Abstract:
An apparatus and method to twist the field polarization of an electromagnetic wave over a desired frequency band is described. In one embodiment, a transmission twister rotates the polarization of a linearly-polarized incident field to produce a transmitted field. In one embodiment, the transmission twister includes a resonant polarization-twisting array between two linearly-polarized arrays. In one embodiment, the transmission twister rotates the polarization by 90 degrees. In one embodiment, the transmission twister produces low reflection of a desired incident polarization. In one embodiment, the transmission twister has a transmission coefficient (with respect to the desired incident field polarization and a correspondingly rotated transmitted field polarization) close to unity.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]    The present application claims priority benefit of U.S. Provisional Application No. 60/349,927, filed Jan. 17, 2002, titled “ELECTROMAGNETIC-FIELD POLARIZATION TWISTER.” 
     
    
     
       BACKGROUND  
         [0002]    Description of the Related Art  
           [0003]    A polarization twister is typically described as a device that rotates the polarization of a linear incident field by some angle (e.g., by an angle of 90 degrees). These devices are constructed using multiple non-resonant layers, each layer having an array of infinite wires. The layers are typically separated by quarter-wavelength foam spacers. The polarization of each array of infinite wires is rotated a fixed number of degrees from its preceding neighbor. Each wire grid re-radiates the component of incident E-field that is co-polarized with the grid. The polarization of the first layer is orthogonal to the incident E-field. The polarization of the next layer is slightly rotated so that a fraction of the incident field is twisted and then reflected back or transmitted forward. Since the grids are separated by a distance of ¼ wavelength, the reflected components tend to cancel, somewhat.  
           [0004]    For many systems, where polarization purity and low reflection are desired, this crude approach is not sufficient. The performance of such polarization twisters, even when several layers are used, is inadequate for many applications. The poor performance of these devices results in the production of unwanted field components such as, for example, partial reflection of the incident field, incomplete rotation (e.g., rotation less than or greater than 90 degrees), poor transmission through the layers, etc.  
         SUMMARY  
         [0005]    The present invention solves these and other problems by providing an improved apparatus and method to twist the field polarization of an electromagnetic wave, with good transmission and low reflection over a desired frequency band. In one embodiment a linearly polarized field is rotated by 90 degrees. The improved apparatus is typically thinner and less costly than the prior art because fewer layers are needed to twist the polarization while maintaining good performance characteristics.  
           [0006]    In one embodiment, a transmission twister rotates the polarization of a linearly-polarized incident field to produce a transmitted field. In one embodiment, the transmission twister rotates the polarization by 90 degrees. In one embodiment, the transmission twister produces low reflection of a desired incident polarization. In one embodiment, the transmission twister has a transmission coefficient (with respect to the desired incident field polarization and a correspondingly rotated transmitted field polarization) close to unity.  
           [0007]    In one embodiment, a reflection twister rotates the polarization of an electromagnetic wave having a linearly-polarized incident field to produce a reflected field with a polarization rotated with respect to the incident field. In one embodiment, the transmission twister rotates the polarization by 90 degrees.  
           [0008]    In one embodiment, the reflection twister operates in a desired frequency band. In the operating band, an incident field (e.g., an incident E-field) is rotated from-a first polarization to a second polarization with high efficiency, producing little reflected field co-polarized with the incident field. In one embodiment, the reflection twister uses a resonant polarization-twisting Frequency Selective Surface (FSS) layer above a ground plane. In one embodiment, each element of the polarization-twisting FSS includes two crossed dipoles that are connected so that one dipole loads the other dipole near its center.  
           [0009]    It is known that a ground plane reflects Right-Hand Circular Polarization (RHCP) as Left-Hand Circular Polairzation (LHCP), and vice versa. In one embodiment, the reflection twister reflects RHCP as RHCP, and reflects LHCP as LHCP.  
