgrade, Serbia, E-mail: snezanastef@telekom.rs

M. Potrebić and D. Tošić are with the School of Electrical Engineering, University of Belgrade, Bulevar kralja Aleksandra 73, 11120 Belgrade, Serbia, E-mail: milka_potrebic@etf.rs, tosic@etf.rs

Z. Cvetković is with the Faculty of Electronic Engineering, University of Niš, Aleksandra Medvedeva 14, 18000 Niš, Serbia, E-mail: Zlata.Cvetkovic@elfak.ni.ac.rs

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Fig. 3. It can be noticed that the resonant frequency moves toward lower values by increasing the resonator length, while the bandwidth remains practically unchanged. On the other hand, the variation of the resonator width, for the chosen length, primarily introduces the change of the bandwidth.

Fig. 2. WIPL-D model of the single-band bandstop filter using quarter-wave resonator.

(a)

(b)

Fig. 3. Comparison of filter responses for various values of: (a) resonator length (c = 0.2 mm, p = 0.45 mm, lstr = 0.5 mm), (b) resonator width (d1 = 1.55 mm, d2 = 3.1 mm, p = 0.45 mm, lstr = 0.5 mm).

The filter response is also analyzed for various

positions of the resonator. Here is considered filter using resonator with f0 = 11 GHz, as in Fig. 1. It is concluded that the bandstop response can be obtained only for two positions of the quarter-wave resonator, i.e. when it is attached to the top and bottom waveguide wall (Fig. 4, positions 1 and 2). Actually, for these two positions of the same resonator, the filter responses are practically overlapped. On the other hand, if the resonator is attached to either of the side walls (Fig. 4, positions 3 and 4), the waveguide with the printed-circuit insert does not operate as a bandstop filter, so these two positions will not be considered further.

p

1d2d

c

strl

strl

p c

2d1d

pc2d

1dstrl strlc1d2d

p

(a)

(b)

Fig. 4. Bandstop filters with various positions of the resonators: (a) printed-circuit inserts, (b) comparison of filter responses.

Finally, the position of the resonator attached to the top waveguide wall, as in Fig. 1, has been varied related to the central position and the filter behavior has been observed (Fig. 5). This variation of the resonator position causes the resonant frequency shift and the bandwidth becomes narrower.

Fig. 5. Comparison of filter responses for models with resonators moved away from the central position.

III. DUAL-BAND AND TRIPLE-BAND BANDSTOP FILTERS USING QUARTER-WAVE RESONATORS

By properly combining quarter-wave resonators of

different dimensions, attached to the top and bottom waveguide wall on the printed-circuit insert, dual-band and triple-band bandstop filters are modeled. These models actually represent the examples of multi-mode resonator application for bandstop waveguide filter design. The characteristic of the proposed filter design is the possibility to independently tune each of the resonators, and therefore the frequency bands can be easily controlled. For each

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model, the printed-circuit insert with quarter-wave resonators is placed in the transverse plane of the standard rectangular waveguide, as described in the previous section.

A. Dual-Band Filters

First, dual-band bandstop waveguide filter, for resonant frequencies f01 = 9 GHz and f02 = 11 GHz, is designed. These resonant frequencies are of interest for all dual-band filter models presented in this section. Two quarter-wave resonators are used for filter realization. One resonator is attached to the top waveguide wall and the other one to the bottom wall. The models with two different orientations of these resonators are considered, as shown in Fig. 6a (models DB-1a and DB-1b). The similar models of dual-band bandstop filters are made by moving the resonators away from the central position, for the same distance of s1 = s2 = 2 mm (Fig. 6b, models DB-2a and DB-2b). The dimensions of the resonators are the same for all four models and they are given in Table I. The comparison of filter responses for all presented models is given in Fig. 6c.

str1l

 

11d  

12d1p 1c

2c2p

 

22d 

21dstr2l

str1l

 

21d

 

11d

 

22d

12d1p 1c

2p2c

str2l

(a) str1l

 

21d

 

11d

 

22d

12d1p 1c

2p2c

str2l

1s2s

str1l

 

21d

 

11d

22d

12d1p 1c

2p2c

str2l

1s2s

(b)

(c)

Fig. 6. Dual-band bandstop filters using quarter-wave resonators: (a) models DB-1a and DB-1b, (b) models DB-2a and DB-2b, (c) comparison of filter responses.

According to the obtained filter responses, the

following conclusions can be made. By switching the orientation of the second resonator, the change of the bandwidth occurs: one band becomes narrower, the other

one becomes wider. The comparison of filter responses for models DB-1a and DB-1b and models DB-2a and DB-2b confirms this. Further, by moving both resonators away from the central positions, while keeping their orientations unchanged, more significant change of the bandwidth is noticeable, in the same manner as previously described. This is concluded based on the comparison of filter responses for models DB-1a and DB-2a and models DB-1b and DB-2b.

