US2948865A - Mode ellipticity correcting device - Google Patents

Description

I Aug. 9, 1960 Filed July 25, 1956 F. W. SMITH MODE ELLIPTICITY CORRECTING DEV ICE 2 Sheets-Sha e: 1

H63 FIG.4

INVENTOR FREnERmwARREN 9mm BY y AGENT Aug. 9, 1.960 F. w. SMITH ,9 8,

MODE ELLIPTICITY CORRECTING DEVICE Filed July 25, 1956 2 Shets-Sheet 2 FIG.5I

FIG-6 INVENTOR AGENT U it d states P t n 2,948,865 I MODE ELLIPTICITY CORRECTING DEVICE Frederick Warren Smith, Brixton, London, England, as-

sign'or, by mesne assignments, to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed July 25, 1956, Ser. No. 600,125 Claims priority, application Great Britain Aug. 8, 1955 V '3 Claims. (Cl. 333-21) This invention relates to wave guides for the transmission of high frequency polarised electro-magnetic waves.

Centimetric and millimetric electromagnetic waves having plane polarisation are commonly utilised in microwave systems, and a metallic wave guide system comprising a symmetrical guide coupled to a rectangular guide is often employed to convey the waves, the rectangular guide constituting a polarised input or output device (the expression symmetrical guide is used herein to denote a guide of circular, square, or other crosssectio'n capable of supporting waves at the same frequency polarised in more than one plane). It is also frequently necessary to provide a symmetrical guide with a longitudinal slot or a side arm or other means for extracting waves polarised in a predetermined plane. In such cases it is frequently important that the waves should have accurate plane polarisation, iLe. should be free from ellipticity or components having a space quadrature relationship to the component in the desired plane: this is advantageous not only to ensure maximum extraction of energy into a desired channel, but also to minimise flow of energy to an undesired channel which might otherwise occur owing to the presence of such ellipticity.

An elliptically polarised wave may be converted into a plane polarised wave by absorption of the energy of the unwanted component, but it is preferable, where possible, to phase-shift the unwanted component so as to bring it into phase with the Wanted component. The

latter action may be carried out with the aid of phaseshifting reactive means which may for example take the form of diametral plates or rods within the guide. Any phase-shifting element of course only introduces the correct phase shift at one frequency although 'it is capable of improving matters at other frequencies. Thus phaseshifting methods are applicable to relatively narrow band or single channel system, for example radar systems; on theother hand, where a highly stable oscillator is required and a high degree of isolation with respect to load variations is therefore necessary, these methods are ideal.

To correct for ellipticity, such phase-shifting means may be arranged normally to the desired plane, but there is the disadvantage that removal of the ellipticity involves rotation of the preferred plane of the wave. This is inconvenient in devices such as circulators or gyrators employing Faraday rotation, which devices may require several adjustments to establish a wave accurately polarised in a desired plane. Correction 'for ellipticity disturbs such setting and requires revision of the previous adjustments and this in turn may re-introduce further ellipticity.

In the drawing: 7

Figs. 1, 2, 3 and 4 are vector diagrams for use in explaining the principles of the invention;

Fig. 5 is a sectional view of a preferred embodiment of the invention; and

Fig. 6 is an end view of the embodiment of Fig. 5. An example of the aforesaid difliculty will be explained more fully with reference to Figures 1 and 2. of the accompanying diagrammatic drawing, in which vectors A and B represent the best known of the various methods of resolving anelliptically polarised wave into 2 two virtual components. In this method the virtual components A and B are selected to be in mutual spatial quadrature, have unequal amplitude and be separated in time by a phase difference of 90 (the latter condition is represented in the drawing by the notations 6:0 and 0=90). As a consequence, it is found that said components are aligned with the major and minor axes of the ellipse respectively. In these circumstances it is possible to shift the phase of component B by 90 so as to bring it into phase with component A and thus obtain plane polarisation. This is represented in Figure 2 by a new vector B (0:0). The plane polarisation is obtained, however, in the plane of the resultant vector C, which is at an angle 4; to the major axis of the original ellipse.

It is an object of the present invention to provide improved means for compensating for ellipticity in the polarisation of a wave guide without rotating the axes of the corresponding ellipse.

