Patent Application
20100099370
1. Field of the Invention
The present invention relates to radio frequency electronics. More specifically, the present invention relates to electronically scanned array antennas for satellite communications.
2. Description of Related Art
Conventional satellite communication antennas have typically relied on mechanical steering approaches using a “dish” antenna to establish and maintain a link with a satellite. A dish antenna typically includes a parabolic reflector dish and a feed element that couples RF (radio frequency) signals between the reflector dish and a modem. The modem modulates data onto a carrier signal to provide a signal to be transmitted to the satellite by the antenna, and also demodulates a signal received from the satellite to extract encoded data.
For “communications on the move” or mobile applications in which the antenna is located on a moving platform such as a ground vehicle, airplane, or ship, the antenna needs to be capable of scanning in different directions in order to locate and then follow a satellite as the platform moves. This is typically accomplished by mounting the dish antenna on a gimbal and mechanically steering the gimbal to point the antenna in the desired direction.
When it is desired to communicate with a satellite from a vehicle that is moving, the use of mechanically steered dish antennas presents a variety of mechanical problems related to the motion of the vehicle over rough roads and uneven terrain, or during periods of high maneuverability. Stabilization techniques are commonly used that place the antenna on a platform that is mechanically stabilized; however, these approaches often can not provide the stability required in highly dynamic maneuvers on uneven terrain, and also add cost and complexity to the system.
Mechanically steered antennas also include gimbal mechanisms, such as mechanical servos, drive motors, gears, drive belts, etc., that typically require significant amounts of time and expense for maintenance and may also break when subject to erratic movement. In addition, conventional dish antennas are typically large and bulky, making them more visible to radar detection.
An alternative to the conventional dish antenna is an electronically scanned array (ESA) or phased array antenna. An ESA includes an array of several individual radiating antenna elements whose relative phases are controlled such that the overall beam from the array radiates in a particular direction due to constructive and destructive interference between the individual elements. Phased arrays are typically low profile, robust to movement, and are capable of switching beam directions in fractions of a millisecond. However, conventional ESA antennas, which have been used predominantly in radar applications, are typically not suitable for use in mobile satellite communications applications due to their large size, heavy weight, and high cost.
Prior attempts at adapting ESA antennas for satellite communications have used passive ESAs in which the entire antenna array is driven by, and interfaces with a modem through the use of intermediary single interface elements such as, a low noise amplifier (LNA), a high power amplifier (HPA), and a diplexer. These external elements are typically large and costly, and create a single point of failure for the system in that failure of one of these elements renders the passive ESA antenna unusable.
Hence, a need exists in the art for an improved antenna for on-the-move satellite communications that offers low profile, smaller size, and lower cost than prior approaches.
The need in the art is addressed by the electronically scanned array antenna of the present invention. The novel antenna includes a first planar array of antenna elements and one or more side planar arrays of antenna elements, each side array adjacent to the first array and tilted at a predetermined angle relative to the first array. In an illustrative embodiment, the antenna also includes a plurality of transmit/receive modules, each module coupled to one antenna element and including a receive circuit and a transmit circuit. Each receive circuit includes a low noise amplifier adapted to receive a first channel enable control signal and in accordance therewith amplify a signal received from the antenna element, and a first phase shifter adapted to receive a first phase control signal and in accordance therewith vary a phase of the received signal. Each transmit circuit includes a high power amplifier adapted to receive a second channel enable control signal and in accordance therewith amplify a transmit signal for transmission by the antenna element, and a second phase shifter adapted to receive a second phase control signal and in accordance therewith vary a phase of the transmit signal. In an illustrative embodiment, a processor provides individual phase and channel enable control signals for independently controlling the phase shifters and amplifiers, respectively, of each module.
a is a simplified three-dimensional diagram of an antenna designed in accordance with an illustrative embodiment of the present invention.
b is a cross-sectional side view of the illustrative antenna of
a is a simplified block diagram of an integrated antenna/circuit module designed in accordance with an illustrative embodiment of the present invention.
b is a simplified cross-sectional diagram of an integrated antenna/circuit module designed in accordance with an illustrative embodiment of the present invention.
a is a three-dimensional view of a subarray antenna/circuit module designed in accordance with an illustrative embodiment of the present invention.
b is an exploded view of a subarray antenna/circuit module designed in accordance with an illustrative embodiment of the present invention.
