Wireless power transmission system and method

A wireless power transmission system including at least one source of electromagnetic radiation, a plurality of wireless power receivers that receive radiated electromagnetic energy, a beacon collocated with each wireless power receiver, wherein the beacon generates and radiates a pilot signal when the beacon is in an active state, and an array of transmitting antennas connected to the source of electromagnetic radiation that radiates the electromagnetic radiation in the direction of the beacon in the active state. The electromagnetic radiation can be electronically steered from one wireless power receiver to another by activating and deactivating the beacons collocated with each wireless power receiver.

FIELD OF THE INVENTION

The present invention generally relates to the generation, transmission, and reception of wirelessly-transmitted power, and more specifically, to wireless power transmission systems capable of efficiently illuminating multiple dispersed wireless power receivers.

DESCRIPTION OF THE RELATED ART

Wireless power transmission is of increasing interest for both military and commercial applications. Potential applications include power transmission from space to earth and microwave-powered aircraft, whose applications include communications, reconnaissance, surveillance, and remote sensing (for a general discussion, see “The History of Power Transmission by Radio Waves” by William C. Brown, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-32, No. 9, pp. 1230-1242, September 1984).

Some applications require the use of multiple dispersed wireless power receivers (WPRs). A broad-beam illumination of all receivers simultaneously can be implemented with a wide-beam antenna fed by a high-power radio frequency (RF) source. Broad-beam illumination, however, is a highly inefficient method of illumination because most of the radiated power falls between the receivers and is wasted. Narrow-beam illumination can be used to efficiently illuminate a single receiver, but very little power is delivered to receivers outside the beam width of the transmitting antenna.

In the context of solar power satellites, Arndt and Kerwin (see “Multiple Beam Microwave Systems for the Solar Power Satellite” by G. D. Arndt and E. M. Kerwin, Space Solar Power Review, Vol. 3, pp. 301-315, 1982) have proposed a multiple-beam system by which an orbiting solar power satellite beams power to multiple receiving sites on the earth at the same time. This type of system requires a large phased array antenna having an illumination function that is the complex sum of the individual illumination functions required to generate each separate beam. As a result, many array elements may be required to operate at less than full output power. Because the efficiency of many microwave power devices (both solid state and vacuum electron devices) falls off dramatically when operated at less than full power, this approach may not provide enough power to allow the system to operate with a high efficiency.

Improvements over such wireless power transmission systems would generally be desirable.

SUMMARY OF THE INVENTION

To overcome at least the drawbacks of other wireless power transmission systems, a system for wireless delivery of electromagnetic energy to multiple dispersed wireless power receivers (also referred to as “WPRs”) is provided. According to the system described below, multiple dispersed receivers can be illuminated with minimum energy falling in the space between receivers.

The system sequentially illuminates a set of dispersed wireless power receivers with a single narrow beam of radio-frequency (RF) radiation at one frequency (also referred to as an “illumination frequency”) from an array of transmitting antennas, which may, for example, be a retrodirective antenna array (also referred to as an “RDA”). Each WPR is collocated with an RF beacon. When activated, the beacon transmits a pilot signal at another frequency (also referred to as a “pilot signal frequency”), which may be the same or different from the illumination frequency.

Each element in the RDA includes a receiver that receives the pilot signal. Each receiver includes phase conjugating circuitry that extracts the phase from the pilot signal at the pilot signal frequency and forms a conjugated signal at the illumination frequency. The conjugated signal is then amplified by an amplifier and directed to an array of transmitting antennas. Each element of the RDA radiates an amplified beam whose polarization is substantially the same as that radiated by all other elements relative to a Cartesian coordinate system common to all elements of the RDA. By conjugating the phase extracted from the pilot signal, each element of the RDA radiates an amplified beam that is in phase with those beams radiated by all other elements of the RDA at the location of the activated beacon.

The amplified signals from each element in the RDA converge at the activated beacon. Because the amplified signals are of substantially the same polarization and carry the conjugate of the phase extracted from the pilot signal, the amplified signals combine in phase or collectively accumulate to yield an electromagnetic field whose amplitude is about, or nearly, N times greater than the electromagnetic field generated by a single element, where N is the number of elements in the RDA. The power density, therefore, is about N2times greater than the power density due to a single element of the RDA.

If the system includes multiple WPRs, each is equipped with a beacon that transmits a pilot signal at a pilot signal frequency when activated. By activating and deactivating the different beacons, the beam of electromagnetic energy at the illumination frequency from the RDA is steered electronically from one WPR to another. Furthermore, no mechanical steering is required as long as all WPRs reside within the beam width of each array element, which may be a 3 dB beam width, for example. Any number of WPRs can be illuminated in any order, and the time for which the beam dwells on each WPR can be chosen by an operator, calculated based upon the power needs and power received by each WPR, or based upon a schedule.

It will be appreciated that the elements in the RDA also may be moved mechanically or steered to point at the target, for example, if the target is moving over a large area or if the WPRs are widely dispersed. The target object(s), therefore, may include a tracking beacon that radiates a tracking signal indicative of the current location of the object or the active beacon. The elements in the RDA accordingly can be moved such that they are always aimed at the target object.

The elements in the RDA may be phased array antennas whose beams can be electronically steered to point at the target, for example, if the target is moving over a large area or if the WPRs are widely dispersed. The tracking beacon radiates a tracking signal indicative of the current location of the object or the location of the active beacon. With target location information derived from the tracking signal, the beams radiated by the phased-array elements in the RDA can accordingly be steered electronically such that they are always aimed at the target object. Furthermore, electronic and mechanical steering can be combined so that if the target object lies outside the field of view of one or more elements of the RDA, those elements can be mechanically steered to point at the target object.

