This invention relates generally to antennae, and, more particularly, to an antenna with a low profile high aspect ratio reflector, focal point less than 12″, a surface suitable for Ka-band transmit and receive operations, and a feed with stacked septum polarizers that provide high gain and low sidelobes.
Constructed of material that minimally attenuates an electromagnetic signal transmitted or received by an antenna, an aircraft radome provides a radiolucent (i.e. rf transparent) weatherproof aerodynamic fairing. Radomes often appear as dome-like blisters on the fuselage of aircraft. To minimize drag, such radomes typically have a low profile, providing extremely limited space for an antenna and its related equipment. A contained antenna may be used for air to ground, satellite or aircraft-to-aircraft communication.
Heretofore, antennae having parabolic reflectors have been avoided in such aircraft radome applications, due to space constraints. Such limited space makes it difficult to illuminate the entire reflector surface, when the aspect ratio of the reflector is high and the feed is positioned only a short distance away.
Forgoing antennae having parabolic reflectors, antenna arrays comprising an array of flat panels have been devised to fit within aircraft radomes. While such high aspect ratio antennas provide adequate gain to close a communication link, they experience low aperture efficiencies and high sidelobe levels and grating lobes in some locations. High sidelobes and grating lobes are undesirable features of the solutions, with sidelobes representing unwanted radiation in undesired directions becoming a potential cause of interference and reduced signal to noise ratio. A spatial aliasing effect may cause some sidelobes to become substantially larger in amplitude, and approaching the level of the main lobe. Such sidelobes are called grating lobes. The efficiency of these antennas can be lowered due to the losses of beamformer networks required to feed the array elements.
By way of example, in-flight broadband connectivity and wireless in-flight entertainment for commercial aircraft, including Southwest Airlines, is provided using a Ku-band antenna system, comprised of a flat panel array, mounted atop the aircraft fuselage and encased within an RF-transparent radome. Such antenna systems communicate with geostationary satellites, allowing uninterrupted in-flight Wi-Fi service over water and on airlines' routes virtually anywhere in the world. Low aperture efficiencies, high sidelobes, grating lobes and losses of beamformer networks, and heavy weights compromise performance of such antenna systems.
The invention is directed to overcoming one or more of the problems and solving one or more of the needs as set forth above.
To solve one or more of the problems set forth above, in an exemplary implementation of the invention, an antenna assembly for an aircraft radome includes a reflector, a feed with a stacked array of septum polarizers, a mount, and a positioner. The feed, which includes bandpass and low pass filters, and broadwall branchline couplers, and the stacked array of septum polarizers, together determine polarization of the waves, attenuating unwanted signals and illuminating the entire reflector. The reflector comprises a portion of a parabolic reflective surface.
Focusing on an exemplary feed for a low profile antenna according to principles of the invention, an array of septum polarizers, namely, at least a first septum polarizer and a second septum polarizer, is provided. The first septum polarizer has a first stepped septum in a first feed horn port, and the second septum polarizer has a second stepped septum in a second feed horn port. The second feed horn port is adjacent and parallel to the first feed horn port. The first stepped septum is orthogonal to the second stepped septum.
A manifold is operably coupled to the first feed horn port and the second feed horn port. The manifold has a left transmit port permitting left circular polarization transmission, a right transmit port permitting right circular polarization transmission, a left receive port permitting left circular polarization reception and a right receive port permitting right circular polarization reception. The left receive port and left transmit port are coupled to the first feed horn port by a first left communication path including a first left waveguide. The right receive port and right transmit port are coupled to the first feed horn port by a first right communication path including a first right waveguide.
Filters separate transmit and receive frequencies. A left stub loaded bandpass filter is operably coupled to the left receive port. The left stub loaded bandpass filter filters (i.e., removes) transmit frequencies. A right stub loaded bandpass filter is operably coupled to the right receive port. The right stub loaded bandpass filter filters transmit frequencies.
A left tee junction and a right tee junction are also provided. The left transmit port is coupled to the first left waveguide by the left tee-junction. The right transmit port is coupled to the first right waveguide by the right tee-junction. A left low pass filter between the left tee junction and left transmit port filters receive frequencies communicated through the left tee junction before reaching the left transmit port. Similarly, a right low pass filter between the right tee junction and right transmit port filters receive frequencies communicated through the right tee junction before reaching the right transmit port.
