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Metal-insert filters with improvedcharacteristics
J. Bornemann and F. Arndt, Sen.Mem.I.E.E.E.
Indexing terms: Waveguides and waveguide components
Abstract: A computer-aided design is introduced for E-plane filters with single and double metal inserts withina reduced-width or increased-width waveguide section, respectively. This design achieves filter performanceswith high peak attenuation in a broad second stopband. The theory described includes both the higher-ordermode interaction of all discontinuities and the finite thickness of the inserts. The step-wall discontinuity effect isincluded in the optimisation process as an additional parameter to be optimised. Ka-band (26.5-40 GHz) designexamples demonstrate the good stopband attenuation behaviour of this type of filters. Moreover, this designonly requires thin strips, which are very appropriate for technical realisation.
1 Introduction
All-metal inserts integrated in the E-plane of rectangularwaveguides yield low-cost mass-producible millimetre-wave filters with low passband insertion loss [1-6].However, as the filter resonators are coupled by way ofevanescent fields along the inductive strips, high attenu-ation specifications in the second stopband are difficult tosatisfy, especially by filter performances designed for pass-bands near the upper end of the waveguide band. This isdue to unwanted direct coupling of modes along the stripsections with progressing frequency if the distancebetween the strips and the waveguide side walls is nolonger negligible compared with the guide wavelength. Toalleviate this problem, the waveguide width in the vicinityof the strips may be reduced, as has been suggested by theauthors [7]. However, in Reference 7 linear taper sectionsare used to match the waveguide sections of differentwidth. This requires additional mechanical efforts and thusmay offset the low-cost advantage of the E-plane inte-grated circuit filters. Moreover, the taper sections compen-sate the inductive junction effects of the discontinuities,which may be advantageously utilised as an additionaloptimisation parameter.
In this paper, therefore, an abrupt discontinuitybetween the waveguide sections in question (Fig. 1) is used.This is more appropriate for a convenient mechanical con-struction than a smooth section. The step-wall disconti-nuity effect is utilised directly as an additional parameterfor designing filters with high stopband attenuation. More-over, for all designs with improved stopband attenuation,only thin strips are required which are very appropriatefor production by photoetching techniques.
The abrupt step-wall discontinuity E-plane filter typemay also be advantageously utilised to improve the stop-band attenuation for filters with passbands near the lower
Paper 4450H (El2), received 7th May 1985The authors are with the Microwave Department, University of Bremen, KufsteinerStrasse NWI, Postfach 33 04 40, D-2800 Bremen 33, West Germany
IA-11A-
S S 1 ii-A -
SW G
\\\1
1I
-A-
\ ^
i
sMI Ii-A-
sWG
\I I
j e t c . |-A A - l
S 2
x = 0
r"
y
z = 0
Fig. 1 E-plane metal-insert filters in waveguides with abrupt step-walldiscontinuitya Waveguide an d metal insertb Abruptly reduced waveguide width; high stopband attenuation filter type for
passbands in the near of the upper band end of the original waveguide (width a);scattering matrices:551 discontinuity change in waveguide width (broad to small)Swc> homogeneous waveguide section
S*" metal-insert section of length /,- (including th e discontinuities waveguide toE-plane bar section an d back to waveguide)
552 discontinuity change in waveguide width (small to broad)c Abruptly increased waveguide width; high stopband attenuation filter typefor
passbands in the near of the lower band end of the original waveguide (width a)d Configuration for the field theory treatm ent of the step-wall discontinuity
IEE PROC EEDINGS, Vol. 133, Pt. H, No. 2, APRIL 1986 103
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band end of the original waveguide. For this design,however, the side-wall distance of the waveguide housingin the vicinity of the strips needs to be increased (Fig. lc).This is due to the nonlinear relation between guide-wavelength and frequency which may be favourably influ-enced by a suitable reduction of the cutoff frequency of thefundamental mode within the filter resonators. High stop-band attenuation over the whole waveguide band isachieved by utilising the double-insert filter type [5, 7] forthe filter section within the waveguide of increased width(Fig. lc).
As in References 3-5 and Reference 7, the design of opti-mised filters is based on a rigorous field expansion intoincident and scattered waves at all discontinuities. Thisallows direct inclusion of higher-order mode coupling,finite strip thickness and the step-wall discontinuity effectsin the optimisation process to make the filter performancesatisfy the given specifications.
