Patent Application
20060250317
1. Field of the Invention
This invention generally relates in general to conformal antennas and more particularly, to a satellite-communication antenna that is integrated with a curved horizontal-driveshaft cover of a helicopter.
2. Description of the Related Art
Wireless communication is accomplished through use of a radio connected to an antenna. An antenna is an impedance-matching device used to absorb or radiate electromagnetic waves into the atmosphere. The function of the antenna is to “match” the impedance of the propagating medium, which is usually air or free space, to the source of the radio waves, i.e., output of the radio.
Antennas are available in many different shapes and sizes. The particular shape and size of an antenna designed for a particular application depends on many factors, such as the operating frequency range, the expected environment the antenna will endure, size limitations, shape of the structure the antenna is to be installed upon, adjacent structures and materials, power efficiency, power limitations, impedance requirements, Voltage Standing Wave Ration (VSWR) requirements, application particulars, and many more.
One common use of antennas is on vehicles, either airborne or terrestrial. An antenna can be placed on various locations on the body of the vehicle, providing communication between the vehicle and other radio-wave-receiving entities, such as handhelds, base stations, other vehicles, and more. The communication links include ground to air, air to ground, air to air, and ground to ground.
All vehicles, whether airborne or terrestrial, have a finite amount of exterior surface area, often referred to as “real estate,” in which antennas can be placed. Antennas that are installed on the exterior of these vehicles must withstand heavy torque from wind and debris, resist moisture, withstand extreme and rapid temperature changes, heavy vibrations, and other environmental hazards. Additionally, the shape of the body, especially in airborne vehicles, provides functionality. Therefore, it is advantageous for antennas mounted on airborne vehicles to not alter the shape of the body.
Helicopters are commonly-used non-fixed-wing aircraft that are able to take off and land vertically and travel at high speeds while airborne. A helicopter consists of a main body section that holds the pilot and other passengers, the engine and the main rotor. The main rotor is a blade that spins generally parallel to the ground and produces a spinning torque on the main body section opposite the direction of rotation of the rotor. To counter the torque produced by the rotor, helicopters have a tail section with a second rotor that is oriented generally perpendicular to the ground and perpendicular to the main rotor. The second rotor spins and produces a force that counteracts the spinning torque of the main rotor and prevents the main body from spinning out of control.
Connecting the engine to the second rotor is a drive shaft. The drive shaft travels above and along the length of the tail section to the second rotor. Because the driveshaft spins, it is advantageous to provide a cover over the shaft to prevent dirt, moisture, and debris from contacting the shaft. To serve this purpose, a driveshaft cover, which consists of one or more sections, is attached to the tail section on either side of the driveshaft so that it encloses the driveshaft. The driveshaft cover is generally made of fiberglass or other composite material and provides a relatively large amount of surface area or “real estate.” However, until now, the space along the driveshaft cover has not been utilized other than for simply covering the driveshaft.
Communication with those on the ground is most easily accomplished with radiating elements commonly called “monopoles” or “dipoles.” A dipole has two elements of equal size arranged in a shared axial alignment configuration with a small gap between the two elements. Each element of the dipole is fed with a charge 180 degrees out of phase from the other. In this manner, the elements will have opposite charges and common nulls. A monopole, in contrast, has only one element, but operates in conjunction with a ground plane, which mimics the missing second element. The physics of monopoles and dipoles are well known. Monopoles and dipoles, however, are efficient only for line-of-sight (LOS) communication. Obstructions such as mountains, or great distances, relative to the curve of the earth's surface, between the transmitter and receiver can prevent the reception of these signals. The relative positions of the transmitter and receiver, as well as the power output of the transmitter thus control whether the LOS signal will be received. Additionally, because of the radiation pattern of an LOS transmitter/receiver, communication between two positions that have a great variance in altitude, such as from the ground to the sky above, are not efficient with a monopole or dipole.
To overcome the effect of LOS obstacles, satellite communication (SATCOM) has been developed. Satellites are transceivers that orbit the Earth and can relay communications back and forth from any position near the earth, whether to terrestrial or airborne, or to other satellites, allowing communication virtually anywhere between the Earth's surface and the orbiting satellites.
One of the characteristics of antenna transmission is “polarization,” which describes what physical plane the signal is being transmitted in. A dipole or monopole oriented in a vertical position (perpendicular to the earth's surface) radiates signals with a vertical polarization. For a second antenna to receive maximum signal strength, it too must have a vertical orientation. As the receiving antenna is rotated away from vertical, its maximum receive power diminishes to −3 dB (50%) at 45° and 135° and −∞ dB (0%) at horizontal orientation (perpendicular to the transmit antenna).
