US3019399A

US3019399A - Circular waveguide diameter transformer

Info

Publication number: US3019399A
Application number: US797667A
Authority: US
Inventors: Daniel A Lanciani; Arthur A Blaisdell
Original assignee: Microwave Associates Inc
Current assignee: MA Com Inc; Microwave Associates Inc
Priority date: 1959-03-06
Filing date: 1959-03-06
Publication date: 1962-01-30
Anticipated expiration: 1979-01-30

Description

Jan. 30, 1962 D. A. LANCIANI ETAL CIRCULAR WAVEGUIDE DIAMETER TRANSFORMER Filed March 6, 1959 2 F|G.l

FIG.2

. JNVENTORS DANIEL A. LANCIANI ARTHUR A. BLAISDE'LL ATTORNEY Uite States atent 3,019,399 Patented Jan, 30, 1962 fire 3,019,399 CIRCULAR WAVEGUIDE DIAMETER TRANSFORMER Daniel A. Lanciani, West Medford, and Arthur A. Biaisdell, Naticir, Mass., assignors to Microwave Associates,

Inc., Burlington, Mass., a corporation of Massachusetts Filed Mar. 6, 195), Ser. No. 797,667 11 Claims. (Cl. 33334) This invention relates to the propagation of electromagnetic wave energy in circularly symmetrical modes in round hollow pipe waveguide, and more particularly to transformers from small to large waveguide and vice versa for a circularly symmetrical wave.

In the propagation of electromagnetic wave energy in round hollow pipe waveguides it is well known that the energy can propagate in one or more modes, or characteristic field configurations, depending on the cross-sectional size, or internal diameter, of the particular waveguide, and on the operating frequency, and that the larger the internal diameter is made the greater is the number of modes in which the energy can propagate at a given operating frequency. It is usually desirable to confine propagation of the energy to one particular mode, depending upon the application which is involved. If the desired mode is also the so-called dominant mode, the diameter of the waveguide can be restricted so that no other modes can be propagated in it. The TE mode, in connection with which the present invention is described, is frequently chosen for its ability to propagate over long waveguide distances with less attenuation than other modes. It is, however, not the dominant mode, and the diameter of a waveguide in which this mode will propagate must be so large that other modes can also be propagated in itthat is, the waveguide is a multimode circular waveguide.

In certain instances it is desirable to effect a change in the diameter of a multimode circular waveguide intended for propagation of a given mode, such as the TE mode. For example, it is desirable to do this to transform the impedance of the waveguide from one value to another. When a change in the diameter is effected, it is also desirable to do so with a minimum of mode conversion.

A commonly used technique to solve this problem is to resort to the use of a conical tapered section of waveguide between the two sections of smaller and larger diameter waveguide. Since the effective mode conversion is an inverse function of the length of the tapered section, mode conversion can be minimized by the use of a long tapered section. This usually results in a structure which is longer than is desirable or tolerable for many applications.

Another technique has been described by S. P. Morgan, Jr., in U.S. Patent No. 2,762,982, which uses dielectric mode converters to effectively cancel spurious modes which have been generated in passing from a smallerdiameter waveguide to a larger-diameter waveguide through a tapered section. This method, too, since it involves the use of a tapered section of waveguide in addition to the dielectric mode converters, usually results in a structure which is undesirably or intolerably long for many applications.

It is an object of the present invention to provide a diameter transformer for round hollow pipe waveguides which is of greatly reduced length compared to a tapered pipe section or to devices involving dielectric mode converters, and which preserves the mode purity of electromagnetic waves propagating in it with an efficiency as good as that of a tapered section of much greater length. Other objects are to provide such a transformer of simple design, which is not expensive or complex to fabricate. or difficult to use and which is inherently of rugged and durable construction.

