US2934725A

US2934725A - Waveguide components

Info

Publication number: US2934725A
Application number: US692890A
Authority: US
Inventors: Gumbrell Michael
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1956-10-26
Filing date: 1957-10-28
Publication date: 1960-04-26
Anticipated expiration: 1977-04-26
Also published as: DE1081520B; GB831503A

Description

Ap 1960 M. GUMBRELL 2,934,725

WAVEGUIDE COMPONENTS Filed Oct. 28. 1957 INVENTOR Win 161. 6: 4 aRGLL FITTORNE Y5 United States Patent WAVEGUIDE COMPONENTS Application October 28, 1957, Serial No. 692,890

Claims priority, application Great Britain October 26, 1956 9 Claims. (Cl. 333-98) This invention relates to waveguide components.

'If a longitudinal slot is provided in the wall of a waveguide through which electromagnetic energy is being propagated, the direction of the electric field within' the waveguide in the vicinity of the slot being across the slot, then it can be shown that some of that energy is radiated from the slot .and thereby lost unless steps are taken to prevent such radiation.

One object of the present invention is to provide a waveguide component in which there is a} longitudinal slot in the wall of a waveguide but which is arranged so as to prevent any appreciable energy being lost through the slot. a 1

Another object is to provide such a slotted waveguide in which the loss of energy from the waveguide is prevented over a wide range of wavelengths of the electromagnetic wave being propagated through the Wave guide.

Various waveguide components have been proposed in which a body of ferrite material is disposed within a Waveguide of circular cross-section, the ferrite body being subjected to a magnetic field in a direction parallel to the longitudinal axis of the waveguide. Some such components, for example, attenuators and gyrators, depend for their operation on the Faraday rotation of the plane of polarisation of the electromagnetic wave within the waveguide. The said magnetic field is usually produced by a coil embracing the waveguide in the region of the ferrite body and the transmission properties of the waveguide component may be changed by varying the current carried by the coil. If the waveguide component is required to present two different conditions of transmission periodically and alternately, this may be done by supplying a suitable electric signal to the coil. In some applications, for example in .a radar system for the purpose of switching between transmitting and receiving conditions of the system, it is desirable to be able to change between these two conditions rapidly and frequently and the current supplied to the coil then contains a component of relatively high frequency. Since, however, the wall of the circular waveguide .efiectively constitutes a short-circuited turn in which power is dissipated, the adverse effect of the short-circuited turn increasing with frequency, steps have to'be taken to reduce the power that would otherwise be required to energise the coil. One suggested way of doing this is to reduce the thickness of the waveguide wall so as to increase the resistance thereof at the switching frequency but there is a limit to the amount the thickness of the waveguide wall can be reduced without'producing leakage of the electromagnetic energy which is propagated through the waveguide.

A further object of the present invention is to ro.- vide a waveguide component in which the difliculty dis-. cussed in the last paragraph is overcome.

According to the present invention, a waveguide component comprises a waveguide having a longitudinal slot in a wall thereof and a member lying outside the wave guide and adjacent to the said slot so that the said member and the waveguide together form a transmission line which is arranged to be excited in an evanescent mode when an electromagnetic wave is being propagated along the waveguide so that no appreciable portion of the electromagnetic energy is lost through the slot.

According to 'a' feature of the present invention, a waveguide component comprises a waveguide which is of circular cross-section, a body (which may consist of a pluralityof separate portions) of ferrite material. dis.- posed within the waveguide, a coil or coils surrounding the waveguide tor the purpose of subjecting the ferrite body to amagnetic field at least a componentlof which is in a direction parallel to the longitudinal axis of the waveguide, the waveguide wall having a longitudinal slot therein which is approximately co-terminous with the fer.- rite body and the waveguide being arranged so that it does not constitute a short-circuited turn embracing the said body, and a member which lies outside the waveguide and adjacent to the said slot and which together with the waveguide forms a transmission line which is arranged to be excited in an evanescent mode when an electromagnetic wave is being propagated along the waveguide so that no appreciable portion of the electromagnetic energy is lost through the slot.

