A right circular cylindrical body of an isotropic dielectric such as a cross-linked styrene copolymer, has respective pluralities of mutually parallel grooves formed in its axial end faces, spaced apart by an intermediate portion whose dimension c is a half wavelength. The axial lengths a, b of the grooves are such that when a wave passes through the body, a quarter wavelength phase difference is produced between a component of a wave having its E-vector parallel to the grooves and a component of the wave having its E-vector orthogonal to the grooves. Alternatively the plate may consist of two or more discrete bodies whose grooves are dimensioned to produce a total differential phase shift of one quarter wavelength.
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1. A quarter wave plate, comprising:
at least one body of dielectric material, said at least one body having respective first and second end faces on opposite sides thereof, and including i); the first and second end faces each having a respective reflection coefficient, said at least one body including:
a respective first portion for providing a phase shift between first and second orthogonal components of an electromagnetic wave traversing the plate, and including a respective plurality of first number of parallel grooves extending inwardly of said respective first end face; ii)
a respective second portion for providing a phase shift between the first and second orthogonal components of the electromagnetic wave, and including a respective plurality of second number of parallel grooves extending inwardly of said respective second end face and aligned with the grooves of said respective plurality of first number of parallel grooves; and iii)
a respective third portion defined between said respective first and second portions,
wherein respective depths of the plurality of first and second parallel grooves are configured to produce the phase shifts of an odd integer multiple of quarter wavelengths between said first and second orthogonal components of the electromagnetic wave traversing the plate, the first component having an E-vector parallel to the plurality of first and second parallel grooves, and the second component having an E-vector perpendicular to the plurality of first and second parallel grooves,
wherein the dielectric material has a relative dielectric constant less than 5, and
wherein a length of the third portion is selected to differ from a half-wavelength by an amount to adjust the respective reflection coefficient at the first and second end faces.
0. 2. The quarter wave plate as claimed in
3. The quarter wave plate as claimed in claim 2 1, wherein the respective grooves of each said respective first plurality of grooves has a depth which of each first parallel groove is equal to a depth of the grooves of each said respective second plurality of grooves each second parallel groove.
4. The quarter wave plate as claimed in
5. The quarter wave plate as claimed in
0. 6. The quarter wave plate as claimed in
0. 7. The quarter wave plate as claimed in
8. The quarter wave plate as claimed in
9. The quarter wave plate as claimed in
10. The quarter wave plate as claimed in
0. 11. The quarter wave plate as claimed in claim 1, wherein the plate has a cylindrical structure.
0. 12. The quarter wave plate as claimed in claim 1, wherein the at least one body comprises a continuous region that establishes a perimeter of each end face.
0. 13. The quarter wave plate as claimed in claim 1, wherein the length of the third portion is an integer of one half wavelength, wherein said integer is an integer other than one.
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1. Field of the Invention
This invention relates to quarter wave plates. It particularly relates to quarter wave plates for use at radio frequencies.
2. Description of the Related Art
As is known to those skilled in the art, a quarter wave plate is a component which produces a phase shift of π/2 radians, i.e. one quarter wavelength (or an odd integer multiple thereof) between orthogonal components of electromagnetic radiation.
Applications of such quarter wave plates include the conversion of unpolarized radiation to circularly-polarized radiation and conversion of plane-polarized radiation to helically-polarized radiation.
It is known to construct a quarter wave plate for use at radio frequencies by using a dielectric. material having an anisotropic relative dielectric constant. Two parallel faces are made on a piece of the anisotropic material. The distance between the faces is such that, in traversing the thickness of the plate, for radiation at the nominal frequency at which the plate is to be used, components in the direction parallel to the axis of the greater dielectric constant undergo a phase shift of one quarter wavelength relative to components in an orthogonal axis having the lesser dielectric constant. One type of material having the necessary anisotropic properties is sapphire. While such plates have been found to produce the necessary phase shift, they suffer a number of disadvantages. Sapphire is relatively “hard” material, i.e., it has a relatively high dielectric constant relative to air. This results in losses by reflection due to the mis-match between free space and the relatively high dielectric constant sapphire. The problem of this mis-match has been addressed by providing anti-reflecting coatings in the conventional manner. While this approach has generally proved satisfactory, problems have arisen from poor adhesion of the coatings to the sapphire. The resulting structure has also been found to have a relatively narrow bandwidth.
