Patent Publication Number: US-2011050166-A1

Title: Method and system for powering an electronic device via a wireless link

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Claim of Priority Under 35 U.S.C. §120 
     This application claims priority under 35 U.S.C. Section 120 to U.S. patent application Ser. No. 11/408,793 entitled “Method and System for Powering an Electronic Device Via a Wireless Link” filed on Apr. 21, 2006, which claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 60/760,064 entitled “Method and Systems for Charging an Electronic Device Via a Wireless Link” filed on Jan. 18, 2006, both of which are incorporated by reference herein in their entireties. 
    
    
     BACKGROUND 
     1. Field 
     The present invention relates generally to methods and systems for powering or charging an electronic device. 
     2. Background 
     Recent developments in technology enable certain electronic devices, such as notebook computers, cell phones, and PDAs (personal digital assistant), to run various multimedia applications. However, these new multimedia applications require a large amount of power to run. A good solution to this challenge may be a system which may charge these electronic devices without having to plug them into the electric outlet. There is also a significant benefit in convenience and safety when any of such devices, for example a cell phone, is kept adequately charged without the need to connect a power wire. 
     SUMMARY 
     In one aspect, a system configured to provide power to a chargeable device via radio frequency link is provided. The system comprises a transmitter configured to generate a substantially unmodulated signal for powering or charging the chargeable device. The system further comprises a transmit antenna configured to receive the substantially unmodulated signal from the transmitter and radiate a substantially unmodulated radio frequency (RF) signal to the chargeable device. 
     In another aspect, a system configured to provide power to a chargeable device via a radio frequency link is provided. The system comprises a first transmitter configured to transmit a first signal via a first antenna for powering or charging the chargeable device. The system further comprises a second transmitter configured to transmit a second signal via a second antenna for powering or charging the chargeable device, wherein the combination of the first and second signals power or charge the chargeable device. 
     In another aspect, a method of providing power to a chargeable device via radio frequency link is provided. The method comprises generating a substantially unmodulated signal. The method further comprises radiating a substantially unmodulated radio frequency (RF) signal to the chargeable device via a transmit antenna based on the substantially unmodulated signal. The method further comprises powering or charging the chargeable device with power delivered by the substantially unmodulated RF signal. 
     In another aspect, a method of providing power to a chargeable device via radio frequency link is provided. The method comprises transmitting a first signal via a first antenna to a chargeable device. The method further comprises transmitting a second signal via a second antenna to the chargeable device. The method further comprises powering or charging the chargeable device with power delivered by the combination of the first and second signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an overview of one exemplary embodiment of a system for powering or charging an electronic device via a wireless link; 
         FIGS. 2A ,  2 B, and  2  C illustrate examples of an electric signal that may be used by the transmitter  12  in  FIG. 1  to transmit power; 
         FIG. 3  illustrates an overview of one exemplary embodiment of a system communicating a radio frequency signal for carrying and delivering energy from an antenna to a device; 
         FIG. 4  is a block diagram illustrating one embodiment of a chargeable device  14  shown in  FIG. 1 ; 
         FIG. 5  illustrates an overview of another exemplary embodiment of a system for powering a device or charging an electronic device via a wireless link; 
         FIGS. 6A ,  6 B, and  6  C illustrate how two in-phase signals interfere constructively; 
         FIG. 7  is a block diagram illustrating an embodiment of a system transmitting two radio frequency signals to power or charge a chargeable device concurrently; 
         FIG. 8  is a flowchart describing a method of using a radio frequency signal carrying energy to power or charge an electronic device via a wireless link. 
         FIG. 9  is a flowchart describing a method of using two radio frequency signals carrying energy to power or charge an electronic device via a wireless link. 
         FIG. 10  is a flowchart describing a method of adjusting phase difference between two radio frequency signals such that they arrive at an electronic device in phase. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is directed to certain specific embodiments of the invention. However, the invention may be embodied in a multitude of different ways as defined and covered by the claims. In this description, reference is made to the drawings wherein like parts are designated with like numerals throughout. 
     The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. 
     Certain embodiments related generally to methods and systems for charging a portable power source, such as a battery, of an electronic device, are disclosed. More particularly, these embodiments relate to supplying power to the electronic device via a wireless link, such as by using radio frequency (RF) waves. 