According to the invention, these objects are attained by providing a transformer in which the internal diameter is increased in a series of steps. In multimode circular waveguide an abrupt diametral change will cause mode conversion to occur; that is, a transfer of a portion of energy propagating in the desired mode to other modes of propagation takes place. The step transformer of the present invention makes use of the mode generation properties of a diametral step and of the progressive phase shift of the generated mode or modes to reduce the net mode conversion to zero or a negligible value in a relatively short length of transmission line. Thus, for example, with essentially pure TE mode energy propagating in a round hollow waveguide, the diameter is increased in steps, which are circularly symmetrical, and of such small size that substantially the only impurity excited by each step is the TE mode. The steps are arranged to excite controlled magnitudes, for example, equal magnitudes of energy in the TE mode, and are appropriately phased to produce an effective cancelling of the TE impurity.

In one practical embodiment, transformation from small to oversize (for the TE mode) circular waveguide is achieved by the use of a series of quarter wavelength (i.e., guide wavelength) steps in the TE mode up to the maximum critical diameter which is below cut-off for the TE mode. From this point, the step discontinuities are made as small as practical but equal in regard to TE mode excitation. Then by proper spacing of the steps the phase relations of the TE waves which are excited are adjusted for effective cancellation.

Other and further objects and features of the inventionare set forth in the detailed description of certain embodiments thereof which follows. The description refers to the accompanying drawings, wherein:

FIG. 1 illustrates in side section a step transformer made of a single piece of solid material; and

FIG. 2 illustrates in side section a step transformer con-.

structed of telescoping sections of separate tubing.

In FIG. 1, the transformer according to the invention is comprised of a series of circular bores of progressively increasing internal diameters D D D D D and D respectively, in a single piece of metal 10. Each of these bores constitutes a section of circular wave-guide of uniform diameter, and the series of waveguide sections constitutes in effect a single circular waveguide of which the internal diameter progressively increases in discrete steps from the smallest diameter D, to the largest diameter D The smallest diameter D and the next three langer diameters D D and D,, are all below cutoff for the TE mode at the operating frequency, the' largest of these diameters D being the maximum critical diameter which is below cut-off for that mode. The wave-- adjacent diameters, that is, the ratios D /D D /D and D1 2/D are related to each other according to a binomial distribution, as well be more fully explained below.

The next larger diameter D is able to support the T13 mode at the operating frequency. The step from diameter D to diameter D is made sufficiently small so that when pure TE mode energy enters the larger guide section substantially the only impurity excited by the discontinuity is the T13 mode. Ability to achieve this result is enhanced by the fact that the discontinuity has circular symmetry. A consideration which restricts to some degree the minimum step size that can be used is the need to make the first step beyond the maximum critical diameter D sufiiciently 'large to permit propagation of the TE mode at the lowest frequency in the operating band. The succeeding step from diameter D to diameter D, is chosen to excite TE energy, from the TE energy incident upon it, equal in magnitude to the TE energy excited by the step from diameter D to diameter D These two steps, from D to D and from D to D are spaced with respect to the guide wavelength for the TE mode so that these equal magnitudes of step-excited T5 mode energy are in phase opposition-i.e., they differ in phase by 180". This produces effective cancellation of the TE impurity.

The design considerations for the step sizes and spacings in the sections of diameters D D and D are as follows. If a pure TE mode wave is incident at the plane A between the two guide sections of diameters D and D the energy content of the Wave is divided into two component modes, 13 and E where E E When the E component reaches the plane B between the two guide sections of diameters D and D its energy content will again divide into two component modes, E' and E' The total T15 mode component at plane B is, therefore, E0g +E 02 In order for this summation to be equal to zero, the following two conditions must be satisfied:

|E |=]E (Condition 1) and where N=any odd integer (Condition 2) Condition 1 is approximately satisfied if D /D =D /D Condition 2 is satisfied if the distance L between the planes A and B has a magnitude computed from the following relation:

Where N=any odder integer. (Relation 1) In Relation l, the expression k flD means the guide Wavelength for the TE mode in the section of diameter D and the expression A (D means the guide wavelength for the TE mode in the same section.

