US3089104A - Device for independent control of ellipticity and orientation of polarized electromagnetic waves - Google Patents

Description

StAHL; mum

w w ems? May 7, 1963 P. J. ALLEN 3,089,104

DEVICE FOR INDEPEND T CONTR OF ELLIPTICITY AND ORIENTATION OF POL ZED E TRQMAGNETIC WAVES Filed Oct. 51, 1960 INVENTOR PHILIP J. ALLEN ATTORNEY United States Patent 3,039,104 DEVICE FOR INDEPENDENT CONTROL OF ELLIP- TlClTY AND ORIENTATION 0F POLARIZED ELECTROMAGNETIC WAVES Philip J. Allen, 8060 Marion St., North Forestville, Md.

Filed Oct. 31, 1960, Ser. No. 66,362 9 Claims. (Cl. 33311) (Granted under Title 35, US. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to polarization control of electromagnetic waves and in particular to devices for controlling the spatial or-ientation, the polarization sense, and the axial ratio of transmitted and received signals to maintain desired polarization for transmission and reception, control of spatial orientation being independent from control of axial ratio and sense of polarization.

In discussing space transmitted electromagnetic waves, such waves are generally considered to be polarized in one way or another. Electromagnetic waves can be linearly polarized in any transverse direction relative to the direction of propagation, the so-called horizontal and vertical polarizations relative to the horizon being commonly used for polarization identification, they can be circularly polarized and in either right hand or left hand sense as viewed along the line of propagation, or they can be elliptically polarized with any axial ratio, any axis orientation, and of either right or left handed sense.

It is not generally realized that all polarizations are closely related and frequently may be analyzed with greater facility by interchanging the polarization designations. Thus at times it may be helpful to consider a linearly polarized wave as actually two circularly polarized coherent waves of opposite sense and equal amplitude. Such a consideration is helpful for example in examining elliptically polarized waves which may be considered as being composed of two circularly polarized waves of opposite sense and unequal amplitude. The elliptically polarized wave may be mathematically analyzed as the ellipse itself, namely as having two orthogonally related axes, the major axis having an amplitude dependent upon the sum of the amplitudes of the circularly polarized components, the minor axis having an amplitude dependent upon the difference in the amplitudes of the circularly polarized components.

It is characteristic of such polarized waves that they can be produced or received to optimum advantage with a device which couples properly to the particular form of polarization employed whether it be linear, elliptical or circular, of the proper spatial orientation, and of the proper circular sense.

Although some forms of coupling devices such as a conical horn for microwave, can couple to all forms of polarization, other devices do not couple to all forms of polarization. A half wave dipole oriented in the horizontal plane would produce or receive a horizontally polarized wave but not a vertically polarized wave. Similarly, transducers having circular polarization coupling characteristics are not necessarily interchangeable as to sense of polarization. For example a transducer such as a spiral antenna which may be of such configuration as to produce a right hand polarized signal would not respond with equal ease to incoming signals of either right or left hand polarization, but would actually be blind to the inappropriate polarization. As to intermediate polarization angles and forms such as a horizontally or a vertically linearly polarized wave cooperative with a 45 oriented linear antenna, and elliptically polarized signals cooperative with circularly polarized antennas,

3,089,104 Patented May 7, 1963 coupling would be existent to some extent but would not necessarily be optimum until the polarization characteristics and angles of the various signals and waves were proportioned properly.

When employing circular polarization, if a circularly polarized signal of one sense is transmitted and reflected by an isotropic target such as a sphere or a flat plate, the refiected signal will be of the opposite sense of polarization. Thus the transmitting antenna without some special provision would actually be blind to the return signal.

The foregoing discussion may thus be seen to lead to a realization that by carefully planned utilization of various polarizations it is possible to optimize reception from targets of selected characteristics. Such a thesis may be carried a step further by actually using polarization and varieties of polarization as a means for classifying targets, that is, of determining what the nature of a radar target is. It also goes without saying that acceptance or reception by an antenna of an incoming signal from some distant independent source can be facilitated by optimizing the polarization characteristics thereof to match the polarization characteristics of the incoming signal. Thus there is a tremendous advantage to be obtained by utilizing a device such as will be subsequently described which provides complete control over the polarization of an outgoing wave as well as complete control over the polarization sensitivity of the device to all incoming wave.

It is accordingly an object of the present invention to provide a polarization control device for controlling the polarization of an outgoing signal.