       FIG. 1  illustrates an overview of one exemplary embodiment of a system for powering or charging an electronic device via a wireless link. In the exemplary embodiment, the system includes one or more transmitters  12 , each in communication with one or more transmitting antennas  18 . One or more electronic devices  14  are shown in  FIG. 1  in communication with the one or more transmitters  12 . 
     The transmitter  12  generates signals carrying power or energy and send such signals to the transmitting antenna  18  through a feed line (not shown) connecting the transmitter to the antenna. In certain embodiments, signals carrying power or energy may comprise radio frequency (RF) signals. In one embodiment, the transmitter  12  may comprise a radio frequency signal source and an amplifier. The radio frequency signal source generates a radio frequency signal of limited power at specified frequencies. The amplifier then amplifies the signal generated by the signal source and feeds the amplified signal to the transmitting antenna via an appropriate interface (e.g., RF cable). 
     In one embodiment, the transmitting antenna  18  may be omni-directional or directional. Omni-directional antennas radiate radio signals substantially all round the antenna, while directional antennas concentrate radio signals in a particular angle, e.g., an angle of less than 180 degrees. The angle of signal coverage provided by an antenna is typically measured by beamwidth. In another embodiment, it is desirable to use a directional antenna as the transmitting antenna  18 , such as a directional antenna with a beam-width between 0.1-20 degrees. For example, the beam-width may be selected at about 0.05, 0.1, 0.2, 0.25, 0.3, 0.5, 0.75, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 degrees or more. In addition, the transmitting antenna  18  is selected to operate at the frequencies of signals to be radiated within reasonable gain. 
     In certain embodiments, it is desirable to select an antenna that has high power gain as the transmitting antenna  18  so that sufficient power is transmitted to the chargeable device  14  (see  FIG. 1 ). In one embodiment, the power gain of the transmitting antenna  18  may be about 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, or 16 decibels (dBd) or more. In this document, the term dBd describes a well-known logarithmic ratio of the power intensity at beam center relative to the power intensity of an otherwise comparable half-wave dipole antenna. When using an antenna with 12 decibel power gain, for example, the transmitting antenna  18  may concentrate the signal it receives so that the power intensity is about 16 times the power intensity from a simple half-wave dipole antenna. 
     As noted above, the transmitting antenna  18  receives radio frequency signals carrying power or energy from the transmitter  12  and radiates such signals to the electronic devices  14  via a wireless link  16 . The electronic devices  14  may be any chargeable or non-chargeable devices comprising at least one of a media player, a personal data assistant (PDA), a portable computer (e.g., a notebook personal computer), a mobile or cellular phone, a clock, an electronic display, or any other device that utilizes electric power, optionally from a portable source, such as a rechargeable battery. Description of typical systems and methods of using received energy to power or charge an electronic device  14  may be found in at least US patent publication no. 2005/0194926 and U.S. Pat. Nos. 6,127,799 and 7,012,405, which are incorporated herein by reference. 
       FIGS. 2A ,  2 B, and  2  C illustrate examples of a signal waveform that may be used by the transmitter  12  in  FIG. 1  to transmit power.  FIG. 2A  is a two-dimensional graph of a pure (e.g. substantially unmodulated) sinusoidal wave signal. The vertical axis represents the amplitude of the pure sinusoidal wave signal while the horizontal axis represents the time. For any of the waveforms discussed here, depending upon the context, the amplitude may represent electric voltage (measured in volts), electric field intensity (measured in volts per meter), electric current (measured in amperes), or magnetic field intensity (measured in amperes per meter). As shown, the pure sinusoidal wave signal is a periodic function of the time.  FIG. 2B  is a two-dimensional graph of a square wave signal. The vertical axis represents the amplitude of the square wave signal while the horizontal axis represents the time. As shown, the square wave signal is a periodic function of the time.  FIG. 2C  is a two-dimensional graph of a frequency modulated sinusoidal wave signal. The vertical axis represents the amplitude of the frequency modulated sinusoidal wave signal while the horizontal axis represents the time. The frequency modulated sinusoidal wave is shown as a function of the time. In  FIG. 2C , the frequency of the frequency modulated signal during the period 0-t 1  varies from the frequency during the period t 1 -t 2 . Signals of other waveforms including, for example, a continuous-wave (CW) single-frequency signal, a modulated sinusoidal wave signal other than the frequency modulated signal shown in  FIG. 2C , and other periodic signals may also be used to carry and deliver the electric power to the electronic devices  14  (see  FIG. 1 ). 
