Communications: radio wave antennas – Antennas – Antenna components
Reexamination Certificate
2002-08-30
2004-02-17
Wimer, Michael C. (Department: 2821)
Communications: radio wave antennas
Antennas
Antenna components
Reexamination Certificate
active
06693605
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to a wave plate, system and methods of making and using same, and more particularly, to a variable quasioptical wave plate that is particularly useful for millimeter wave frequencies, as well as a system including same, and methods of making and using such a wave plate and system.
BACKGROUND OF THE INVENTION
Wave plates are devices that utilize an anisotropy to change the polarization (direction of the electric field vector) of an incident electromagnetic wave, i.e., to change the direction of the electric field vector. Wave plates frequently are used in optical systems, to induce a change in the polarization of an incident electromagnetic wave, such as light. Such optical wave plates typically are constructed from a birefringent material, i.e., one having different indices of refraction for different incident polarizations. For example, an optical wave plate may be a dielectric plate having different refractive indices in orthogonal x and y directions (with the z direction normal to the surface of the plate in an orthogonal coordinate system). More specifically, a wave plate may be made from a birefringent material such as calcite, for example, whose optic axis is parallel to the surface. In terms of the polarization of the incident wave directed to a wave plate, there are two cases of interest: when the electric field vector of the incident electromagnetic wave is parallel to the optic axis, and when it is perpendicular to the optic axis. The component of the incident wave whose electric vector is parallel to the optic axis is known as the extraordinary wave (or e-wave for short), and the wave component whose electric vector is perpendicular to the optic axis is known as the ordinary wave (o-wave). Each wave or component sees a different index of refraction, n
e
for the extraordinary wave, and n
o
for the ordinary wave. As a result, upon propagation through a plate of thickness L, the difference in phase between the two wave components is given by
&Dgr;&phgr;={fraction (2&pgr;/&lgr;)}(
n
e
−n
o
)
L,
where &lgr; is the free-space wavelength. An incident wave that is linearly polarized and whose electric field vector makes an angle of 45° relative to the optic axis may be considered to have e-wave and o-wave components having equal magnitudes and phases.
If L is chosen so that &Dgr;&phgr;=2&pgr;, the device, known as a full-wave plate, has no effect on the polarization of the transmitted electromagnetic wave. If L is chosen so that &Dgr;&phgr;=&pgr;, the polarization of the transmitted electromagnetic wave is rotated about an axis parallel to the direction of propagation by 90° relative to that of the incident wave, and the resulting device is known as a half-wave plate. If L is chosen so that &Dgr;&phgr;=&pgr;/2, the wave plate induces a 90° phase shift between the initially in-phase electric-field components of the incident wave, resulting in a circularly polarized transmitted wave. Such a wave plate is known as a quarter-wave plate. Finally, if L is chosen so that &Dgr;&phgr;=&pgr;/4, the plate induces a 45° shift in phase between the initially in-phase components of the incident wave. Such a wave plate is known as an eighth-wave plate.
Wave plates may be used in quasioptical millimeter-wave systems. However, the wave plates described above are seldom used at millimeter-wave frequencies (frequencies at which the wavelength is between 1 and 10 millimeters) for two primary reasons. First, accurate measurements of the anisotropic properties of dielectrics at millimeter-wave frequencies are very limited. Second, most materials have relatively high losses at millimeter-wave frequencies, limiting their usefulness. This problem is compounded for anisotropic materials, since the loss also tends to be anisotropic, producing an absorption that is dependent on the electric-field polarization.
Wave plates also have been made from isotropic dielectrics by inducing an artificial anisotropy. For example, half- and quarter-wave plates have been made from Rexolite® (a low loss polymer available from C-LEC Plastics, Inc. of Beverly, N.J., U.S.A.) by machining a series of parallel grooves in the surface of a plate. An incident wave whose electric field is polarized parallel to the grooves will see a different index of refraction than an incident wave whose electric field is polarized perpendicular to the grooves. This technique is effective at low power levels; however, at high power levels the low thermal conductivity of Rexolite® causes excess heat to accumulate until the plate fails.
For high power levels, wave plates have been constructed of metal plates by fabricating periodic arrays of rectangular or elliptical slots in the plates. For example, see Paul F. Goldsmith,
Quasioptical Systems: Gaussian Beam Quasioptical Propagation and Applications
(1998). Each slot acts like a waveguide. For a rectangular waveguide of width W and height H (where W>H), for example, the phase shift per unit length for the TE
10
mode (electric field polarized parallel to H) is different from that for the TE
01
mode (electric field polarized parallel to W). A similar relationship exists for an elliptical waveguide. By properly choosing the slot dimensions, the thickness of the plate, and the periodicity of the slots, the desired relative phase shift can be imposed between the orthogonally-polarized components of a normally-incident wave, and the reflected power for each component of the incident wave can be minimized. Such a wave plate, being of all-metal construction, can handle very high power levels, particularly if it is actively cooled around the edges, or includes internal cooling channels.
A slotted metal or metallic wave plate like that just described is difficult and expensive to fabricate, however, as the slots for millimeter-wave wavelengths are too small to be made by conventional machining techniques. Typically, the rectangular or elliptical slots have to be formed using some form of electron-discharge machining (EDM). If wire EDM is used, a hole first is machined where each slot is to be placed, then the EDM wire is threaded through the hole. After cutting the slot to the desired dimensions, the wire is cut and is manually threaded through the next hole. As this technique is very labor intensive, it is not cost effective if more than one or two wave plates are to be constructed.
Another form of EDM uses a mandrel. To construct a wave plate of the type described above, the mandrel has a “waffle” pattern of raised rectangular protrusions extending from its surface. The mandrel is then used to “burn” the desired pattern into the metal plate. This type of EDM results in gradual deformation of the mandrel, which has to be trimmed after burning part way through the plate and eventually has to be replaced. A wave plate constructed in this manner would likely be less expensive than one constructed using wire EDM, but generally is still cost-prohibitive.
SUMMARY OF THE INVENTION
The present invention provides a wave plate that is based on a perforated metallic plate. However, unlike the slotted metallic plates previously used in highpower applications, the wave plate provided by the present invention has circular through-holes and induces an anisotropy by forming holes of a predetermined size in a predetermined pattern. More specifically, by proper choice of the hole diameter and plate thickness, and by different spacing of the holes in respective orthogonal x and y directions, the desired relative phase difference between orthogonally-polarized components is achieved while minimizing the reflected power for both polarization components and maximizing power transmission through the plate.
The present invention replaces the rectangular or elliptical slots with circular holes, eliminating the need for EDM and resulting in significantly lower manufacturing costs. A wave plate made with circular holes can be made with conventional machining techniques, eliminating the need for EDM and resulting in a signi
Crouch David D.
Rattray Alan A.
Finn Thomas J.
Lenzen, Jr. Glenn H.
Raytheon Company
Saralino Mark A.
Wimer Michael C.
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