According to one aspect of the invention, a wave guide arrangement for the transmission of high-frequency polarised electro-magnetic waves comprising a symmetrical guide and reactance means associated with said guide in a plane extending through the longitudinal axis of the guide, is used for modifying ellipticity present in said polarised waves the major and minor axes of the corresponding ellipse being substantially at 45 to said plane of said reactance means.

. According to another aspect of the invention, a wave guide arrangement for the transmission of high-frequency polarised electro-magnetic waves comprises a symmetrical guide coupled to a rectangular guide and reactance means associated with the symmetrical guide in a plane extending through the longitudinal axis of the symmetrical guide and substantially at 45 to the major and minor axes of the cross-section of said rectangular guide.

Acording to a further aspect of the invention, a wave guide arrangement for the transmission of high-frequency polarised electro-magnetic waves comprises a symmetrical guide, branch coupling means such as an aperture or a probe with its axis in a plane passing through the longitudinal axis and reactance means associated with the symmetrical guide in a plane extending through the longitudinal axis of the glide and substantially at 45 to said plane of the branch coupling means.

The invention is based on the fact that an alternative and less well known method of resolving an ellipse may be adopted by which the two virtual components are selected so as to be again mutually in spatial quadrature but to have equal amplitudes and be separated in time by a phase angle 6 related to the ellipticity. With this method of resolution it is found that said two components are each spatially at 45 to either axis of the ellipse. Since any ellipse can thus be represented, with appropriate values of the difierence in the phases 0, it follows that the phase angle of one or both components can be varied without shifting the orientation of the axes of the ellipse. By this operation the ellipticity can be reduced down to the extreme case of plane polarisation on a plane containing the original major axis.

This is illustrated by the diagram of Figures 3 and 4. In Figure 3 an ellipse is shown resolved into two vector components D, E of equal amplitude on either side of, and at 45 to, the major axis of the ellipse. If vector D is taken as the datum, as represented by the notation 6:0, then the ellipticity can be expressed by the notation 0: ix applied to the vector E. All that is required is the phase-shifting of component B in the appropriate direction so as to reduce x to zero. When this is done, the resultant of components D and E be come a plane polarised wave in a plane containing the original major axis of the elipse, and such wave is represented as a vector F in Figure 4. This correction can be carried out in accordance with the invention by means of one or more reactive phase-shifting elements P, P (Figure 3) at 45 to the major axis. The amount of correction required is in practice small as compared with the method of Figures :l 2 since the phase error is always 90 in the latter case however small the ellipticity, whereas in Figures 3-4 the phase error at is related to the degree of ellipticity, ie in many practical cases something of the order of a few degrees or even as small as a fraction of a degree.

Assuming in conventional manner that the vectors shown are the electric vectors of the electro-magnetic wave in the wave-guide, if vector E is leading vector D in phase (i.e. x is positive), then correction can be applied by an inductive element P affecting vector E or a capacitive element at P affecting vector D or bycorrection applied at both positions. If x is negative, inverse reactances are required.

This phase-shifting correction is preferably achieved by means of a diametral rod-like conductive plunger provided at P or P, the length of plunger projecting into the guide determining whether a positive or negative reactance is produced and the magnitude thereof.

A preferred embodiment of the invention will now be described by way of example with reference to Figures 5 and 6 of the accompanying drawings as applied to a four-arm circulator employing a ferrite Faraday rotator element, Figure 5 being an axial section while Figure 6 is an end view of the output end.

Referring now to Figure 5 and 6, the circulator shown comprises a circular two-part guide 1 composed of two cylindrical parts abutting at the centre of the device. An iris transition 2 having a vertical major axis constitutes an input terminal for connection to a rectangular guide, and a second iris transition 3 constitutes an output terminal and is provided for connection to a rectangular output guide having its major axis at 45 to the input guide. A first side arm 5, acting as a terminal of the system, is provided adjacentthe iris transition 2 and in the plane of the major axis thereof. The other side arm, which also acts as a terminal, is indicated at 6 and is mounted in the same plane as the major axis ofthe output iris transition 3 and adjacent the latter.