     It should be noted that modulation refers to the process of varying a measurable property (such as amplitude, frequency or phase, or some combination thereof) of a carrier signal (e.g., a sinusoidal signal) in order to communicate information. The resulting varied signal is referred to as modulated signal. 
     In certain embodiments, the transmitter  12  is configured to generate substantially unmodulated signals to carry the charging energy via the wireless link  16  (see  FIG. 1 ). Examples of substantially unmodulated signals may be, but not limited to, a pure sinusoidal wave signal as shown above in  FIG. 2A . In one embodiment, a pure (e.g. substantially unmodulated) sinusoidal wave signal is used to carry and deliver the charging power. A pure sinusoidal wave signal is characterized by a relatively narrow bandwidth centered on a substantially single fundamental frequency. In another embodiment, other periodic wave signals such as square, pulse, triangular, sawtooth or irregular signals made up of a base sinusoidal wave and at least one harmonic sinusoidal wave may be used. Typically, the base sinusoidal wave signal has a lowest frequency, called the fundamental frequency, and which typically has the largest amplitude. The harmonic sinusoidal wave signal has a frequency which is an integer multiple of the fundamental frequency and typically has an amplitude lower than the base sinusoidal wave signal. Because other periodic wave signals contain at least one harmonic sinusoidal wave signal, they have a bandwidth wider than a pure sinusoidal wave signal. A frequency modulated (FM) sinusoidal signal such as the one shown in  FIG. 2C  also has a wider bandwidth than a pure sinusoidal wave signal, because it contains sinusoidal waves of substantially different frequencies. Using a pure sinusoidal wave signal to carry energy provides many advantages over other types of signals and therefore, may be chosen over other alternatives in certain embodiments. 
     In one embodiment, the transmitter  12  may advantageously achieve high power transfer efficiency using a pure sinusoidal signal. First, a pure sinusoidal wave signal has a narrow frequency bandwidth, which enables antennas and other devices to be matched precisely in frequency and achieve high power transfer efficiency. Second, the single-frequency purity of the transmitted beam enables a collimated transmission, limits beam divergence, and leads to a high power transfer efficiency. 
     Another example is that using a pure sinusoidal wave signal simplifies the system design and reduces the manufacturing cost, because no modulation is required. Further, using a pure sinusoidal wave signal keeps the interference effects to a minimum because a pure sinusoidal wave signal has a narrow frequency bandwidth. 
     The signals used for delivering energy may be selected at any desired frequency and power level suitable for carrying and delivering power sufficient to charge the chargeable device  14 . Generally, an exemplary radio frequency signal has a frequency between 3 MHz to 30 GHz. For example, the signal used for delivering energy may be of a frequency of about 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 800, 900 MHZ, or 1 GHz. 
     Many factors, technical and non-technical, such as the human RF exposure allowed by the FCC may impact the search to find a frequency for signals carrying and delivering power. For example, it is also desirable for the receiving antenna&#39;s equivalent aperture be large. The effective aperture of an antenna increases significantly at lower frequencies because it is proportional to the square of the wavelength. As a result, the delivered power increases. The equivalent aperture of an antenna, measured in square meters, is the ratio of the power (in watts) received by that antenna from an incoming radio wave, to the power intensity of that wave (in watts per square meter). Use of a lower frequency gives us a larger effective aperture, but on the other hand, at lower frequencies, the size of a receiving antenna, such as a dipole antenna, may become cumbersome for applications such as mobile handset. In one embodiment, the signal may be selected with a frequency between 1 GHz to 40 GHz, and preferably between 12 GHz to 36 GHz. In another embodiment, the signal may be selected with a frequency between 30 MHz to 300 MHz, and preferably between 88 MHz to 108 MHz. The frequency band ranging from 88 MHz to 108 MHz, used worldwide for FM broadcasting. This band is divided into 100 channels with 200 kHz spacing. It is possible to apply for dual use as a single-frequency transmission in the spacing between channels because the transmission involved in the invention would not interfere with existing FM channels. For example, the transmission discussed here may be made at a frequency of 100.2 MHz, which is 100 kHz away from each of the neighboring channels of 100.1 MHz and 100.3 MHz. 
       FIG. 3  illustrates an overview of one exemplary embodiment of a system communicating a radio frequency signal for carrying and delivering energy from an antenna to a device. In the exemplary embodiment, a transmitting antenna  18  sends a pure sinusoidal wave radio frequency signal  17  to a receiving antenna  148  of the chargeable device  14 . The transmitting antenna  18  may be directional or omni-directional. 
