Communications: radio wave antennas – Antennas – With spaced or external radio wave refractor
Reexamination Certificate
2001-09-27
2004-07-27
Wilmer, Michael C. (Department: 2821)
Communications: radio wave antennas
Antennas
With spaced or external radio wave refractor
C343S909000
Reexamination Certificate
active
06768468
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to a reflecting surface for synthesis of reflected wavefronts therefrom for use in reflecting antennas and mirrors, for example. More particularly, the present invention is related to a system and method of making and using such a reflecting surface that is particularly useful for reflecting millimeter-wave frequencies.
BACKGROUND OF THE INVENTION
Reflecting antennas and mirrors, such as those used in beam-waveguide systems, tend to be difficult and expensive to build for millimeter-wave frequencies because the mechanical tolerances required to achieve the best signal are difficult to attain. For example, as a general rule the reflecting surface of a parabolic reflector must conform to the ideal paraboloid to within approximately one-fiftieth of a wavelength. At a frequency of 100 GHz, this corresponds to a tolerance of approximately 2 mils (about 50.8 &mgr;m). As the frequency and/or the size of the reflector increases, holding the required tolerance becomes more difficult. A regular curved surface, such as a paraboloid or hyperboloid, is particularly difficult to manufacture to a high degree of precision.
As difficult as it can be to manufacture a regular curved surface, some applications require an irregular curved surface in order to produce a desired far-field pattern, or an irregular reflecting surface (in a beam-waveguide system, for example) to correct the phase of the incident beam. Depending on the frequency and the required degree of irregularity, such a curved surface may be cost prohibitive to machine and in some cases impossible to manufacture with current manufacturing techniques.
Flat Parabolic Surface (FLAPS) antenna technology attempts to solve this problem by using an array of dipoles separated from a ground plane by a dielectric layer. The local phase shift imparted to the wave reflected from the FLAPS surface is determined by the geometry of nearby dipoles. By proper variation of the dipole geometry and spacing as a function of location on the FLAPS surface, the properties of a conventional curved reflecting antenna can be emulated.
Unfortunately, however, dielectrics generally are not environmentally rugged, and must be protected from the weather. In addition, in some applications (experimental inertial-confinement fusion reactors, for example) the beam may carry more than a megawatt of power at frequencies exceeding 100 GHz. Dielectrics tend to be lossy at millimeter-wave frequencies, and are poor conductors of heat, both of which are serious disadvantages in high-power applications. Therefore, use of a dielectric layer to support the dipoles in a FLAPS system generally precludes its use in high power applications.
SUMMARY OF THE INVENTION
Unlike prior systems, the present invention provides a reflecting surface in the form of a plate having cavities of varying dimensions and/or spacing to achieve a desired local phase shift across the reflecting surface, thereby eliminating the need to use dielectric materials. The surface of the plate can be flat or curved. A plane wave incident on the plate undergoes a shift in phase upon reflection, with the local phase shift depending on the dimensions and spacing of the cavities. By properly choosing the cavity dimensions as a function of position on the plate, the wavefronts reflected from the plate can be made to mimic a wavefront reflected from an equivalent curved reflector. In other words, the present invention provides a reflecting structure having a desired surface geometry that can emulate the electromagnetic behavior of an arbitrarily curved surface. For example, a reflecting structure that emulates a parabolic reflector can be embedded in a cylindrical surface, e.g., the skin of an aircraft. Reflecting antennas and mirrors based on this technology offer significant advantages in cost and performance over their conventional-shape counterparts.
More particularly, the present invention provides a wavefront transformer suitable for transforming an incident electromagnetic wavefront having a given shape to a reflected wavefront having a different shape. The wavefront transformer includes a substrate having a conductive surface for reflecting the incident electromagnetic energy, and a plurality of openings in the conductive surface. Each opening is formed by a respective one of a plurality of discrete cavities extending from the conductive surface, and has a selected position on the conductive surface with respect to the focal point to induce a propagation phase shift over the distance to the focal point. Each cavity also includes a local phase shift in the reflected electromagnetic energy as a function of a selected dimension of the cavity. The combined propagation phase shift and local phase shift from the plurality of cavities places the reflected electromagnetic energy in phase at the focal point.
Other features encompassed by the present invention include a wavefront transformer wherein the substrate is a metal plate; wherein the plate is substantially flat; wherein the plate includes a first plate overlying a second plate, wherein the first plate has a plurality of through-holes therein that form the cavities and the second plate forms a flat bottom surface of the cavities; wherein the plate has a substantially uniform thickness; wherein one or more properties of the cavities varies with position with respect to the focal point; wherein the properties that vary include dimensions of the cavities and spacing between neighboring cavities; wherein the dimensions of the cavities include cross-sectional dimensions that include one or more of width, depth and radius; wherein the plurality of cavities form a periodic array; wherein only the positions of the cavities and the selected dimension of the cavities varies, and the dimension of each cavity is selected such that the total phase shift at the focal point of an electromagnetic wave reflected from each cavity is equal, so that
φ
(
r
)
=
φ
(
0
)
+
2
π
λ
(
r
2
+
f
2
-
f
)
,
where r is the distance of the cavity from a reference point in the plane of the conductive surface, &phgr;(r) is the local phase shift imposed on an incident electromagnetic wave at r by the flat reflecting surface, f is the focal length of the reflector, &lgr; is a desired wavelength of the reflected electromagnetic energy, and &phgr;(
0
) is the local phase shift imposed on an incident electromagnetic wave by a cavity at the reference point having a dimension a(
0
,
0
).
Other features include a wavefront transformer having a focal length of about four and a half inches (about 11.4 cm); wherein a dimension of the central cavity, a(
0
,
0
), is a radius of a circular opening formed by a cylindrical cavity; wherein a(
0
,
0
) is about 44.5 mils (about 254 &mgr;m); wherein the cavity dimension is selected for frequencies greater than about 20 GHZ; wherein the cavity dimension is selected for a frequency of about 95 GHz; wherein the cavities have a uniform depth of about 100 mils (about 2.54 mm); wherein the nearest-neighbor distance between adjacent cavities is uniform; wherein the nearest-neighbor distance between adjacent cavities is about 105 mils (about 2.67 mm); wherein the cavities have a depth that is less than a local thickness of the plate; wherein the openings are circular; wherein the cavities are cylindrical; and wherein the plurality of cavities are arrayed in an equilateral-triangular arrangement.
The present invention also provides a reflector suitable for focusing incident electromagnetic energy at an operating wavelength on a focal point, including the wavefront transformer, and an antenna including the reflector and a waveguide feed located at the focal point.
The present invention also provides a method of making a reflector suitable for focusing incident electromagnetic energy at an operating wavelength on a focal point, the wavefront transformer having a substrate with a conductive surface for reflecting the incident electromagnetic energy, and a plurality of openings in the
Crouch David D.
Dolash William E.
Berestecki Philip P.
Finn Thomas J.
Raytheon Company
Renner Otto Boisselle & Sklar
Wilmer Michael C.
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