According to an aspect of the invention, a switched-beam wireless power transmission system includes at least one electromagnetic radiation source, a plurality of wireless power receivers each collocated with a beacon that radiates a pilot signal when the beacon is in an active state, and an array of transmitting antennas that receives the pilot signal from the active beacon and radiates the electromagnetic radiation from the at least one electromagnetic radiation source in the direction of the wireless power receiver collocated with the beacon in the active state. The direction of the radiated electromagnetic radiation is switched among the wireless power receivers based upon which beacon is in an active state.

According to another aspect, a method of wirelessly transmitting power from a retrodirective array to an array of wireless power receivers includes: (i) activating a beacon on a wireless power receiver by radiating a pilot signal with the beacon; (ii) receiving the pilot signal, extracting the phase from the pilot signal, and forming a conjugated signal with a receiving unit on each element in the retrodirective array; (iii) amplifying each conjugated signal with an amplifier; (iv) radiating each amplified signal in the direction of the activated beacon with a transmitting antenna on each element in the retrodirective array; (v) receiving the amplified signal with the wireless power receiver that is collocated with the activated beacon; (vi) deactivating the activated beacon by extinguishing the pilot signal and repeating steps (i)-(v) for a beacon on a different wireless power receiver.

According to another aspect, a switched-beam wireless power transmission system includes one or more amplifying sources of electromagnetic radiation, an array of transmitting antennas corresponding to each amplifying source, a transmission line that transports the electromagnetic radiation from each source to each corresponding transmitting antenna, a plurality of wireless power receivers that receive the electromagnetic energy radiated by the transmitting antennas and convert the electromagnetic energy to DC power, and a control system that controls the transmitting antennas to direct electromagnetic radiation at different wireless power receivers for varying lengths of time according to a schedule.

According to another aspect, a wireless power transmission system includes at least one electromagnetic radiation source, at least one wireless power receiver collocated with a beacon that radiates a high frequency carrier signal, wherein a pilot signal is impressed or carried on the high frequency carrier signal, and at least one transmitting antenna that includes a receiving unit that receives the high frequency carrier signal and includes circuitry to extract the pilot signal from the carrier signal. The at least one transmitting antenna radiates electromagnetic radiation from the at least one electromagnetic source in the direction of the pilot signal to transmit power to the wireless power receiver.

The foregoing and other features of the invention are hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawings setting forth in detail several illustrative embodiments of the invention, such being indicative, however, of but a few of the various ways in which the principles of the invention may be employed.

DETAILED DESCRIPTION

Referring initially toFIG. 1, a simplified block drawing is used to illustrate the basic operation of a switched beam wireless power transmission system10. The system10includes at least one source of electromagnetic energy, such as an array of transmitting antennas12, such as a retrodirective array of antennas (collectively referred to as an “RDA”), which may be, for example, parabolic reflecting antennas, phased array antennas, etc. The power generated by the at least one source of electromagnetic radiation can be divided among a plurality of antenna elements. For simplicity, the operation of a single transmitting antenna12is described.

Each element12in the RDA is used to illuminate or to transmit energy to one or more dispersed WPRs14, which may be, for example, one or more rectennas or rectifying antennas.

Each WPR14is collocated with a beacon16. The beacon16radiates a pilot signal18at a first frequency when the beacon16is in an active state. The pilot signal18is received by each element12in the RDA to direct electromagnetic radiation at a second frequency to the WPR14that is collocated with the beacon16in the active state. By extinguishing the beacon associated with one WPR and activating a beacon associated with a different WPR, the beam of electromagnetic radiation radiated by the RDA is electronically steered from one WPR to another. In this manner, each WPR can be illuminated as needed to meet the average power requirements of the WPR.

Each element12in the RDA receives the pilot signal18from the beacon16that is collocated with the WPR14that is to be illuminated with electromagnetic radiation. Each element12in the RDA includes a receive antenna20for receiving the pilot signal18and a receiver/phase conjugator22. The pilot signal18at a first frequency is received by the receive antenna20and transmitted to the receiver/phase conjugator22. The receiver/phase conjugator22extracts the phase from the pilot signal18and forms a phase-conjugated signal at a second frequency that it transmits to an amplifier24. The amplifier24amplifies the phase conjugated signal and transmits the amplified signal to a transmit antenna26. The amplified signal28is radiated from the transmit antenna26. The polarization of the amplified signal28radiated by each element12of the RDA is substantially the same, and may be measured relative to a Cartesian coordinate system common to all elements of the RDA, for example. The amplified signal28may be linearly polarized or circularly polarized. If the amplified signal28is linearly polarized, then the rectenna is a dual-linearly polarized rectenna. If the amplified signal28has a circular polarization, then the rectenna is a circularly-polarized rectenna whose polarization matches that of the amplified signal28.

Because the phase of the amplified signal28is the conjugate of the phase extracted from the pilot signal18and because the polarization of each amplified signal28is substantially the same, the amplified signal, which is the collective sum of all the amplified signals28radiated by the individual elements of the RDA, converges or collectively accumulates such that the electric-field vectors are aligned and add in phase at the site of the active beacon16.

As illustrated inFIG. 1, the receive antenna20and transmit antenna26of each element12in the RDA may be separate from one another to maintain a high isolation between the path of the transmit signal28and the path of the pilot signal18. It will be appreciated, however, that the receive antenna20and the transmit antenna26may be combined as a transceiver as may be desired.

