Patent GB-795862-A (GB)
Title: Improvements in or relating to transmission line coupling devices
IPC: H01J23/52
Filed: 1955-10-28
Abstract: 795,862. Directional couplers. WESTERN ELECTRIC CO., Inc. Oct. 28, 1955 [Oct. 29, 1954; Oct. 29, 1954], No. 30904/55. Class 40 (8). A coupling device for transmission lines, e.g. waveguides, comprises two lines whose fields are coupled over a coupling section, transfer of energy from one line to the other being achieved by variation in the propagation parameters, in particular the coupling coefficient and phase propagation constant, over the coupling section. The invention is based upon the fact that over a coupled section in which all characteristics remain constant there are two so-called " normal modes " into which the energy propagated in the section can be resolved. For one of these modes the field intensities in the two lines are in phase and propagate at a lower phase, velocity than the normal wave in either of the lines alone. For the other mode the intensities are out of phase and the phase velocity is greater than that of the normal single-line mode. Fig. 1 shows a waveguide coupler in which variation of the phase propagation constant along the coupled guides 21, 22 is achieved by gradual variation of the wider transverse dimension, the guides being coupled by an aperture 27 which extends from the point of minimum width to the point of maximum width of the guide 21. The aperture may be divided by wires in the manner described in U.S.A. Specification 2,701,342. The electrical intensity distribution in the two guides of the " in phase " (low velocity) mode at sections a, b, c, e (c is at the centre of the coupling section) are shown in Figs. 2A, 2B, 2C, 2E, whence it is apparent that the relative amplitudes of the mode in the two guides at any point is a function of the difference in phase velocities (i.e. in the width of the guides) at the point, the whole of the energy in guide 21 at a being transferred to guide 22 at e. The coupling coefficient k may be represented by the amplitudes of the fields in the coupled guides at the point of overlap and it is seen that for the desired distribution the coupling coefficient should increase as the difference in phase velocities between the guides decreases. It is stated in the Specification that optimum performance for the shortest coupling interval may be obtained by making ##<SP>2</SP>+k<SP>2</SP> constant along the interval, where # is equal to half the difference in the phase constants of the guide. This may be achieved by using the values # = #a-#b/2 cos 6 and k = #a-#b/2 sin # where # a  and # b  are the phase constants of the guides at their points of greatest difference and # is a parameter proportional to the distance along the coupling interval (see Fig. 4). The variation in k is achieved by shaping the slot 27 in the manner shown. A wider bandwidth may be obtained by making # a non-linear function of distance which varies more rapidly in the centre of the coupling than at the ends. If the input energy is applied to the terminal 25 of guide 21, transfer of energy to guide 22 will take place in the higher velocity " out of phase " mode since at the start of the coupling section the guide 21 has minimum width and supports a wave of high-phase velocity. The energy coupled into the guide 22 is out of phase with the input energy. Thus terminals 25, 26 are in antiphase relation and whilst terminals 23, 24 are in phase. Any desired distribution of power can be made by using a portion only of the coupling section. Thus an equal division can be effected by terminating the coupling section at the point c where the amplitudes of the intensities in either guide are equal. The required variation in phase constant can be achieved by dielectric or permeability loading of the guides instead of a variation in dimensions. Thus the guides may be everywhere identical in cross-section and loaded by pieces of dielectric which gradually increase or decrease in an axial direction (Fig. 5, not shown). In Fig. 7 is shown a modification in which coupling is achieved by variation in the coupling parameter only. The guide 71 is dimensioned to have a larger phase constant than the guide 72 and the two guides arranged as shown, are coupled by a pair of oblique slots 73, 74, each with one end near an edge of the narrow face of guide 72 in a region of high coupling and the other end on the common centre line of the contiguous faces of the guides in a region of low coupling. As in the construction of Fig. 1, for energy applied to the near end of guide 71 the coupling is zero at the point where the whole of the energy is in the guide of larger phase constant. The coupling becomes a maximum at the end of the slot 73 where it changes sign and thereafter decreases to zero at the end of the slot 74. It is stated that the reversal of the coupling coefficient at the centre of the coupling section is effective to transfer the whole of the energy in the guide 71 into the guide 72. In a further method of obtaining a reversal of the coupling coefficient (Fig. 9, not shown), the two guides are arranged with their axes at an angle and their broad faces contiguous over the coupling section. A single coupling slot runs near and parallel to the edge of the broad face of one guide and obliquely across the broad face of the other guide. With this arrangement the coupling coefficient runs from a positive maximum at one end to the slot, through zero at the centre, to a negative maximum at the other end. In the construction shown in Fig. 12, transfer of power is achieved by a variation in the difference of the phase propagation constants of the guides 110, 111 from a positive maximum at one end of the coupling to a negative maximum at the other end. This may be done either by a variation of the transverse dimensions of the guides or by progressively loading the guides with dielectric or magnetic material 123, 124, Fig. 13. The coupling coefficient remains constant over the coupling interval in these constructions. The principles of the invention may be applied to waveguides of the all-dielectric type described in Specification 731,473. The distance between two such waveguides 60, 61, Fig. 6, may be arranged so that coupling is greatest on the line 62 and decreases to a minimum at either side of this line. At the point of maximum coupling the dielectric waveguides are of symmetrical (circular) section, this circular section being progressively deformed to a non- symmetrical (elliptical) section to either side of this point in the manner illustrated. Since the phase velocity of the propagated energy is inversely proportional to the thickness of the dielectric measured in the direction of the electric vector (in this case the horizontal vector E) it is apparent that with the arrangement shown, maximum phase velocity difference is achieved at the points of minimum coupling and zero phase velocity difference at the point of maximum coupling. This is the desired distribution of parameters shown in Fig. 4 for directional coupling of energy. In the embodiment shown, branch 65 is coupled to branch 68 and branch 66 to branch 67. The invention may be applied to the coupling of energy to and from travelling-wave electrondischarge tubes. This application is described and claimed in Specification 795,863.