A second left waveguide and a second right waveguide are provided. The second left waveguide is joined to the first left waveguide by a left splitter. The second right waveguide is joined to the first right waveguide by a right splitter. The left and right splitters may comprise left and right broadwall couplers, respectively.
The second waveguides have terminated ends. Specifically, the second left waveguide has a left port end extending to the second feedhorn port and a loaded terminated end opposite the left port end. Similarly, the second right waveguide has a right port end extending to the first feedhorn port and a loaded terminated end opposite the right port end.
The feed, which has a phase center, may be used with a single reflector, or with multiple reflectors, such as in a Gregorian or Cassegrain configuration. When used with a single reflector, the phase center of the feed is located at about the focal point of the reflector. The reflector is less than twelve inches, preferably about nine inches, from the feed. The feed and reflector may be contained in a low profile aircraft radome. An exemplary reflector has a reflective surface shape comprising a portion of a parabola having a ratio of focal length to diameter equal to about 0.269 and a diameter equal to about 33.5″.
Dimensional changes may be made to the waveguides to accommodate signals having different frequency bands. For example, in one exemplary implementation, the waveguides may be generally rectangular and have a 2:1 aspect ratio, i.e., a broad wall that is twice the dimension of the narrow wall, or very nearly so. The greater the size of the waveguide, the lower the frequency it communicates effectively. A rectangular waveguide having dimensions of approximately 0.622×0.311 inches may be suitable for Ku band communications operating at 12.4 to 18.0 GHz, while a waveguide having dimensions of approximately 0.280×0.140 inches may be suitable for Ka band communications operating at 26.5 to 40.0 GHz. In the case of a circular waveguide, the diameter may be about 0.500 to 0.688 inches for Ku band (12.4 to 18.0 GHz), and 0.219 to 0.315 inches for Ka band (26.5 to 40 GHz), depending upon the particular frequency.
In an implementation with two reflectors, including a primary reflector and a secondary reflector, the secondary reflector is in front of and aimed at the primary reflector, and the feed is behind the primary reflector and aimed at the secondary reflector. The secondary reflector may be a concave or convex reflector.
The foregoing and other aspects, objects, features and advantages of the invention will become better understood with reference to the following description, appended claims, and accompanying drawings, where:
Those skilled in the art will appreciate that the figures are not intended to be drawn to any particular scale; nor are the figures intended to illustrate every embodiment of the invention. The invention is not limited to the exemplary embodiments depicted in the figures or the specific components, configurations, shapes, relative sizes, ornamental aspects or proportions as shown in the figures.
The feed arm 105 is designed for feed 120 focusing and optimization. The feed arm 105 is a mechanical coupling to which the feed 120 and reflector 110 are attached. A positioner 130 connects the feed 120 to the feed arm 105. The feed arm 105 positions the phase center of the feed 120 at the focal point of the reflector 110.
RF energy from the feed 120 located at the reflector's focal point, reflects from the reflector 110 as narrow beams through parallel paths. All reflected rays from the feed 120 to a common plane have the same path length, create a coherent beam, and arrive in-phase. Characteristics of the incoming rays, are the reciprocal of the behavior of the reflected rays. The exemplary reflector 110 comprises an upper portion from a specific area of a larger parabolic surface, to optimize the RF performance and minimize RF blockage. More specifically, the exemplary reflector 110 comprises a portion from a larger parabolic surface. In one embodiment, the surface has a ratio of focal length to diameter (F/D) equal to about 0.269 and a diameter equal to about 33.5″, to avoid blockage and reduce unwanted scattering. The F/D and diameter could be varied for a particular set of constraints. The overall reflector geometry is shaped to fit within the space available in the radome of an aircraft including azimuth and elevation travel ranges. The phase center of the feed 120 is located at the focal point of the reflector 110.
In the perspective view of
Referring now to
Now, with reference to
A pair of waveguide broadwall couplers 230, 235 are provided. Two waveguides (i.e., branchlines) share a broad wall on each side (LH and RH) of the manifold. Holes are provided in the broad wall to couple the waveguides. In an exemplary implementation, the holes may be ¼ wave apart. In a forward case the coupled signals add, in the reverse they may subtract (180 apart) and disappear. A right hand waveguide broadwall coupler 230 couples branchlines 231 and 232. A left hand waveguide broadwall coupler 235 couples branchlines 236 and 237.
