US2859418A - High power transmission line filters - Google Patents

Description

Nov 4, 1958 J, VQGELMAN 2,859,418

HIGH POWER TRANSMISSION LINE FILTERS Filed June '21 1955 INVENTOR. ddS'P/l bi. V06 (Mi/V 2,859,418 HEGH POWER TRANSMESSION LINE FILTERS Joseph H. Vogelinan, Rome, N. Y.

Application dune 21, 1955, Serial No. 517,102

8 Claims. (Cl. 333-73) (Granted under Title 35, U. S. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to me of any royalty thereon. g

This invention relates to high power waveguide filters capable of handling powers equal to that of the waveguide or coaxial transmission line with which the filter is used.

There are numerous devices in the prior art utilizing constricted waveguide filtering techniques which are unsuitable for high power applications by their general nature, and in addition are either of the high pass filter type which relies upon the cutofi. frequency of a waveguide or are of the resonant cavity variety.

Tapered transmission lines, both of the waveguide and coaxial type have long been known and the characteristics have been determined and have been fully set forth at pages 2954 of the Wave Guide Handbook of the Radiation Laboratory Series of the Massachusetts Institute of Technology, first edition, published by the McGraw-Hill Book Company, Inc.

The filter of this invention is ofthe high power band pass type and relies solely upon impedance discontinuities in a transmission line to achieve filtering action. The filter consists of a series of symmetrical discontinuities of size distributed along a transmission line which may be of the waveguide or coaxial type or of any other type derivable therefrom. These discontinuities consist of changing from a uniform transmission line to a tapered section and from a taper of one angle to a taper of another angle. The characteristic of the impedance at the junctionof a rectangular and a tapered or radial guide together with the equivalent circuit is set forth at pages 322'and323 of the Wave Guide Handbook above noted. These variations are limited to changes which affect the impedance of the transmission line while keeping the Waveguide wavelength constant. Furthermore, constrictions of the standard transmission line can not be used for high power applications so that all of the tapers are such that the minimum diameter of any filter section is that of the associated transmission line. The impedance mismatch properties resulting from such tapers are used to obtain the desired filtering action. This type of filter considerably improves the operation of high pulsed power multi-channel systems using a common antenna system.

It is therefore an object of this invention to provide a filter the power handling capability of which is equal to that of the transmission line into which it is inserted and which will have a zero insertion loss at the design frequency when the parameters are properly selected.

It is a further object of this invention to provide for the synthesis of a Wide variety of filter characteristics by providing a suflicient number of variable parameters to achieve the desired results and to provide a filter design which can be readily scaled from one frequency band to another and from one transmission line size to another.

These and other objects and advantages will be apparent to those skilled in the art from the following de- .ates atent ice .2 tailed specificationand attached 'drawing forming apart thereof and wherein: I

Figure 1 is a schematic cross section of a filter inserted in, a waveguide transmission line.

Figure 2 is a schematiecrosssection of afilter inserted.

in. a coaxial transmissionline.

Figure 3 is an equivalent'circuit for-the filter'of Figures 1 and 2.

Turning now to the drawings it will be seen that the waveguide, whichxmaynhave any. suitable crosssection such: as rectangular or cylindrical, filter of Figure land the coaxial line filter of'Figure 2' both are shown as consisting of three sections. Section I consists of an expand-' ing transmission line producing a discontinuity. at plane 1 as a result of the junction-eaused-by the expanding transmission line and theuniform linesW or W respectively.

Thediscontinuity at plane lihas an impedance XI shown in the equivalent circuit of: Figure. 3. which impedance is' determined by the angle. 0 or 0 respectively. At plane 2 an impedance discontinuity results. from the abrupt change in going fromsection Ixtosection II and the magnitude of this discontinuity has an impedance X which is a function of the angle. 0 uor 0 respectively. Section II of either the waveguide or coaxial type can consist of a larger size uniform transmission line or an expanding or contracting transmission line propagating high order modes of operation'asmaybe desirable for the tion of lineof one dimension to another section of line This, however, requires tapered of another dimension. sections several times the wavelength at the operating frequency in order to avoid impedance discontinuities. In

' the present filter, each tapered section is deliberately made of the. same order of magnitude. as. the wavelength at the center frequency of the pass band in: orderto introduce desired impedance discontinuities.

