US2759156A - Waveguide mode filter - Google Patents

Description

14, 5 E. L. YOUNKER WAVEGUIDE MODE FILTER Filed Nov. 19, 1945 INVENTOR ELMER LEROY YOUNKER BALW ATTORNEY Patented Aug. 14, 1956 "it C WAVEGE MQDE FILTER Elmer L. Younker, Cambridge, Mass, assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Application November 19, 1945, Serial No. 629,673

2 Claims. (Cl. 333-98) This invention relates to an electric wave transmission system and in particular, to a mode filter in such a system.

In hollow wave guides designed to propagate electric waves, it is well known that several modes may be set up. It is, in fact, difiicult or impossible to set up in a hollow guide a single pure mode of oscillation, and thus one or more unwanted modes may exist in the guide together with the desired mode. The presence of these unwanted modes leads to various undesirable results.

In some cases it is possible to select the size of the guide so that unwanted modes are suppressed; this will be possible where the frequency used is above the cutofi frequency for the wanted mode but below the cutoff frequency for the unwanted modes. In other cases, however, this may be an impossible or undesirable arrangement, and it is necessary to provide some sort of mode filter to accomplish the desired end.

Such situations arise with cylindrical wave guides, where axial symmetry of the electric field is a desirable feature. Of the lower modes in such a guide, the two having this desirable feature of axial symmetry are the TM01 and the TE01 modes, and, as between these two possible choices, the TE01 mode is often chosen for work at ultra high frequencies because of its more favorable attenuation properties. (For the configuration of the electric field pattern in these and other herein mentioned modes, see Schelkunoif, Electromagnetic Waves, D. Van Nostrand Company, New York, 1943, pages 395, 396, or Brainerd, Koehler, Reich, and Woodruff, Ultra- High-Frequency Techniques, D. Van Nostrand Company, New York, 1942, page 473.)

There is, however, one unfortunate feature connected with transmission in the T1501 mode. For transmission in this mode, the diameter of the wave guide must be at least 112A, which is the cut-off diameter for this mode (A being the free space wave length of the energy being carried). Since the cut-01f diameters for the TE21, TE11, the TM01, and the TM11 modes are all less than or equal to 1.22%, such a system may carry any of these modes, of which the "Him is particularly apt to be excited and the TE21, TMn, and TEn, modes are particularly undesirable, since they destroy the axial symmetry of the electric field.

All modes are characterized by the fact that they induce currents on the inner wall of the wave guide carrying them, and whereas the currents induced by the unwanted modes described above have components in the direction of the axis of the cylindrical wave guide, wall currents of the desired TEm mode are wholly circumferential. This invention is directed to removing or filtering out modes of energy having a field configuration while produces currents in the walls of the waveguide running in a direction parallel to the axis of the wave guide. The means and methods hereinbelow set forth are specifically directed to a cylindrical wave guide, but it will be apparent to those skilled in the art that these methods and means may be applied to other types of transmission line systems and wave guides.

Various methods have been used for providing such a filter. Among these may be mentioned the use of a number of thin metal wires disposed in the tube in such a way as to coincide with the lines of electrical force in the unwanted mode. Reference is made also to a copending patent application, Serial Number 667,501, filed on November 19, 1945 by Edward M. Purcell for 8. Mode Filter For Wave Guide, now abandoned, assigned to the same assignee as this invention, which outlines a solution involving resonant cavities surrounding the guide and olfering high impedance to the passage of unwanted modes having longitudinal components of electric field and/ or wall currents. These and other existing methods are subject to the difliculty that they involve somewhat elaborate mechanical constructions, a feature with respect to which the present invention represents a considerable improvement.

It is accordingly an object of this invention to provide in a wave guide system a mode filter that is operable independently of the transverse dimensions of the wave guide in the system.

It is a further object of this invention to provide in a wave guide system a mode filter for electric waves that will provide a high attenuation to those modes having electrostatic field vectors and/ or currents tending to flow in a direction parallel to the axis of the wave guide.

it is a still further object of this invention to provide, in a cylindrical wave guide system, a mode filter for those modes having electric field vectors with components in a direction parallel to the axis of the wave guide and/or currents that tend to flow in a direction parallel to the axis of the wave guide.

It is a still further object of this invention to provide in a wave guide system means for dissipating energy in those modes having currents that tend to flow in a direction parallel to the axis of the wave guide.

It is a still further object of this invention to provide in a cylindrical wave guide system adapted to carry energy in the TEor mode, a filter for removing or providing a high attenuation to energy in lower modes than the said TEOI mode.

It is another object of this invention to provide such a filter which will be simple and easy to manufacture without the addition of complicated mechanical constructions, and which is not frequency critical.

Other objects and features of the present invention will become apparent upon a careful consideration of the following detailed description when taken together with the accompanying drawing, which illustrates a typical embodiment of this invention.

This drawing is a perspective view of a filter in accordance with my invention, incorporated in a cylindrical wave guide.

