US2619539A

US2619539A - Mode changer

Info

Publication number: US2619539A
Application number: US620127A
Authority: US
Inventors: Roberto M Fano
Original assignee: Individual
Current assignee: Individual
Priority date: 1945-10-03
Filing date: 1945-10-03
Publication date: 1952-11-25
Anticipated expiration: 1969-11-25

Description

NOV. 25, R. M. FANO MODE CHANGER Filed Oct. 5, 1945 2 SHEETS--SHEET 1 Fl G.l FIG2 FIG.3

INVENTOR. ROBERTO M. FANO W4 Q, ATTORNEY.

R. M. FANO MODE CHANGER Nov. 25, 1952 2 Sl'IEETS--Sl-IEET 2 Filed Oct. 5, 1945 INVENTOR. Fflflf70 M FA /Va 601114 Patented Nov. 25, 1952 MODE CHANGER Roberto M. Fano, Boston, Mass, assignor, by mesne assignments, to the United States of America as represented by the Secretary of War Application October 3, 1945, Serial No. 620,127

Claims. (Cl. 178-44) This invention relates to transmission lines and more particularly to inclosed transmission lines such as hollow-pipe wave guides and coaxial transmission lines.

It frequently becomes desirable to propagate electromagnetic energy through an inclosed transmission line which is circularly symmetrical; that is, symmetrical about its longitudinal axis so that any cross section of the transmission line exhibits circular conducting lines. More briefly, by a circularly symmetrical inclosed transmission line is meant either a circular wave guide or a concentric coaxial cable the interior of which substantially is closed from the outside atmosphere. In cross-section these display a circle as the inner part of the outer conductor and a concentric inner circle as the outer part of the inner conductor if one exists. In some systems wherein such propagation is desirable, it is also desirable that the mode of propagation be one in which the electric vectors assume a substantially uniform, symmetrical, and radial configuration, as will appear more particularly hereinafter in this specification for the hollowpipe wave guide and coaxial transmission line respectively. Such radial symmetry permits the inclusion of rotating joints in the system for turning an antenna or other purposes, which is insensitive in polarization of the transmitted waves to the angle through which the rotating joint may be turned. However, when the inclosed transmission line has dimensions permitting the propagation of two modes the problem arises of causing attenuation of one mode, and propagating the desired mode alone.

Among objects of this invention therefore are to provide means for propagating electromagnetic energy in a mode with radial symmetrical electric vectors in an inclosed transmission line of circular symmetry, the dimensions of which would otherwise permit the propagation of other modes; to provide means for propagating the TMai mode and attenuating the 'I'E1,1 mode in circular wave guides, the dimensions of which would otherwise permit the propagation of both of those modes; to provide means for propagating the TEM mode, and suppressing the TE1,1 mode in coaxial cables, the dimensions of which would otherwise permit the propagation of both these modes; to provide coupling junctions between inclosed transmission lines of circular symmetry and wave guides of rectangular cross-section such that energy transference therebetween will be from the TEu,1 mode in the rectangular guide to a modeexhibiting radial symmetry in the inclosed transmission" line of circular 'symmetry; to provide a coupling junction between a wave guide of circular symmetry and a wave guide of rectangular cross-section such that energy transference therebetween will be with the TMo,1 mode in the circular wave guide; to provide a coupling junction between a coaxial transmission line of concentric, radially symmetric coaxial cable such that the energy transference therebetween will be with the TEM mode in the coaxial cable.

Other objects, novel features, and advantages of the invention will be apparent from the description contained herein. I

In the drawings I Fig. 1 illustrates the configuration of electric vectors in the dominant or lowest mode, the'TE1,1 mode, in a hollow-pipe wave guide of circular cross-section;

Fig. 2 illustrates the configuration of electric vectors in the next to the lowest, or next dominant mode, the 'IMo,1 mode in such wave guides;

Fig. 3 illustrates the configuration of electric vectors in the dominant mode, which is the TEM mode, in a coaxial transmission line;

