A NOVEL WAVEGUIDE BANDPASS FILTERS WITH AN ASYMMETRICALATTENUATION POLE

Jun-Seok Park and Dal Ahn

Division of Information Technology Engineering, Soonchunhyang Univ., R. 0. Korea

E-mail: jsparkgramrec.sch.ac.kr

ABSTRACT

A novel dominant mode waveguide bandpass filterwith attenuation pole is proposed to obtain improvedattenuation performance. An extra-shorted cavity resonatorachieves the attenuation pole. The shored cavity iscombined to ordinary waveguide bandpass filter by meansof H-or E-plane T-junction. The attenuation pole can beplaced lower or upper stopband. Experimental results aregiven to show the improved stopband characteristics.

I. INTRODUCTION

Microwave and millimeter-wave applications ofconventional waveguide filters with various configurationssuch as fin lines, metal-inserted, inductive window, and soon have drawn considerable attention over the past fewyears. [1] - [4] These waveguide filters for the lower end ofthe waveguide band have relatively long resonators due tothe rapid increase of the guided wavelength toward cutofffrequency. Therefore, their harmonic resonance can occurwithin the single-mode range of the waveguide. Moreover,because of the high waveguide attenuation close to cutoff,the resonators are resonators are relatively lossy. On theother hand, bandpass filters designed for the band-edge ofwaveguide have poor second stopband attenuation becausethat an increasing amount of power is carried by parasitic orhigher order modes across the coupling sections between thesepta and the waveguide walls, even before the onset of thesecond-harmonic passband. [1], [2]

Several researches have been reported and introducedto improve attenuation characteristic of dominant modewaveguide filters by using full-wave analysis and computeroptimized procedures. [2] - [4] This paper presents a noveldominant mode waveguide bandpass filter with attenuationpole to obtain improved attenuation performance. The filterportion with inductive windows is operated withfundamental-mode. This conventional filter section isdesigned by filter synthesis method based on mode-matching method. [1] The attenuation pole can be achievedby implementing an extra-shorted cavity resonator. Theshored cavity is combined to ordinary waveguide bandpassfilter by means of H-or E-plane T-junction. The attenuationpole can be placed lower or upper stopband. Experimental

results are presented to show the improved stopbandcharacteristics by an attenuation pole location.

II. FILTER CONFIGURATIONS WITH EXTRASHORTED CAVITY

One of various methods for the implementation ofasymmetrical attenuation pole that is normally encounteredin the design of the proposed bandpass filters has beeninvestigated. Fig. 1 shows the configuration of theproposed waveguide bandpass filter for the investigation.The proposed waveguide bandpass filter is composed ofthe standard waveguide bandpass filter section, which hasinductive window coupling structure in this paper, the E-plane or H-plane T-junction, and the extra-shortedwaveguide cavity resonator. It has been well known thatthe shorted waveguide cavity resonator having a half-wavelength at certain frequency may be represented aseries LC resonator. [5] Hence, the resultant series LCresonator provides a transmission zero at the frequencycorresponding to half-wavelength. However, physicalimplementation of this shorted waveguide cavity resonatoris valid through the E-plane or H-plane T-junction, whichhas junction capacitances at each port. [6] In this paper, wehave considered only the H-plane T-junction case, whichhas in-phase characteristics of output E-fields. Therefore,one would expect that the junction capacitance at the portof bandpass filter could give some influences toperformances of bandpass filter. These influences due tothe presence of the extra-shorted waveguide resonator andthe H-plane T-junction are investigated based on the filtersynthesis theory.

Fig. 1 Proposed waveguide bandpass filter configuration.

Fig.2 shows the equivalent circuit of the proposedwaveguide bandpass filter with an extra-shorted cavity. Theequivalent circuit has a series LC resonator, which iscorresponding to the shorted cavity, and two paralleljunction susceptances as well as a waveguide filter sectionas shown in Fig.2 (a).

ol / J01 A

y = -j

-jBArYO

(2)

By using the previous formula, the simplified equivalentcircuit can be converted to Fig.3 (b), which has pure realsource admittance level. By considering (2), we can findout that the original first inverter value and the susceptanceofthe first resonator should be changed as follows

it ~ JO01 0

011B j)

(3)

(a)

Yt22B',,1=Bi - 0A

1rl = rl, L- A

= Brl -BAr

(4)

(b)Fig.2 Equivalent circuit representation of the proposed filterwith H-plane T-junction.

Meanwhile, the series LC resonator may be transformed toparallel LC resonator by employing the inverter seen inFig.2 (b). Then, the input admittance toward to source isgiven by

I~~~~~~~Yn = YO + I(BA2 B ,)

= YO + jBA

A notable thing from (3) and (4) is that the presence of aseries LC resonator and the junction susceptances due tothe H-plane T-junction has an effect only on the firstinverter and resonator of the bandpass filter. In other words,to get the proper filter performance with the proposed filterconfigurations, the first inverter and resonator of theoriginal bandpass filter should be decreased.

(1)

where BA means the equivalent parallel susceptanceincluding the junction capacitance and the series LCresonator section.

Hence, the equivalent circuit seen in Fig.2 (b) can befurther simplified as shown in Fig.3 (a). In order to achievethe bandpass filter performances, the source and loadadmittances should be pure real. However, the sourceadmittance has imaginary part due to the parallelsusceptance. This parallel susceptance should be removedby proper transformation. This transformation may beachieved by considering the first inverter section of thewaveguide bandpass filter. The input admittance to sourcethrough the first inverter is given by as follows:

(a)

(b)Fig.3 Modified equivalent circuit model of the proposedbandpass filter.