An e-plane waveguide tee junction 207, 242 connects each branchline 232, 237 to a transmit port, LHCP TX port 127 and RHCP TX port 129 via waveguide transitions 250, 255. One tee junction 207 is provided on the left side and the other 242 on the right side.
A waveguide stub loaded bandpass filter 240, 245 is attached to each receive port 140, 145 to reject transmit (TX) frequencies. Specifically, filter 240 filters transmit frequencies for port 140, while filter 245 filters transmit frequencies for port 145.
Waveguide transitions 250, 255, each comprise a waveguide squeeze section low pass filter for each transmit port 129, 127 to reject receive (RX) frequencies. Specifically, filter 250, filters receive frequencies for port 129, while filter 255 filters transmit frequencies for port 127.
Branchlines 231 and 236 lead, at one end, to loaded terminated ports 200, 205, respectively. At the opposite ends, the branchlines 231, 236 lead to interfaces 210, 215, respectively. Thus, the feed 120 provides two left hand and two right hand radiating elements, including one left hand and one right hand radiating element for each orthogonal septum polarizer.
Now referring to
The waveguides may be sized and shaped to accommodate signals having different frequency bands. For example, in one exemplary implementation, the waveguides may be generally rectangular and have a 2:1 aspect ratio, i.e., a broad wall that is twice the dimension of the narrow wall, or very nearly so. The greater the size of the waveguide, the lower the frequency it communicates effectively. A rectangular waveguide having dimensions of approximately 0.622×0.311 inches may be suitable for Ku band communications operating at 12.4 to 18.0 GHz, while a waveguide having dimensions of approximately 0.280×0.140 inches may be suitable for Ka band communications operating at 26.5 to 40.0 GHz. In the case of a circular waveguide, the diameter may be about 0.500 to 0.688 inches for Ku band (12.4 to 18.0 GHz), and 0.219 to 0.315 inches for Ka band (26.5 to 40 GHz), depending upon the particular frequency.
A stacked septum polarizer and feed for a low profile reflector according to principles of the invention may be utilized in single reflector (e.g.,
As another non-limiting example,
Reflectors according to principles of the invention may be comprised of any suitable materials, including, but not limited to aluminum and graphite composite. By way of example, a light-weight graphite composite reflector may be utilized to facilitate positioning. Such a reflector may comprise a lightweight core, such as of honeycomb material, having graphite composite fabric reflector layers bonded to the opposed faces of the core. Such a reflector may constructed by laminating thin composite fabric reflector layers to a central reinforcing core of conventional lightweight honeycomb material, such as paper fiberboard, heat-resistant plastic, aluminum alloy, etc., by means of curable adhesive layers. The graphite composite reflective layers are comprised of graphite encapsulated within a cured plastic composition, such as polycyanate ester resin, epoxy resin or other curable polymer resin conventionally used to form fiber-reinforced composite fabrics conventionally used in the aviation industry. The various layers are superposed and heat-bonded to form a reflector laminate. The honeycomb core is formed in the desired size, shape or curvature, whereupon the outer reflector layers will conform to the surface shapes of the core to form the reflector. In alternative embodiments, a structural core may be comprised of laid up layers of graphite composite or other structural material suitable for fiber-reinforced composite fabrics construction in the aviation industry.
While an exemplary embodiment of the invention has been described, it should be apparent that modifications and variations thereto are possible, all of which fall within the true spirit and scope of the invention. With respect to the above description then, it is to be realized that the optimum relationships for the components and steps of the invention, including variations in order, form, content, function and manner of operation, are deemed readily apparent and obvious to one skilled in the art, and all equivalent relationships to those illustrated in the drawings and described in the specification are intended to be encompassed by the present invention. The above description and drawings are illustrative of modifications that can be made without departing from the present invention, the scope of which is to be limited only by the following claims. Therefore, the foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents are intended to fall within the scope of the invention as claimed.
This application is a non-provisional and claims the benefit of priority of U.S. provisional application 62/026,600 entitled “Stacked Septum Polarizer and Feed for a Low Profile Reflector.”
Number | Date | Country | |
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62026600 | Jul 2014 | US |