Because satellites orbit the earth and transmit to receivers in multiple directions and orientations, single plane transmission is not efficient. Therefore, satellites transmit signals in a “circular” polarization. In this manner, the signal is transmitted in a continuous right-hand rotating orientation.
A circularly polarized antenna transmits and receives signals in a circular polarization. The antenna has two dipoles arranged orthogonal to one another. The dipoles alternate “firing” with a positive charge rotating sequentially around the four individual elements and a negative charge on its axially oppositely aligned second element. When viewed on a three-dimensional time vs. polarization graph, the circularly polarized signal resembles a helix.
The transmission path of the circularly polarized signal radiated from the two linear dipoles is substantially perpendicular to the intersecting axis of the crossed dipoles. In other words, the beam width of the crossed dipoles is relatively narrow. As the receiving antenna moves away from this perpendicular alignment relative to the dipoles, i.e., the narrow radiation beam, its maximum receive power diminishes. Therefore, these types of antennas suffer from the shortcomings of having narrow-angle maximum communication efficiency ranges.
Accordingly, a need exists to overcome the shortcomings with the prior art and to provide a dual-band/dual-element antenna with a single connector that also provides adequate isolation between frequency bands.
Briefly, in accordance with the present invention, disclosed is an antenna assembly that includes an antenna assembly having an arcuate-shaped dielectric substrate formed to fit a horizontal-driveshaft cover of a rotor assembly in a non-fixed-wing aircraft. The antenna assembly has a set of arcuate-shaped dipoles disposed on the dielectric substrate to conform to the arcuate shape thereof. The arcuate-shaped dipoles transceive an electromagnetic radiation pattern at a larger angle from a line perpendicular to a lengthwise dimension of the horizontal-driveshaft cover than an electromagnetic radiation pattern transceived by a set of substantially co-planar dipoles of a length equal to a length of the arcuate dipoles and placed in a plane parallel to the lengthwise dimension of the horizontal-driveshaft cover.
In one embodiment of the present invention, the antenna assembly is attached to a concave inside-surface of the horizontal-driveshaft cover. In another embodiment, the antenna assembly is attached to a convex outside surface of the horizontal-driveshaft cover. In yet another embodiment of the present invention, the arcuate-shaped dipoles are disposed within a layer of the horizontal-driveshaft cover.
In an exemplary embodiment of the present invention, the antenna assembly includes an electrical connector, an impedance-matching circuit, and a quadrature hybrid circuit disposed between the electrical connector and the impedance-matching circuit
In further embodiments of the present invention, the antenna assembly includes a balun assembly in electrical communication with the impedance-matching circuit
The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the present invention, in which:
While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures, in which like reference numerals are carried forward.
The present invention, according to an embodiment, overcomes problems with the prior art by providing an antenna assembly that conforms to, and becomes part of, the body of an aircraft thereby conserving space and having little or no negative effect on the aerodynamics of the aircraft. Additionally, the shape of the antenna provides an advantageously wide cross-polarized radiation pattern.
Described now is an antenna configuration, according to an exemplary embodiment of the present invention. The present invention is a conformal antenna assembly that is integrated either into or onto a portion of a horizontally oriented rear driveshaft cover for a helicopter. The present invention can also be used for other similar applications. The antenna assembly includes a set of orthogonal dipole elements etched on a circuit board or other suitable organic or inorganic medium and sandwiched by fiberglass, composite, or other suitable outer shells. The shape of the assembly is concaved so that the antenna can fit on or beneath a cover that fits around a driveshaft. The two elements are excited by a signal fed by a 90° hybrid and a feed network and matching circuitry. Additionally, the concaved shape advantageously provides a broader radiation pattern for the antenna.
With reference to
Also shown in
As is known in the art, the length of the dipoles is dependent on the intended frequency range of the antenna. Typically, elements are chosen to be ¼ or ⅛ of a wavelength of the center frequency within a band of intended frequencies. The present invention is intended to be operated in the Satellite Communication (SATCOM) frequency range, which is 244-318 MHz.
For illustrative purposes, a radiation pattern 201 of a dipole antenna, such as dipole 106 of
Continuing further,
When placed in an orthogonal orientation, as shown in
Specifically, and with reference to the element placement shown in
Turning now to
Referring back to
Located between the impedance-matching circuit 114 and radio/antenna interface 116 is a 90° quadrature hybrid 118 and a terminating load. Quadrature hybrids are circuits that separate a single input signal into two output signals with a relative phase difference, in this case, 90°. Hybrids are well known in the art. Therefore, terminations, hybrids, and particulars of such circuits will not be further discussed herein.