Illustrative embodiments and exemplary applications will now be described with reference to the accompanying drawings to disclose the advantageous teachings of the present invention.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
The present invention provides a novel antenna for satellite communications that uses an active electronically scanned array (ESA), or phased array. Unlike dish antennas that use mechanical servos and drive motors to steer the dish antenna to the desired angle, a phased array steers the transmit/receive beam by independently controlling the phase relationships of the active radiating elements of the array. Because phased array antenna beam patterns can be switched in fractions of a millisecond, the antenna can lock onto a satellite channel and maintain lock even if the antenna is mounted on a vehicle that is moving across uneven terrain or performing highly dynamic maneuvers.
The novel antenna design of the present teachings provides a thin, flat antenna (nominally less than two inches in height) that can maintain coverage over nearly an entire hemisphere without any moving parts in a low profile package that greatly reduces visibility as compared to conventional satellite dishes.
In a preferred embodiment, the novel antenna is adapted for use in satellite communications. In an illustrative embodiment, the antenna is designed for use at L-band frequencies appropriate for communicating with the INMARSAT I-4 satellite network. The novel antenna array is a full duplex, single aperture antenna allowing for simultaneous receive and transmit through the use of frequency multiplexing, and fully active, providing independently controlled transmit and receive channels for each radiating element. This allows the antenna to receive and transmit in different directions at the same time, consistent with satellite architecture.
a is a simplified diagram showing a three-dimensional view of an antenna 10 designed in accordance with an illustrative embodiment of the present invention. The novel antenna 10 is an ESA having a unique “carapace” design comprised of five sections: a top, center section 12, and four side sections 14A, 14B, 14C, and 14D that are adjacent to the center section 12 and tilted relative to the center section 12. The center section 12 includes a flat, planar (two-dimensional) array of patch antenna elements 20. In the illustrative embodiment of
The center section 12 is surrounded on all four sides by a side section 14. Each side section 14 includes a smaller (relative to the center section 12) two-dimensional planar array of patch antenna elements 20, and each side section 14 is tilted at a particular angle φ relative to the center section 12.
In an illustrative embodiment suitable for L-band communications, each radiating element 20 is a square patch having sides of approximately 3″. The center section 12 is therefore about 12″ square, each side section 14 is approximately 12″×6″ (l=12″ and w=6″ in
The angle φ is chosen such that the overall antenna 10 provides sufficient coverage for the desired application. The amount of coverage needed depends on where the antenna is located and the relative position of the satellite 16 to the antenna. In an illustrative embodiment, the antenna 10 is designed to cover the near full upper hemisphere such that it can connect to the INMARSAT satellite network from almost anywhere in the world. In an illustrative embodiment, the top section 12 with its planar array alone (without the arrays of the side sections 14) can communicate with a satellite 16 that is at an elevation θ of 30° above the horizon or higher using active electronic beam steering. The addition of an array in a side section 14 increases the coverage of the antenna resulting from a combination of the increased number of aperture elements and the tilt angle φ of the section 14. For example, a side section 14 tilted at an angle φ of 45° will increase coverage of the antenna 10 by nearly 30°. In a preferred embodiment, each side section 14 is tilted at an angle φ of 45° relative to the center section 12 such that the overall antenna 10 can communicate with any satellite approximately 5 degrees above horizon level (near full upper hemisphere coverage), consistent with a satellite having line of sight access to the antenna.
All of the antenna elements 20 may not be in use at the same time. In an illustrative embodiment, only the elements 20 in the center section 12 and the elements 20 in up to two side sections 14 are operating at any given time. Thus, if the center section 12 includes sixteen elements and each side section 14 includes eight elements, only thirty-two or fewer elements are operating at any given time. Which antenna elements 20 are turned on is dependent on the location (elevation and azimuth) of the antenna relative to the fixed satellite 16 location. If the satellite 16 has an elevation θ of 30° or higher above the horizon relative to the antenna 10, then the antenna 10 can communicate with the satellite 16 by using only the elements 20 in the center section 12 (the antenna elements 20 in the side sections 14 are turned off). If the satellite 16 has an elevation θ less than 30° above the horizon and an azimuth aligned with one of the side sections 14, then the antenna elements 20 in the center section 12 and in that particular side section 14 are turned on (the antenna elements 20 in the other side sections 14 are turned off). If the satellite 16 has an elevation θ less than 30° above the horizon and an azimuth between two of the side sections 14, then the antenna elements 20 in the center section 12 and in the two particular side sections 14 are turned on (the antenna elements 20 in the other side sections 14 are turned off).
