US 3287729 A
Description (OCR text may contain errors)
Nov. 22, 1966 J. R. MARK ETAL. 3,287,729
'POLARISERS FOR VERY HIGH FREQUENCY ELECTRO-MAGNETIG WAVES Filed Nov. 26, 1962 W .37 mz #4 ATTO RNEYS United States Patent 3,287,729 POLARISERS FOR VERY HIGH FREQUENCY ELECTRO-MAGNETIC WAVES John Richard Mark and Ronald Kenneth Walker, Chelmsford, England, assignors to The Marconi Company Limited, a British company Filed Nov. 26, 1962, Ser. No. 240,038 Claims priority, application Great Britain, Dec. 14, 1961, 44,87 2/ 6 1 4 Claims. (Cl. 343756) This invention relates to polarising arrangements for very high frequency electro-magnetic waves and more particularly to polarising arrangements providing, at will, either linearly or elliptically polarised waves. The term elliptically is used in this specification in a broader sense than is customary to include circularly which is regarded, in this specification and from the viewpoint of this invention, as a special case of elliptically. The terms polarising arrangements and polarisers as used herein include arrangements or devices which will serve to affect the direction and/ or nature of the polarisation of electormagnetic waves.
It is common practice in radar and other microwave systems to use in conjunction with a radio reflector a tapered radio horn, fed from a suitable waveguide system. It is also a common requirement that said horn should be capable of providing, at will, either linearly or circularly polarised radiation. Such horns are, of course, normally of tapered rectangular section and, in designing the horn, the aspect ratioi.e. the ratio of length to breadth of the horn aperture-is chosen to suit the reflector with which the horn is to be used so as to provide best illumination of the reflector. With such rectangularly sectioned horn, the taper is not usually the same in the two mutually perpendicular planes which are parallel, respectively, to the directions of length and beadth of the horn aperture. cause of this, when such a horn is employed for circularly polarised waves, it introduces different phase shifts for the two mutually perpendicular components of the circularly polarised wave. It is customary to correct for phase error arising in this way by providing in a rectangularly sectioned waveguide leading to the horn a suitable compensating element which introduces the requisite compensating phase shift between the two mutually perpendicular components of the circularly polarised wave. Such a compensating element may consist, for example, of a metal or dielectric vane projecting into said Waveguide from one wall thereof. There is usually provided, leading to the guide length containing the compensating element, a polariser, consisting, for example, of a length of rotatable circularly sectioned guide with a suitably shaped diametrically situated dielectric vane in it, and adapted to provide, in dependence on the position of the circularly sectioned guide, either a linearly or circularly polarised wave. Thus a typical known arrangement would comprise in the order stated (a transmitting arrangement is assumed here though, of course, the same arrangement will serve for reception) a rectangularly sectioned feeder fed with linearly polarised waves; a circularly sectioned rotatable polariser containing a dielectric vane or other known polariser element and fed from the feeder via a rectangular-to-circular transistion guide length of known form; a rectangularly sectioned phase compensator containing a phase compensation element and fed from the polariser via a circular-to-rectangular transition guide length of known form; and a tapered radio horn fed from the compensator. In this type of arrangement the polariser has a limited frequency bandwidth over which it will operate satisfactorly and where the polariser and compensator are so designed as to produce a correctly Bev "ice circularly polarised wave at the mouth of the horn, at one predetermined frequency, the phase errors produced by the polariser at other frequencies will not be compensated by the compensator due to the fact that, when arranged to provide circularly polarised waves, the dielectric vane of this polariser is at an angle to the sides of the horn and the waveguide incorporating the compensating element. Consequently the known arrangement suffers from the defect that it will operate satisfactorily over only a relatively narrow band of frequencies. The principal object of the present invention is to overcome this defect.
