Patent Application
20180175508
Not Applicable
Not Applicable
Steerable beam antennas that allow for the transmission and/or reception of a highly directional electromagnetic signals are well-known in the art, as exemplified by U.S. Pat. No. 6,750,827; U.S. Pat. No. 6,211,836; U.S. Pat. No. 5,815,124; and U.S. Pat. No. 5,959,589. These exemplary prior art antennas include a waveguide feed that provides the evanescent coupling of electromagnetic waves between a waveguide feed including an elongate (typically rod-like) dielectric element, and a cylinder or drum spinning or rotating on an axis parallel to the axis of the dielectric element, and then radiating the coupled electromagnetic energy in directions determined by a diffraction grating provided by surface features (such as, for example, grooves) of the drum. By defining rows of features, wherein the features of each row have a different period, and by rotating the drum around an axis that is parallel to that of the dielectric element, the radiation can be directed in a plane over an angular range determined by the different periods.
As noted above, the typical waveguide feed used with a spinning drum antenna includes an elongate dielectric rod, typically of quartz. Such rods are inherently fragile, and their placement usually requires some manual labor to obtain the needed precision. Furthermore, the spinning drum with a groove pattern creates air flows for which the dielectric rod presents a bluff body that creates air vortices, thereby causing rod vibrations, and otherwise degrading dynamic antenna parameters.
Accordingly, it would be advantageous to provide a steerable beam antenna in which the problems associated with a rod-like dielectric coupling element are minimized or substantially reduced.
Broadly, a steerable beam antenna in accordance with this disclosure comprises a cylindrical drum rotatable around a longitudinal axis and having a surface providing a diffraction grating, a bifurcated waveguide feed comprising first and second parallel waveguide feed portions extending longitudinally (axially) along the length of the drum, and an opposed pair of dielectric coupling elements, each of which is configured as flush-mounted dielectric strip extending longitudinally along an inner surface of each of the waveguide feed portions.
In one aspect, the steerable beam antenna comprises a rotatable drum having a drum surface configured as a diffraction grating; a bifurcated waveguide feed comprising first and second conductive metal bases plates extending longitudinally (axially) along the length of the drum, each of the bases having an inner surface opposed to and spaced from the corresponding inner surface of the other base, and a proximal surface spaced from the drum surface by an air gap. First and second conductive metal plates extend distally from the first and second bases, respectively. The first and second plates are parallel to each other and define respective inner surfaces separated by an inter-plate space. The first and second plates are thereby advantageously configured as an output horn. First and second flush-mounted dielectric strips on the inner surface of each of the first and second metal bases, respectively. Each dielectric strip extends longitudinally (axially) along the inner surface of its respective base. In specific embodiments, the proximal surface of each of the first and second bases may have one or more longitudinally-extending “choke” grooves.
Performance optimization may be achieved, in some embodiments, with a dielectric strip width (distance between its proximal edge and its distal edge) of approximately one-half wavelength (λ/2) of the transmitted beam. Preferably, the thickness of each of the dielectric strips is substantially less than the wavelength λ. An inter-plate space of approximately one-half wavelength (λ/2) is considered optimum, although not critical. The width of the air gap between the proximal edges of the plates and the drum surface should preferably not exceed λ/4, to optimize evanescent coupling between the diffraction grating of the drum surface and the dielectric waveguide provided by the dielectric strips. In embodiments having one or more choke grooves in each of the plates, the optimum width and depth of each groove are both preferably approximately λ/4.
As will be appreciated from the detailed description below, steerable beam antennas in accordance with this disclosure provide efficient evanescent coupling between the rotating or spinning diffraction grating on the drum surface and the dielectric strips, without the aforementioned disadvantages of quartz rod dielectric coupling elements. For example, the dielectric strips are easily fabricated and attached to the plates that form the antenna output elements, thereby simplifying the fabrication process. Furthermore, configured as thin flat strips flush-mounted on the inner surfaces of the plates, the dielectric elements do not exhibit the aerodynamic problems and vibrational tendencies to which the rod-like elements are prone, as noted above. These and other advantages will be apparent from the detailed description that follows.
Referring to the drawings, a steerable beam antenna 10 in accordance with aspects of this disclosure is shown. The antenna 10 comprises a cylindrical drum 12 that is rotatable around a longitudinal axis A, a bifurcated, conductive metal waveguide feed comprising first and second parallel waveguide feed portions 14 extending axially (longitudinally) along the length of the drum 12, and flush-mounted dielectric strips 16 extending longitudinally along an inner surface of each of the waveguide feed portions 14.
The drum 12, which may be of a conventional type for a steerable beam antenna, is provided with a diffraction grating 18 on a major portion of its outer surface. The diffraction grating 18 may have any suitable configuration well known in the art, such as, for example, a pattern of annular grooves that define desired periodicities suitable for the range of wavelengths to be transmitted and/or received. Thus, although the grating grooves are illustrated (for the sake of simplicity) with a substantially uniform periodicity, in actuality the periodicity of the grooves will typically be varied at different angular positions along the length of the drum 12, whereby the groove pattern forms a diffraction grating 18 designed to provide the desired beam shape.
