US2943280A

US2943280A - Wave filter

Info

Publication number: US2943280A
Application number: US662623A
Authority: US
Inventors: Melville D Brill
Original assignee: Bell Telephone Laboratories Inc
Current assignee: AT&T Corp
Priority date: 1957-05-31
Filing date: 1957-05-31
Publication date: 1960-06-28
Anticipated expiration: 1977-06-28
Also published as: US3004201A

Description

June 28, 1960 M. D. BRILL 2,943,280

WAVE FILTER Filed May 31, 1957 INVENTOR By M. 0. BRILL WfW ATTORNEY United States PatentO phone Laboratories, IncorporateQNew York, N.Y. a;

corporation ofiNew York y Filed May 31, 1957, SenNo. 662,623 s c1. 3334-13 This invention relates to wave transmission networks and more particularly to a wave-guide filter of the varying-impedance type with impedancecorrecting means.

The principal object of the invention-is to improve the impedance and decrease the insertion loss in the transmission band of a waveguide filter'of the varying-impedance type. A further objectis to maintain high attenuation in" the suppression-band of thefilter while reducing the loss in the transmission band.

A waveguide has a natural high-pass cut-E frequency. If a low-pass structure with a higher cut-off frequency is associated with the wave guide, a band-pass filter is obtained. "The low-pass; structure may be a filter of-the varying-impedance type built into'the wave guide. The filtencomprises tandem-connected sections of guide having characteristic impedances which are alternately lower and higher, or alternately higher and lower, than the terminal impedances between which the filter is designed to operate. If the wave guide is of the hollow-pipe type, with a rectangular cross section and a fixed width, the desired impedances may be obtained by making the height alternately less and greater than the height of the guide into which .the filter is inserted. Band-pass wave-guide filters of this type are disclosed in Very High-Frequency Techniques, compiled by the Stafl of the Radio Research Laboratory of Harvard University and published by McGraw-Hill Book Company, Inc., New York and London, 1947, volume II, pages 731 to 736.

In such a filter, each change in height of the wave guide produces a field disturbance. transverse electric mode (TE which is ordinarily used, each discontinuity is equivalent to the parasitic capacitive shunt susceptance. Within the filter, the effects of these susceptances may be annulled by modifying the lengths of the low-impedance sections of guide. The discontinuity susceptances at the ends of the filter, however, are not amenable to this treatment. Their presence increascs'the loss and degrades the impedance match in the transmission band of the filter.

In accordance with the present invention, these end susceptances are compensated for, and the impedance match in the band improved, by adding an annulling susceptance at one or both ends of the filter.- These preferably take the form ofv capacitive screws or probes inserted into the guide through a wider wall. For best performance, the probes are symmetrically placed at points optimally spaced from each end of the filter. In order to prevent the generation of unwanted highersorder modes, which would decrease the attenuation in the suppression region of the-filter, each susceptance comprises a pair of probes projecting inwardly from a wider wall of the wave guide in the same transverse plane and each spaced from a side wall approximately one-third of the width of the section.

The nature of the invention and its various objects,

For the dominant 1 ice pedance-correctingnaeans in accordance with the present invention.

h 1ow-pass fi e 1 s buil into awave g ded. ha ing a rectangular cross section of width a and height I). The wider walls of the guide 2v are formed by an; nppenblock 3 and a lower block 4. The narrower side walls are constituted by the plates 61 and 7 whichmay be secured to the blocks 3 and 4 in any suitable manner. Electromagnetic waves are introduced into the guide 2 at one :end, as indicated by the arrow 8, pass through-the filter and arep pagatcdtn a suitable load, as indi a ed by the arrow 9.

' The filter 1 comprises seven tandern connected sections.

of wave guide, numbered 11 through. 17, having char,-

a'cteristic irnpedances which are alternately lowerand' the dominant mode at some frequency f explained in t e abovcre e enc t fi t rl s des ned as aicw: p ss st uc re. with a cut tr q en v f2 hi h r: han-f1- The conrbination of wave guide and filter thus a band-pass characteristic with the transmission band, eggtending' between and f 7 v 1 p A change in the heig t-of a r angul r wave g ide as from b to c or from): o d is qu a ntto a par 's shunt capacitive susceptance. {Each of the parasitic uss n ance app a n be we n tw jacent sta on of the fil er 'anfl c mpens ed-by prop r y djust n th lengths of thev associated low-impedance section, For example, the increase in height from c to d at the junction of the sections 11 and 1.2 may be taken care of by modifying the length of section 1 1.- However, the-dis,- continuity between the guide '2 and-the low-impedance sections 11 and '17 at, the ends of the filter 1 can not be treated in this way.

In accordance with the present invention, impedance- -c.orrecting means. are added to take care of the end parasitic susceptances and also to improve the impedance match between the filter 1 and the guide 2 in the trans mission band. As shown, these means comprise a pair of probes or screws 20, ZI-at the input end and a

similar pair

22, 23 at the output end. It will be understood that a single pair, at either end of the filter l ,'may he sed, ut the tw pa provide symmet requ r compensation at each end, and give a better impedance match between the filter and the guide. Each of the

probes

20, 21, 22, and 23 projects inwardly through a tapped hole in ,a avider wall 3,015 the guide 2. The

I probes forming each pair are in the same transverse plane,

wave-glide filter of the varying-impedance type with imare spaced a distances from the end of the filter 1, and have approximately equal projections.