           [0010]    In one embodiment, the transmission polarization twister operates in a desired frequency band. In the operating band, an electromagnetic wave having an incident field (e.g., an incident E-field) is twisted from a first polarization to a second polarization with good efficiency, producing little or no undesired reflected field and little transmitted field co-polarized with the incident field. In one embodiment, the transmission twister uses three Frequency Selective Surface (FSS) layers arranged as a middle layer with two outer FSS layers (one on either side of the middle layer) and, optionally, two spacers. In one embodiment, the two outer FSS layers are linearly-polarized arrays (e.g., linearly-polarized wires or slots), and the middle layer is a polarization-twisting FSS array. In one embodiment, the two outer FSS layers are dipole arrays, and the middle layer is a polarization-twisting FSS array. In one embodiment, one or both of the two outer FSS layers are slot arrays, and the middle layer is a polarization-twisting FSS array of slots or wire elements. In one embodiment, one or both of the two outer FSS layers are non-resonant grids, and the middle layer is a polarization twisting FSS array. In one embodiment, each element of the polarization twisting FSS includes two crossed dipoles that are connected so that one dipole loads the other dipole near its center. In one embodiment, the middle layer is a polarization twisting FSS array comprising loop-type elements. In one embodiment, the middle layer is a polarization twisting FSS array comprising bowtie loop-type elements. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0011]    The advantages and features of the disclosed invention will readily be appreciated by persons skilled in the art from the following detailed description when read in conjunction with the drawings listed below.  
         [0012]    [0012]FIG. 1 shows a five-layer polarization twister using non-resonant wire grids (sometimes called “infinite” wire grids).  
         [0013]    [0013]FIG. 2 shows a reflection twister.  
         [0014]    [0014]FIG. 3 shows a transmission twister.  
         [0015]    [0015]FIG. 4A shows the first FSS layer of a three-layer polarization twister using three FSS layers, where the middle layer comprises bent dipole-type elements.  
         [0016]    [0016]FIG. 4B shows the second FSS layer of a three-layer polarization twister using three FSS layers, where the middle layer comprises bent dipole-type elements.  
         [0017]    [0017]FIG. 4C shows the third FSS layer of a three-layer polarization twister using three FSS layers, where the middle layer comprises bent dipole-type elements.  
         [0018]    [0018]FIG. 5 shows an equivalent-circuit model of the three-layer polarization twister shown in FIGS.  4 A- 4 C.  
         [0019]    [0019]FIG. 6 shows the predicted and measured performance of the five-layer polarization twister shown in FIG. 1.  
         [0020]    [0020]FIG. 7 shows the predicted and measured performance of the three-layer polarization twister shown in FIGS.  4 A- 4 C.  
         [0021]    [0021]FIG. 8A shows the first FSS layer of a three-layer polarization twister using three FSS layers, where the middle layer comprises bowtie loop-type elements.  
         [0022]    [0022]FIG. 8B shows the second FSS layer of a three-layer polarization twister using three FSS layers, where the middle layer comprises bowtie loop-type elements.  
         [0023]    [0023]FIG. 8C shows the third FSS layer of a three-layer polarization twister using three FSS layers, where the middle layer comprises bowtie loop-type elements. 
     
    
     DETAILED DESCRIPTION  
       [0024]    [0024]FIG. 1 shows a prior art polarization twister having five non-resonant layers of wires  101 - 105  (sometimes called an “infinite” wire grid because the wires are long with respect to the wavelength of the incident field). The layers are non-resonant in that they do not exhibit significant resonance effects in the desired operating band. The first layer  101  is cross-polarized to the desired incident field. Each successive non-resonant layer  102 - 105  is rotated with respect to its preceding layer such that the final non-resonant layer  105  is co-polarized with the incident field.  
         [0025]    A reflection twister is shown in FIG. 2. The reflection twister has a polarization-twisting FSS  201  (such as, for example, the polarization-twisting FSS layers shown in FIGS.  4 B and/or  8 B) located above a groundplane  202 . The polarization-twisting FSS layer  201  rotates the polarization of an incident field to produce transmitted and reflected fields where the polarization of at least a portion of the incident field has been rotated by a desired rotation. The polarization-twisting FSS layer  201  can be constructed using FSS elements such as loaded dipoles (or slots), V dipoles (or slots), bent dipoles (or slots), asymmetrical loops (wires or slots), rectangular loops (wires or slots), dipoles (or slots) rotated by some angle (e.g., 45 degrees) with respect to the incident field, etc. In one embodiment, each polarization-twisting FSS element of the array  201  is a dipole loaded with a cross-polarized dipole. At resonance, the dipole is matched by the cross-polarized dipole load. In one embodiment, each polarization-twisting FSS element is a slot loaded with a cross-polarized slot. In one embodiment, a dielectric spacer is placed between the FSS and the ground plane. In one embodiment, the FSS  201  and/or the ground plane  202  are bonded to the dielectric spacer.  