Another object of the present invention is to provide a polarization control device for controlling the polarization sensitivity of a space coupling device to an incom ing signal.

Another object of the present invention is to provide a polarization control device which provides independent control of the axial ratio, and of the spatial orientation of the polarized wave whether as an outgoing signal or an incoming signal.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 shows a typical embodiment of the features of the present invention wherein two separate signal coupling ports are coupled to an omnipolarization antenna system.

FIG. 2 shows schematically a typical polarization control device constructed in accordance with the teachings of the present invention.

FIG. 3 shows a variant embodiment of the polarization control device of FIG. 2.

FIG. 4 shows an additional variation of the polarization control device.

FIG. 5 shows an end view of the device of FIG. 1 which is useful in understanding the operation of the various preceding figures.

In accordance with the basic teachings of the present invention, a combination of birefringent elements such as plates of differential planar phase length is made within a suitable transmission line whereby electromagnetic energy therein can be converted to any desired polarization by proper orientation of the various plates involved. Where energy is incident at one end of the control device and exits at a second end, the device typically contains first a half wave element, secondly a quarter wave element and thirdly a half wave element all arranged in sequence so that the incident electromagnetic energy must traverse the elements in reaching the exit port. By

controlling the orientation of the first half wave element relative to the plane of polarization of an incident linearly polarized wave it is possible to control the variation in axial ratio produced by the device whereas by controlling the relative orientation of the second half wave element it is possible to adjust the relative spatial orientation of the polarization of the output energy.

In further discussion of the basic principles of the present invention the device is also possessive of desirable characteristics wherein the control elements are of such structure as to be of a nonreciprocal nature such as a Faraday rotator so that it has ditferent polarization characteristics for energy propagating through the device in different directions.

With reference now to FIGS. 1 and 5 of the drawings, the apparatus therein which is typical for the microwave region of the electromagnetic wave spectrum contains a two-mode transducer and an omnipolarization horn 11 separated by a control portion of cylindrical waveguide containing a half wave plate 12, a quarter wave plate 13 and a second half wave plate 14, in that order, disposed therein. The two-mode transducer 10 has two output ports 15 and 16 disposed in such relationship as to couple to orthogonally related components of linearly polarized energy contained therein or to excite such wave energy. Typcally the orthogonal relationship is such that the ports 15 and 16 are at 90 angular relationship to each other. The elements or plates 12, 13 and 14 may be considered birefractive elements in that the velocity of propagation of a linearly polarized wave in one plane through a waveguide or other transmission medium containing them is ditferent from that in an orthogonal plane. For the microwave case these elements or plates typically are slabs of dielectric material such as polystyrene, possibly of varying or stepped thickness for optimum irnpendance matching purposes. These plates are characterized by their property of altering the phase of energy in one plane by a different amount from that experienced by energy propagating therethrough in an orthogonally related plane. Thus a wave electrically polarized in the plane of the diaelectric slab will be phase delayed in transmission more than a wave whose electric field propagates perpendicular to the slab. Where the plate is identified as a half wave plate the phase difference is 180 whereas the phase diiference is 90 for the quarter wave plate. Typically for the microwave case the plates themselves can be made of polystyrene whereas for higher frequencies such as those in the optical region plates made of mica or other suitable birefractive materials would be more appropriate.

When the half wave plates 12 and 14 are rotated rnechanically or electrically as would be the case if birefringent ferrite type half wave plates were used about the axis of propagation they will introduce spatial rotation of a polarized wave entering the plate at one end thereof through twice the angle through which the half wave plate is rotated. If the half wave plate is rotated 10 the polarized wave transferred through it will rotate 20. This applies to each of the elements 12 and =14. A further property of such a half wave plate where it receives a circularly polarized wave is that it will reverse the polarization sense of the wave applied to it. Thus for example, if a right hand circularly polarized Wave is applied to the left end of plate 12 of FIG. 1, it will emerge from the right hand end of plate 12 as a left hand circularly polarized wave. In analyzing the device for elliptical polarization it is helpful to consider elliptically polarized waves as being a combination of two circularly polarized waves having unequal amplitudes and opposite sense. As each circularly polarized component is reversed in sense by the half wave plate 12 the wave will evolve as an elliptically polarized wave of reversed sense. In short then, such an elliptically polarized wave having traversed through the half wave plate will in general experience a change in its spatial orientation about the axis of propagation and will emerge with reversed sense.