The beacon16broadcasts the pilot signal18of the form:
VB(t)=Acos(ωt+θ0)   (Eq. 1)
where ω=2πf, f is the frequency, A is the signal amplitude, and θ0is an arbitrary phase. The signal received by the kthelement of the RDA at time t is proportional to that radiated by the beacon16at an earlier time t−rk/c, where rkis the distance from the beacon16to the kthreceive antenna20and c is the velocity of light (c=2.9979×108meters/second in vacuum):
VRk(t)=Bcos[ω(t−rk/c)+θ0]=Bcos(ωt+φt),   (Eq. 2)
where φt=−ωrk/c+θ0is the accumulated phase of the received signal and B is the signal amplitude. The received pilot signal18is processed by phase conjugating circuitry in the receiver/phase conjugator22. The phase conjugating circuitry extracts the phase from the pilot signal18and forms a phase-conjugated signal at a second frequency and transmits the signal to the amplifier24, such as a high-power microwave amplifier.

If the first frequency of the pilot signal18and the second frequency of the phase-conjugated signal are the same, then the phase conjugating circuitry may assume a form like that illustrated inFIG. 2. InFIG. 2, the pilot signal18is received by the receive antenna20, which transmits the received signal18to a mixer29. The mixer29multiplies the received signal with a reference signal R whose frequency is 2ω=4πf and whose phase is φREFto generate a signal containing the desired phase-conjugated signal;

The amplified or high-power signal28incident on the WPR14at time t from the kthelement of the RDA is that transmitted at an earlier time t−rk/c, so that

VHPk⁡(t)=⁢D⁢⁢cos⁡[ω⁡(t-rk/c)+ϕREF-ϕt]=⁢D⁢⁢cos⁡[ω⁡(t-rk/c)+ω⁢⁢rk/c+ϕREF-θ0]=⁢D⁢⁢cos⁡(ω⁢⁢t+ϕREF-θ0).(Eq.⁢5)
where D is the amplitude of the signal incident on the WPR14.

Because the phase of the incident signal VHPk(t) is independent of the array index k, the signals from each array element arrive at the WPR14with the same phase. If the polarization of the amplified signal transmitted by each transmitting antenna26is substantially the same, then the electromagnetic fields radiated by each array element will add vectorially to yield a power density that exceeds that from a single element by about a factor of N2, where N is the number of array elements;

If the pilot signal18and the amplified phase-conjugated signal28are of the same frequency, feedback from the transmit antenna26to the receive antenna20may result. Feedback may interfere with the proper operation of the system10. Two means by which feedback may be prevented will be described. The first means utilizes at least one very high-frequency beam of electromagnetic radiation such as a laser beam as a carrier for the pilot beam signal. If amplitude modulation is utilized to carry the pilot beam signal, the amplitude of such a signal may be of the form
e(t)=[a+bcos(ωRFt+θ0)] cos ω0t,(Eq. 8)
where a and b are constants, ωRF=2πfRFis the RF frequency, ω0=2πf0is the carrier frequency, and θ0is an arbitrary phase. The transmitted signal described in Eq. 8 may be expressed as a sum of monochromatic signals;

etrans⁡(t)=a⁢⁢cos⁢⁢ω0⁢t+b2⁢{cos⁡[(ω0+ωRF)⁢t+θ0]+cos⁡[(ω0-ωRF)⁢t-θ0]}.(Eq.⁢9)
The signal incident on the kthelement of the RDA at time t is proportional to that radiated by the beacon16at an earlier time t−rk/c, where rkis the distance from the beacon16to the kthreceive antenna20and c is the velocity of light (c=2.9979×108meters/second in vacuum):

At each element of the RDA, the modulated carrier beam is received by a detector. It is well known to those skilled in the art that the output of a square-law detector is proportional to the average value of erec2(t), where the averaging is performed over a few periods of the high-frequency carrier, eliminating components of the signal at frequencies ω0, 2ω0, ω0±ωRF, etc., but preserving the RF components at frequencies ωRFand 2ωRF. The detector output signal produced by this averaging process is

This signal contains frequency components at dc, at ωRF, and at 2ωRF. If only the component at ωRFis desired, the undesired components at dc and 2ωRFmay be eliminated via filtering. For example, by first passing the signal through a high-pass filter having a cutoff frequency well below ωRFand then through a low-pass filter having a cutoff frequency between ωRFand 2ωRF, both the dc component and the RF component at 2ωRFmay be eliminated. The filtered signal assumes the form

vfiltk⁡(t)=C⁢⁢cos⁡[ωRF⁡(t-rkC)+θ0+ϕfiltk],(Eq.⁢12)
where φfiltkis the phase imposed upon the detected signal by the filter chain of the kthreceive element. By matching the filters comprising the filter chains associated with each element of the RDA, all values of φfiltkcan be made to fall within a narrow range, in which case φfiltkcan be treated as a constant independent of k;

The beacons may radiate or transmit a single high-frequency carrier beam that encompasses all of the elements in the antenna array, or N-high-frequency carrier beams, where N is the number of elements comprising the RDA and each beam carries the same pilot signal to a single element of the RDA.

Those skilled in the art will appreciate that the carrier beam may occupy different portions of the frequency spectrum and modulation techniques other than those described here may be utilized.

