Communications: radio wave antennas – Antennas – With aircraft
Reexamination Certificate
2001-04-24
2002-06-18
Ho, Tan (Department: 2821)
Communications: radio wave antennas
Antennas
With aircraft
C343S878000
Reexamination Certificate
active
06407711
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to an apparatus for mounting a RF antenna array to a non-planar surface. More particularly, the present invention relates to an apparatus for attaching a plurality of discrete antenna elements to a surface of curvature such that the antenna array is made to conform to the contour of a complex three-dimensional surface.
While the invention disclosed herein may be used in a wide variety of RF sensing applications in which discrete antenna elements are mounted in conformity to nonplanar mounting structures, the preferred embodiment is directed to the antenna elements of a RF sensing apparatus of an aircraft, sensor pod, missile or surface array having a substantially cylindrical or conical array configuration. The antenna array may be coupled to an Anti-Radiation Homing (ARH) subsystem, for example, and the RF sensing apparatus used to detect a target and track its position using signals received in the form of energy emitted by or reflected from the target. The RF sensing apparatus embodying the present invention includes a passive antenna array comprising a plurality of individual broadband antenna elements, each generating a voltage when excited by an electromagnetic waveform emanating from the target. The elements are connected to a broadband receiver where the signals are processed and the signal information passed to a guidance processing unit for performing various guidance functions. For example, the guidance processing unit may perform angle-of-arrival determinations in which the direction of the source of a signal within the array's field of view is located using signal information derived from the voltages sensed by the elements of the array.
A conventional RF sensing apparatus employs a plurality of RF antenna elements mounted on a stationary device or moving surface such as the nose of an aircraft, missile, sensor pod or other airborne apparatus. In more recent missile applications, the antenna elements have been confined to a structure aft of the nose section, which may house additional sensors. The antenna elements may then be distributed in one or more ring-like configurations protectively concealed below the skin of the cylindrically or conically shaped RF sensing apparatus. A low profile antenna array made compliant to its mounting surface while preserving the overall aerodynamic configuration of the airborne vehicle, or the surface continuity of the mounting surface, is generally referred to as a conformal antenna.
Positioning forward-looking conformal antenna elements behind a fairing, radome or similar protective and electromagnetically compatible mounting structure creates a formidable set of problems. First, the individual antenna elements, distributed circumferentially around the body of the RF sensing apparatus, are substantially shielded from signals originating on the opposing side of the RF sensing apparatus. Where the emitter signal is obliquely incident on the vehicle, the vehicle body shields as many as half the individual elements composing the array. This can significantly impair the performance of a direction-finding system using an array of conformal antenna elements. The antenna elements not shielded must then be capable of acquiring a minimum number of signals to generate independent phase and/or amplitude measurements at sufficiently high signal-to-noise ratios to resolve angular ambiguities and measure the angles-of-arrival accurately. The problem is further complicated by the polarization diversity of the antenna elements in the case of a cylindrical distribution of elements introduced in the preferred embodiment below. To this end, it is desirable to efficiently arrange a large number of compact elements in a dense array configuration.
As a second problem, the nose section of the RF sensing apparatus obstructs the antenna elements aft of it from signals originating from the direction immediately in front of the nose. The conformal nature of the element therefore conflicts with the preference for an end-firing antenna array. The challenge is then to design an array having a large effective field of view that is sensitive to both off-axis signals originating from the broadside of the RF sensing apparatus, as well as signals propagating along the vehicle's centerline axis. Maintaining this degree of sensitivity across the field of view is achieved in part by mounting the antenna elements as close as practically possible to the surface of the RF sensing apparatus.
Ideally, an antenna element of a RF sensing apparatus is of high gain and provides reliable and uniform electrical performance over a wide range of frequencies. There are many such broadband antennas, including the spiral, log-periodic and traveling wave antennas, but few can be made small enough to satisfy the particular criteria necessary for missile and compact sensor suite applications. The antenna elements must lend themselves to being mounted in non-planar configurations and in sufficient number and density to acquire the signals necessary for performing direction-finding without producing significant electrical coupling between adjacent antenna elements. At the same time, an antenna element for missile ARH subsystems and other rugged, portable applications more generally, must be designed to withstand a range of demanding environmental conditions including severe shock, vibration, humidity, pressure and temperature variations.
One example of a suitable conformal antenna element is the microstrip antenna manufactured with printed-circuit technology. A typical microstrip antenna comprises a metal radiator and a ground plane separated by a dielectric layer with a thickness on the order of a tenth of a wavelength. The microstrip is then fed by a transmission line feed. While these microstrip antennas are small in volume and afford great variation in the number of elements and the array configuration, the manner of mounting or conforming the microstrip antennas to non-planar surfaces poses a challenge.
One fabrication technique for applying a microstrip antenna to a substantially curved surface involves constructing an antenna assembly from a sheet of dielectric material, then deforming the assembly to conform to the curved surface. The method as described is unsuitable for complex, curved surfaces, particularly those subject to stressing environments, because the various layers of the microstrip are under differing levels of tension/compression and are thus predisposed to delamination.
In a second method described in U.S. Pat. No. 4,816,836 to Lalezari, the fabrication of the microstrip is achieved in a two-step process in which a thicker first layer of dielectric is made to adhere to the curved surface and a second thinner layer of dielectric including the antenna circuit is shaped and secured to the first layer. Not only can the antenna element suffer from delamination, but the resulting antenna element possesses a substantially curved forward profile that gives rise to unacceptably large variations in the polarization orientation across the face of the antenna. This antenna as well as the previously described antenna and method of construction are therefore less desirable for use in stressful airborne vehicle applications.
In addition to the manufacture and installation of the individual antenna elements, a challenge remained to develop a RF sensing apparatus that exhibits the geometric and electrical uniformity necessary for implementing high-quality direction-finding. One prior art method of constructing a ring-shaped conformal array for mounting within the confines of a recessed channel in a missile system involves a three-step process. In the first step, the antenna elements with electrical connectors are pre-assembled in the shape of a ring. In the second step, the antenna elements are embedded in a compliant material such as epoxy in the shape of a ring that is severed at one point in the circumference. In the third step, the ring is expanded and the entire epoxy-embedded array slipped into the
Bonebright Mark
McNaul Bill
Rerecich Tony
Sinclair Rodney
Weatherington Jon Duff
Brooks Michael Blaine
Brooks & Fillbach
Ho Tan
Naglestad Andrew Steven
Science and Applied Technology, Inc.
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