     The radio frequency signal  17  may be either modulated or substantially unmodulated. In certain embodiments, the radio frequency signal  17  is substantially unmodulated. Due to imperfections in the system, the signal may have small deviations in its amplitude, frequency or phase which do not detract seriously from its applicability to the present invention. In other embodiments it is desirable to intentionally modulate the amplitude, frequency or phase of the signal briefly from time to time, for purposes of legal identification of the transmitter(s) or for identifying which transmitter produces a particular radio signal for installation, adjustment or troubleshooting purposes. Legal identification of the radio transmitter may be required, in certain embodiments, by the FCC or other government agencies. For example, legal identification may be implemented by means of a brief series of interruptions in the radio signal transmission to provide a Morse code representation of the call letters of that transmitter. In the exemplary embodiment, a pure sinusoidal wave radio frequency signal  17  is used. 
     The receiving antenna  148  is included in the transmitter  14  shown in  FIG. 1  respectively. Alternatively, the receiving antenna  148  may be attached to the chargeable device  14  externally. In case the chargeable device  14  has an antenna for data communication, the receiving antenna may or may not be the same antenna used for data communication. In certain embodiments, the receiving antenna  148  is configured to be omni-directional thus allowing the user to place the chargeable device in one of multiple orientations. The chargeable device  14  will be described in further detail in connection with  FIG. 4 . 
     A radio frequency signal (also known as an electromagnetic wave) is a combined transverse radiated wave resulting from an electric field wave and a magnetic field wave. The electric or voltage wave (electric field E measured in volts/meter) is generated when a voltage difference exists between two parts of an antenna, for example the two conductive rod parts of a dipole antenna. The magnetic or current wave (magnetic field H measured in amperes/meter) is generated when a current travels through any parts of the antenna, for example current flow along the length axis of the two rods in a dipole antenna. The product of the electric field E and magnetic field H gives the power intensity of the radio frequency wave (measured in watts/meter 2 ). Generally, polarization of an electromagnetic wave refers to the spatial orientation of the electric field component of the electromagnetic wave. The polarization of an antenna is the polarization of an electromagnetic wave radiated by the antenna. When the polarization direction of a receiving antenna is parallel to the electric field orientation of an incoming electromagnetic wave, the maximum power is delivered from the wave to the antenna, compared to other orientations of the antenna. The concept of polarization of radio frequency waves is disclosed in at least U.S. Pat. No. 5,936,575, which is incorporated herein by reference. 
     In certain embodiments, the polarizations of the transmitting antenna  18  and of the receiving antenna  148  are aligned for maximum power transfer. Since it is desirable to allow the user to place the chargeable device  14  placed in a desired orientation, the transmitting antenna  18 &#39;s polarization may be adjusted to match alignment by rotating the electric field of the radio frequency signal  17 . 
     In one embodiment, both the transmitting antenna  18  and the receiving antenna  148  are directional antennas such that a fixed point-to-point wireless link is established for transmission. 
       FIG. 4  is a block diagram illustrating one embodiment of a chargeable device  14  shown in  FIG. 1 . The device  14  may comprise a receiver unit  142  and a rechargeable battery  146 . The rechargeable battery  146  may be any rechargeable power storage unit configured to supply power to the chargeable device  14 . The receiver unit  142  is configured to receive signals carrying charging power and charge the rechargeable battery  146  with the received power. Though the receiver unit  142  may be integrated in the chargeable device  14  in the exemplary embodiment, the receiver unit  142  may be a stand-alone unit which may be attached via wire or cable to a variety of types of chargeable devices  14  and deliver the charging energy to the chargeable device  14  through the link established by wire or cable. 
     The chargeable device  14  comprises a receiving antenna  148  which gathers some of the beamed radio frequency power radiated by the transmitting antenna  18  (see  FIG. 1 ) and delivers these AC signals to a rectifier  152 . The rectifier  152  then coverts the AC electrical energy from the receiving antenna  148  to a unidirectional pulsating signal and/or ultimately into a DC signal suitable for charging the rechargeable battery  146 . An exemplary rectifier  152  may comprise a Germanium-based rectifier characterized by a low barrier or threshold voltage (i.e., low on-power rectifier), to allow activation of the rectifier  152  in the event of receiving a low level signal. The rectifier may also be fabricated using Silicon, Gallium Arsenide, and other semiconductor materials as well. The rectifier  152  may also be characterized as a passive RF power sensor to minimize the use of power by the rectifier  152  from the chargeable device  14 . 