The first and second waveguide feed portions 14 include first and second conductive metal bases 20, respectively. Each of the first and second bases extends longitudinally (axially) along the length of the drum 12. Each of the first and second bases 20 has a proximal (with respect to the drum) surface that is spaced from the outer surface of the drum 12 by an air gap G (see
The distal portions of the plates 24 form an output horn 26 that forms a beam shape in a plane orthogonal to the drum axis A. In some embodiments, such as shown in the drawings, the output horn 26 has an outwardly-flared configuration. Other configurations for the output horn are known, and will readily suggest themselves to those skilled in the art.
The waveguide feed portions 14 are secured to each other at the transition portions 22, one at each of the opposite first and second ends of each of the first and second bases 20, as described above. The transition portions 22 of one waveguide feed portion 14 may be secured to the transition portions 22 of the other waveguide feed portion 14 by any suitable attachment or fastening means (not shown), such as, for example, screws, bolts, welding joints, rivets, etc. The transition portions 22 at opposite ends of each of the bases 20 are spaced from the ends of the drum 12 so as to provide a clearance that accommodates the rotation of the drum 12. The structure and configuration of the waveguide feed portions 14, including their respective transition portions 22, as well as the space S between the plates 24, are such that the plates 24 have a mirror symmetry with respect to an imaginary plane located between the plates 24 and parallel to them.
Each of the waveguide feed bases 20 has an inner surface spaced from and parallel to the inner surface of the other waveguide enclosure base 20. First and second dielectric strips 16 extend longitudinally (axially) along the inner surface of the first and second waveguide feed bases 20, respectively, adjacent the proximal surface thereof. As mentioned above, the dielectric strips 16 are flush-mounted on their respective bases, and they may be secured to their respective bases 20 by, for example, a suitable adhesive. As best shown in
In an antenna operable to transmit and/or receive an electromagnetic signal of a defined wavelength λ, an inter-plate space S having a width of approximately one-half wavelength (λ/2) of the transmitted/received signal is considered optimum, although not critical. The width of the air gap G between each of the waveguide feed bases 20 and the surface of the drum 12 should preferably not exceed λ/4, to optimize evanescent coupling between the diffraction grating 18 of the drum surface and the dielectric strips 16, while allowing clearance for the rotation of the drum 12. Performance optimization may be achieved, in some embodiments, with a dielectric strip width (distance between the proximal edge and distal edge of each dielectric strip 16) of approximately one-quarter wavelength (λ/4) to one-half wavelength (λ/2) of the transmitted/received beam. Preferably, the thickness of each of the dielectric strips 16 is substantially less than the wavelength λ. For use of the antenna 10 to transmit/receive radiation in the millimeter wavelength band, an exemplary thickness of about 0.5 mm is suggested, although this specific thickness is not critical.
In specific embodiments, the proximal surface of each of the first and second bases 20 will advantageously have one or more longitudinally-extending “choke” grooves 32, essentially parallel with the dielectric strip 16 attached to each base 20. As is known in the art, the choke grooves 32 reduce leakage of scattered signal through the air gap G between the waveguide feed bases 20 and the drum 12, thereby increasing signal propagation through the output horn 26. The optimum width and depth of the choke grooves 32 are both preferably approximately λ/4.
As mentioned above, antennas of the type described herein are typically used in steerable beam antenna systems for the transmission/reception of electromagnetic radiation in millimeter wavelengths, such as the W waveband (75-110 GHz). Such systems typically use an external waveguide 30 at one or both ends of the antenna 10. To match the impedance ZF of the external waveguide(s) 30 with the impedance ZA of the antenna 10, an impedance-matching transformer 34 is typically installed between each external waveguide 30 and the transition portions 22 at each end of the antenna 10 that is coupled to an external waveguide 30. The transition portions 22 are specifically designed, in accordance with an aspect of this disclosure, to provide, in conjunction with the impedance-matching transformer(s) 34 and the tapered end portions 27 of the dielectric strips 16, a gradual transition of the impedance from ZF (the first impedance) to ZA (the second impedance), thereby avoiding the creation of parasitic modes of the radiation coupled to or from the antenna 10 through the external waveguide(s) 30.
One specific exemplary embodiment of a transition portion 22 in accordance with an aspect of this disclosure is illustrated in
It will be appreciated that, in some embodiments, the structure shown in
The above-described description of the transition portions 22, as illustrated in the drawings, is exemplary only. In practice, the specific geometry and construction of the transition portions 22 and the dielectric strips 16 may be dictated by such factors as the operational frequency of the antenna, the bandwidth of the antenna beam, the materials used, and the specific antenna geometry. The object in all cases is to minimize reflection of waves at the external waveguide/antenna interface and to provide single mode operation (i.e., minimizing parasitic modes).
While exemplary embodiments have been described above and illustrated in the drawings, it will be appreciated that variations and modifications of these embodiments may suggest themselves to those skilled in the pertinent arts. Thus, as noted above, such aspects as the configuration of the waveguide feed (including, for instance, the output horn), the structure and configuration of the transition portions (including their internal structure and configuration), and the configuration of the dielectric strips may be varied or modified without departing from the spirit and scope of the disclosure. Any dimensions set forth above are, likewise, exemplary only and not limiting. Such variations and modifications, and any equivalents thereof, are to be considered within the scope of this disclosure.