TABLE I

DIMENSIONS OF THE QUARTER-WAVE RESONATORS USED FOR DUAL-BAND FILTER DESIGN

Dimension di1 [mm]

di2 [mm]

ci [mm]

pi [mm]

lstri [mm]

1st reson. (i=1) 1.95 3.90 0.20 0.45 0.50 2nd reson. (i=2) 1.55 3.10 0.20 0.45 0.50

str2l

12d11d

1c2p

22d21d2c

1p

str1l

1s 2s

str2l

12d 11d

1c 2p

 

22d 

21d2c

1p

str1l

1s 2s

str2l 

12d 

11d

1c2p

 

22d 21d2c

1p

str1l

1s 2s

1

1

12

2

2

DB-3a

DB-3c

DB-3b

(a)

(b)

Fig. 7. Dual-band bandstop filters using quarter-wave resonators: (a) models DB-3a, DB-3b and DB-3c, (b) comparison of filter responses.

Next, models with both resonators attached to the top

waveguide wall are developed. The dimensions of the resonators are given in Table I. Each of the resonators is moved away from the central position for the distance of s1 = s2 = 4 mm, according to Fig. 7. The filter responses for these models are compared and this is shown in the same figure. As can be seen, model DB-3c provides the lowest values of the return loss beyond the rejection bands, compared with the other presented configurations, except the lower resonant frequency is slightly moved away from

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the desired value. Therefore, additional tuning of the dimensions of the first resonator is needed. With d11 = 1.8 mm, d12 = 3.6 mm and the other parameters unchanged, more accurate value of the lower resonant frequency (f01 = 9 GHz) is achieved, as shown in Fig. 7b. B. Triple-Band Filters

Triple-band bandstop waveguide filter is designed

using three quarter-wave resonators. The resonant frequencies of interest are f01 = 9 GHz, f02 = 10 GHz and f03 = 11 GHz. Actually, this model is obtained by adding the third resonator on the same printed-circuit insert, along with the existing resonators used in the models for dual-band filter design. Various positions of these three resonators are employed and here are presented some of the possible combinations. Since the configuration of the printed-circuit insert marked as DB-3c in the previous section provides the best selectivity, it is used for triple-band filter design, as shown in Fig. 8. The dimensions of the resonators are given in Table II. The distances of the first and the third resonator from the central position are s1 = s3 = 4 mm. The filter responses for the chosen models are also given in Fig. 8.

TABLE II DIMENSIONS OF THE QUARTER-WAVE RESONATORS USED FOR

TRIPLE-BAND FILTER DESIGN

Dimension di1 [mm]

di2 [mm]

ci [mm]

pi [mm]

lstri [mm]

1st res. (i=1) 1.80 3.60 0.20 0.45 0.50 2nd res.(i=2) 1.75 3.50 0.20 0.45 0.50 3rd res.(i=3) 1.55 3.10 0.20 0.45 0.50

 

12d 

11d

1c 1p

str1l

1s

str2l

2p 

22d 

21d

str3l

32d31d3c

3p3s

2c

12d 

11d

1c 1p

str1l

1s

str3l

32d31d3c

3p3s

2c 

22d

str2l

 

21d2p

(a)

(b)

Fig. 8. Triple-band bandstop filters using quarter-wave resonators: (a) models TB-1a and TB-1b, (b) comparison of filter responses.

According to the obtained results, model TB-1b provides better selectivity for the second rejection-band (f02 = 10 GHz), compared with the model TB-1a, while the other two rejection-bands (f01 = 9 GHz and f03 = 11 GHz) are not degraded. Therefore, the proposed configuration of the resonators, marked as TB-1b, can be adopted as valid solution for triple-band filter design.

IV. CONCLUSION

Novel design of multi-band bandstop waveguide

filters, using quarter-wave resonators, is proposed. The filter response has been analyzed for various dimensions of the resonators and their positions on the printed-circuit inserts, so the influence of these parameters has been accurately investigated. By choosing the proper positions of quarter-wave resonators, multi-band bandstop filters have been developed. There is possibility to independently tune each of the resonators, so each frequency band can be independently controlled, which is important for multi-band filter design. As examples of the proposed design, dual-band and triple-band bandstop filters are presented. The advantages of the proposed method for filter design are simplicity, applicability for the filters with multiple frequency bands and ease of implementation.

ACKNOWLEDGEMENT

This work was supported by the Ministry of

Education, Science and Technological Development of the Republic of Serbia under Grant TR32005.

REFERENCES

[1] I. C. Hunter, Theory and Design of Microwave Filters,

London: The Institution of Engineering and Technology, 2006.

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[3] S. Fallahzadeh, H. Bahrami, and M. Tayarani, “A novel dual-band bandstop waveguide filter using split ring resonators”, Progress in Electromagnetics Research Letters, 2009, vol. 12, pp. 133-139.

[4] S. Lj. Stefanovski, M. M. Potrebić, D. V. Tošić, and Z. Ž. Cvetković, “Design and Analysis of Bandstop Waveguide Filters Using Split Ring Resonators”, in Proc. 11th

International Conference on Applied Electromagnetics PES 2013, Niš, Serbia, 2013, pp. 135-136.

[5] S. Stefanovski, M. Potrebić, and D. Tošić, “Novel realization of bandstop waveguide filters”, Technics, special edition, 2013, pp. 69-76.

[6] M. N. M. Kehn, O. Quevedo-Teruel, and E. Rajo-Iglesias, “Split-ring resonator loaded waveguides with multiple stopbands”, Electronics Letters, 2008, vol. 44, pp. 714-716.

[7] WIPL-D Pro 10.0, 3D Electromagnetic Solver, WIPL-D d.o.o., Belgrade, Serbia, 2012.