Fig. 4 illustrates the configuration of electric vectors in the next dominant, or next to the lowest, mode, which is the TE1,1 mode, in a coaxial transmission line Fig. 5 is a cross-section of a preferred embodiment of the invention;

Fig. 5A is an orthogonal perspective view of the coupling junction or adapter at the junction of the rectangular wave guide and the coaxial transmission line shown in Fig. 5 and embodying the present invention;

Fig. 5B is the field configuration in an end of rectangular cross section of the adapter shown in Fig. 5A excited in the 'IEo,1 mode;

Fig. 6 is a top view in partial cross-section of the embodiment illustrated in Fig. 5;

Fig. 6A is a variation of the embodiment illus trated in Fig. 5;

Fig. 7 is a sectional view of another preferre embodiment of the invention; and

Fig. 8 is a top view in partial cross-section of the embodiment illustrated in Fig. 7.

Referring now to Fig. 1, there is illustrated the TE1,1 mode in circular wave guide. It is apparent from the illustration that this configuration has only one axis of symmetry, indicated by dotted line In. Fig. 2 illustrates the TMo,1 mode in circular wave guide. vectors of Fig. 2 show this mode to have the characteristic herein termed circular symmetry, and further, the electric vectors are radial, emanat ing from the center of the guide. The TE1,1 mode The pattern of the electricis that variously termed dominant, fundamental, or lowest, because, for any given diameter of a circular wave guide, the electromagnetic waves of lowest frequency which can be propagated therein will be propagated in that mode. To state it another way, it is the mode having the lowest cut-01f frequency, so that lower frequency waves are attenuated only, and not propagated. That mode variously termed the next to the lowest, the next highest, or the next dominant mode is the one having the next highest cut-off frequency.

Referring now to Fig. 3, there is illustrated the dominant, or TEM mode, in coaxial cable. This mode has radial, symmetric electric vectors. Further, as is well known, the TEM mcde in coaxial cable has no cut-off frequency; any frequency, however low, may be propagated within a coaxial cable in the TEM mode. The next higher mode in coaxial cable is the TE1,1 mode illustrated in Fig. 4, which has a higher cutoff frequency, of course, than the TEM mode.

It-will be observed that the TE'1,1 mode in circular wave guide, and the TE1,1 mode in coaxial cable have certain features in common. The general configuration of the electric vectors with respect to the outer conductor is the same. In particular, each pattern has only one axis of symmetry, that indicated by line If! in Fig. 1, and that indicated by line H in Fig.- 4. Further, for a given phasing, the electric vector along the axisof symmetry has a specified direction, and there is no vectorial component of any electric vector in either Fig. 1 or Fig. 4 which is opposite the direction specified thereby.

Likewise, the configurations of the TIVIOJ mode displayedin Fig. 2 and the TEM mode displayed lnFig. 3 have certain features in common. Both configurations have radial electric vectors (which may extend radially inward instead of outward forv adifierent phase condition), and are circularly symmetrical. A consequence of the circular symmetry displayed by these patterns is that any .two orthogonal axes through the center of the figure are alsoaxes of symmetry, in contrast with theTE l modes wherein there is only one axis of symmetry. This difference in symmetry of the TEM mode and TMo,1 mode in coaxial cable and circular wave guide respectively on the one hand and the TE1,1 modes in coaxial cable and circular wave guides on the other hand is utilized in the present invention to suppress the TE1,1 modes and propagate the TEM or TMO,1 modes, by exciting the wave guide of circular symmetry in a manner symmetrical with two orthogonal axes.

Referring now to Figs. 5 and 6 there is illustrated a preferred embodiment of the invention suitable for achieving only the TEM mode in coaxial cable or the TMo,1 mode in circular wave guides even though the cable or Wave guide is proportioned to permit propagation of both of the two lowest modes therein at the operating frequency. The transmission line junction or transducer device in Figures 5 and 6 comprises a box at the junction of a coaxial conductor l5 and a rectangular wave guide 25. The interior of the box is partitioned and compartmented for the continuous propagation of electromagnetic energy between the-coaxial conductor I5 and the wave guide 28. The coaxial cable leis excited at its junction with two rectangular wave guides l6 and H Whose longitudinal axes |9 and 26 respectively are aligned near the junction. Furthermorathe energy in wave guides 16 and IT,