The quarter wave plate 13 has entirely different properties from those previously described for the half Wave plates 12 and 14. The quarter wave plate introduces a differential phase shift between orthogonal linearly polarized components transmitted therethrough. With an input of circularly polarized energy at one end of the quarter wave plate 13, the quarter wave plate 13 will convert the circularly polarized wave to a linearly polarized wave by causing a 90 degree delay in one linear component over the orthogonal linear component.

To further analyze the quarter wave plate and its eifect upon incident linearly polarized waves it might be desirable to now consider a linearly polarized Wave as being made up of two circularly polarized components, one in the plane of the plate and one orthogonally related thereto. The quarter wave plate will cause a delay in one linear component which is 90 greater than that of the other orthogonal linear component efiectively placing the two components in phase quadrature, the amplitude relationship of the two components being dependent upon the orientation of the plane of the input polarization relative to the plane of the quarter wave plate. As a consequence the output of the quarter wave plate will have a variable axial ratio depending upon the orientation of the plane of the linearly polarized wave applied to the input of the quarter Wave plate 13.

When the action of the second half wave plate 14 is added to that basically described for the half wave plate 12 and the quarter wave plate 13, in the general case there will be an elliptically polarized Wave in the output of the quarter Wave plate 13 for some arbitrary relative orientation of the half wave plate 12. The two circularly polarized components which comprise this elliptically polarized wave have their senses reversed by the half wave plate 14 and are then in efiect recombined as an elliptically polarized wave having the same axial ratio as that which entered the half wave plate 14 at the left end but with a reversed sense. If the half wave plate 14 is physically rotated about the axis of propagation, the effect Will be to change the orientation of the major axis of the ellipse of the polarization of the output at the right end of the half wave plate 14 so that the effect is merely a changing in the relative spatial orientation of the polarization of the output at the right end of the half wave plate 14 which is delivered to the omnipolarization horn 11 for radiation thereby.

In summary of the individual action of the three plates 12, 13 and 14 upon the application of linearly polarized energy to the left end of plate 12 for radiation by the device 11, the half wave plate 14 can be called an orientation control which simply controls the orientation of the polarized output wave whereas element 12 is an axial ratio control and also determines sense of the polarized Wave, the two elements operating indpendently so that element 12 will vary the axial ratio independent of the setting of the orientation control 14 and vice versa.

Normally the form of the wave existing at the left end of the half wave plate 12 will be a linearly polarized Wave as obtained by the excitation of either port 15 or 16 from a rectangular waveguide. This signal can be changed from the linearly polarized form at the left end of 12 to either a left or a right sense circularly polarized signal at the right end of the quarter wave plate 13 or may be retained as a linearly polarized signal by the appropriate adjustment of the orientation of the half wave plate 12. Furthermore any intermediate condition as represented by elliptical polarization of either sense can also be readily obtained by the appropriate adjustment of the orientation of the half wave plate 12. Although rotation of the half wave plate 14 is not effective as an orientation control if the output is a circularly polarized wave, it will control the phase at any point in the far field and hence could be advantageous in certain desired conditions. Where the output from the quarter wave plate 13 is either of a linearly polarized nature or of an elliptically polarized nature however the half wave plate 14 can be adjusted to provide any desired orientation of the major axis of the polarization in the radiated wave from the antenna 11.

The apparatus of FIG. 1 as thus far described is reciprocal in that it can be used for transmission by applying energy to the left hand end of the half wave plate 12 and pointing the antenna 11 in some desired direction or it can be used for reception by pointing the antenna 11 again in a desired direction and connecting the left end of the half wave plate 12 to some receiver device for amplification of the received signal. With the two terminals 15 and 16 being rectangular waveguide and oriented as shown from the output transducer it is possible to couple these ports independently to orthogonally related states of polarization at the antenna device 11. Thus it may be possible for example by the proper orientation of the various plates 12, 13 and 14 to cause the terminal 15 to be coupled to a horizontally polarized field and the other port to be coupled to a vertically polarized field. Actually the device of FIG. 1 would have utility with only one port 15 or 16 however with the two shown it is readily possible to independently analyze the operation in orthogonal planes without requiring readjustment of the plates 12, 13 and 14.

The device of FIG. 1 as described is many-fold, in a radar device it permits the complete control of the polarization of radiated and received energy providing a means of determining polarization sensitivity of varous targets. Optcally such a device could be used for identifying the character of various objects by determining their selective polarization characteristics.