III. IMPLEMENTATION AND EXPERIMENTS

In order to design the proposed waveguide bandpassfilter, the six-section inductive waveguide bandpass filterwas designed by using filter synthesis method based on themode-matching field analysis. [3] The designed bandpassfilter has a fractional bandwidth of 1.4% at a centerfrequency of 25.8GHz. Example designed in this paper usedthe WR-42 waveguide. The simulated S-parameters usingGSM are plotted in Fig.4. [4] To achieve the asymmetricalattenuation pole with the proposed filter configuration, thedesigned results should be modified based on (2) and (3)due to the presence of the extra-shorted waveguideresonator and the H-plane T-junction. As a modified designresults, both the inductive window gap dimension and theresonator length of the first section of the pre-designedbandpass filter were decreased because that both thecoupling and the resonator susceptance in (2) and (3)decrease, respectively. The conventional waveguide filtersections have been designed accurately by using the mode-matching calculations for the determined design values.Meanwhile, to compute the modified inverter value andsusceptance, the full-wave calculation on the extra-shortedwaveguide resonator and the H-plane T-junction must betaken into account in the proposed filter design. However,this design for the proposed filter not included the full-wavecalculation. To compensate the design errors due to theomitted-computational efforts, the tuning process was used.The tuning procedures are relatively time consuming toobtain desired performance. In order to overcome thisproblem, we plan to carry out a 3-D full-wave computationon the proposed waveguide bandpass filter.

the stopband performance of each fabricated bandpassfilters compared to that of a conventional waveguidebandpass filter. The attenuation pole location can be easilycontrolled by varying the length of the shorted-cavitysection, which is done by simple tuning efforts as alreadymentioned above. Furthermore, the passband performancesof both measurements show good agreement with thetheoretical results except a small center frequency shifting,which is probably caused by fabrication tolerances. Bothfabricated bandpass filters with an asymmetricalattenuation pole perform excellent matching characteristicsalong with good return losses.

0

0

z0

Lzz

-10

-20

-30

-40

-50

-60

-70

-80

-90

-10023.73

Insertion LossReturn Loss

0

-5

-10

-15 m

7n-20 00

z-25 t

DLU

-30 t

-35

-40

-4524.40 25.05 25.71 26.37 27.03 27.70

Freq. [GHz]

(a)

0

0

20

40

7 60

no 80z- 100

LU0 120z

140

160

Insertion lossReturn loss

0

10

20

0

30 Un0

z40 tDL5

50

60

180 7023.00 23.48 23.96 24.44 24.92 25.40 25.88 26.36 26.84 27.32 27.80

Freq. [GHz]

Fig.4 Simulations on the designed inductive-windowwaveguide bandpass filter without attenuation pole.

The measured results on the fabricated waveguide bandpassfilters with an extra-shorted waveguide cavity are plotted inFig.5. Measurements shown in Fig.5 show the asymmetricalattenuation pole at lower side and upper side of passband,respectively. These attenuation poles noticeably improved

-20

C 40

0z -600

LU-1 80

-100

-1 2023.73 24.40 25.05 25.71 26.37

Freq. [GHz]

0

-5

-10

co-15 2

-0

020zD

-25 LU

-30

-35

-4027.03 27.70

(b)Fig.5 Measurements on the fabricated waveguide bandpassfilter with asymmetrical attenuation pole. (a) Realization ofan asymmetrical attenuation pole at lower stopband. (b)Realization of an asymmetrical attenuation pole at upperstopband.

Grauerholz, " E-plane integrated circuit filters withimproved stopband attenuation," IEEE Trans. onMicrowave Theory and Techniques, Vol. MTT-32,No.10, pp.1391-1394, October 1984.

[3] R. Vahldieck and W. J. R. Hoefer, " Finline andMetal insert filters with improved passbandseparation and increased stopband attenuation,"IEEE Trans. on Microwave Theory andTechniques, Vol. MTT-33, No.12, pp.1333-1338,

(a) December 1985.

[4] D. Budimir, " Optimized B-plane bandpass filterswith improved stop band performance," IEEETrans. on Microwave Theory and Techniques, Vol.MTT-45, No.2, pp.2 12-220, February 1997.

[5] G. L. Matthaei, Leo Young, E. M. T. John"Microwave Filters, Impedance MatchingNetworks and Coupling Structures", ArtechHouse, pp. 214-217, 1980.

(b) [6] L. Lewin, "Theory of waveguides, Techniques forthe solution of waveguide problems," London

Fig.6 Photographs of the fabricated waveguide bandpass newness-Butterworths, pp.291-301, 1975.filter with an asymmetrical attenuation pole. (a) Side view(b) Top view.

IV. CONCLUSION

In this paper, we have presented a novel waveguidebandpass filter structure and an implementing method of theasymmetrical attenuation pole for implementation ofexcellent stopband performance by using only dominantwaveguide mode. Measurements on the designed bandpassfilters, which have very narrow bandwidths, demonstratethat the proposed waveguide bandpass filter is extremelyeffect in improving the stopband performance of aconventional waveguide bandpass filter with simpleimplementing method. The passband characteristics such asinsertion loss and return loss are also excellent. Moreaccurate design procedures based on the 3-D full-wavecomputation will be included in future design in order togive a more precise design results and a reduction of tuningefforts.

REFERENCES

[1] J, B, Lim, C. W. Lee, and T. Itoh, " An accurateCAD algorithm for E-plane type bandpass filtersusing a new pass correction method combined withthe synthesis procedures," IEEE MTT-S Int.Microwave Symp. Dig., pp. 1 179-1182, June 1990.

[2] F. Arndt, J. Bomemann, R. Vahldieck and D.