respectivelyJs in phase at equal distances from the ;center l8 of coaxial cable 15 measured along axes I 9 and 20. Aligned axes l9 and 20 projected toward each other intersect axis l8 at right angles, and a line 2| perpendicular to axes I9 and 2G and I8 through their intersection is thereby uniquely determined. Since energy is in phase in wave guides l6 and I! at equal distances from axis I9 and wave guides I6 and I? are of rectangular cross section and excited in the TEO,1 mode with electrical vectors perpendicular to the plane of Fig. 6, so that their amplitude distribution will be a half sinusoid along any line in wave guides l5 and H perpendicular to axis H3 or 2!], it is clear that aligned axes l9 and 29 and axis 2| form orthogonal axes of symmetry for the mode generated in transmission line I5.

To assure the in-phase condition required along aligned axes I9 and 29 of wave guides it and I7, these may communicate through bends or turns 22 and 23 at the ends of the septum or partition 3d away from axis i 3 with

wave guides

24 and 25 respectively.

Wave guides

24 and 25 are also of equal lengths and excited by a common source in a symmetrical fashion so that each absorbs theoretically half the energy therefrom. In this instance, a rectangular wave guide 26 is used as a common source; from it energy is fed through two windows 2'! and 28 proportioned for impedance matching purposes. Different shaped windows or other impedance matching devices may be utilized, however, as the case demands. For example, an adjustable terminating plunger, aligned with the longitudinal axis of wave guide 25 or one perpendicular thereto and extending opposite axis |8 of wave guide l5 may be utilized.

Fig. 6A illustrates another embodiment of the invention in which wave guides fli' and H are excited by forming them as an extension of curvilinear wave guides 25 and 25' communicating with a common source wave guide such as 26. In order for excitation of wave guide |5' to be symmetrical with respect to two mutually perpendicular axes in a cross section thereof, the longitudinal axes I9 and 20 of wave guides I5 and I! are aligned inthe neighborhood where they join with wave guide l5 and these axes pass through axis l3. As before, aligned axes l9 and 2D and axis 2| perpendicular thereto at the intersection with axis l8 are two perpendicular'axes of symmetry for the excitation of wave guide !5 which is thereby excited in the TEM mode. Impedance matching windows may be used in guides 26 or 26'.

Referring again to Figs. 5 and 6, energy propagated though rectangular wave guide 26 may be radiated through

windows

21 and 28, formed by

projections

21 and 23 to provide a good impedance match, into

guides

24 and 25 which are so dimensioned that only the TEo,1 mode may be propagated therein, with electric vectors parallel to the plane of Fig. 5 and perpendicular to that of Fig. 6. propagate only the TEo,1 mode at the contemplated frequencies. Therefore, excitation of coaxial cable !5 will be symmetrical with respect to two orthogonal axes. First it Will be symmetrical with respect to axis 2| because the distance through which the energy passes approaching from one side of axis 2| after'leaving common source wave guide 26 is equal to the length traveled in approaching the other side of axis 2|. Second, it will be symmetrical with, respect to aligned axes-l9 and 29 because the propagation of energy-through Waveguides I6 and I1 is sym! metrical with this axis (and likewise the. plane Likewise wave guides l6 and I1 canv through Which'the view of Fig. 5 is taken). Because of the symmetryand the equal phasing on each side of axis 2| due to equal path lengths which the energy has traveled, it is impossible for'an electric field to result which has a configuration of the TE1,1 mode. Thus, it is impossible for the TE1,1 mode to be excited, even thou h the radii b and a are such as to permit propagation thereof. The base 29 provided for central conductor 30 of wave guide I5 is for impedance 1 matching purposes. If inclosed transmission line I5 is of the hollow wave guide type, central conductor 30 is lacking. A base such as 29 may be provided, or other impedance matching means may be utilized therewith. Base 29 may have a different shape, to which the outer walls may conform, as indicated by dotted lines A.