With reference now to FIG. 2 of the drawing the apparatus shown therein is a schematic presentation of the half wave plates 12 and 14 as well as the quarter wave plate 13 indicated in the apparatus of FIG. 1. A similar schematic presentation for a variant embodiment of the basic apparatus is shown in FIG. 3 wherein a Faraday rotator 22 is substituted for the first half wave plate 12, other portions of the structure of FIG. 1 being unchanged except for the placement of a coil 22a around the'portion of the apparatus in the region of the Faraday rotator 22 to apply a magnetic field parallel to the axis of propagation of energy through the ferrite material of the rotator.

The basic performance of the apparatus in the configuration of FIG. 3 is similar to that of the FIGS. 1 and 2 it being possible in this device to vary the intensity of the magnetic field to achieve a similar effect to that obtained by mechanically rotating the half wave plate 12. This similarity is with regard to one-way passage of electromagnetic energy through the device because by virtue of the nonreciprocal rotational properties of a Faraday rotator the effect of that device upon the energy in passage therethrough is different for energy traveling from left to right in comparison to energy traveling from right to left. The nonreciprocal properties of the Faraday rotator can be such as to result in radar return energy from an isotropic target being received not at the port 15 but rather at the port 16 for any transmitted polarization. It is thus apparent that what is obtainable in this device is a radar duplexer in which a transmitter for example may be connected to the port 15 and a receiver to the port 16, the orthogonal relationship in the two mode transducer providing for an absence of direct coupling between ports 15 and 16.

Another very important practical advantage of the apparatus of FIG. 3 is in the action of the overall device when it is used with elliptically polarized or circularly polarized output signals. It is characteristic of a radar when used with elliptically polarized or circularly polarized energy and operated against an isotropic target such as a fiat plate or a sphere that the sense of the return signal is reverse to that emitted. For example, an ordinary circularly polarized antenna system used for both transmission and reception as in a conventional pulse-echo radar arrangement would be blind to such a signal returned by an isotropic target since an antenna which will couple to one sense of circular polarization will not couple to the opposite sense. When the apparatus of FIG. 1 is adjusted to transmit circular polarization responsive to input linearly polarized energy to the terminal 15, the return signals of opposite polarization sense will come out port 16 as linearly polarized signals.

With the apparatus of FIG. 3, the Faraday rotator provides essentially the same control over the axial ratio as was obtained by the half wave plate 12. Thus when such rotation is considered it is apparent that the apparatus of FIG. 3 is in reality considerably more flexible than the basic apparatus of FIGS. 1 and 2 in that not only is it able to provide the desired control over axial ratio and orientation provided by FIG. 1 but it is possible to obtain duplexer action from it as well, so as to permit a single antenna horn 11 to be used for both transmission and reception of variably polarized signals in a radar system operative against isotropic targets.

A further variation of the basic apparatus of the present invention is shown schematically in FIG. 4 which contains a Faraday rotator 22, in addition to the quarter wave plate 13 and a second Faraday rotator 23 the latter being a substitution for the half wave plate 14 of FIGS. 1, 2 and 3, axial magnetic fields being applied by the coils 22a and 23a. The Faraday rotator 22 is substantially the same as that of the same number previously described in connection with FIG. 3. As to the operation of this device when the Faraday rotators are activated by the application of magnetic fields parallel to the axis of propagation of the energy therethrough it is apparent that for one-way transmission through the device say from left to right as indicated in FIG. 4 by proportionment of the Faraday rotators as to rotational characteristics, length, and strength of magnetic field it is possible to obtain results comparable to that of the basic apparatus of FIGS. 1 and 2. That is, the Faraday rotator 22 can be adjusted electrically by controlling the strength of the magnetic field to produce control over the axial ratio of the signal leaving the quarter wave plate 13. Likewise the Faraday rotator 23 can be adjusted to obtain for one-way transmission the orientation control obtained by the half wave plate 14 of FIGS. 1 and 2. Thus for oneway transmission from left to right through the apparatus of FIG. 4 results equivalent to those of the apparatus of FIGS. 1 and 3 can be obtained with the additional advantage in some instances of being able to produce the control over the variables merely by varying electronic devices or variable potentiometers rather than requiring physical rotation of the half wave plates 12 and 14 as in FIGS. 1 and 2. Additionally the Faraday rotators being fairly high speed responsive devices as contrasted to the physical inertia devices 12 and 14, can respond much more rapidly in instances where high speed of response is desired.