The cylindrical wave guide 3 has a plurality of parallel circumferential slots 1 each extending through the wall 2 and having its length in a direction transverse to the axis of the guide. Each of these slots has a length L which is approximately equal to one half the free space wave length of the energy being propagated in the guide. The slots are arranged in regular fashion, so that they form longitudinal rows 4, grouped around the circumference of the cylinder, and running down the guide parallel to the axis and to each other. Proceeding down the guide, one group of rows, arranged circumferentially as described, is followed by another group, the rows of which are circumferentially staggered with respect to the preceding group in such a way as to intercept lines running between the rows of the preceding group. While the width w of the slots 4 is not critical, it must be small compared to the length L. It is also very desirable, in any circumferential group of rows, to have the corresponding slots approximately lined up with each other around the circumference of the guide,

as in this embodiment, so that, between slots, a plane passed through the guide normal to the axis would not intersect any slot. The spacing between the slots will be ,equal to a small fraction in the order of one eighth dr llesslof a'free space wavelength of the energy being propagated in the wave guide, in order to permit a maximum number of slots-to be cut in a given section of waveguide and thus maximize 'dissipation of the energy of theunwantedmodes.

" Thewave guide 3 ihas an'inner diameter d large enough to make possible propagation therein of electric waves in 'the 'TEmmode. Thus the diameter d is desirably 1122 times as great as the length of such waves in the wave guide 3 .or greater. This mode is'characterized by circular. electrostatic field vectors lying in a plane transverse .to the axis of the wave guide. There are substantialy no' electrostatic field vectors lying in the direction parallel to the said axis. 'In the'TEur mode, currents will flow on the inner surfaceof the wave guide Wall 2 in a circumferential direction, transverse to the axis of the wave .guide.(as indicated-by arrows'5). There will be substantially no current in an axial direction. However, due to 'thesize of the diameter d, it is possible that there will be 'other modes of oscillation in the wave guide 3, such as, for example, the -TE21, TE11, and TMui, and the -TMu modes. These modes are undesirable, particularly the TEzg-th TEu, and the TM11 modes, all of which tend to'destroy the axial symmetry which is a primary desirable quality of the TEor mode. The TEZI mode is particularly troublesome, since it is apt to be excited by the same type' of probe orientation which will excite the TEM mode. A'llof the unwanted modes have fields which tend toinducecurrents flowing in the wall of the wave guide 3 with components lying in a direction parallel to the axis of this waveguide. The electrostaticfield of the TEm mode will not pass through the slots 1, inasmuch as the direction of this field, as well as df'the wall currents 5 produced by it, is parallel to the direction of these slots 1. However, since the interfering modes produce currents in the wall of the waveguide 3 which have a componentparallel to the axis of the guide, these undesired modes 'will excite the slots 1, which will radiate away the energy of the said undesired modes. Because of this fact, the slots 1 will cause attenuation tending to prevent propagationofthe undesired modes along the waveguide 3, and will thus actas a filter tending to pass the desired mode and reject the undesired niodes.

Even if the slots 1 are not of a length equal to one half of the free-space wave length, as hereinbefore specified, they will still interfere with the propagation of the undesired modes along the Wave guide, since they will introduce series reactances and set up reflections of energy of the undesired modes, and there may also be some radiation of energy through the slots, if their 3. lengths are not greatly difierent from one half of the free-space wavelength. Radiation of the energy of the' undesired mode through. the slots 1 is desirable, to prevent the building up of standing Waves of large amplitudes in these undesired modes, and thus it is desirable that the length L of the slots 1 should be approximately equal to half of the free'space wavelength of the energy being propagated, in order to obtain bestradiation and minimum standing waves. It should be apparent, however, 'from the foregoing discussion that slots of other dimensions will serve the purpose of rejecting unwahted modes.

It is apparent that this invention may be used in any shape of wave guide, as desired, to filter out modes having currents that will 'flow in a longitudinal direction. Therefore, it is preferred that this invention be limited only by the piror art and the scope of the appendedclaims.

What is claimed is:

l. A mode filter, comprising: a cylindrical wave guide; and a plurality of parallel slots extending through the wall of said Wave guide and extending lengthwise in a direction transverse to the axis of said wave guide, said slots being so arranged as to form longitudinal rows grouped 'aroundthe circumference of said cylindrical Wave guide, each group of rows being circumferentially staggered with respect to the adjacent group, said slots having lengths half the free space wave length of energy inth'e guide to radiate the energy of the rejected mode, and being axiallyspaced a small fraction-of the Wavelength of this'energy.

2. A mode filter, comprising a cylindrical wave guide and a plurality of parallel slots extending through the wall of said wave guide and extending lengthwise in a direction transverse to the axis of said -Wave guide, said slots being so arranged as to form longitudinal rows grouped about the circumference of saidcylindrical Wave guide, each group of rows being circumferentially staggered with respect to the adjacent group, said slots having lengths substantially-half the free space wave length of energy in the guide to radiate the energy propagated in modes producing current flow in a direction parallel tosaid axis and to act as series reactance to reflect energy propagated in certain modes, said slots being axially spaced a' small fraction of the wave length of this energy. "7

'zReterencesCited in the file of this patent UNITED STATES PATENTS 2,088,749 King Aug. 3, 1937 2,151,118 King 'Mar. 21, 1939 2,206,923 SouthWorth July 9, 1940 2,405,242 Southworth Aug. 6, 1946 2,500,417 'Kinzer Mar. 14, 1950

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