     In one embodiment, the receiver unit  142  comprises a voltage regulator  154 . The voltage regulator  154  may be integrated with or in addition to the rectifier  152  to regulate or limit the voltage supplied to the rechargeable battery  146  at a pre-determined level. The voltage regulator  154  may operate particularly when the physical movement of the chargeable device  14  causes the voltage of signals received by the receiving antenna  148  to vary significantly. This variation may be due to the variation in the geometric signal path from the transmitting antenna  18  to the receiving antenna  148 . 
     In one embodiment, the receiver unit  142  also comprises a pair of diodes  144  and  156 , which allow the rechargeable battery  146  to be charged by either a wire charging unit  158  or signals received by the receiving antenna  148 . The rechargeable battery  146  is charged by the wire charging unit  158  whenever the wire charging unit is connected via wire to an AC power source such as a standard AC power outlet. The rechargeable battery may be charged by signals received by the receiving antenna  148  when the wire charging unit does not provide charging power. Examples of the wire charging unit  158  may be found in most rechargeable electronic devices such as a cell phone. 
     In one embodiment, the receiver unit  14  may further comprise a signal power detector  162  for detecting the power intensity of the signal received at the receiving antenna  148 . The signal power detector may be connected directly to the receiving antenna  148 , to the rectifier  152 , or the regulator  154 . In one embodiment, the signal power detector  162  is connected to detect the signal output from the rectifier  152 . 
     As will be described in connection with  FIG. 7 , a transmitting antenna  164  then transmits a signal indicative of the power intensity of the charging signal received to the transmitter  12  (see  FIG. 1 ). The transmitting antenna  164  may be directional or omni-directional. The transmitting antenna  164  may be integrated with or separate from the receiving antenna  148 . In case the chargeable device  14  has an antenna for radio communication, the transmitting antenna  164  may or may not be the same antenna used for data communication. Numerous other alternative means are suitable to convey signals reporting the delivered radio signal strength. For example, such information may be reported by means of visible or non-visible light (infra red or ultra violet light), by means of sound or acoustic signals either audible to humans or not, or by means of connecting wires. 
       FIG. 5  illustrates a schematic overview of another exemplary embodiment of a system for powering or charging an electronic device via a wireless link. In this embodiment, the system comprises at least two transmitters (not shown in this figure) coupled to at least two transmitting antennas  18   a  and  18   b  respectively, each communicating an substantially unmodulated radio frequency signal for carrying and delivering energy to charge an electronic device. A first transmitting antenna  18   a  sends a first radio frequency signal  17 A to a receiving antenna  148  of a chargeable device  14 . A second transmitting antenna  18   b  sends a second radio frequency signal  17 B to the receiving antenna  148 . These radio frequency signals  17 A and  17 B may be selected to be similar to the signals used for transmitting charging power discussed above in relation to  FIGS. 2A ,  2 B, and  2 C. These radio frequency signals  17 A and  17 B may be either modulated or substantially unmodulated. In this exemplary embodiment, pure sinusoidal wave radio frequency signals  17 A and  17 B are used. In other embodiments, more than two transmitters may be used, e.g., 3, 4, 5, 6, 7, 8, 9, 10, or more transmitters may be used concurrently. 
     In certain embodiments, it is desirable that the first and the second signals  17 A and  17 B reach the receiving antenna  148  substantially in phase to maximize the power received by the chargeable device  14  and achieve efficient power transfer. Two signals are said to be in phase when they have a phase difference of about 0 degrees. In one embodiment, the first and second signals  17 A and  17 B are substantially the same, except that there is a phase offset between them when transmitted by the transmitting antennas  18   a  and  18   b . The phase offset may be calculated such that the first and second signals  17 A and  17 B, each traveling though a different wireless link after transmission by its respective transmitting antenna, arrive at the receiving antenna  148  with a phase difference of about 0 degrees. In another embodiment, the first and second substantially unmodulated signals  17 A and  17 B are pure sinusoidal radio frequency signal of the same single frequency. 