According to this invention a polarising arrangement adapted to provide at will either elliptically (including circularly) or linear polarised electro-mangetic from input linearly polarised waves comprises a length of circularly sectioned waveguide having an odd number of successive substantially diametrically arranged conductor units, said units extending substantially across the interior of said guide and arranged at angles to one another, means, operable at will, for causing said conductor units to impart an overall change of either 45 or zero in the direction of linear polarisation of waves propagated through said waveguide and means for relatively phase delaying, to an extent to provide the required degree of ellipticity of polarisation, components of the output waves from said waveguide which are respectively parallel and perpendicular to the end conductor unit at the input end of said waveguide.
Where it is desired to provide either circularly or linearly polarised waves the extent of the phase delay will be 90".
In a preferred embodiment of the invention, said phase delaying means comprises a length of rectangularly sectioned waveguide having a phase compensating element therein and, connected to said waveguide, a tapered rectangular radio horn, one pair on opposite sides of said rectangularly sectioned guide being parallel to said input end conductor unit, and said rectangularly sectioned guide and radio horn together providing a relative phase delay of 90 to said wave components.
In one form of embodiment the conductor units are comprised of spaced diametral metallic conductors, the angle between the central conductor unit and the input end unit being 22 /2 and the output end unit, together with the remaining units (if any) between the central unit and the output end unit, being rotatable about the axis of the guide so that the output end unit may, at will, be brought either parallel or at 45 to the input end unit. Although each conductor unit may consist of a single conductor this is not preferred and preferable each unit consists of a plurality of parallel spaced metallic conductors, all the conductors in any one unit being coplanar. Thus there may be, for example, three equally spaced conductor units each consisting of a number of coplanar diametral conductors, for example, three. It will be seen that when the two end units are at 45 to one another the input waves will have their direction of polarisation turned through 45 but when the two end units are parallel there will be no effective change in the direction of polarisation.
The invention is illustrated in the accompanying drawings in which FIGURE 1 is a schematic perspective view of a polarising arrangement in accordance with this invention employing a tapered radio horn, and FIGURE 2 shows a form which one of the elements of FIGURE 1 may take. In each of the figures the arrow represents the electric vector of the input linearly polarised wave.
Referring to FIGURE 1, the installation therein generally shown may be regarded as consisting of four units, namely (assuming the installation to be employed for transmission) an input feeder unit generally designated A, a polariser unit generally designated B, a phase compensator unit generally designated C and a radio horn D.
The input feeder unit consists of a length of rectangular sectioned waveguide A1 unitarily formed with a rectangular-to-circular waveguide transistion A2 as known per se. The polariser unit B consists of a polariser B1 and a circular-to-rectangular waveguide transition B2. The polariser is represented only in outline by a cylinder in FIGURE 1, but one form which the polariser may take is shown in FIGURE 2. The unit C consists of a length of rectangularly sectioned waveguide C1 having its sides parallel to the corresponding sides of waveguide A1, with a metallic or dielectric vane or similar phase compensating element C2 as known per se. The horn D is a normal tapered horn of rectangular section and an aspect ratio which will in normal practice be determined by the shape and arrangement of a reflector (not shown) which the horn is to illuminate. The phase compensating element C2 is so arranged, in accordance with known practice, that the unit C and horn D in combination provide a relative phase delay of 90 between two perpendicular wave components which are parallel to adjacent sides of the waveguide C1.
FIGURE 2 shows one form of polariser suitable for use at B1. The polariser comprises two colinear, coaxial lengths of circular waveguide 1B and 2B arranged in any convenient manner so that the length 2B can be rotated with respect to the length 1B about the common axis. The two ends of the rotatable length of guide are indicated in FIGURE 2 by broken line circles.