Those skilled in the art will further appreciate that the component of the signal described in Eq. 11 at frequency 2ωRFmay be utilized to transmit the pilot signal information if 2ωRF=ωRF′, in which case only a high-pass filter is needed to extract the desired component of the detected signal. The filtered signal assumes the form

vHPk⁡(t)=C⁢⁢cos⁡[ωRF′⁡(t-rkc)+2⁢θ0+ϕHPk](Eq.⁢14)
where φHPkis the phase imposed by the high-pass filter upon the signal detected by the kthreceive element. By matching the high-pass filters associated with each element of the RDA, all values of φHPkcan be made to fall within a narrow range, in which case φHPkcan be treated as a constant independent of k;

A second means by which feedback may be prevented is to utilize a pilot signal of a first frequency and an amplified phase-conjugated signal28of a second frequency distinct from the first frequency. For example, if the first frequency of the pilot signal18is ω1=2πf and the second frequency of the phase-conjugated signal is ω2=2ω1=4πf, then the phase conjugating circuitry may assume a form like that illustrated inFIG. 3, and, for example, as described in Rodenbeck, C. T.; Ming-yi Li; Kai Chang, “A phased-array architecture for retrodirective microwave power transmission from the space solar power satellite,”Microwave Symposium Digest,2004IEEE MTT-S International, vol. 3, no., pp. 1679-1682 Vol. 3, 6-11 Jun. 2004, which is incorporated herein by reference in its entirety. InFIG. 3, the pilot signal18is received by the receive antenna20, which transmits the received signal18to a first mixer30. The first mixer30mixes the received signal18with a reference signal R whose frequency is ω2=4πf and whose phase is φREFto generate an intermediate signal of the same form as that obtained by processing the signal from Eq. (3) with a low-pass filter. The intermediate signal is then applied to a second mixer31where it is mixed with itself to yield

The amplified or high-power signal28incident on the WPR at time t from the kthelement of the RDA is that transmitted at an earlier time t−rk/c, so that

VHPk⁡(t)=⁢D⁢⁢cos⁡[2⁢ω⁡(t-rk/c)+2⁢ϕREF-2⁢ϕt]=⁢D⁢⁢cos⁡[2⁢ω⁡(t-rk/c)+2⁢ω⁢⁢rk/c+2⁢ϕREF-2⁢θ0]=⁢D⁢⁢cos[2⁢ω⁢⁢t+2⁢ϕREF-2⁢θ0).(Eq.⁢18)
where D is the signal amplitude.

Because the phase of the incident signal VHPk(t) is independent of the array index k, the signals from each array element arrive at the WPR with the same phase. If the polarization of the amplified signal transmitted by each transmitting antenna26is substantially the same, then the electromagnetic fields radiated by each array element, will add vectorially to yield a power density that exceeds that from a single element by about a factor of N2, where N is the number of array elements;

As will be appreciated by those skilled in the art, different pilot-beam conjugated-signal frequency combinations may be utilized.

FIG. 4shows an illustrative embodiment of a switched beam wireless power system in the context of a High-Altitude Long Endurance (HALE) microwave-powered aircraft32. Such aircraft are of interest as long-endurance platforms for surveillance, remote sensing, and communications. Due to the limited amount of power available for propulsion, HALE aircraft, microwave-powered and otherwise, utilize ultralight construction and typically have long, high-aspect ratio wings for aerodynamic efficiency. The microwave-powered HALE aircraft32of the illustrative embodiment has a wingspan of 90 meters and a chord of 3.5 meters, flies at an altitude of 60,000 feet in a circle of radius 60,000 feet centered on an RDA34and requires delivery of 100 kW to the rectennas or WPRs to meet aircraft propulsion and payload power requirements. Although illustrated with respect to a HALE aircraft, the concepts described herein are applicable to wirelessly power any remote target, as will be appreciated by one of skill in the art.

The aircraft32is powered by microwave radiation36transmitted from an RDA34on the ground. The electromagnetic energy36is received by one or more WPRs on the aircraft32, which receive the electromagnetic energy36and convert it to DC power, which may be used by power-conditioning electronics and other components on the aircraft32. The DC power delivered to the power-conditioning electronics on board the aircraft32will be reduced by rectenna scan losses and other dissipative losses, as will be appreciated.

In one embodiment, the WPRs are located on the underside of a wing of the aircraft32, which provides a relatively large surface on which multiple WPRs may be placed. It will be appreciated, however, that the WPRs may be located on other areas of the aircraft, as may be desired.

The RDA34illustrated inFIG. 4has sixteen elements arranged in a 4×4 grid. The system operates at a frequency of 8 GHz, which assures a low-loss propagation path in all weather conditions. It will be appreciated, however, that a different frequency may be utilized as may be desirable. It also will be appreciated that the RDA34may include more or fewer elements and that the elements may be arranged in configurations other than a 4×4 grid, for example, the elements may be in any array of two or more elements. In some applications it may be advantageous to add an element of randomness to the arrangement of the elements of the RDA34(e.g., such that the elements are not arranged in a grid), to eliminate or to reduce grating lobes.

If the elements are mechanically steered, the spacing between elements in the RDA34must be sufficiently large to eliminate the possibility of blockage from adjacent elements, e.g., the elements must be spaced far enough apart from one another such that each element is able to maintain a line of sight with the aircraft32as it moves in the sky. If the element aperture diameter is D, the minimum spacing between elements is L, and the elevation angle is θ, no blockage between adjacent elements occurs when the following condition is satisfied:

In the exemplary embodiment ofFIG. 4, if D=8 meters and the elevation angle θ≧45°, there will be no blockage if L≧11.3 meters. Thus, the spacing between the adjacent elements in the RDA34may be any length greater than 11.3 meters to avoid blockage. In one embodiment, the spacing between elements may be about 15 meters. As will be appreciated, Eq. 21 may be used to calculate the appropriate distance between different sized elements or apertures and/or for different elevations.

In one embodiment, each element in the RDA34is a high-gain 8-meter diameter parabolic reflecting antenna that is fed by a high-power microwave source such that the radiated power per element in the RDA34is 100 kW and the total radiated power from the 4×4 array is 1.6 MW.