       FIGS. 6A ,  6 B, and  6 C illustrate how two in-phase signals interfere constructively.  FIGS. 6A and 6B  show two identical sinusoidal radio frequency signals where the amplitude of the signal is a periodic function of time. The amplitude of each signal is indicative of the strength of the electric field generated by the signal. These two signals, when arriving at the same point, interfere with each other.  FIG. 6C  shows the resulting signal of such interference. As shown in  FIG. 6C , the resulting signal has amplitude twice the amplitude of the each original signal as shown in  FIGS. 6A and 6B . Since the power intensity of the radio frequency signal is proportional to the square of the electric field strength, the power intensity of the signal in  FIG. 6C  is four times the power of either of the two signals shown in  FIGS. 6A and 6B  considered individually. Although sinusoidal signals are used in the example, similar result may follow as to other types of modulated or substantially unmodulated signals. Also, although the exemplary signals shown in  FIG. 6A  and in  FIG. 6B  are the same, they do not have to be of the same amplitude in order to interfere constructively with each other. 
       FIG. 7  is a block diagram illustrating an embodiment of a system transmitting two radio frequency signals to power or charge a chargeable device concurrently. The system  31  comprises a clock signal generator  32  which generates a common clock signal and sends the clock signal to a controller  34 . In one embodiment, the clock signal generator  32  may be an oscillator. There may be various embodiments of the controller  34 . In one embodiment, the controller  34  is a processor which may be any suitable general purpose single- or multi-chip microprocessor, or any suitable special purpose microprocessor such as a digital signal processor, microcontroller, or a programmable gate array. As is conventional, the processor may be configured to execute one or more programmed instructions. 
     The controller  34  coordinates generating two radio frequency signals  17 A and  17 B by transmitters  12   a  and  12   b  based on the common clock signal such that both signals are on a common time frame. Each transmitter is connected to a separate transmitting antenna which transmits the received radio frequency signal to the chargeable device  14 . The radio frequency signals  17 A and  17 B arriving at the chargeable device  14  then interfere with each other and generate a combination signal. The energy carried in the combination signal is then received by the chargeable device  14 . Characteristics of the radio frequency signals  17 A and  17 B may be similar to those of the signals  17 A and  17 B discussed in  FIG. 5 . 
     The signals  17 A and  17 B travel to the chargeable device  14  via their own paths, respectively. Because the paths taken by the signals  17 A and  17 B are often different, the time it takes for these signals to travel from the transmitting antennas  17 A or  17 B to the chargeable device  14  may be different. Therefore, even if the signals  17 A and  17 B are transmitted by transmitting antennas  12   a  and  12   b  with a phase difference of 0 degrees, there may be a phase difference between the signals  17 A and  17 B when arriving at the chargeable device  14 . Such a phase difference varies depending, at least in part, on the lengths of the paths between the transmitting antennas  12   a ,  12   b  and the chargeable device  14 . The controller  34  may cause the transmitters  12   a  and  12   b  to introduce a phase offset between the signals  17 A and  17 B to compensate for the phase difference introduced by traveling in different paths so that these signals arrive at the chargeable device  14  substantially in phase. 
     In certain embodiments, the controller  34  is able to track the signal strength of the charging signal at the chargeable device  14 . As above described, the chargeable device  14  may comprise a signal power detector  162  and a transmitting antenna  164  (see  FIG. 4 ). The signal power detector  162  detects the signal strength of the charging signal received by the chargeable device  14  and sends a feedback signal indicative of such signal strength via the transmitting antenna  164 . The system  31  further comprises a receiving antenna  38  connected to a receiver  36 . The receiving antenna  38  and the receiver  36  receive the signal indicative of signal strength at the chargeable device  14 , and forward the same signal to the controller  34 . As already noted, the signal from the device to the controller may be implemented using light, sound or other means than radio. 
     In certain embodiments, the appropriate value of the phase offset between the signals  17 A and  17 B at the transmitting antennas  18   a  and  18   b  may be determined by incrementally adjusting the phase offset through a range, and monitoring corresponding signal strength of the charging signal received by the chargeable device. In one embodiment, the radio frequency signal  17 B is the same as the radio frequency signal  17 A except that there is a phase difference between these signals before these signals get radiated. 
     In one embodiment, the feedback signal from the chargeable device  14  is also used to adjust the polarization of the transmitting antennas  18   a  and  18   b  such that it aligns with the polarization of the receiving antenna  148  (see  FIG. 3 ). As discussed with regard to  FIG. 3 , the power transfer between a transmitting antenna and a receiving antenna is maximized when the polarization of both antennas align with each other. The controller  34  incrementally adjusts the polarization of the transmitting antenna  18   a  by rotating the orientation of the electric field of the electromagnetic wave generated by the transmitting antenna  18   a , for example, from 0 to 90 degrees. The feedback signal from the device  14  is monitored to determine at which angle maximum power transfer is achieved. At first the angle may be adjusted in increments such as 10 degrees to find an approximately optimal angle. Once the approximately optimal angle is determined, the angle may be adjusted in increments such as 0.5 degrees to find an angle much closer to the optimal angle. Once the polarization of the transmitting antenna  18   a  is adjusted to match the polarization of the receiving antenna  148 , the same process may be repeated to adjust the polarization of other transmitting antennas such as  18   b.    