The longitudinal slot of a waveguide component in accordance with the present invention may be parallel to the longitudinal axis of the Waveguide but it is to be understood that this feature is not necessary, the expression longitudinal slotfmerely implying that the-direction of the slot has a component parallel to the longitudinal axis. v

The manner of operation of a waveguide component in accordance with the present invention will now be discussed with reference to Figurel of the accompanying diagrammatic drawing which shows a cross-sectionalview of a waveguide which is assumed to be of infinite length. The waveguide, which has the reference 1 in the drawing, is of uniform circular cross-section; along its length and is filled with a homogeneous lossless material, therefractive index of this material to electromagnetic waves being N A slot 2 is provided in the wall 3 of this waveguide 1 in a direction parallel to the longitudinal axis of: the waveguide, this slot also being assumed to be of infinite length, while outside the waveguide 1 and adjacent to the slot 2 along the whole of its length there is provided an arcuate metal member '4,. This metal member 4; lies equally on either side of the slot 2 and the adjacent surfaces Sand 6 of the waveguide wa1l3 and the member 4 are spaced a uniform distanceapart, ,The material in the space 7 between the member 4 and the waveguide Wall 3 has a refractiveindex N If now an electromagnetic Wave which is circularly polarised in the dominant H mode 'is propagated along the waveguide 1, it is convenient to consider such a wave as being made up of two plane-polarised waves which are in time and space quadrature, one of these component waves being plane polarised in the plane of the slot 2 that is to say parallel to the arrow 8, while the other-is Patented Apr. 26, 1960 polarised in a plane perpendicular to the plane of the slot, that is to say parallel to the arrow 9. Of these two components, only the one that is polarised in the plane parallel to the arrow 9 is coupled through the slot 2 to the parallel plate transmission line formed by the member 4 and the waveguide wall 3. The amplitude of this coupling varies sinusoidally along the length of the slot and the pattern of excitation in the transmission line moves in the direction parallel to the longitudinal axis of the waveguide (that is to say at right angles to the plane of Figure 1') at the speed of propagation of the wave in the waveguide 1.

The traveling pattern of excitation in the transmission line formed by the member 4 and the waveguide wall 3 can be considered as being the resultant of two standing wave patterns which are of equal amplitude and which are spaced a quarter of a wavelength apart and in time quadrature. Since, however, the factors effecting the propagation in the said transmission line resulting from each of these standing wave patterns is the same, it is only necessary to consider the propagation resulting from a single standing excitation pattern. If it is assumed that the spacing of the member 4 and the waveguide wall 3 is small enough to ensure that modes of excitation involving components of electric field in directions parallel to the conductors of the said transmission line cannot be propagated, it follows that the only possible mode of propagation excited by this particular standing wave pattern is one in which the electric lines extend between the two conductors so as to be normal to the surfaces 5 and 6 while the magnetic lines are orthogonal to the electric lines and form loops which are disposed in a succession of rows, each of these rows being parallel to the 'slot 2 in a direction perpendicular to the plane of Figure l. p

The transmission line is arranged (as hereinafter described) so that if it were to propagate a Wave in the mode discussed in the last paragraph, the wavelength of that wave in the transmission line would be greater than the wavelength of the exciting wave in the waveguide 1. This is, of course, not possible with the result that the transmission line cannot be excited in the mode discussed to propagate a wave and it is therefore excited in an evanescent mode. Thus any wave coupled into the trans- 1nission line through the slot 2 is rapidly attenuated and does not reach the open ends 10 and 11 of the line. qccordingly substantially no energy is lost through the "s ot 2.

The transmission line formed by the member 4 and waveguide wall 3 is excited in an evanescent mode, as aforesaid, if the'following inequality is satisfied.

2 g o where a is the wavelength measured longitudinally in the waveguide of the electromagnetic wave being propagated through the'wavelength 1 and A is the free space wavelength of that wave. In other words A must be less than the natural wavelength where h is the cut-ofi wavelength for the waveguide 1 the H11 mode.

Although the above discussion is based on the assumption that the Waveguide 1 is filled with a homogeneous material of refractive index N the inequality (1) still holds if the waveguide 1 is partially filled with a body,

such as a ferrite rod which is shown by a broken outline inclined to the longitudinal axis of the Waveguide.

shaped member 17 of polystyrene.

4 12, having one refractive index while the rest of the waveguide is filled with material having a different refractive index.

Furthermore, although the above discussion has assumed the waveguide 1 to be passing an electromagnetic wave that is circularly polarised, it is equally applicable to plane-polarised wave, which may have a continuously rotating plane of polarisation along its direction of propagation, since a plane-polarised wave can be considered as being formed by two circularly-polarised Waves of equal amplitude and opposite senses of rotation.