Power can be delivered efficiently to the rectenna on the aircraft32by dividing the power needs of the aircraft32into two or more sections and equipping each section with a beacon, such as an RF beacon, that radiates a pilot signal to the RDA34. The beacon causes the electromagnetic radiation transmitted from the RDA34to be electronically steered from one section of the wing to another. The different sections of the wing, therefore, can be illuminated by a narrow beam from the RDA34, which may reduce the amount of electromagnetic energy that falls between the WPRs and improve the overall energy transfer efficiency of the system.

The underside of a wing40from the aircraft32is illustrated schematically inFIGS. 5A-5C. The wing40is divided into three sections42,44,46based upon the power needs of the aircraft32. A first beacon48is located on the first section42, a second beacon50is located on the second section44and a third beacon52is located on the third section46. As will be appreciated, each beacon48,50,52is collocated with a respective WPR. It will be appreciated that the system may include more or fewer sections and beacons than those illustrated inFIGS. 5A-5C. Also illustrated inFIGS. 5A-5Care several individual elements34a-dfrom the RDA34. For simplicity of description, four elements34a-dare shown, however, It will be appreciated that the RDA may include any number of elements, each operating in the same manner as those illustrated inFIGS. 5A-5C.

InFIG. 5A, microwave power is delivered to the first section42of the wing40by activating the first beacon48and deactivating the second beacon50and third beacon52. The first beacon48radiates a pilot signal54that is received by each element34a-din the RDA34. As described above with respect toFIG. 1, each element34a-dconjugates the phase of the pilot signal62. The phase conjugated signal is then amplified and radiated from each element34a-din the direction of the active beacon, which, in this example, is in the direction of the first beacon48, e.g., along the general path of the pilot signal54received by each element34a-d. The amplified signals56radiated from each element34a-din the RDA34converge around the first beacon48and the first section42of the wing40. The electric-field vectors of the amplified signals56are in phase and of the same polarization and therefore add vectorially to power the WPR located at the first section42with a signal that is greater than the signal emitted from any one element in the array.

InFIG. 5B, the microwave power is delivered to the second section44of the wing40by activating the second beacon50and deactivating the first beacon48and the third beacon52. When activated, the second beacon50radiates a pilot signal58that is received by each element34a-din the RDA34. As described above with respect toFIG. 1, each element34a-dconjugates the phase of the pilot signal58. The phase conjugated signal is then amplified and radiated from each element34a-din the direction of the second beacon50, e.g., along the general path of the pilot signal58received by each element34a-din the RDA34. The amplified signals60radiated from each element34a-din the RDA34converge around the second beacon50and the second section44of the wing40. The electric-field vectors of the amplified signals60are in phase and of the same polarization and therefore add vectorially to power the WPR located at the second section44.

InFIG. 5C, the microwave power is delivered to the third section46of the wing40by activating the third beacon52and deactivating the first beacon48and the second beacon50. When activated, the third beacon52radiates a pilot signal62that is received by each element34a-din the RDA34, which conjugates and amplifies the pilot signal62and radiates the amplified signal64from each element34a-din the general direction of the third beacon52, e.g., along the path of the pilot signal62. The amplified signals64radiated from each element34a-din the RDA34converge around the third beacon52and the third section46of the wing40. The electric-field vectors of the amplified signals64are in phase and of the same polarization and therefore add vectorially to power the WPR located at the third section46.

In this manner, microwave power can be delivered to any number of rectenna sections on the underside of the wing40. As will be appreciated, as long as the aircraft remains within the beam of the RDA, the process by which the position of the beam is switched from one section to another is entirely electronic and no mechanical beam steering is required. Furthermore, this same process may be used to electronically steer microwave power to any number of dispersed WPRs as long as all of the WPRs lie within the beam radiated by a single element of the RDA. As described in more detail below with respect toFIG. 12, if all of the WPRs do not lie within the beam radiated by a single element of the RDA, for example, if the WPRs are widely dispersed or if the WPRs are moving over a large field, then mechanical steering may be used to mechanically aim each element at the target object.

In general, the equipment to which each WPR delivers power requires constant average power at its input. In order to deliver a constant level of power to the input, each WPR in the switched-beam wireless power transmission system must implement power storage and processing functions, as shown schematically in the power processing module66ofFIG. 6. The power processing module66receives pulses of power68from the RDA with a rectenna70. The pulses of power68are accepted, stored and processed with a power processor72. The power processor72outputs a constant DC power supply that can be used to drive an electrical load74which may consist of electronic equipment, electric motors, etc. Thus, the power processor66converts the pulses of power received from the RDA into a constant DC power supply.

The results of applying the switched-beam technique to the microwave-powered aircraft32of the illustrative embodiment are shown inFIG. 7. The graph inFIG. 7illustrates the calculated power density as a function of the position on the wing when the first beacon48, second beacon50and third beacon52are located at y=−30 meters, y=0, and y=+30 meters along the wing centerline, respectively. The graph is a line plot of the power density along the wing centerline for each successive illumination of the beacons as described above with respect toFIGS. 5A-5C.

InFIG. 7, curve80corresponds to the power density profile of the wing40when the first beacon48is illuminated (or activated) and the second and third beacons50,52are not activated, e.g., as described above with respect toFIG. 5A. Curve82corresponds to the power density profile of the wing40when the second beacon50is illuminated or activated and the first and third beacons48,52are turned off, e.g., as described above with respect toFIG. 5B. Curve84corresponds to the power density profile when the third beacon52is activated or illuminated and the first and second beacons48,50are not activated, e.g., as described above with respect toFIG. 5C.