Referring now to Figs. 7 and 8, there is illustrated another preferred embodiment of the invention. A rectangular wave guide 40 forms a junction with an inclosed transmission line M which is illustrated as a coaxial cable having a central conductor 42. If energy is propagated through wave guide 40, all modes applicable to transmission line 4| will be generated, at the junction. However, it is presumed that the dimensions of transmission line 4| are such that the dominant and next dominant modes may be propagated therein, other modes bein subject to attenuation only.

A septum of

parts

43 and 43 made of conductive substance and preferably thin is inserted diametrically in line 4|. It is substantially in the plane defined by the axes of the two transmission lines near the junction. Excitation in wave guide 40 is symmetrical with respect to axis 44 (which lies in the plane along which the view of Fig. '7 is taken). Therefore, this plane is one of symmetry which is not disturbed by septum 43-43. In fact, the septum may be considered as forming two semi-annular wave guides having a dominant mode with a cut-off frequency the same as the cut-off frequency of the TE1,1 mode in a coaxial line of the same dimensions if the partition is infinitely thin. The electric lines of the TE1,1 modes are normal to a diametric plane (as c of Fig. 4) so that this plane can be materialized in a thin conducting septum or wall without disturbing an existing field of TE1,1 configuration, because the new boundary conditions are automatically satisfied since there is no tangential component of electric field at the septum. V

- Since they are equally excited, waves propagated 'insemi-circular guides 4| and 4|" will be equal in amplitude. As explained, they will be alike in configuration and phase, so that the vectors all extend in a general way outwards or inwards, as illustrated by the arrowed lines in Fig. 8. Therefore, the electric field at the input of coaxial line 4| where septum 43-43 termi- ..--nates will have two-plane symmetry, one along the plane of Fig. '7 and another perpendicular thereto through the axis of line 4|. Thus, no TE1,1 mode will be generated in line 4!. The sections of semi-annular guides 4| and 4| "must be sufficiently long to attenuate to a negligible amplitude higher modes, that is the local field, generated at the junction With the rectangular guide. A local field is also present at the termination of septum 43-43 into the coaxial line.

proper. Therefore, a good circular symmetry of an electrical field is obtained only at a certain distance from the junction. The septum may taper symmetrically toward the longitudinal axis of cable 4| to improve {impedance match and reduce reflections, and its top portions 45' and 45 viewed in the section perpendicular to that of Fig. '7, may be rounded to avoid arc-over if high power carrying capacity is desired. Again, a base 45 is provided for central conductor 42 of line 4| for impedance matching purposes. Also a stub 46 is included beyond the junction for like purposes. This may be adjustable in length although it is shown as fixed in the drawing. If the principle of'the invention is to be applied to a circular wave guide instead of coaxial cable 4|, central conductor such as 42 is absent, and the septum 43 and 43' may be extended completely" across the wave guide as one member. Thus, the embodiment illustrated in Fig. 8 may be considered as comprising two semi-circular wave guides adapted to feed a circularly sym metrical inclosed transmission line with twoplane symmetry.

The description herein has been directed to the transfer of energy from a rectangular or semi-circular wave guide to an inclosed trans mission line of circular symmetry. As is well known in the art, a like transfer of energy may be accomplished in the reverse direction; that is, the energy of the desired mode of radial symmetry will be transferred into the rectangular or semi-circular wave guides with substantially no transfer of energy between other modes. It is not absolutely necessary that a rectangular wave guide be used as shown in the preferred embodiments.

It will be apparent to those skilled in the'art that there are many variations of the invention which do not depart from its scope and spirit,

What is claimed is:

1. A transducer comprising an inclosed substantially circular transmission line, a first and a sec-0nd straight rectangular wave guide communicating therewith and axially opening into each other at one end and closed at their opposite ends, said two wave guides forming with said transmission line a common junction in the neighborhood of which said transmission line has circular symmetry, said first and second wave guides being so disposed that their longitudinal axes lie along a straight line that intersects the axis of said transmission line at substantially a right angle, a third and fourth wave guide disposed parallel to and having a common wall with said first and second Wave guides respectively, said wall terminating short of the closed ends for coupling energy from said first wave guide to said third wave guide and from said second wave guide to said fourth wave guide, and a fifth rectangular wave guide communicating with said third and fourth wave guides, said third and fourth Wave guides forming with said fifth wave guide a common junction, the longitudinal axes of said third and fourth wave guides lying along a straight line intersecting the longitudinal axis of said fifth wave guide at substantially right angles whereby there may be obtained a transference of electromagnetic energy propagated in said transmission line in a mode having radial symmetrical electric vectors and electromagnetic energy propagated in said fifth wave guide in the TEo,1 mode.