         [0026]    If a conjugate-matched element is located above a ground plane, then most (theoretically all) of the energy will end up in the load. In this case, the load is the cross-polarized dipole (or slot). Therefore, when the twister FSS  201  is properly located above the ground plane  202 , then most of the reflected signal will be rotated 90 degrees from the incident polarization.  
         [0027]    A transmission twister  300  is shown in FIG. 3. The transmission twister  300  includes a first FSS layer  301 , a second FSS layer  302 , and a third FSS layer  303 . The polarization of the elements of the first FSS  301  is orthogonal to the polarization of the incident field (the input polarization) such that at least a portion of the incident field can pass through the first FSS layer  301 . The elements of the second FSS  302  are polarization-twisting elements. The polarization of the elements of the third FSS  303  is orthogonal to the desired transmitted polarization (the output polarization) such that at least a portion of the transmission field can pass through the third FSS layer  303 . The second FSS  302  is disposed between the first FSS  301  and the third FSS  303 . In one embodiment, one or more dielectric spacers are used between the FSS layers  301 - 303 . In one embodiment, one or more of the FSS layers  301 - 303  are bonded to the dielectric spacers. The elements of the first FSS layer  301  can be resonant or non-resonant wires (e.g., dipole-type elements, “infinite” wires, etc.), resonant or non-resonant slots, and the like. The elements of the second FSS layer  302  can be resonant wires, slots, and the like. The elements of the third FSS layer  303  can be resonant or non-resonant wires, resonant or non-resonant slots, and the like. The first, second, and third FSS layers  301 - 303  need not use the same type of FSS elements. Thus, some of the FSS layers  301 - 303  can use slot elements and some of the FSS layers  301 - 303  can use wire elements (e.g., dipoles).  
         [0028]    In one embodiment, the first FSS layer  301  is a linearly-polarized array having elements that are cross-polarized with respect to the incident field (that is, elements that allow the desired incident polarization to pass through relatively unattenuated) and co-polarized with respect to the transmitted field (that is, elements that reflect the desired transmitted polarization). In one embodiment, the second FSS layer  302  is a polarization-twisting layer that rotates the polarization of the incident field. In one embodiment, the third FSS layer  303  is a linearly-polarized array having elements that are co-polarized with respect to the incident field (that is, elements that reflect the desired incident field polarization) and cross-polarized with respect to the transmitted field (that is, elements that allow the desired transmitted polarization to pass through relatively unattenuated). The polarization-twisting FSS layer  302  can be constructed using FSS elements such as loaded dipoles (or slots), V dipoles (or slots), bent dipoles (or slots), asymmetrical loops (wires or slots), rectangular loops (wires or slots), dipoles (or slots) rotated by some angle (e.g., 45 degrees) with respect to the incident field, etc.  
         [0029]    In one embodiment, a first dielectric spacer is placed between the first FSS layer and the second FSS layer. In one embodiment, a second dielectric spacer is placed between the second FSS layer and the third FSS layer. In one embodiment, one or more of the FSS layers are bonded to the dielectric spacers.  
         [0030]    [0030]FIG. 4A shows one embodiment of the linearly-polarized array  301  as a dipole FSS  401 . FIG. 4B shows one embodiment of the polarization-twisting array  302 , where the polarization-twisting array  302  comprises bent dipole-type elements in an FSS  402 . FIG. 4C shows one embodiment of the linearly-polarized array  303  as a dipole FSS  403 . The arrays shown in FIGS.  4 A- 4 C can be used to rotate a linearly-polarized incident field by 90 degrees. FIGS. 4A and 4C show linearly-polarized dipole arrays (FIGS. 4A and 4C show dipoles, but resonant slots, non-resonant wires, or non-resonant slots can also be used). FIG. 4B shows a polarization-twisting FSS array  402  comprising bent dipole-type elements. In one embodiment, the linearly-polarized FSS layers  401 ,  403  is placed on each side of the polarization-twisting FSS  402 . The polarization-twisting FSS array  402  comprises bent dipole-type elements arranged to form elements that can be considered to be a dipole loaded with a crossed dipole. Alternatively, the polarization-twisting FSS layer  402  can be viewed as two L-shaped elements with a gap in the center of each group of two L shaped elements. In each dipole pair the vertical dipole loads the horizontal dipole and visa versa.  