FIG. 7also illustrates a dwell time based weighted average of the power density profile along the centerline of the entire wing40at curve86. The dwell time is the amount of time which a particular beacon is activated, e.g., the amount of time that the particular section of the wing is illuminated with electromagnetic radiation. The curve86represents the weighted average obtained by applying a set of weights to the three curves80,82,84based on the fraction of time for which each beacon is active. In the example illustrated inFIGS. 5A-5C, the weights are chosen to equalize the average power delivered to the three 30-meter sections42,44,46into which the aircraft wing40is divided in the example. As will be appreciated, the dwell times may be selected or controlled by an operator, or may be based on a schedule.

The instantaneous power levels delivered when first, second and third beacons48,50,52are active are denoted below by P1, P2, and P3, respectively. Let α represent the dwell time fraction for the first section42, i.e., the fraction of time that the first beacon48is active. Assuming the same dwell time for the third beacon52, the time-average power level delivered to each section42,44,46is
P1avg=αP1
P2avg=(1−2α)P2(Eq. 22)
P3avg=αP3
and the total average power delivered to the aircraft is
Ptotavg=αP1+(1−2α)P2+αP3.   (Eq. 23)

If the size and location of each WPR is symmetric with respect to the wing axis parallel to the direction of flight, it is reasonable to assume that P1≈P3, in which case the value of α is determined by setting P1avg=P2avgand solving the resulting equation for α; one obtains

Numerical experiments confirm that P1≈P3and that nearly equal power is delivered to each section42,44,46when the beam dwells on the first section42and the third section4634.8% of the time (α=0.348) and on the second section4430.4% of the time. Under these conditions, 44.0 kW is delivered to the first, second and third sections42,44,46. The total power delivered to the aircraft32is 132.1 kW, which is 8.26% of the 1.6 MW radiated by the RDA34.

The transmission efficiency, e.g., the ratio of the total power delivered to the aircraft32to the total power radiated by the RDA34can be increased by increasing the diameter of each element in the RDA to narrow the width of the beam or by increasing the number of elements in the RDA.

FIG. 8shows the effect of varying aperture size on the performance of the 4×4 RDA34of the illustrative embodiment. InFIG. 8, the power transfer efficiency is calculated as a function of aperture diameter for a 4×4 array of antennas, each separated by 15 meters in an X direction and by 15 meters in a Y direction. The dwell time weights for each data point are calculated using Eq. 24 to equalize power transfer to each of the three aircraft sections42,44,46.

As illustrated by the graph inFIG. 8, the power transfer efficiency is increased by increasing the size of each aperture. As discussed earlier, there is a limit on the maximum aperture diameter imposed by geometry. The limits are quantified by Eq. 19. For a 15 meter separation, the maximum aperture diameter is 10.6 meters.

FIG. 9shows the impact of increasing the number of elements in the RDA on the power transfer efficiency. As in the illustrative embodiment, the array consists of 8-meter apertures with an aperture-to-aperture separation of 15 meters in an N×N configuration. The results of varying the total number of array elements N2are plotted inFIG. 9. As shown inFIG. 9, the power transfer efficiency rises rapidly initially as elements are added to the array, however, the power transfer efficiency effectively plateaus once N2>500.

In the illustrative embodiment of the HALE aircraft32, substantially the same amount of power is constantly delivered to each of three WPRs. It will be appreciated, however, that delivery of different power levels to each of the three WPRs and different time-varying power levels, i.e., dynamic power allocation, is possible. Implementation of dynamic power allocation requires a feedback loop between the transmitter and each WPR, as will be appreciated.

For example, in the context of the illustrative embodiment, suppose that each of the three WPRs aboard the microwave-powered aircraft32have time-varying power requirements. Let the dwell time fractions for each be denoted by α1, α2, and α3. Power-sensing circuitry incorporated into the power-conditioning electronics fed by each WPR can monitor the instantaneous and time-averaged power outputs of the corresponding WPR. This information, along with the average power requirement, is transmitted to the RDA34. The RDA34incorporates one or more receivers to receive the signals, and circuitry to calculate an error function based on the difference between the power output of each WPR and the power requirement of the corresponding power-conditioning electronics.

For example, let the error function for the kthWPR be δk=Pkrequire−Pkout, where Pkoutis the average output power of the kthWPR and Pkrequireis the required average power. If the received average power is more than required, δk<0, and the corresponding dwell time fraction αk(where k=1, 2, or 3) is reduced until |δk|≦δthreshold, where δthresholdis a positive constant denoting the maximum acceptable value of |δk|. If the received power is less than required, δk>0, and the corresponding dwell time fraction αk(where k=1, 2, or 3) is increased until |δk|≦δthreshold. Those skilled in the art will appreciate that other error functions may be used and other types of feedback loops implemented between each WPR and the RDA.

The long-endurance aspect of HALE aircraft requires a wireless power transmission system of very high reliability. The switched beam wireless power transmission system described herein satisfies this requirement by distributing the microwave power generating function among many separate power-generating elements in the RDA34. Each element of the RDA34generates and radiates its own power. Thus, if the performance of one element degrades, if the element fails completely, or if the element is blocked or otherwise prevented from transmitting power to the WPR, the system performance slowly degrades.

As described above,FIG. 7shows that 132.1 kW is delivered to the aircraft32when all array elements34a-dfunction normally. Thus, the power delivered to the aircraft exceeds the power delivery requirement of 100 kW. The excess capacity of the system (approximately 32.1 kW) will be needed if one or more elements fail.

The power density in the event of a single random element failure is illustrated inFIG. 10. In this particular instance, the total power delivered in the event of a single element failure is 117.5 kW, as indicated by curve86′. As indicated inFIG. 10the total power86′ is divided such that 39.0 kW is delivered to the first section42(curve80′), 39.7 kW is delivered to the second section44(curve82′), and 39.1 kW is delivered to the third section46(curve84′) using the same dwell time schedule as used to generateFIG. 8.