In operation, the phase of each antenna element 20 is varied by control electronics to steer the transmit and receive beams of the overall antenna 10 resulting in electronic beam steering. In accordance with the present teachings, the electronics for controlling and driving the antenna elements 20 are located directly beneath the radiating elements 20 and integrated with the antenna patches 20 to form a compact, integrated antenna/circuit module.
a is a simplified block diagram of a single integrated antenna/circuit module 18 designed in accordance with an illustrative embodiment of the present invention. The antenna/circuit module 18 includes a transmit/receive (T/R) circuit 30 coupled to an individual antenna radiating element 20 for controlling and driving the radiating patch 20. In accordance with the present teachings, a separate T/R module 30 is coupled to each radiating element 20 of the antenna array 10.
In a preferred embodiment, the T/R module 30 includes independently controlled receive and transmit channels 32 and 34, respectively, allowing the overall antenna receive and transmit beams to be pointed in different directions at the same time (allowing, for example, the antenna 10 to transmit data to one satellite while receiving data from a different satellite consistent with satellite architectures and operating frequencies). A diplexer 36 couples both the receive channel 32 and transmit channel 34 to the radiator element 20. The diplexer 36 implements frequency multiplexing such that signals in a first frequency band are coupled between the radiator 20 and the receive channel 32 while signals in a second frequency band are coupled between the radiator 20 and the transmit channel 34. This provides a full duplex system that can receive and transmit signals simultaneously. In an illustrative embodiment, the diplexer 36 is compatible with the transmit and receive frequency bands of the INMARSAT satellite network.
The receive channel 32 includes a phase shifter 40 for actively controlling the phase of a received signal from the radiating element 20. The phase shifter 40 also receives a control signal, labeled Rec. Phase in
The receive channel 32 also includes a low noise amplifier (LNA) 42 for amplifying a signal received from the radiator 20 (after filtering by the diplexer 36). After traveling the significant distance between the satellite and the antenna, a received signal is typically at a very low level and should be amplified by an LNA before being demodulated. In accordance with the present teachings, the LNA 42 is connected directly to the diplexer 36, as close to the radiating element 20 as possible in order to reduce system noise and provide the highest G/T (the ratio of antenna gain G to noise equivalent temperature T), thereby allowing for a smaller overall antenna size (given a desired G/T). Optionally, the receive channel 32 may also include a driver amplifier 44 connected in series with the LNA 42 between the diplexer 36 and the phase shifter 40. In the illustrative embodiment, the LNA 42 and driver amplifier 44 are both coupled to a voltage supply (a+5 V supply is shown in
The transmit channel 34 includes a phase shifter 50 for actively controlling the phase of the transmitted signal from the radiating element 20. The input to the phase shifter 50 is the signal to be transmitted, which is provided by an RF distribution board that splits the transmit signal (provided by a modem) and sends the same signal—the same in both amplitude and phase—to each of the T/R modules 30 of the array 10. The phase shifter 50 also receives a control signal, labeled Tx; Phase in
The transmit channel 34 also includes a high power amplifier (HPA) 52 for amplifying the phase shifted signal output from the transmit phase shifter 50 to a power level appropriate for transmission. The amplified transmit signal output by the HPA 52 is coupled to the radiator 20 by the diplexer 36. In accordance with the present teachings, the HPA 52 is connected directly to the diplexer 36, as close to the radiating element 20 as possible in order to reduce loss in the system. Optionally, the transmit channel 34 may also include a driver amplifier 54 connected in series with the HPA 52 between the diplexer 36 and the phase shifter 50. In the illustrative embodiment, the HPA 52 and driver amplifier 54 are both coupled to a voltage supply (a+5 V supply is shown in
In a preferred embodiment, the radiator patch 20 is aperture coupled to the T/R module 30, providing a connector-less integration with the T/R module 30.
Returning to
In a preferred embodiment, the antenna 10 also includes a serial to parallel interface 94 for coupling control signals (such as Tx. Phase, Rec. Phase, Tx. Enable, and Rec. Enable) to each T/R module 30A-30N. A computer or processor 96 provides the control signals via a serial input/output (to minimize the number of control leads). The serial to parallel interface 94, which may be implemented, for example, using a plurality of serially connected shift registers, then sends the control signals to the T/R modules 30A-30N in parallel. In a preferred embodiment, the serial to parallel interface 94 is implemented as part of the circuit board containing the T/R modules to reduce the number of connectors between different parts of the system 100.