In the guide sections of internal diameters D D D and D which are all below cut-off for the TE mode, it is only necessary to space the steps gtll 4 apart. In the present embodiment, which employs three steps to go from the smallest diameter D to the maximum critical diameter D the step sizes are binomia-lly related, so that the TE mode disturbance created at the intermediate plane X will be twice the magnitude of the T E mode disturbance created at each of the outer planes Y and Z; the latter disturbances are of equal magnitude. The larger TE mode disturbance is then cancelled half by the TE mode disturbance created at plane Y and half by the TE mode disturbance created at plane Z.

An impedance transformer designed for use with a TE mode circular waveguide of internal diameter 0.634 inch intended for operation at a frequency of approximately 35,000 mc./sec., has been built with the following dimensions (referring to FIG. 1):

D =0.634 inch inch inch D =0f700 inch D =0.835 inch D =0.995 inch L =0.382 inch L =0.694 inch L =0.80 1 inc-h L.;=0.9l1 inch From these dimensions, the step ratios of the guide sections of critical diameter (D and less are:

which are practically identical.

Tests of mode conversion from TE to TE in the output of two transformers constructed according to FIG. 1 with the foregoing dimensions, employing a signal centered at 34,860 mc./sec. (i.e., 34.86 kmc./s.), yielded the following results:

Mode (TE to Frequency Conversion, TE

Sample Sample No. 1 No. 2

Percent Percent 34.85 kmo/s O. 3 0. 4 34.00 kinC./s 1. 8 2. 4 36.00 kmcJs 0 5 0.5

These results are substantially identical with those which can be achieved using a tapered round hollow waveguide one foot long to go uniformly from D to D They are attained with step transformers according to the present invention which are less than one inch long (L =0.9ll inch).

FIG. 2 illustrates a construction or" a step transformer according to the invention utilizing a series of telescoping sections of circular waveguide. A first section 21 of relatively the smallest inner diameter fits telescopically into a second section 22, which fits telescopically into a third section 23, and so forth to

sections

24, 25 and 26. The wall thicknesses of the various sections are chosen to provide the desired step sizes, for example according to the design considerations described above with reference to FIG. 1, and the desired axial lengths of the effective waveguide sections of the various inner diameters are achieved by relative telescopic adjustment of contiguous pairs of the original waveguide sections. In a step transformer constructed in this manner which changed from an inner diameter of 0.634 inch of the first section 21 to an inner diameter of 0.995 inch of the last section 26, in six steps, the result achieved was mode conversion to TE in the output, from essentially pure TE mode energy introduced via the first section 21, on the order of two. to three percent. The size of each step in this transformer was arbitrary, however, being governed by the wall thicknesses of available commercial tubing. Employing tubing fabricated with wall thicknesses chosen according to the design considerations described above, results similar to those set forth above can be achieved. This latter consideration, however, introduces the need for a fabrication step, namely that of drawing or machining the individual tubing sections to desired sizes, unless available tubing can be found which satisfies all the desired design parameters. In addition, the concentricity of eachsection is critical, if circular symmetry of all the steps is to be preserved. For these reasons, embodiments according to FIG. 1 are the preferred embodiments of the invention.

It will be recognized that embodiments of the invention can be constructed having three or more steps in the sections of diameter above cut-oif for the T13 mode. In such cases it may be advantageous to choose step ratios which provide a step-generated TE mode amplitude distribution which is other than equal for all the steps. For example, a binomial step distribution can be used. In the case of three steps, the step-generated T15 mode amplitudes would desirably be distributed with the relative values 121, as described above with reference to the three steps in the region of diameters which are below the critical diameter for the TE mode. In the case of four steps, the step-generated TE mode amplitudes would desirably be distributed with the relative values 1-3-34, and for five steps a desirable relative amplitude distribution would be 1464-l. Alternatively, a T chebyscheff step distribution may be employed. In any case, it is preferred that the diameters of the waveguide sections which will support the TE mode (D and D in FIG. 1, for example) be below cut-off for the TE mode.