     There may be many ways to adjust the polarization of an antenna. In one embodiment, the transmitting antennas  12   a  and  12   b  are mechanically rotatable controlled by signals sent by the controller  34 . In another embodiment, each of the transmitting antennas  12   a  and  12   b  comprises a vertically mounted radiating element and a horizontally mounted radiating element. By incrementally dividing and reversing the voltage applied to the antenna between the vertically mounted element and the horizontally mounted element, the polarization of the antenna may be adjusted from 0 to 90 degrees. 
     It will be appreciated that the embodiments discussed above of a method of aligning polarization of a transmitting antenna and of the receiving antenna may be incorporated in the embodiment illustrated in  FIG. 3 . 
       FIG. 8  is a flowchart describing a method of using a radio frequency signal carrying energy to power or charge an electronic device via a wireless link. The method is performed using the system for charging an electronic device as described above with regard to  FIGS. 1 ,  3 , and  4 . 
     The method starts at a block  810 , where the transmitter  12  generates an electrical signal and sends the same to the antenna  18  (see  FIG. 1 ). As discussed in  FIG. 1 , the antenna  18  may be either omni-directional or directional. Next at a block  820 , the antenna  18  receives the electrical signal and radiates a radio frequency signal to a chargeable device  14  (see  FIG. 1 ). The radio frequency signal is discussed above with regard to the  FIGS. 2A ,  2 B, and  2 C. The radio frequency signal may be either modulated or substantially unmodulated. The radio frequency signal may be of a single frequency. In one embodiment, the radio frequency signal may be a pure sinusoidal wave signal. 
     Moving to a block  830 , the receiving antenna  148  of the chargeable device  14  receives the radio frequency signal and converts the signal into an electrical AC signal. Next at a block  840 , the rectifier  152  converts the electric AC signal into a power signal. The power signal can be a pulsating unidirectional signal or a DC signal suitable for powering the device and/or charging the rechargeable battery, as discussed above in  FIG. 4 . 
     Next at a block  850 , the voltage regulator  154  regulates the voltage level of the power signal if necessary, as discussed above in  FIG. 3 . It will be appreciated that block  850  may be removed in certain embodiments. Last at a block  860 , the power signal is applied to charge the rechargeable battery  146  of the chargeable device  14 , as discussed above in  FIG. 3 . 
       FIG. 9  is a flowchart describing a method of using two radio frequency signals carrying energy to power or charge an electronic device via a wireless link. The method is performed using the system for charging an electronic device as described above with regard to  FIGS. 5 ,  6 , and  7 . 
     The method starts at block  910 A, where the first transmitter  12   a  generates a first electrical signal and sends the signal to the first antenna  18   a . Next at a block  920 A, the first antenna  18   a  receives the first electrical signal and radiates the first radio frequency signal  17 A to the chargeable device  14 . Similarly, the method provides blocks  910 B and  920 B, which are preformed substantially concurrently with blocks  910 A and  920 A. At blocks  910 B and  920 B, the second transmitter  12   b  and the second antenna  18   b  radiates the second radio frequency signal  17 B to the chargeable device  14 . The transmitters, antennas, and the RF signals are the same as discussed in  FIGS. 5 ,  6 , and  7 . 
     Next at a block  930 , the chargeable device  14  receives a combination of the first and second RF signals. In Blocks  940 ,  950 ,  960 , and  970 , the received combination RF signal is used to charge the device  14 , similarly to the discussion above in  FIG. 8 . 
       FIG. 10  is a flowchart describing a method of adjusting phase difference between two radio frequency signals such that they arrive at an electronic device in phase. In the exemplary method, the appropriate value of the phase offset between the signals  17 A and  17 B at the transmitting antennas  18   a  and  18   b  may be determined by incrementally adjusting the phase offset and monitoring corresponding signal strength of the charging signal received by the chargeable device. The phase offset enabling the signals  17 A and  17 B to arrive at the chargeable device  14  in phase corresponds to the highest or near highest signal strength at the chargeable device  14 . In the exemplary embodiment, the method is applied in the system  31  of transmitting two RF signals to charge a chargeable device as illustrated in  FIG. 7 . 