2 T h e o r y a n d des ign
Because the field-theory treatment of the metal-insert filterstructure within the length / (Figs. \b , c) has already beendescribed in detail in References 3 and 7, respectively, thetheory in this paper may be restricted to the derivation ofthe scattering matrix of the step-wall discontinuity at z = 0(Fig. \d).
The fields in the subregions i = I, II
where the coefficients of
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insert filter with reduced-width waveguide.Th e input an doutput waveguide (Fig. Ib) has standard WR 28-dimensions (a = 7.112 mm, b = 3.556 mm, see Table 1),
100
50
30 36 42f.GHz
48 60
38.5I.OHl
40.0 40.5
Fig . 2 Computer optimised Ka-band three-resonator metal-insert filtersdesigned for a midband frequency of 39.37 GHz and 1.8% bandwidth
Metal-insert thickness r = 190 /
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considerably degrades the stopband behaviour. Secondly,for passbands at the lower band end, double metal-insert
25 30 35 40 4 5 50f.GHz
26.6 26.8 f . G H z 27.0 27.2 27.4
Fig. 5 Computer optimised Ka-band three-resonator metal-insert filtersdesigned for a midband frequency of 27 GHz and about 1% bandwidthMelal insert-thickness f 190 nm (for design data see Table 1)a Insertion loss (1/| S 21 1) in decibels as a function of frequency
curve 1: conven tional E-plane metal-insert filtercurve 2: E-plane double metal-insert filter with abruptly increased width wave-
guide step-wall discontinuity (included in the optimisation)b Insertion loss (1/|S
2 1|) and return loss (1 /| S ,, |) in decibels as a function of
frequency for the filter with step-wall discontinuity (curve 2 of Fig. 5a )
structures within an increased-width waveguide, whichreduces the guide wavelength of the filter resonators, yieldhigh stopband attenuation over the whole original wave-guide band.
As the step-wall discontinuity effect is utilised directly asan additional parameter to be optimised, the filters may bedesigned for low passband insertion losses and highattenuation levels in the second stopband, which, in addi-tion, is much broader than the conventional metal-insert
filters. Moreover, for all improved stopband designs, onlythin strips (e.g. t = 190 nm ) are required, which are veryappropriate for production by photoetching techniques.
5 References
1 TA JIM A, Y., and SA WAY AMA , Y.: 'Design and analysis of awaveguide-sandwich microwave filter', IEEE Trans., 1974, MTT-22,pp. 839-841
2 K ON ISH I, Y., and UE NA KA DA , K.: 'The design of a bandpass filterwith inductive strip-Planar circuit mounted in waveguide',ibid., 1974,MTT-22, pp. 869-873
3 VAHLDIEC K, R., BOR NEM ANN , J. , ARN DT, F. , and GRAU ER-HOLZ, D.: 'Optimized waveguide E-plane metal insert filters formillimeter-wave applications', ibid., 1983, MTT-31, pp. 65-69
4 VAHLDIEC K, R., BOR NEM ANN , J. , ARN DT. F. , and GRA UER-HOLZ, D.: 'W-band low-insertion-loss E-plane filter', ibid., 1984,MTT-32, pp. 133-135
5 BOR NEM ANN , J. , ARN DT, F. , VAHL DIECK, R., and GR AUE R-HOLZ, D.: 'Double planar integrated millimeter-wave filter', 13thEuropean Microwave Conference Niirnberg, W. Germany, September5th-8th , 1983, pp. 168-173
6 SHIH, Y.C.: 'Design of waveguide E-plane filters with all-metal inserts'.IEEE Trans., 1984, MTT-32, pp. 695-704
7 ARNDT, F. , BOR NEM ANN , J ., VAHLDIECK , R., and GR AUER -HOLZ, D.: 'E-plane integrated circuit filters with improved stopbandattenuation', ibid., 1984, MTT-32, pp. 1391-1394
6 Appendix
6.1 Scattering coefficients in eqn. 5
(7)
where (W ) = {U) + {LH)(LE) an d {U ) is the unit matrix.The matrix coefficients of (L) are given by
bil)'Vim
a(g - c ) C j f V amnsin | x
sin l ~^ (x - c) dx ] (8)
- (sn)(2)
(s22)(1)
r
= (521)(2)[(C7) -
( S 2 1 )( 1 >
when (U ) is the unit matrix.
IEE PROC EEDINGS, Vol. 133, Pt. H, No. 2, APRIL 1986 107
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