The foregoing principles are valid also for transitions from diameters which are below cut-off for the TE mode to diameters above such cut-oifi.e., for transformers from T15 to TE Q mode sized waveguide, and in general for designing step transformers from TE to TE +1) mode sizes. More generally, these principles are valid for diametric transformation from a size limited to any mode having circular symmetry to a size able to propagate the next higher order of the same mode, or vice versa.

The embodiments of the invention which have been illustrated and described herein are but a few illustrations of the invention. Other embodiments and modifications will occur to those skilled in the art. No attempt has been made to illustrate all possible embodiments of the invention, but rather only to illustrate its principles and the best manner presently known to practice it. Therefore, while certain specific embodiments have been described as illustrative of the invention, such other forms as would occur to one skilled in this art on a reading of the foregoing specification are also Within the spirit and scope of the invention, and it is intended that this invention include all modifications and equivalents which fall within the scope of the appended claims.

What is claimed is:

1. Diameter transformer for circular waveguide intended for propagation of electromagnetic wave energy in a prescribed first mode having circular symmetry to one or more larger diameters capable of propagating energy in a second mode which is the next higher order of said first mode, comprising a circular waveguide the diameter of which, in a given axial direction, progressively increases stepwise at each of a plurality of axially-spaced transverse planes, each section of said waveguide between adjacent pairs of said planes being of substantially constant diameter, the diametral changes each being dimensioned to generate a prescribed amount of said second mode energy from the first mode energy at the operating frequency propagating through it, the magnitude of each diametral change being restricted so that substantially no energy in a mode higher than said second mode is generated from said first mode energy, the planes defining said diametral changes being spaced apart relative to said second mode energy at the operating frequency so that said amounts of second mode energy generated by the diametral change at each of said planes due to incidence thereat of first mode energy propagating through said waveguide in a given direction substantially cancel each other.

2. Diameter transformer for circular waveguide intended for propagation of electromagnetic wave energy in the TE mode, where n is an integer, to one or more larger diameters capable of propagating energy in the T E mode, comprising a circular waveguide the diameter of which, in a given axial direction, progressively increases stepwise at each of a plurality of axially-spaced transverse planes, each section of said waveguide between adjacent pairs of said planes being of substantially constant diameter, the diametral changes each being dimensioned to generate a prescribed amount of TE mode energy from TE mode energy at the operating frequency propagating through it, the magnitude of each diametral change being restricted so that substantially no energy in a mode higher than said TE mode is generated from said TE mode energy, the planes defining said diametral changes being spaced apart relative to TE mode energy at the operating frequency so that said amounts of TE mode energy generated by the diametral change at each of said planes due to incidence thereat of TE mode energy propagating through said waveguide in a given direction substantially cancel each other.

3. Diameter transformer for circular waveguide intended for propagation of electromagnetic wave energy in the TE mode comprising a circular waveguide the diameter of which progressively increases stepwise at each of a plurality of consecutive axially-spaced transverse planes, each section of said waveguide between adjacent pairs of said planes being of substantially constant diameter, the diametral changes each being dimensioned to generate a prescribed amount of "SE mode energy from TE mode energy at the operating frequency propagating through it, the magnitude of each diametral change being restricted so that substantially no energy in a mode higher than said TE mode is generated from said TE mode energy, the planes defining said diametral changes being spaced apart relative to TE mode energy at the operating frequency so that said amounts of TE mode energy generated by the diametral change at each of said planes due to TE mode energy propagating through said waveguide in a given direction substantially cancel each other.

4. Diameter transformer for circular waveguide intended for propagation of electromagnetic wave energy in the TE mode comprising a circular waveguide the diarneter of which progressively increases stepwise at each of a plurality of consecutive aXiallly-spaced transverse planes, each section of said waveguide between adjacent pairs of said planes being of substantially constant diameter, one of said diametral increases being from a section of diameter below cut-off for the TE mode at the operating frequency to a section above cut-off for said mode, said one diametral increase being followed by at least one further diametral increase, said one and said following diametral increases each being dimensioned to generate a prescribed amount of TE mode energy from TE mode energy at the operating frequency propagating through it, the magnitude of each of said one and said following diametral increases being restricted so that substantially no energy in a mode higher than said TE mode is generated from said TE mode energy, the planes defining said one and said following diametral increases being spaced apart relative to TE mode energy at the operating frequency so that said amounts of TE mode energy generated by the diametral change at each of said planes due to inci dence thereat of TE mode energy propagating through said waveguide in a given direction substantially cancel each other.