2. A transducer comprising an inclosed substantially circular transmission line, a first and a second straight rectangular wave guide communicating therewith and axially opening into each other at one end and closed at their opposite ends, said two wave guides forming with said transmission line a commonjunction in the neighborhood of which said transmission line has circular symmetry, said first and second wave guides being so disposed that their longitudinal axes lie along a straight line that intersects the axis of saidtransmission line at substantially a right angle, a third and a fourth wave guide disposed parallel to said first and second Wave guides respectively, a broad wall common to said first and third wave guides on opposite sides thereof and common to said second and fourth wave guides on opposite sides thereof, said wall terminating short of the closed ends for coupling energy from said first wave guideto said third wave guide and from said second wave guide to said fourth wave guide, and a rectangular transmission line'communicating with said third and said fourth wave guides, said third and fourth wave guides forming with said transmission line a common junction, the longitudinal axes of said third and fourth wave guides lying along a straight line intersecting the longitudinal axis of said transmission line at substantially a right angle, whereby there may be obtained a transference of electromagnetic energy propagated in said first transmission line in a mode having radial symmetrical electric vectors into electromagnetic energy propagated in said rectangular transmission line in the TEo,1 mode.

3. Apparatus in accordance with claim 2 wherein said two transmission lines are disposed at equal distances from said two common junctions.

4. A transducer comprising an in-closed transmission line, a first and a second straight rectangular wave guide communicating therewith and axially opening into each other at one end and closed at their opposite ends, said two wave guides forming with said transmission line a common junction in the neighborhood of which said transmission line has circular symmetry, said first and second wave guides being so disposed that their longitudinal axes lie along a straight line that intersects the axis of said transmission line at substantially a right angle. a third and a fourth wave guide disposed parallel to said first and second wave guides respectively, a fifth rectangular wave guide communicating with said third and said fourth wave guides, said third and fourth wave guides forming with said fifth wave guide a common junction, the longitudinal axes of said third and fourth wave guides lying along a straight line intersecting the longitudinal axis of said fi-fth wave guide at substantially a right angle, the point of intersection of said line and said axis lying on a line colinear agated in said fifth wave guide at the TEo.1,

mode.

5. An adapter at the junction of a coaxial line and a rectangular wave guide, comprising a coaxia'l line having an axis and an outerconductor and an inner conductor, a first pair of coaxial rectangular wave guides opening into each other and into said coaxial line at their inner ends and closed at their outer ends and having aligned axes incident to and in angular relation with the axis of said coaxial line, a second pair of coaxial rectangular wave guides parallel to .said first pair of wave guides and opening into each other and into said rectangular wave guide at their inner ends and closed at their outer ends and having aligned axes incident to and in angul-ar relation with the axis of said rectangular wave guide to comprise a rectangular adapter to the adjacent outer walls of which adjacent ends of said coaxial line outer conductor and said rectangular wave guide are attached, a partition shorter than the inner length of and supported within said adapter and against which an end of said coaxial line inner conductor is positioned, said partition constituting one wall of each of said pairs of coaxial rectangular wave guides, and said rectangular wave guide having an axis incident to and in angular relation with the axes of said coaxial line and said second pair of coaxial wave guides.

ROBERTO M. FANO.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,129,669 Bowen Sept. 13, 1938 2,396,044 Fox Mar. 5, 1946 2,401,751 Friis June 11, 1946 2,408,033 Beck Sept. 24, 1946 2,410,840 Samuel Nov. 12, 1946