The power density in the event of two random element failures is illustrated inFIG. 11. For this particular instance of two random element failures, the total power delivered to the wing40, as indicated by curve86″ is 103.9 kW. As indicated inFIG. 11, 34.2 kW is delivered to the first section42(curve80″), 35.4 kW is delivered to the second section44(curve82″), and 34.2 kW is delivered to the third section46(curve88″). Thus, the exemplary system has sufficient built-in redundancy that it can continue to meet the delivered-power requirement if two random elements in the RDA34fail.

Referring now toFIG. 12, a schematic diagram of a complete switched-beam wireless power transmission system100in the context of a microwave-powered aircraft101is illustrated. The underside of a wing102on the aircraft101includes three beacons104,106,108. Each beacon104,106,108is associated with a different WPR on the aircraft101. InFIG. 12, the first beacon104is active and the second beacon106and third beacon108are inactive. The first beacon104radiates a pilot signal110that is received by all elements of the RDA112.

Although only two exemplary elements112a,112bfrom the RDA are shown, it will be appreciated that the RDA may include any number of elements and, for example, may be a 4×4 array of elements. Each element112a,112bincludes a low-gain receiving antenna114for receiving the pilot signal110and a pilot signal receiver and phase conjugator116(also referred to as a “receiving unit”) having circuitry to receive and derive a phase-conjugated signal from the pilot signal110, for example, as described above with respect toFIG. 2.

The pilot signal receiver/phase conjugator116includes a phase-conjugating circuit (also referred to as the “PCC”). The PCC requires a phase reference signal to conjugate the phase extracted from the pilot signal110. The phase reference signal is supplied by a reference phase distribution circuit122. The reference phase distribution circuit122receives the pilot signal110from the activated beacon104by way of a receiving antenna111. The reference phase distribution circuit122amplifies and processes the pilot signal110, and then divides and distributes the amplified and processed reference signal to each element112a,112bin the RDA112. The processed reference signal may oscillate at twice the frequency as the pilot signal110as described above with respect toFIGS. 2 and 3, for example. Thus, for an array of N elements, the amplified pilot signal is divided into N different signals, each directed to a different element in the RDA. The reference signals are transmitted to a phase reference port of the pilot signal receiver/phase conjugator116, such that the reference signal arrives at each respective PCC with the same phase. The reference signal therefore provides a baseline for the conjugation of the phase extracted from the pilot signal110received at each individual element in the RDA.

The PCC for each element112a,112bof the RDA is located at the pilot signal receiver and phase conjugator116. The length of each transmission line with the phase reference signal from the distribution unit122to each pilot signal receiver and phase conjugator116must therefore be equal to one another or within a fraction of a wavelength to provide a consistent reference signal to each element in the RDA. This may prove difficult for a large array in which the elements are dispersed over a wide area.

As an alternative, the PCC for each element can be located near the reference phase distribution unit122. This type of arrangement is known as “central phasing” as described in Chernoff, R. C., “Large Active Retrodirective Arrays for Space Applications,” IEEE Trans. Antennas and Propagation, Vol. AP-27, No. 4, pp. 489-496, July, 1979, which is incorporated herein by reference in its entirety. When using a central phasing phase-distribution scheme, it is not necessary to equalize the lengths of the transmission lines carrying signals between the PCCs and the array elements. In addition, the line-length discrepancies between the reference phase distributor122and the PCCs are compensated for by the PCC in the same way as path-length differences between individual array elements and the active beacon; in essence, the length of the transmission line carrying the received pilot signal from the receiving antenna114to the pilot-signal receiver and phase conjugator116is added to the distance rkbetween the active beacon and the receiving antenna114for the kthelement of the RDA.

The phase conjugated signal is transmitted to an amplifier118. The amplifier118may be a klystron amplifier, a traveling-wave tube amplifier, a solid-state amplifier, a magnetron directional amplifier, a gyroklystron amplifier, or another amplifier, as will be appreciated by one of skill in the art.

The amplifier118amplifies the conjugated signal and transmits the amplified signal to a high-gain antenna120via a transmission line124. The transmission line may be a waveguide or a coaxial transmission line, or another type of transmission line depending on the frequency and the power level of the amplified signal. In general, a conventional waveguide is used when large amounts of power are to be transmitted. A coaxial transmission line may be used if the power transmission requirement is not too high, up to a few kilowatts. At frequencies in W-band (e.g., 75-111 GHz) and beyond, a corrugated waveguide or beam waveguide may be appropriate, as will be appreciated by one of skill in the art.

The high-gain antenna120radiates the amplified phase-conjugated signal126back towards the section of the aircraft with the activated beacon, which in the example ofFIG. 12is beacon104. Each high-gain antenna120radiates in the direction in which it is pointed in the case of a mechanically-steered antenna, or in the direction in which the beam is electronically steered in the case of a phased array antenna. In order to deliver a significant portion of the radiated power126to the aircraft, the antenna120may be a high-gain, narrow beam antenna. The amplified phase-conjugated signals126from the RDA converge at the active beacon104with the same phase and polarization and add vectorially, or collectively accumulate, at the beacon104to wirelessly power the WPR associated with that section of the wing102.

In order for the aircraft101to receive power, the aircraft101should remain within the beam radiated by the RDA112. For example, if the aircraft101moves out of the beam radiated from an element in the RDA112, the power delivered to the aircraft101may be insufficient to maintain flight or to operate the systems on the aircraft101. Each element in the array112a,112b, therefore, is supported by a movable pedestal130, which is capable of moving the transmission antenna120according to tracking information received from the aircraft101.