The above discussion is concerned with a waveguide having a longitudinal slot in the waveguide wall that is parallel to the longitudinal axis of the waveguide but this limitation as to the direction of the slot is not a necessary feature of the invention as previously mentioned. In fact, if the waveguide is required to pass a circularlypolarised wave, the longitudinal slot may preferably be The reason for this is that if the slot is in the form of arighthanded helix which has a pitch of 25 radians per unit length while the wave itself has a left-hand polarisation (using the terminology of section 3.12 of Microwave Antenna Theory and Design, volume 12 of the Massachusetts Institute 'of Technology Radiation Laboratories Series, edited by S. Silver), the inequality (1) above is replaced by panying drawing which shows an isometric view of the component. For the purpose of showing the construction of the component, it is shown in Figure 2 partially in exploded form and partially sectioned with the outer layers removed.

Referring now to'Figure 2, the component comprises a waveguide 13 of circular cross-section in which is supported a ferrite'rod 14 which is made up of a plurality of likemain portions 14a and two like end portions 14b and the rest of the waveguide is filled with polystyrene. In

fact the waveguide 13 is formed by silvering the outer surface of a tube 16 of polystyrene by a suitable chemical process and then electroplating the silver layer so as to build up the waveguide Wall 15 with either silver or copper to I a thickness of approximately five thousandths 'of an inch.

The portions 14a of the ferrite rod are of uniform diameter which is slightly less than the inner diameter of the polystyrene tube 16 while the end portions 14b are tapered so as, in known manner, to minimise the impedance discontinuities presented by the rod 14. During manufacture, the rod 14 is pushed into position within the tube 16 and each end of this tube is plugged with a When so assembled, the-end 18 of the member 17 is fiush with the end 19 of the polystyrene tube 16. A slot 20 which lies parallel to the longitudinal axis 21 of the waveguide is provided in the waveguide wall, this slot being approximately coterminous with the ferrite rod 14. This slot is formed during manufacture of the waveguide by sticking a filament or thread (not shown) along the polystyrene tube "16 during the silvering process and then stripping off this filament-or thread prior to the electroplating. In similar manner two circumferential slots (of which only the slot 22'is shown in Figure 2) are provided in the waveguide wall 15 at each end of the longitudinal slot 21, the two circumferential slots being connected to the longitudinal slot so that the portion of the waveguide wall 15 embracthe-ferrite rod 14 does not; constitute a short-circuited i The circumferential "slot 22,,fo1, example, lies'in the plane containing the edge 23. i A rectangular metal plate 24 which is bent to an arcuate shape is mounted over the longitudinal slot 20. The spacing between the adjacent surfaces of the plate 24 and the waveguide wall 15 is uniform and the dielectric inthe transmission line formed thereby is mainly air. This plate 24 is in fact spaced from the waveguide wall 15 by thin strips 25 of a suitable solid dielectric material such as polyethylene, these strips partially embracing the waveguide 13. Alternatively strips of polyethylene may, for this purpose, form loops completely embracing the'waveguide 13 or be wound round the waveguide 13 in helical manner. The plate 24 is held in position against these strips 25 by tapes 26 which are wound round the plate 24 and the waveguide 13 together.

The magnetic field in which the ferrite rod 14 lies during use of the component being described, is provided by a coil 27. This coil 27 is wound between cheeks 28 over the assembly of the waveguide 13 and the plate 24. The coil 27 may be wound on a former (not shown) which is placed over the assembly.

Metal coupling flanges 29 are provided at each end of the waveguide 13, these flanges being electrically connected to the waveguide wall 15.

Typical dimensions of the waveguide component dercrfibed above and the parameters of the materials are as o ows:

The length of the rod 14 and thus of the slot 20 and plate 24, is not of course critical, as far as the present invention is concerned, and is chosen to give the component the desired electrical characteristics.

The waveguide component may be a gyrator and in that case the coil 27 is excited, during use, so that a wave passing through the waveguide 13 in one direction is subjected to a phase shift of 180 while a wave passing in the opposite direction does not undergo any phase shift. The phase shift to which waves passing through the waveguide 13 in the two directions are subjected may be interchanged by reversing the flow of energizing current through the coil 27. For this purpose switching means (not shown) are connected in circuit with the coil 27.