It is an object of the present invention to provide a quarter wave plate in which the disadvantages of the prior art are ameliorated.
The present invention provides a quarter wave plate comprising at least one body of dielectric material, each said body having respective first and second faces on opposite sides thereof; each such body consisting of a respective first portion consisting of a respective first number of parallel grooves extending inwardly of said respective first face; a respective second portion consisting of a respective second number of parallel grooves extending inwardly of said respective second face and aligned with the grooves of said respective first number of grooves; and a respective third portion defined between said respective first and second portions.
The invention will now be described by way of non-limiting example only, with reference to the drawings in which
Before describing the embodiments it should be made clear for the avoidance of doubt that, when referring to the relative dielectric constant of a material, “soft” refers to materials having a low dielectric constant, and “hard” refers to materials having a high dielectric constant. For the purposes of this specification, a soft material is one having a relative dielectric constant less than 5 and a hard material is one having a relative dielectric constant greater than 5. The terms “hard” and “soft” in this context do not necessarily mean that the materials in question are also hard or soft in a physical sense.
Referring to
The first plurality of lands 1 and grooves 2 constitute a first region delimited by lines A-A and B-B and having an axial length a equal to the depth of the grooves 2. The second plurality of lands 11 and grooves 12 constitute a second region delimited by lines C-C and D-D and having an axial length b equal to the depth of the grooves 12.
The third region delimited by lines B-B and C-C constitutes a third region having an axial length c.
The sum of axial lengths a and b is such that a wave traversing the distance a+b through the isotropic dielectric exhibits a quarter wave length phase shift with respect to a wave traveling the distance a+b through the medium filling the grooves. In the present embodiment this medium is air. In the present embodiment the wave plate is completely reflection symmetric about its center and the first region is identical with the second region. Thus the impedance of the first region at plane B-B is the same as the impedance of the second region at plane C-C. The length c of the third region is nominally one-half wavelength of the design frequency. A half wavelength structure has the property that, whatever impedance is presented to one end, that impedance appears unchanged at the other end and thus the half wave central region effectively couples B-B directly to C-C. As the impedance at plane B-B is the same as the impedance at plane C-C, theoretically a perfect impedance match results, with no loss by reflection at surfaces B-B or C-C. By designing the input impedances of the first and second structures for minimum reflection loss at surfaces A-A or D-D, the loss by reflection of energy traversing the quarter wave plate can be minimized. The reflectivity for input waves whose E-vector is parallel to the grooves is preferably as close as possible to the reflectivity for input waves whose E-vector is orthogonal to the grooves. This preserves the amplitude relationship between orthogonal components. By allowing plane polarized radiation to impinge on the structure with its E-vector at 45 degrees to the axis of the grooves, the two orthogonal components will emerge with equal amplitudes, thereby ensuring that circular (not elliptical) polarized radiation results.
Details of the procedure for determining the dimensions of the first and second sections will now be given.
A known method of providing a substantially reflection-free transformation between media having different characteristic impedances Z1, Z2 involves the provision between the media of a quarter-wavelength section (i.e., a section having a length of one quarter wavelength at the design frequency) having a characteristic impedance Z3 which is the square root of the product of the two impedances, i.e.,
Z3=√{square root over (Z1Z2)}
The publication “The Design Of Quarter Wave Matching Layers For Dielectric Surfaces” by R. E. Collin and J. Brown, (Proc, IEE Part C Vol. 103, 1956, pp. 153-158), teaches the design of structures having an electrical length of one quarter wavelength for providing a good impedance match between free space and a dielectric by providing slots in the surface of the dielectric at its interface with free space. The design techniques described in this prior art to construct impedance transformers, can be used to design the radial dimensions of the grooves of quarter wave plates in accordance with the present invention.
The first step is to determine the dimensions of the grooves which would be necessary to construct a quarter wave matching layer between free space and the dielectric material of which the quarter wave plate is to be constructed, using the design criteria given in the Collin et al. paper, supra.