To obtain a filter of given characteristics itis necessary to, solve the equivalent transmission line circuit of Figure 3. For any. given .waveguide or transmission line within the frequency band of interest each'of the discontinuities X through X, are experimentally measured individually as a function of the angles 0 through 0 respectively. The impedances and line lengths L through L, of Figure 3 are selected to give the desired resonances at the specified frequency; that is, terminating the terminals 7, 8 in the characteristic impedance Z should result in an input impedance at terminals 5, 6 which is equal to Z In practice this is accomplished by utilizing standard and well-known network synthesis methods to obtain an equivalent circuit having the desired frequency characteristics. From the equivalent circuit for the various frequencies in the band under consideration the appropriate discontinuities and line lengths are determined. From the data giving the discontinuity impedances resulting from variation of 0 the proper arrangement and physical embodiment of discontinuities can then be determined.

An alternative method of synthesis of this type of filter would consist in determining the resonances and antiresonances desired of the filter. Then, by standard transmission line techniques, the input impedances expressed as a function of the discontinuities at the planes 1, 2, 3, and 4 and lengths L L and L could be determined. The values of the discontinuities and the line lengths could In. the. coaxial type of filter shownthen be selected to give the desired resonances and antiresonances in the frequency region under consideration. For example: to obtain a filter resonant at f and antiresonant at and about f determine the. transmission line equation Z =F(X X X X4, 'L L L appro-' priate to the particular transmission line system and design frequency. This eq'uationis solved for values f f and to giverthe resonance and antiresonances desired and relationships are obtained between X, and L From the experimental .data describing variation of X as a function of 0 it is then possible to obtain values of angles 6 and lengths L to give a physically realizable structure.

, While a specific preferred embodiment of the invention has been described for purposes of illustration it is under-' stood that the invention is defined solely by the appended claims.

o What I claim is: i

1. A high power handling band pass filter comprising, a uniform transmission line having a. first and second section, a plurality of contiguous tapered-line filter sections inserted in said uniform 'line' between said first and second sections, said tapered-line sections producing a plurality of impedance discontinuities, each of said filter sections having a lengthwhich is of theLsame order to magnitude as the operating wavelength at 'the center of the pass band and, having a minimum cross sectional dimension which is at least as great as the minimum cross sectional dimension of said uniform transmission line, said impedance discontinuities being a function of the'.angles formed'by the junctions of said tapered sections.

2. Apparatus as in claim 1 .wherein said uniform transmission line isa waveguide and wherein the first of said plurality of tapered filter sections is an expanding waveguide section and the last of said plurality of tapered filter sections is a contracting waveguide section.

3. Apparatus as in claim 1 wherein said uniform transmission line is a coaxial cable and wherein the first of said plurality of tapered filter sections is an expanding section of coaxial cable having an outer and an inner conductor both of 'Which have an expanding taper, and wherein the last of said plurality of tapered filter sections is a contracting section of coaxial cable having an outer and inner conductor both of which have a contracting taper.

4. A high power band pass filter system comprising a first and second section of uniform transmission line, a series of radial sections of transmission line connected between said first and second sections, each of said radial sections having a length which is of the same order of magnitude as the operating wavelength of the system, each of said radial sections having a minimum cross-sectional dimension which is at least as great as the crosssectional dimension of said first and second sections of uniform transmission line.

5. For use in a high power transmission line having a first and a second Wave guide section of uniform cross section, a high power handling band pass filter comprising a first expanding radial wave guide filter section, a second contracting radial wave guide filter section, the minimum cross section of each of said radial wave guide filter sections being coextensive with the cross section of the respective wave guide transmission line sections, an intermediate wave guide filter section connecting said first and second radial wave guide'filter sections, each of said filter sections having a length which is of the same order of magnitude as the operating wavelength of the center of the pass band, said radial wave guide filter sections producing an impedance discontinuity at each junction.

guide filter section connecting said first and second radial Wave guide filter sections, said intermediate filter section being a radial wave guide, each of said filter sections having'a length which is of the same order of magnitude as the operating wavelength of the center of the pass band, said radial wave guide filter sections producing an impedance discontinuity at each junction.

7. Apparatus as in claim 5 wherein the filter section between said expanding and contracting filter sections is enlarged to propagate high order modes to produce the filtering effect.

8. Apparatus as in claim 5 wherein the intermediate filter section is a uniform line section, the TE mode being excited therein.

References Cited in the file of this patent UNITED STATES PATENTS 2,533,239 Gent et al. Dec. 12, 1950 2,737,630 Miller Mar. 6, 1956 2,738,468 Miller Mar. 13, 1956 2,738,469 Miller Mar. 13, 1956 2,747,184 Kock May 22, 1956

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