5. Diameter transformer for circular waveguide intended for propagation of electromagnetic wave energy in the TE mode comprising a circular waveguide the diameter of which progressively increases stepwise at each of a plurality of consecutive axially-spaced transverse planes, each section of said waveguide between adjacent pairs of said planes being of substantially constant diameter, the first diametral increase being from a section of the maximum critical diameter which is below cut-off for the TE mode at the operating frequency to a section above cut-off for said mode, the diametral increases each being dimensioned to generate a prescribed amount of TE mode energy from TE mode energy at the operating frequency propagating through it, the magnitude of each diametral increase being restricted so that substantially no energy in a mode higher than said TE mode is generated from said TE mode energy, said planes being spaced apart relative to TE mode energy at the operating frequency in the sections between them so that said amounts of TE mode energy generated by the diametral change at each of said planes due to incidence thereat of TE mode energy propagating through said Waveguide in a given direction substantially cancel each other.

6. Diameter transformer for circular waveguide intended for propagation of electromagnetic wave energy in the TE mode comprising a circular waveguide the diameter of which progressively increases stepwise at each of two consecutive axially-spaced transverse planes, each section of said waveguide between adjacent pairs of said planes being of substantially constant diameter, the first diametral increase being from a section of diameter below cut-off for the TE mode at the operating frequency to a section above cut-off for said mode, the diametral increases each being dimensioned to generate substantially equal amounts of TE mode energy from TE mode energy at the operating frequency propagating through them in the direction of increasing diameters, the magnitude of each diametral increase being restricted so that substantially no energy in a mode higher than said T13 mode is generated from said TE mode energy, said planes being axially spaced apart relative to T E mode energy at the operating frequency in the waveguide section between them so that said amounts of TE mode energy generated at each of said planes due to incidence thereat of TE mode energy propagating through said waveguide in said direction substantially cancel each other at the increase to the largest diameter.

7. Internal diameter transformer for hollow round waveguide intended for propagation of electromagnetic wave energy in the TE mode comprising a round hollow waveguide the internal diameter of which progressively increases stepwise at each of a plurality of consecutive axially-spaced transverse planes, each section of said waveguide between adjacent pairs of said planes being of substantially constant diameter, one of said diametral increases being from a section of internal diameter below cut-01f for the T E mode at the operating frequency to a section above cut-oil? for said mode, said one diametral increase being followed by at least one further internal diametral increase, said one and said following internal diametral increases each being dimensioned to generate a prescribed amount of T13 mode energy from T13 mode energy at the operating frequency propagating through it, the magnitude of each of said one and said following diametral increases being restricted so that substantially no energy in a mode higher than said TE mode is generated from said TE mode energy, the planes defining said one and said following diametral increases being spaced apart elative to TE mode energy at the operating frequency so that said amounts of TE mode energy generated by the diametral change at each of said planes due to incidence thereat of TE mode energy propagating through said waveguide in a given direction substantially cancel each other.

8. Internal diameter transformer for hollow round waveguide intended for propagation of electromagnetic wave energy in the TE mode comprising a round hollow waveguide made of a single piece of electrically conductive material the internal diameter of which progressively increases stepwise at each of a plurality of consecutive axially-spaced transverse planes, each section of said waveguide between adjacent pairs of said planes being of substanitally constant diameter, one of said diarnetral increases being from a section of internal diameter below cut-off for the TE mode at the operating frequency to a section above cut-off for said mode, said one diametral increase being followed by at least one further internal diametral increase, said one and said following internal diametral increases each being dimensioned to generate a prescribed amount of TE mode energy from TE mode energy at the operating frequency propagating through it, the magnitude of each of said one and said following diametral increases being restricted so that substantially no energy in a mode higher than said TE mode is generated from said T5 mode energy, the planes defining said one and said following diametral increases being spaced apart relative to TE mode energy at the operating frequency so that said amounts or" TE mode energy generated by the diametral change at each of said planes due to incidence thereat of TE mode energy propagating through said waveguide in a given direction substantially cancel each other.