The processor 96 includes software for determining the receive and transmit phases of each antenna element 20 and providing the appropriate control signals (Tx. Phase, Rec. Phase). Separate control signals are provided for each antenna element 20. Thus, the processor 96 provides N Tx. Phase control signals (labeled Tx. PhaseA-Tx. PhaseN in
The desired direction of the transmit/receive beams may be controlled manually by the user, or the processor 96 may instruct the antenna 10 to search for the desired satellite, scanning in different directions (by varying the relative phases of the antenna elements) until a signal lock (based on, for example, received signal strength) is found. Alternatively, in a preferred embodiment, the processor 96 may include software for determining the direction of a satellite based on the known location of a satellite and the location and orientation of the antenna 10, which may be obtained using, for example, a GPS (global positioning system) receiver, a tilt sensor, and a north finding module. An illustrative method for determining the relative direction of a satellite using a GPS receiver and orientation sensors is disclosed in a patent application entitled “Method and System for Controlling the Direction of an Antenna Beam”, filed ______, by R. W. Nichols et al. (Atty. Docket No. PD 07E007), the teachings of which are incorporated herein by reference.
The processor 96 may also include software for determining which antenna elements 20 should be on or off at any given time and providing the appropriate control signals (Tx. Enable, Rec. Enable). Separate control signals are provided for each antenna element. Thus, the processor 96 provides N Tx. Enable control signals (labeled Tx. EnableA-Tx. EnableN in
In a preferred embodiment, the antenna array 10 is implemented using a modular design, with a basic module comprising a 2×2 subarray of four radiating elements and associated drive and control electronics.
b is an exploded view of a subarray antenna/circuit module 110 designed in accordance with an illustrative embodiment of the present invention, showing the different layers of the module 110. The antenna/circuit module 110 is implemented using a tile architecture to provide a lower profile and integration of the patch antenna and T/R circuitry. In the illustrative embodiment, the subarray module 110 includes four patch radiators 20 etched on a patch substrate 22 and a printed circuit board 28 mounted parallel to the patch substrate 22. A ground plane 24 (with apertures located beneath each radiator 20) is disposed on a first side of the circuit board 28 (closest to the patch substrate 22), and the drive and control electronics for each radiator 20 are populated on the opposite side of the board 28. In an illustrative embodiment, a foam spacer 112 is placed between the patch substrate 22 and the ground plane 24. The foam spacer 112 provides a “near-air” dielectric to space the patches 20 away from the ground plane 24. Air provides the broadest bandwidth but comes at the cost of maximum height. A higher-dielectric material would lower the height but reduce the bandwidth. An alternative method would be to use stand-offs, but the foam has the advantage of providing more structure and displacing air and its associated moisture.
The electronics on the board 28 include four T/R modules 30 and the aperture coupled transmission lines 60 as shown in
The integrated patch antenna 20 and circuit board 28 are mounted on a modular frame 114, which provides structural support for the assembly. The module 110 may also include a 4 to 1 RF combiner board 82, which combines the received signals from each of the four T/R modules 30 to form one RF output signal, and an RF distribution board 92, which receives an RF transmit signal (from the modem 92) and distributes it to the four T/R modules 30. Thus, in this embodiment, the subarray module 110 has one RF input and one RF output. The module 110 may also include shielding 116 for protecting the antenna circuitry from electromagnetic interference.
A flat sheet of metal 118 provides a back cover for the module 110, and a radome 120 may also be provided to protect the radiator elements 20. In the embodiment of
A plurality of 2×2 subarray modules 110 as shown in
A manifold/aperture feed circuit board 132 is also attached to the support frame 104. The manifold 106 includes RF distribution circuits for receiving an RF signal from a modem 92 and distributing the signal to each of the T/R modules 30 of the antenna/circuit modules 110. The manifold 132 also includes RF combiner circuits for receiving RF signals from each of the T/R modules 30 and combining them to form a single RF signal that is sent to the input port of the modem 92 (as shown in
A flat metal sheet 134 provides a base for the antenna structure 10, and a radome 136 provides a protective cover over the patch antennas of the antenna/circuit modules 110. The antenna 10 may also include a power supply 138, such as a battery, housed in the hollow space above the base 134 for providing power to the various electronic components. The space above the base 134 may also be adapted to house the modem 92. The modem 92 may be connected to a user data terminal (such as a computer or laptop) via, for example, an Ethernet or WiFi connection. The antenna 10 may also include a serial connector for coupling control signals from the user computer or other processor to the antenna/circuit modules 110 as described above with reference to
Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art, and access to the present teachings will recognize additional modifications, applications and embodiments within the scope thereof.
It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention.
Accordingly,