It will be appreciated that in the waveguide component described above, the wavelength A is a function of the amplitude of the magnetic field provided by the coil 27 and the inequality (1) must therefore be satistfied forv the largest value of this wavelength over the desired range of amplitude of the magnetic field. Since however the wavelength changes in this manner, it will be appreciated that it would not be possible to prevent radiation from the longitudinal slot 20 in the waveguide wall 15 by means of a conventional resonant choke arrangement. Such radiation is, however, prevented by providing the transmission line which is formed by the member 24 and the waveguide wall 15 and which is excited in an evanescent mode in accordance with the present invention.

If the component is required for use in a waveguide system which is made up mainly of waveguides of rectangular cross-section, lengths of waveguide for convertwe e 6 ing' from circular to rectangular cross-section may be connected one to each end of the waveguide 13.

For symmetry, it may be convenient in some circum stances to provide more than one longitudinal slot in the waveguide wall 15 and, for example, four equally spaced slots may be provided, each of these slots having its associated transmissionline which is arranged to excited in an evanescent mode. I

I claim:

1. A waveguide component comprising a waveguide which has a longitudinal slot in a wall thereof and which is filled with dielectric material, a member lying outside the waveguide and adjacent to the 'said slot, and dielectric material which is disposed between the waveguide wall and said member and which has a refractive index less than that of the dielectric material filling the waveguide so that the said member and the waveguide together form a transmission line which is arranged to be excited in an evanescent mode when an electromagnetic wave is being propagated along the waveguide so that no appreciable portion of the electromagnetic energy is lost through the slot.

2. A waveguide component according to claim 1 wherein the said waveguide is of circular cross-section.

3. A waveguide component comprising a waveguide which is of circular cross-section, a body of ferrite material disposed within the waveguide, first dielectric material filling the space between the ferrite body and the waveguide wall, a coil or coils surrounding the waveguide for the purpose of subjecting the ferrite body to a magnetic field at least a component of which is in a direction parallel to the longitudinal axis of the waveguide, the waveguide wall having a longitudinal slot therein which is approximately co-terminous with the ferrite body and the waveguide being arranged so that it does not constitute a short-circuited turn embracing the v said body, a member which lies outside the waveguide and adjacent to the said slot, and second dielectric material filling the space between the waveguide wall and said member, the refractive index of the second dielectric material being less than the effective refractive index of the waveguide filling constituted by the first dielectric material and the ferrite body so that the member and the waveguide together form a transmission line which is arranged to be excited in an evanescent mode when an electromagnetic wave is being propagated along the waveguide so that no appreciable portion of the electromagnetic energy is lost through the slot.

4. A waveguide component according to claim 3 wherein at each end of the said longitudinal slot there is a circumferential slot in the waveguide wall so that the portion of the waveguide wall that embraces the said body does not constitute a short-circuited turn.

5. A waveguide component according to claim 1 wherein the said longitudinal slot is parallel to the longitudinal axis of the waveguide.

6. A waveguide component according to claim 1 wherein the said longitudinal slot lies on a helix.

7. A waveguide component according to claim 1 wherein the said transmission line lies on either side of the said slot along the length of the slot.

8. A waveguide component according to claim 1 wherein there is uniform spacing between the said member and the waveguide wall.

9. A waveguide component comprising a waveguide formed by a tube which is of circular cross-section and which has a wall of uniform thickness with an elongated opening in the wall extending longitudinally of the tube, an arcuate member having an uninterrupted electrically conducting surface, and means to mount the arcuate member outside the tube and adjacent to the opening so that along the length of the opening and on either side thereof the said wall and said surface of the arcuate member together form a transmission line and at the frequency of an electromagnetic wave being propagated along the wavegnide the natural wavelength of a wave in the transmission line is greater than that in the waveguide.

References Cited in the file of this patent UNITED STATES PATENTS Schelkunoff Apr. 25, I939 Llewellyn Nov. 11, 1941 Llewellyn Feb. 26, 1946 Carlson July 2, 1946 Kallmann Feb. 8, 1949 Collard June19, 1951 FOREIGN PATENTS v France Jan. 7, 1957 OTHER REFERENCES 19 5 Montgomery: Technique of Microwave Measurements Radiation Laboratory Series, McGraw-Hill 1947, pages 489, 491 and 493. I I

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