9. Internal diameter transformer for hollow round waveguide intended for propagation of electromagnetic wave energy in the TE mode comprising a round hollow waveguide the internal diameter of which progressively increases stepwise at each of a plurality of consecutive axially-spaced transverse planes, each section of said waveguide between adjacent pairs of said planes being of substantially constant diameter, the first diametral increase being from a section of internal diameter below cut-01f for the TE mode at the operating frequency to a section above cut-off for said mode, the diametral increases each being dimensioned to generate a prescribed amount of TE mode energy from T E mode energy at the operating frequency propagating through it, the magnitude of each diametral increase being restricted so that substantially no energy in a mode higher than said TE mode is generated from said TE mode energy, said planes being spaced apart relative to TE mode energy at the operating frequency in the sections between them so that said amounts of TE mode energy generated by the diametral change at each of said planes due to incidence thereat of TE mode energy propagating through said waveguide in a given direction substantially cancel each other.

10. Internal diameter transformer for hollow round waveguide intended for propagation of electromagnetic wave energy in the TE- mode comprising, a first section of round hollow waveguide having a first substantially constant internal diameter which is below cut-off for the TE mode at the operating frequency, a second section of round hollow waveguide having a second substantially constant internal diameter which is greater than said first internal diameter and is above cut-off for the TE mode at the operating frequency, said first and second sections meeting concentrically at a first plane transverse to their common axis to provide in said first plane a first circularly symmetrical stepwise diametral change, a third section of round hollow waveguide having a third substantially constant internal diameter which is greater than said second internal diameter, said second and third sections meeting concentrically at a second plane transverse to their common axis to provide in said second plane a second circularly symmetrical stepwise diametral change, the length of said second section between said first and second planes being determined by the relation where:

N=any odd integer,

and said diameters being so related that D ==said first diameter,

D =said second diameter, and

D =said third diameter, the magnitude of each diametral change from D to D and from D to D respectively, being restricted so that substantially no energy in a mode higher than said TE mode is generated from said 'I'E mode energy passing therethrough.

an internal diameter which is above cut-off for said mode,

said last named section being followed by at least one more section of still larger internal diameter, the diametral increases to internal diameters which are above cutoff for said TE mode each being dimensioned to generate a prescribed amount of TE mode energy from TE mode energy at the operating frequency propagating through it in the direction of increasing internal diameters, the magnitude of each diametral increase being re stricted so that substantially no energy in a mode higher than said TE mode is generated from said T13 mode energy, the planes defining said last named diametral increases being spaced apart relative to TE mode energy at the operating frequency so that said amounts of TE mode energy generated at each of said planes due to incidence thereat of TE mode energy propagating through said waveguide in said direction substantially cancel each other.

References Cited in the file of this patent UNITED STATES PATENTS 2,531,437 Johnson et a1. Nov. 28, 1950 2,643,296 Hansen June 23, 1953 2,762,982 Morgan a- Sept. 11, 1956 2,767,380 Zobel Oct. 16, 1956 FOREIGN PATENTS 147,078 Australia Sept. 9, 1948 OTHER REFERENCES Collin Proceedings of the IRE, Feb. 1955, vol. 43, No. 2, pages 179-185.

Olin Electronics, Dec. 1955, pages 146 and 147.

Unger: Bell System Tech. Journal, vol. 37, No. 4, July 1958, pages 899-912.

Wray: Electronic Engineering, vol. 31, No. 372, Feb. 1959, pages 7679.