Within the fixed length 1B are two spaced diametrically arranged conductor units each comprising of three parallel coplanar diametral metal tubes or rods which are referenced 1B1 in the case of the conductor unit nearest the input feeder and 1B2 in the case of the other conductor unit. In the rotatable length of guide is a similar conductor unit consisting of three diametrical parallel coplanar metal rods or tubes referenced 2B1. The units 1B1, 1B2 and 2B1 are equally spaced along the axis and they are alike. The plane of the unit 1B1 is at 22 /2 to the plane of the central unit 1B2. The plane of the unit 2B1 will obviously depend upon the position of rotation of the rotary length of guide 2B. If circular polarisation is to be produced from the horn D (FIGURE 1) the rotary length of guide is rotated until the plane of the conductor unit 2B1 is at 22 /z to that of the unit 1B2 and 45 to that of the unit 1B1. The plane of the said unit 1B1 is at right angles to the electric vector of the input waves from the input feeder and consequently the output waves from unit 2B1 will have a direction of polarisation at 45 to that of the input waves. If the linearly polarised input Waves are merely to be transmitted by the horn without change in the nature of the polarisation (i.e. as linearly polarised waves) the rotatable guide length 2B is rotated until the plane of the unit 2B1 is parallel to the plane of the unit 1B1. This position is shown in broken lines in FIGURE 2.
It will be seen that when the polariser is set in either of these two positions it produces two angles of twist as respects the incident wave linear vector, each being of 22 /z and the total angle of twist being 45. Accordingly reflections at the successive conductor units will be of equal magnitude and may be made to cancel by suitably choosing the spacing of said units in accordance with known principles. In the one case the two angles of twist are progressive so that there is an overall angle of twist of 45 while in the other case the two angles of twist cancel one another.
Reverting to FIGURE 1 it will be seen that when the polariser B1 is ararnged to provide an output wave of the same polarisation as the input wave, this output wave will be unchanged in passing through the waveguide C1 and horn D and will produce at the mouth of horn D a linearly polarised wave having a direction of polarisation substantially the same as that of the input wave. When the polariser B is arranged to provide an output wave having a direction of plarisation at 45 to that of the input wave, however, the two equal and mutually perpendicular components of the output wave will sufier relative phase delay, in the Waveguide C1 and horn D, of and will accordingly provide, at the mouth of the horn, a circularly polarised wave.
It will be seen that in the illustrated embodiment the bandwidth, as respects the obtaining of true circular polarisation, is determined substantially solely by the Waveguide-horn combination C1-D.
In cases in which the polariser in accordance with the invention is not required to provide output waves at the mouth of a radio horn, the horn is dispensed with and the waveguide C1 arranged above to provide the required phase delay.
1. An adjustable polarizer for providing either elliptically or linearly polarized electromagnetic waves from input linearly polarized waves comprising first and second colinear, coaxial, circularly sectioned lengths of waveguide, said second length of waveguide being rotatable relative to said first length of waveguide about the common axis thereof, means for applying linearly polar.- v
ized waves' to said first length of waveguide, a pair of spaced conductor units extending diametrically within said first length of waveguide at an angle of substantially 22 /2 degrees to each other, a third conductor unit extending diametrically within said second length of waveguide and rotatable therewith between an angular range of plus or minus 22 /2 degrees with respect to the nearest of said pair of spaced conductor units, and means for relatively phase delaying to an extent to provide the required degree of ellipticity of polarization, components of the output waves from said second length of waveguide which are respectively parallel and perpendicular to the conductor unit which is nearest to the input end of said first length of waveguide.
2. A polarizer as claimed in claim 1 which is adapted to provide at will either linearly or circularly polarized waves from linearly polarized waves and wherein the extent of the phase delay is 90".
3. A polarizer as claimed in claim 2 wherein the phase delaying means comprise a length of rectangularly sectioned waveguide having a phase compensating element therein and, connected to said waveguide, a tapered rectangular radio horn, one pair on opposite sides of said rectangularly sectioned guide being parallel to said input end conductor unit, and said rectangularly sectioned guide and radio horn together providing a relative phase delay of 90 to said wave components.
4. An arrangement as claimed in claim '1 wherein each conductor unit consists of a plurality of parallel spaced metallic conductors, all the conductors in any one unit being coplanar.
References Cited by the Examiner UNITED STATES PATENTS 2,159,937 5/1939 Zworykin 343-909 X 2,438,119 3/1948 Fox 333-21 2,735,092 2/1956 Brown 343756 3,031,661 4/1962 Moeller etal 343756 3,076,188 1/1963 Schneider "u 343T756 X ELI LIEBERMAN, Primary Examiner,