Connecting the 90° quadrature hybrid/terminating load 118 and the feed network/matching circuit 114 is two lengths of transmission line 122 and 124, which are preferably semi-rigid coaxial cables, such as part number UT-085, available from Micro-Coax, Inc. at 206 Jones Blvd. Pottstown, Pa. 19464-3465. The transmission lines 122 and 124 are conductive pathways that are insulated from and run within an outer conducting jacket. Coaxial cables are advantageous because they provide high levels of isolation to the signal-carrying center conductors by prevent stray electromagnetic signals from entering or exiting the conductors. Semi-rigid cables also offer the advantage of solderability to their outer jackets. The metallic outer jackets, usually made of aluminum or copper, can be securely affixed to a supporting material by soldering or spot-welding the jacket surface. The center conductor and jacket are isolated from each other by a dielectric insulating material that runs throughout the length of the cable.
Also attached to the feed network/matching circuit 114 and then to the jacket of the transmission lines 122 and 124 is a balun assembly that includes a set of baluns 126 and 128. In an exemplary embodiment, the baluns are each 7.3 inches in length, but other lengths have been shown to be used advantageously with the present invention. Baluns are well known in the art as a way of reducing the voltage standing wave ratio (VSWR) on the transmission lines. Therefore, there is no need to describe any further details of baluns herein.
Referring now to
When the conductive material 604 is energized with a varying voltage signal, electromagnetic energy is radiated from the conductive material (or in the alternative, the electromagnetic energy is collected with it) forming an antenna to enable wireless communication. As is understood in the relevant arts, the receive and transmit characteristics of RF antennas are essentially identical. It is therefore understood that references to or descriptions of either one of the receive or the transmit characteristics of an antenna apply to both the receive and transmit characteristics of that antenna.
As can be seen in
Turning to
As discussed in the preceding paragraph, the elements can be laminated directly onto or into the inner concave surface of the outer shell 704. If the circuit is attached to a circuit supporting material, the dielectric material 606 can be fastened to either of the shells 704 or 706 by using epoxy, rivets, screws, bolts, placing between layers of laminate, or other fastening means. Because the antenna assembly will be subjected to relatively high magnitude vibrations, it is preferable that the circuit 100, the circuit-supporting material 606 (if used) and the shells 704 and 706 are secured as a unit and do not move relative to one another. Any movement relative to one another can result in disconnection of electrical paths or degradation in performance of the antenna and harm to other components on the aircraft.
In one embodiment, the antenna assembly 700 is fastened to the outside surface of a vertical driveshaft cover. In this embodiment, the outside convex surface of the outer shell 704 is exposed to conditions, such as high-velocity wind, rain, sun, dust, debris, and others. Therefore, the outer shell 704 is preferably made of a durable material. Additionally, electromagnetic waves must be able to pass through the outer shell 704 to the dipoles 106 and 112 of circuit 100. Therefore, at least the outer shell 704 is selected from a material that provides a minimum amount of attenuation to the transmission of electromagnetic waves acting as a durable shell. In one embodiment, the outer shell 704 is made of fiberglass. Other suitable materials are plastic and carbon fiber, among others.
In an embodiment of the present invention, as is known in the art, strength is added to the housing 702 by providing a honeycomb material between the two shells. The honeycomb material 802 is shown in
The inventive antenna assembly 700 conforms to the curvature of the driveshaft cover 908 and can be attached, by bolting, screwing, or otherwise, the antenna assembly 700 to the outer surface of the driveshaft cover 908. As can be seen in
In another embodiment of the present invention, the antenna assembly 700 can be built into the driveshaft cover itself instead of the using the shells 704 and 706. In this embodiment, the driveshaft cover is the radome for the antenna assembly 700.
It should be noted that most driveshaft covers 908 are hinged (not shown) on one side to allow access to the driveshaft for repairs and otherwise. In one preferred embodiment, the antenna assembly 700 is installed so that the connector 116 is on the same side of the driveshaft cover 908 as the hinge. This will prevent the connecting cable, which connects a radio to the antenna 700 via the connector 116, from spanning the opening when the driveshaft cover 908 is opened to access the driveshaft 904.
In another embodiment of the present invention, the shells 704 and 706 forming the housing 702 are not separate pieces, but are instead one continuous covering for protecting and containing the circuit 100 and other components. In this case, the term “shells” refers to at least a first and a second surface of the continuous covering. In yet another embodiment of the present invention, each of the shells 704 and 706 are made of sub-pieces or component coverings. Therefore, the shells 704 and 706 are not limited to any number of individual pieces, but instead refer any configuration, material, or device that provides protection for the antenna elements.
As an effect of the curved dipoles 106 and 112, the radiation pattern of the antenna 700 has been broadened, as shown in
A second radiation pattern 1006, is also shown in
It should be clear from the above description that the present invention can be used for transmitting as well as receiving. While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.