     The method starts at a block  1010 , where the antennas  18   a  and  18   b  receive two electrical signals from transmitters  12   a  and  12   b  and radiate two radio frequency signals to the chargeable device  14  (see  FIG. 7 ). At a block  1020 , the chargeable device  14  receives the combined two radio frequency signals. Next at a block  1030 , a signal power detector  162  detects the received signal power p(T) at the chargeable device  14  (see  FIG. 7 ). The transmitting antenna  164  of the device  14  then sends a feedback signal indicative of the measured signal power to a controller  34 . Moving to a block  1040 , a receiver  36  receives the feedback signal via a receiving antenna  38  and sends a signal related to the measured signal power to the controller  34 . As previously noted, other alternative means than radio may be used to convey this feedback signal. 
     At a block  1050 , the controller  34  determines whether or not the optimal phase offset has been achieved, e.g., whether or not the maximum signal strength of the combined RF signal has been received by the chargeable device  14 . The optimal phase offset is the phase offset which causes the two radio frequency signals  18   a  and  18   b  to arrive at the chargeable device  14  substantially in phase. In this block  1050 , p(T) represents the current power measurement, p(T−1) represents the measurement immediately before p(T), and p(T−2) represents the measurement immediately before p(T−1). The controller  34  will conclude that the optimal phase offset has been achieved during the immediately previous measurement, if the immediately previous power measurement p(T−1) is greater than both of its immediate neighbors in time order, p(T−2) and p(T). In one embodiment, the controller  34  may conclude that the optimal phase offset has been achieved during the immediately previous measurement, if the p(T), either is greater than or equals to, both p(T−2) and p(T). For the initial two measurements, the controller  34  is configured to conclude that the optimal phase offset has not been achieved since at least one of p(T−1) and p(T−2) is not available. For example, p(T−1) and p(T−2) may be assigned a default value of 0 if any of them is not available yet. If the optimal phase offset has been achieved, the method proceeds to a block  1080 , where the two transmitting antennas  18   a  and  18   b  continue radiating the two radio frequency signals based on the immediately previous phase settings. In certain embodiments, at block  1050 , the controller  34  may stop the phase adjustment if the current measured signal power is over a pre-determined or desired value, e.g., a signal power value that may be estimated mathematically. 
     If at block  1050 , the controller  34  determines that p[t−1] is not greater than both p[t] and p[t−2], the method moves to a block  1060 . At block  1060 , the controller  1060  stores the current phase setting and corresponding measured signal power for later use. Next at a block  1070 , the controller adjusts the phase setting for these two radio frequency signals. In one embodiment, the phase of one radio frequency signal keeps constant while the phase of the other radio frequency signal is adjusted. The phase of a radio frequency signal may be increased at increments of, for example, 10 degrees. The increment may be bigger or smaller depending on how accurate the phase adjustment needs to be done. 
     In certain embodiments, the chargeable device  14  may move while a user moves, therefore making it necessary for the controller  34  to check whether the two radio frequency signals  18   a  and  18   b  are in phase from time to time. After the controller  34  finds the proper phase setting and continues radiating the two radio frequency signals at that phase setting as shown in block  1080 , the method moves to a block  1080 , where the controller  34  checks whether a time period of a predetermined length T 0  (e.g., 1, 2, 5, 10 or more minutes) has passed since the controller  34  finishes the last phase adjustment. If the answer is no, the method goes back to block  1080 . If the answer is yes, the method moves to block  1030  where the controller  34  starts a new round of phase adjustment. 
     The foregoing description details certain embodiments of the invention. It will be appreciated, however, that the invention may be practiced in many ways. For example, although a workable method is described here for optimizing the phase and the polarization of the electromagnetic waves at the device receive antenna, there may be many other methods for optimization that are applicable to the present invention without departing from the scope and spirit of the invention. It should be noted that the use of particular terminology when describing certain features or aspects of the invention should not be taken to imply that the terminology is being re-defined herein to be restricted to including any specific characteristics of the features or aspects of the invention with which that terminology is associated. Although the present invention has been described with reference to specific exemplary embodiments, it will be evident that the various modification and changes may be made to these embodiments without departing from the broader spirit of the invention. Accordingly, the written description, including any drawings, is to be regarded in an illustrative sense rather than in a restrictive sense.