         [0031]    The linearly-polarized dipole (or slot) FSS layers  401 ,  403  are broad-banded enough such that in the desired frequency band they approximate a ground plane to a first linear polarization and are approximately invisible to a second linear polarization rotated 90 degrees with respect to the first linear polarization. On the input side of the twister, the FSS elements (slots or wires) are cross-polarized to the incident E-field. On the output side of the twister the FSS elements (slots or wires) are co-polarized to the incident E-field.  
         [0032]    As shown in the equivalent circuit model illustrated in FIG. 3, the transmission twister is conceptually analogous to two connected dipole arrays  502 ,  503  backed by polarization-dependent ground planes  501   504 . For convenience, and without limitation to horizontal polarization (H-pol.) and vertical polarization (V-pol.), the two dipole arrays  501 ,  504  will be referred two as the H-pol. array and the V-pol. array. A V-pol. incident E field initially passes through the H-pol. array  501  and is then received by the vertical dipoles  502  of the polarization-twisting array. The energy is then passed from the vertical dipoles  502  to the horizontal dipoles  503  of the polarization-twisting array. The horizontal dipoles  503  of the polarization-twisting array then re-radiate (scatter) the energy forward and backward. The H-pol. ground plane  504  reflects H-pol. fields and thus prevents H-pol. radiation from the horizontal dipole array  503  from being backscattered by the polarization twister. The V-pol. ground plane  501  prevents transmission of V-pol. fields, but passes H-pol. fields with little or no attenuation. Thus, the transmission twister shown in FIG. 4 converts an incident V-pol. field into a transmitted H-pol field. If one or more of the layers can be constructed using slots instead of dipoles as discussed above. In other embodiments, a horizontal slot array can be used in place of the vertical dipole array, and vice versa.  
         [0033]    [0033]FIG. 5 shows predicted and measured performance of the five-layer prior art twister shown in FIG. 1. In FIG. 5, the cross-pole isolation is only 30 dB.  
         [0034]    [0034]FIG. 6 shows the predicted and measured performance of the three-layer polarization twister shown in FIGS.  4 A- 4 C. In FIG. 6, in the operating band, the cross-pole isolation is at least 40 dB down. Thus the three-layer resonant polarization twister produces better performance, with fewer layers, than the five-layer non-resonant polarization twister.  
         [0035]    [0035]FIG. 8A shows one embodiment of the linearly-polarized array  301  as a non-resonant wire FSS  801 . FIG. 8B shows one embodiment of the polarization-twisting array  302 , where the polarization-twisting array  302  comprises bowtie loop-type elements in an FSS  802 . FIG. 8C shows one embodiment of the linearly-polarized array  303  as a non-resonant wire FSS  803 . Either or both of the wire arrays  801 ,  803  can be replaced by non-resonant slots arrays, resonant slot or dipole arrays, etc. The arrays shown in FIGS. 8A through 8C can be used to rotate a linearly polarized incident field by 90 degrees. FIGS. 8A and 8C show non-resonant long wire arrays  801 ,  803  (FIGS. 8A and 8C show non-resonant wires, but resonant dipoles, resonant slots, or non-resonant slots can also be used). FIG. 8B shows a polarization-twisting FSS array  802  comprising bowtie loop-type elements. The polarization-twisting FSS  802  array comprises loops with a generally bowtie shape. In one embodiment, the bowtie elements are similar to the dipole-type elements of FIG. 4B with the ends of the dipoles connected to form a bowtie-shaped loop.  
         [0036]    The linearly-polarized layers  801 ,  803  are broad-banded enough such that in the desired frequency band they approximate a ground plane to a first linear polarization and are approximately invisible to a second linear polarization rotated 90 degrees with respect to the first linear polarization. On the input side of the twister, the wires (or slots) are polarized to allow transmission of the incident field.  
         [0037]    Although the foregoing has been a description and illustration of specific embodiments of the invention, various modifications and changes can be made thereto by persons skilled in the art without departing from the scope and spirit of the invention.