A tapered ferrite rotator 8 or" circular cross-section is mounted coaxially within the guide 1 and is supported therein by two diaphragms 9 of insulating material, for example foamed polythen. Such diaphragms are carried on a metal sleeve it) which is axially compressed between two shoulders on the two halves of guide 1 by a series of peripheral turnbuckles 12.

For operation with relatively large power values, means are provided for cooling the ferrite element 8, such means including an inlet port 13 and outlet ports 14 for circulation of a coolant which may be compressed air. Three notches 15 are provided in the supports 9 around the rotator 8 so as to permit flow of the coolant from one side to the other of the supports. In order to restrict the coolant, discs 11 of mica or other suitable dielectric material are mounted at each end of the ferrite rotator e.g. as shown by clamping the discs between the ends of the sleeve and the corresponding shoulders ion the two parts of guide l.

The magnetic field is produced by solenoids 16, 17 surrounding the guide 1.

In this example two reactive phase-shifting elements 18, 19, sufiiciently short to be capacitive, are provided for experimental purposes at 90 to each other, although, as will be appreciated, only one of such elements is necessary when the phase of the unwanted polarisation component is known. Both elements are in this example cylindrical plungers passing through circular holes in the wall of the guide 1, and adjustment of each plunger is effected from the exterior by screw means constituting a continuation of the plunger. The two elements are shown at 18 and 19 respectively and are in a common plane located between one end of the ferrite 8 and the second side arm 6.

The construction shown is suitable for operation in the X-band of frequencies. Being a circulator, it can be used as an isolator; thus, if arms 5 and 6 are terminated in their characteristic impedances the device will act as an isolator between its ends 2' and 3. Although two plungers 18, 19 have been shown in a transverse plane between arms 6 and the ferrite, a single phase shifting element may be placed either between arm 5 and the rotator element or between the rotator element and arm 6 and its orientation (at 45 on one side or the other of the adjacent arm 5 or 6) would depend on the direction of rotation of the unwanted ellipse, and on whether the phase shifting element introduced a phase advance or lag to the polarisation component in its own plane. The fact that the phaseshifting element or pair of elements may be located at either side of the ferrite rotator is advantageous and is due to the following consideration. With a Faraday rotator giving (as is usual in circulators and isolators) 45 rotation, since ellipticity is due to differential attenuation of the two circularly polarised components of a plane polarised wave, a correction applied at one end of the rotator will be equally effective for both directions of transmission. In effect, whereas the ellipticity is directly reduced towards zero in one direction, in the other direction the reactive means introduc an ellipticity which is equal and opposite to that which will be caused by the rotator so that cancellation ensues.

What is claimed is:

1. A waveguide arrangement comprising a symmetrical waveguide, means for providing a polarized electromagnetic wave in said waveguide, said polarized wave having an undesired ellipticity, and means for correcting said ellipticity, said last-named means comprising at least one reactance member extending inwardly'from the wall of said waveguide and located at an angle or'4s degrees with respect to both the major and minor axes of said ellipticity, said reactance member introducing substantially less than degrees phase shift to said wave.

2. A waveguide arrangement as'claimed claim 1, including a second reactance member extending inwardly from the wall of said waveguide and located at an angle of 9.0 degrees from the first-named reactance member with respect to the axis of said waveguide, said first and second reactance members lying in a plane perpendicular to said waveguide axis.

3. A waveguide arrangement for electromagnetic waves, comprising a symmetrical waveguide, wave input and output terminal means coupled to said waveguide at different points along the axis thereof, a gyromagnetic element positioned in said waveguide between said terminal means, the electromagnetic waves in said wave guide having an undesired ellipticity, and means for correcting said ellipticity, said last-named means comprising at least one reactance member extending inwardly from the wall of said waveguide at a point between said gyromagnetic element and one of said terminal means and located at an angle of 45 degrees from the last-mentioned terminal means with respect to the axis of said waveguide, said reactance member introducing substantially less than 90 degrees phase shift to said waves.

References Cited in the file of this patent UNITED STATES PATENTS 2,557,882 Marie June I9, 1951 2,599,753 Fox June 10, 1952 2,748,353 Hogan May 29, 1956 2,787,765 Fox ApL Z, 19 57 FOREIGN PATENTS 582,856 Great Britain Nov. T29,

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