In one embodiment, each element of the RDA is a conventional phased array antenna (also referred to as an “electronically steered array”), which consists of a large number of small antenna elements whose vertical and horizontal dimensions are on the order of about half of a wavelength to a full wavelength. Due to the small size of the elements, each element radiates a wide beam. The beam radiated by the array can be steered anywhere within the beam of a single element. Therefore, if the target WPR is moving, and the WPR transmits its location to the RDA, each element of the RDA can compute the phases that must be applied to each element of the phased array to steer the beam in the desired direction. These phases, which are separate from and independent of the conjugated phases needed to implement an RDA, generally are different for each element of a given phased array antenna. The conjugated phases are applied to each element of the RDA. If the elements in the RDA are phased arrays, then the conjugated phase is an overall phase that will be the same for all elements of the given phased array.

The position of the aircraft101relative to each element112a,112bin the RDA can be tracked in many different ways. For example, the aircraft101may include on-board electronics that relay the position of the aircraft101to each element in the array. The onboard electronics, for example, may include a tracking beacon132, such as an RF beacon, that emits a tracking signal134, such as radar-echo emulating pulses, that can be tracked by a radar or a tracking receiver138collocated with each array element112a,112b. The onboard electronics also can take the form of a Global Positioning System (GPS) receiver and a data link, which relays accurate positional information to the tracking receiver138, for example, through the tracking beacon132. The onboard electronics may track the position of the aircraft101relative to the RDA by GPS. If the elements of the RDA utilize mechanical steering to point each antenna in the desired direction, the positional information received through the radar-echo emulating pulses134from the aircraft101can be used to derive pointing commands for each element, which are relayed to a pedestal control unit140to move the pedestal130to aim the transmitting antenna120at the aircraft101.

Each element of the RDA is equipped with a local control unit142that is linked to the pedestal control unit140, the high-power amplifier118, and to a central command and control unit144. Each local control unit142monitors the operation of key components of the associated array element112a,112b. In particular, the local control142monitors the pedestal130and pedestal control unit140to ensure proper operation and monitors the operation of the high-power amplifier118. In the event of a failure, a notice is sent to the central command and control unit144, which initiates proper action, such as a shutdown of the array element in question and activation of a redundant array element.

The central command and control unit144includes an antenna146for controlling and communicating with the aircraft101with transmissions148. The central control unit144monitors the operation of each element in the RDA and controls the operation of the aircraft101.

The illustrative embodiment utilizes separate transmit and receive antennas operating at the same frequency. Those skilled in the art will recognize that different frequencies can be used for the pilot signal110radiated by each active beacon and the power-bearing signal126radiated by the switched-beam wireless power transmission system described herein.

As described above with respect toFIG. 12, if each element of the RDA is a conventional phased array antenna, then each element may be equipped with a local control unit linked to a pedestal control unit, at least one high-power amplifier, and central command and control unit. In such an embodiment, mechanical steering is not required to point or aim the elements of the array at the desired target. The control unit may receive the location of the active beacon in three-dimensional space from the pilot signal and derive the appropriate phase for each individual element in the array so that the beam or the electromagnetic radiation from each element in the array converges and adds vectorially at the active beacon.

In one embodiment, the phased array is implemented as an active array, in which each antenna element is powered by a respective RF amplifier, which typically is a solid-state amplifier. In another embodiment, the phased array is constructed using a single RF source and a corporate feed network with phase shifters applying the required phase shifts at the input to each antenna element.

It also will be appreciated that although described primarily with respect to the powering of an aircraft, the switched-beam wireless transmission system may be utilized in any number of environments, for example to transmit power across a body of water to an island or a boat, or to transmit power across a valley or other region that may be difficult to traverse, or to transmit electromagnetic radiation from outer space to one or more ground-based wireless power receivers.

In the example ofFIG. 13, the switched-beam wireless transmission system200is illustrated transmitting power from a first area202to a second area204, for example, from one piece of land202across a body of water206to a second piece of land204. In the illustrated example, the first area includes an RDA208that consists of three elements208a-cand the second area204includes three dispersed WPRs210,212,214, each collocated with a respective beacon216,218,220.

As described above, the respective beacons are activated/deactivated to electronically steer the signal to the WPRs. In the illustrated example, beacon216is “on” or activated and radiates a pilot signal222. The pilot signal222is radiated across the body of water206where it is received by the RDA208. Each element208a-creceives and extracts the phase from the pilot signal222. Furthermore, each element conjugates the extracted pilot-signal phase and forms an amplified conjugated signal224. The amplified conjugated signal224is radiated from each element208a-cof the RDA208back along the path of the pilot signal222in the direction of the active beacon216such that the radiated signals224converge on the active beacon216and are received by the WPR210. The other beacons218,220on the other WPRs212,214can similarly be activated and deactivated to electronically steer the signal to the WPRs212,214, for example, in a similar manner to that described above with respect toFIGS. 5A-5C.

In another embodiment, the wireless power transmission system may be used to transmit electromagnetic radiation from a remote source to at least one wireless power receiver, for example, to wirelessly power an object on the Earth or another planet, the moon, or another space-based object from outer space. As described above, each wireless power receiver may be collocated with a beacon that radiates a high-frequency carrier signal, such as a laser beam, with a pilot signal impressed or carried on the high-frequency carrier signal. The system also includes at least one transmitting antenna. The transmitting antenna includes a receiving unit that receives the high frequency carrier signal and circuitry to extract the pilot signal from the carrier signal, as described above. The transmitting antenna radiates electromagnetic radiation from the electromagnetic source in the direction of the beacon such that the signals transmitted from the transmitting antenna converge and add vectorially at the beacon to transmit power to the wireless power receiver.