If the apparatus of FIG. 4 were used for two-way transmission systems, sueh as radar systems, various unique properties could be obtained dependent upon the overall apparatus associated therewith. For example, the action of the Faraday rotator 22 in place of the reciprocal half wave plate 12 of FIGS. 1 and 2 provides a means of rendering a radar system using circular polarization receptive to return signals by connecting the transmitter and the receiver to separate orthogonally related output coupling ports 15 and 16. This action can also be obtained with the apparatus of FIG. 4 and in addition by virtue of the Faraday rotator 23 it is possible to obtain unique properties when operating with linearly polarized Waves. For example, the Faraday rotator 23 would provide a different linear polarization axis for transmission and reception through a given port, say 15.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. In combination, a dual mode transducer, an omnipolarization coupling device, first and second half Wave birefringent elements disposed between said transducer and said device, and a quarter wave birefringent element intermediate said first and second elements.

2. In combination, a dual mode transducer, a coupling device having selected polarization coupling characteristics, first and second half Wave birefringent element disposed between said transducer and said device, and a quarter wave birefringent element intermediate said first and second elements.

3. In combination, a dual mode transducer having two linearly polarized coupling ports, said ports coupling to orthogonally related fields in the transducer, a coupling device having selected polarization coupling characteristics, first and second half wave birefringent elements disposed between said transducer and said device, and a quarter wave birefringent element intermediate said first and second elements.

4. In combination, a dual mode transducer, 2. coupling device having selected polarization coupling characteristics, first and second half wave plates disposed between said transducer and said device, and a quarter wave plate intermediate said first and second elements.

5. In combination, a dual mode transducer having two linearly polarized coupling ports, said ports coupling to orthogonally related fields in the transducer, a coupling device having selected polarization coupling characteristics, first and second half wave plates disposed between said transducer and said device, a quarter wave plate intermediate said first and second elements, and means for adjusting the orientation of at least one of said plates.

6. In combination, a dual mode transducer having two linearly polarized coupling ports, said ports coupling to orthogonally related fields in the transducer, a coupling device having selected polarization coupling characteristics, a half wave plate disposed between said transducer and said device, a quarter wave plate intermediate said half Wave plate and said transducer, and a Faraday rotator disposed between said quarter wave plate and said trans ducer.

7. In combination, a dual mode transducer having two linearly polarized coupling ports, said ports coupling to orthogonally related fields in the transducer, a coupling device having selected polarization coupling characteristics, a half wave plate disposed between said transducer and said device, a quarter wave plate intermediate said half wave plate and said transducer, a Faraday rotator disposed between said quarter Wave plate and said transducer, and means for adjusting the axial relationship between the plates, the ports of the dual mode transducer and the major axis of the field of the coupling device.

8. In combination, a dual mode transducer having two linearly polarized coupling ports, said ports coupling to orthogonally related fields in the transducer, a coupling device having selected polarization coupling characteristics, a quarter wave plate disposed between the dual mode transducer and the coupling device, a Faraday rotator disposed between the coupling device and the quarter wave plate, and a Faraday rotator disposed between the transducer and the quarter wave plate.

9. In combination, a dual mode transducer having two linearly polarized coupling ports, said ports coupling to orthogonally related fields in the transducer, :1 coupling device having selected polarization coupling characteristics, a quarter wave plate disposed between the dual mode transducer and the coupling device, a Faraday rotator disposed between the coupling device and the quarter wave plate, a Faraday rotator disposed between the transducer and the quarter wave plate, and means for adjusting the axial relationship between the plate, the ports of the dual mode transducer and the major axis of .the field of the coupling device.

References Cited in the file of this patent UNITED STATES PATENTS 2,438,119 Fox Mar. 23, 1948 2,607,849 Purcell et al Aug. 19, 1952 3,052,152 Koester Sept. 4, 1962

Claims (1)

1. IN COMBINATION, A DUAL MODE TRANSDUCER, AN OMNIPOLARIZATION COUPLING DEVICE, FIRST AND SECOND HALF WAVE BIREFRINGENT ELEMENTS DISPOSED BETWEEN SAID TRANSDUCER AND SAID DEVICE, AND A QUARTER WAVE BIREFRINGENT ELEMENT INTERMEDIATE SAID FIRST AND SECOND ELEMENTS.

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