Communications: radio wave antennas – Antennas – Wave guide type
Reexamination Certificate
1999-06-11
2001-04-17
Wimer, Michael C. (Department: 2821)
Communications: radio wave antennas
Antennas
Wave guide type
C343S705000
Reexamination Certificate
active
06219004
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates in general to communication systems, and is directed to a new and improved antenna that may be employed for providing hemispherical coverage for air-to-ground communications, with a radiation/directivity pattern that is readily tailored or optimized to mitigate against sensitivity degradation in the vicinity of the horizon, such as may be associated with multipath, increased range, and rain.
BACKGROUND OF THE INVENTION
A variety of communication platforms, such as an unmanned aerial vehicle (UAV)-mounted system diagrammatically illustrated at
10
in
FIG. 1
, are required to maintain effectively continuous broadbeam communication capability (with a ground station
12
) without having to (physically or electronically) steer the aerial system's antenna
14
. Because both the range and direction of the aerial vehicle-mounted system, relative to the ground station, are dynamic, it is essential that the airborne equipment's antenna
14
provide communication coverage that is at least hemispheric. The antenna should provide somewhat ‘above the horizon’ coverage, and be designed for circular polarization, in order to accommodate changes in aircraft attitude (roll, pitch and yaw). In addition, because of the significant reduction in signal strength, increased probability of multipath and rain fades at the horizon, especially at X band and higher frequencies, it is preferred that the antenna's radiation/directivity pattern exhibit peak gain at or in the vicinity of the horizon.
Unfortunately, existing antenna architectures address only subsets of these requirements. For example, as diagrammatically shown in
FIG. 2
, a biconical antenna
20
exhibits a very narrow, flat pattern
21
, which has a peak gain
22
at the horizon, and is therefore potentially well suited for long range, reduced elevation look angle coverage. Unfortunately, the gain over the remainder of the characteristic drops off very rapidly from the horizon peak and exhibits a null or close to a null over a very substantial portion of coverage on either side of nadir
23
(looking straight down). Even though relatively low gain can be tolerated at nadir, the very significant reduction in gain exhibited by a biconical antenna over a wide portion of intended coverage between nadir and the vicinity of the horizon is not acceptable. A further drawback to a biconical antenna is the need for an external polarizer.
A bifilar helical configuration, such as diagrammaticallly shown at
30
in
FIG. 3
, on the other hand, has a relatively wide beam radiation pattern
32
, which exhibits significant gain not only at and in the vicinity of the horizon
33
, but also over a major coverage look angle that is well displaced from the horizon. However, a major drawback to a bifilar helix configuration is the fact that it has a poor axial ratio for circular polarization. In addition, the upper end of the performance bandwidth of bifilar helical antennas is limited to the neighborhood of 20-25 GHz.
Other conventional antenna architectures that have been proposed for non-steered broad coverage (UAV) applications include circular dipoles (which suffer the same limitations as the biconical approach), patch antennas (which have a null at the horizon), and slot arrays (which suffer reduced gain toward the horizon, require an external polarizer and have unproven performance). A further problem of each of the above conventional approaches is the fact that the antenna pattern cannot be shaped as necessary to provide optimal coverage for a particular application.
SUMMARY OF THE INVENTION
In accordance with the present invention, the above enumerated shortcomings of conventional antenna configurations that have proposed for hemispherical, or quasi-hemispherical, (air-to-ground) coverage are effectively obviated by a new and improved shaped (ring focus subreflector-based) antenna architecture, which exhibits a hemispherical radiation pattern that not only mitigates against sensitivity degradation in the vicinity of the horizon, but which can be tailored or optimized for a specific application.
For this purpose, the antenna of the present invention comprises a shaped ring focus type subreflector (e.g., shaped ellipsoid), that is rotationally symmetric about the boresight axis of a feed horn to which communication equipment of a first communications location (e.g., on board a UAV) is coupled. There is no main reflector associated with the shaped subreflector, as in a conventional ring focus antenna architecture, so that rays emanating from the subreflector (in a generally hemispherical pattern) are not intercepted and redirected by a main reflector.
The generally hemispherical radiation pattern exhibits a peak gain toward the horizon and encompasses a second communications location (e.g., ground station) with which a communications link from the first location is established. Preferably the subreflector is shaped such that the generally hemispherical radiation pattern produced thereby has a peak gain in a peak gain region that extends from a first prescribed elevation differential slightly above the horizon to a second prescribed elevation differential slightly below the horizon.
The feed horn causes a partial blockage of rays emanating directly beneath the antenna (i.e., reflected by the shaped subreflector straight down toward the ground). Although this causes a reduction in antenna gain in the nadir direction, it is quite tolerable in a UAV application, as it will last for only a very abbreviated interval (fraction of second) when the UAV platform passes directly overhead (at which point range-based propagation loss is minimum). Moreover, as the principal theater of deployment of a UAV is over a hostile environment that is geographically remote from the ground station (and therefore at low elevation angle where the directivity pattern has substantial gain and no blockage), rather than directly over the ground station, nadir-associated gain reduction is not a practical problem.
REFERENCES:
patent: 2549143 (1951-04-01), Tinus
patent: 2881431 (1959-04-01), Hennessey
patent: 2921309 (1960-01-01), Elliott
patent: 4458249 (1984-07-01), Valentino et al.
patent: 4520363 (1985-05-01), Wachspress et al.
patent: 5121129 (1992-06-01), Lee et al.
patent: 5486838 (1996-01-01), Dienes
patent: 5654724 (1997-08-01), Chu
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Harris Corporation
Wimer Michael C.
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