The next step is to determine the axial groove length 1 which would be necessary to produce a quarter wavelength phase shift between a wave traveling in the dielectric and a wave traveling the same distance in free space. Halving the length thus determined gives the respective axial depths a, b of the slots, i.e., a=b=½. Dimension c is nominally the length of one half wavelength of the design frequency in the dielectric medium. Applicants found that the making of dimension c exactly equal to one-half wavelength did not produce the minimum reflection in practice. Applicants found that varying dimension c of the third section allowed a fine tuning of the reflection coefficient of the quarter wave plate. An estimation of the exact dimensions can be made by computer modeling, or empirically determined by simply making a number of structures which are identical in all respects other than dimension c, and determining by actual tests the dimension c giving the best reflection coefficient.
The resulting structure may be considered to have an impedance at plane A-A and D-D providing a good match to free space, and impedances at planes B-B and C-C which are a function of the lengths a and b. While these latter impedances will in general not be such as to provide a good impedance match to the dielectric, this does not matter as the half-wavelength third section of length c effectively brings plane B-B coincident with plane C-C, thereby providing an impedance match between the first and second sections. Varying length c allows fine tuning of the reflection coefficients at planes A-A and D-D. The sum of lengths a and b is such as to provide the necessary anisotropic birefringent dielectric properties necessary for the structure to behave as a quarter wave plate.
Additional degrees of design freedom can be obtained by using a compound arrangement consisting of two or more discrete plates, the plates being such that a total differential phase shift of one quarter wavelength (or an odd integer multiple thereof) is imported to orthogonal components of a wave in its passage through the plates. The distance between the plates and the nature of the dielectric therebetween provides additional degrees of design freedom.
One of the eighth-wave plates 40 will now be described with reference to
In a modification, not shown, the medium in the intermediate space 60 is not air but comprises a material of a dielectric constant other than unity. This material may be the same as the material filling the grooves in the facing regions b′, b″.
In a further modification, not shown, a quarter wave plate in accordance with the invention may consist of more than two plates. The differential phase shift contributed by each plate is such that the total differential phase shift is an odd integer multiple of one quarter wavelength. Thus a three plate arrangement could have three identical plates, each producing a one-twelfth wavelength phase shift, or one plate having a one-eighth phase shift in conjunction with two plates each having a one-sixteenth phase shift, or any other combination producing a total differential phase shift of one quarter wavelength. While more complex than a two-plate arrangement, the extra gaps between plates provide extra degrees of design freedom.
While the grooves 2, 12 of the first embodiment are shown as extending entirely across the structure, this is not necessary. It is only necessary for the grooves to extend across that part of the structure through which electromagnetic radiation has to pass. Thus the periphery of each end face may be continuous, providing mechanical support for the ends of lands 1, 11 as in the second embodiment. Similarly, the grooves of the second embodiment may extend completely across the end faces as in the first embodiment.
It is not necessary for the total phase shift provided by the grooved sections to be one quarter wavelength. Any odd integer multiple of quarter wavelengths will suffice.
It is not necessary for the intermediate sections to be one half-wavelength (nominal). Any integer multiple of half wavelengths will suffice.
While the described embodiments employs a “soft” substrate having a low dielectric constant, material of any dielectric constant may be employed.
While the described embodiments provide quarter wave plates for use in air, the invention can also be performed where the dielectric interfaces with a medium other than air and having a relative dielectric constant other than unity, the relevant dimensions being changed according to the dielectric constant of the medium as to give the necessary differential phase shift.
While the described embodiments are quarter wave plates in which the same medium is present at both axial ends, the invention can also be performed where different media are present at opposite ends, e.g., air at one end and oil at the other end. The dimensions of the slots at each end are then of different design so as to provide impedance matching between the respective media and the dielectric. Thus in an embodiment physically consisting of a single plate, the sum of lengths a and b is such to provide the necessary phase shift. It is to be noted that the paths to be compared now comprise on the one hand a path via the dielectric, and on the other hand a path partly in one medium and partly in the other medium. The actual lengths of a and b are chosen so as to present the same impedances at intermediate surfaces B-B and C-C, fine tuning being effected by varying dimension c as before. Similar considerations apply, mutatis mutandis, to arrangements physically consisting of more than one plate.
The grooves may be provided by any convenient method appropriate to the dielectric material used, e.g., milling, casting or grinding.
While the embodiment depicts a circular cylindrical structure, the structure may be any shape appropriate to the application or structure in which the device is to be employed.
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