There exist many combinations of probe penetration and distance s that will provide perfect ,intpeda-ncem tch .at a particular frequency. Ihe combination that nsua lly results in the best impedance match over a specified re.- ai of t er p ba is tha co ina ion. o pen trati n and distanc pro di g p rf ct m h at th c nter r qu ncy, is the r ion, w he a tp be penetration; "The following relations apply .this'case. I

'Themagnitude, and the angle 6, of the complex reflection coefficient at the frequency .f atone end of the uncorrected low-pass filter 1 when terminated in the impedance of the guide Zjrnay be calculated or measured. For a perfect impedance gnatch at f,,, the minimum no ma zed .sys sptauce, h mlet a h pair. of

probes, corresponding to minimum penetration, is given by the expression When b,,,,,,' is known, the required penetration may be determined.

The distance s is found from where A; is the wavelength in the guide 2 at f and n is an integer. The value of n is chosen to make s equal The maximum internal height, d, of the filter is usually madesufiiciently small so that over thefrequency range of interest as few as possible of the higher order modes excited by the discontinuities of the filter will propagate. However, the mode cut-off frequencies for the higher order TE modes, where m is greater than one, depend not on the height but only on the width a of the filter 1 and the guide 2. The dimension a is always large enough to allow the propagation of the higher order TE modes, if these modes are excited by the incident energy. The transmission characteristic of the filter forthese higher modes would be considerably difierent than for the assumed dominant mode. In particular, the attenuation in 'thesuppression band may belowered considerably. The geometry of the filter 1 discourages the conversion of energy of the dominant mode into these higher order TE modes, but, unlessprecautions are taken, the impedance-correeting means may perform such a conversion. Therefore, as a further important feature of the invention, the center of each of the probes of a pair is spaced athird of the width a from a narrower side of the guide 2. Thus, the probe 20 is spaced a/3 from the side 6 and the probe 21 a/ 3 from the side 7. The space between the probes is also a/3. The probes will react as shunt capacitance to the dominant mode but will not couple this mode to any TE modes where m is even or any odd multiple of three. There will be no coupling to the even modes because of the symmetry of the probes with respect to the, transverse plane, and no coupling when m is an odd multiple of three because the probes 20 and 21 are located at points where the electric fields of these modes are zero. Therefore, the first higherorder TE mode to which energy of the incident dominant mode will couple is TE The guide cut-off frequency for this mode is five times f and will almost always be far out in the attenuating region of the filter 1 and in a relatively unimportant frequency range.

What is claimed is:

1. In combination, two end sections of rectangular wave guide having unequal transverse dimensions, a low-pass wave filter inserted therebetween, and impedance-correcting means associated with one of the end sections, the filter comprising a plurality of sections of rectangular wave guide connected in tandem, the filter sections having characteristic impedances which are alternately lower and higher than that of the end sections, said impedance-correcting means comprising a pair of probes projecting inwardly from a wider Wall of the end section in the same transverse plane, and the centerof each of the probes being spaced from a side of the end section a distance approximately equal to one-third of the width of the end section.

2. In combination, two endsections of rectangular Wave guide having unequal transverse dimensions, a lowpass wave filter inserted therebetween, and impedancecorrecting means associated with each of the end sections, the filter comprising a plurality of sections of rectangular wave guide connected in tandem, the filter sections having characteristic impedances which are alternately lower and higher than that of the end sections, each of the impedance-correcting means comprising a pair of probes projecting inwardly from a wider wall of the end section in the,same transverse plane, and the center of each of the probes being spaced from a side of the end section a distance approximately equal to onethird of the width of the end section.

3. A band-pass wave filter comprising a plurality of tandem-connected'sections of rectangular wave guide having the same width and unequal transverse dimensions, the intermediatesections having heights which are alternately less and greater than the height of the end sections, and impedance-correcting means comprising a pair of probes projecting inwardly from a wider wall of one of the end sections in the sametransverse plane, the spacing between the probes and the distance between each probe and the nearer side of the'end section being approximately equal.

4. A band-pass wave filter comprising a plurality of tandem-connected sections of rectangular wave guide having the same width and unequal transverse dimensions, the intermediatesections having heights which are alternately less and greater than'the height of the end sections, and impedance-correcting means associated with each of the end sections, each of the impedance-correcting means comprising a pair of probes projecting inwardly from a wider wall of the end section in the same transverse plane, and the spacing between the probes and the distance between each probe and the nearer side of the end section being approximately equal.

5. A band-pass wave filter comprising a plurality of tandem-connected sections of rectangular wave guide having the same width and unequal transverse dimensions, the intermediate sections having heights which are alternately less and greater than the height of the end sec tions, and impedance-correcting means associated with each of the end sections, each of the impedance-correcting means comprising a pair of probes projecting inwardly from a wider wall of the end section in the same trans verse plane, and the plane being spaced from the adjoining section a distance s chosen to provide an impedance match between the end section and the adjoining section at a selected frequency f in the transmission band for a minimum penetration of the probes.

6. A filter in accordance with claim 5 in which s is equal at least to one-quarter but not more than threequarters of a wavelength at f, in the end section.

7. A filter in accordance with claim 5 in which the center of each of the probes is spaced from a side of the end section a distance approximately equal to onethird of the Width of the end section. v a

' 8. In combination, two end sections of rectangular wave guide having unequal transverse dimensions, a wave transmission network inserted between the sections, the network comprising sections of rectangular wave guide and having a parasitic shunt susceptance at each end, and means associated with each of the end sections for compensating the parasitic shunt susceptance comprising a pair of probes projecting inwardly from a wider wall of the end section in the same transverse plane, the center of each of the probes being spaced from a side of the end section a distance approximately equal to one-third of the width of the end section.

References Cited inthe file of this patent UNITED STATES PATENTS OTHER REFERENCES Microwave Theory and Techniques, Reich et al. 1953 Van Nostrand, pages 336-339 and 375-381.