Communications: radio wave antennas – Antennas – High frequency type loops
Reexamination Certificate
2000-12-07
2003-05-27
Le, Hoanganh (Department: 2821)
Communications: radio wave antennas
Antennas
High frequency type loops
C343S867000, C343S895000
Reexamination Certificate
active
06570541
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to systems and methods for wirelessly projecting power and more particularly to systems and methods for wirelessly projecting power to microelectronic devices.
BACKGROUND OF THE INVENTION
Wireless powering of microelectronic devices is used, for example, for wireless Radio Frequency (RF) powering of Radio Frequency Identification (RFID) tags. RFID tags are used in the Automatic Data Collection (ADC) industry. In particular, printed bar codes are now widely used in the ADC industry. Unfortunately, bar codes may require line of sight reading, may hold limited amounts of information, may need to be read one at a time, may be subject to defacing and/or counterfeiting and may only provide fixed information. In contrast, RFID tags need not require line of sight reading, can hold large quantities of information, can have high transfer data rates, can be read in groups, can be more reliable and more difficult to destroy and/or counterfeit and can update stored information.
RFID tags generally may be classified into battery powered (active) RFID tags and RF powered (passive) tags. Compared to passive tags, active tags may be more expensive, may have a defined shelf life, may deplete with operation, may have potential disposability problems, may be physically larger and may be environmentally constrained due to the presence of a battery thereon. In sharp contrast, passive tags can be less expensive, can have an unlimited shelf life without depletion, can be relatively safe to dispose, can be relatively compact and can withstand harsher operating environments.
Notwithstanding these potential advantages, a major factor that may limit the availability of passive RFID tags is the ability to wirelessly project sufficient power to power the RFID tag.
In particular, RF communication among electronic devices currently is used across the RF spectrum. For example, cellular radiotelephones are widely used. In the United States, the Federal Communications Commission (FCC) regulates usage of electromagnetic radiation.
Unfortunately, the amount of power that is used to operate electronics may be orders of magnitude more than is used to exchange information. Accordingly, notwithstanding the advent of low power microelectronic devices, the ability to transmit enough power to be extracted by a remote microelectronic device may be difficult. In wirelessly projecting power to wirelessly power microelectronic devices, the biggest constraint may be the government regulations concerning permissible RF field strength.
Electromagnetic field emanation from an antenna classically is categorized as “near field” and “far field.” Generally, electronic components that carry RF currents or voltages produce both types of fields. However, the relative amount of each field may vary greatly.
From an RF energy standpoint, near field generally refers to RF energy that is stored in the immediate vicinity of the component and that is recovered at a later time in the alternating RF current cycle. An ideal inductor is a perfect near field only device. Far field generally refers to the energy that radiates or propagates from a component as an electromagnetic wave. Thus, a real world inductor may produce some far field radiation. Conversely, an ideal dipole antenna produces no near field components but produces significant far field radiation. Real world dipole antennas may produce some near field components but generate large amounts of far field radiation.
Thus, the far field is the component of energy that permanently leaves an antenna or any other component, radiating or propagating into the environment as an electromagnetic wave. Conversely, in each cycle, a near field is created and the energy associated with the near field is stored in the space around the antenna. As the near field collapses, the energy is transferred back onto the antenna and drive circuitry.
It will be understood that the terms “near field” and “far field” classically also may be defined relative to the wavelength of the energy under consideration. As used herein, far field denotes energy at distances greater than about one wavelength, for example, greater than about 22 meters at 13.56 MHz and greater than about 31.6 cm at 950 MHz. Conversely, near field refers to energy that is less than about one wavelength in distance. For practical purposes, near field generally may be considered to be a fraction of a wavelength, while far field may generally be considered to be multiple wavelengths so that there may be an order-of-magnitude difference therebetween.
Near field and far field also may be distinguished by the drop-off of power from the antenna. Power in the far field generally drops off from a source antenna without gain as a function of 1/(distance)
2
. In contrast, power in the near field generally may exhibit a more complex relationship. At distances that are far less than one wavelength, the individual current carrying elements of the antenna may produce a near field that decreases, remains constant or may even increase with distance. Moreover, at distances that approach one wavelength, power generally drops off much quicker with distance compared to the far field, with some components dropping off as fast as 1/(distance)
8
, others closer to 1/(distance)
4
.
Antennas generally are designed to communicate over great distances. Accordingly, antennas generally are designed to optimize the far field for a particular application. Accordingly, FCC regulations also generally are written for far field radiation. For example, radiation typically is measured based on FCC standards at a distance greater than one wavelength because it is assumed that near field energy is greatly reduced at that distance. However, there also are FCC guidelines that relate to maximum exposures to electromagnetic radiation that can impact near field intensity limits.
For purposes of wirelessly projecting power to wirelessly power microelectronic devices, it would be desirable to increase the near field component of energy without increasing the far field component of energy sufficiently to violate FCC regulations. Preferably, the near field component also is not increased to the point where maximum exposure as stated by the FCC guidelines occurs too quickly. By increasing the near field component of energy, the microelectronic devices may be powered by the field that is stored in the space around the radiator. By not increasing the far field, the energy that propagates outward and that is not reclaimed may be reduced, and violation of government regulations that govern far field energy may be prevented. Unfortunately, when the near field is increased in order to extend the range at which power may be projected to wirelessly power microelectronic devices, the far field also may increase, thereby increasing the likelihood of regulatory violations.
SUMMARY OF THE INVENTION
The parent application provides an array of in-phase current loops that are disposed adjacent to one another to define a surface and to define a virtual current loop at a periphery of the surface that produces a same direction virtual current while current in adjacent portions of adjacent current loops flows in opposite directions, to thereby wirelessly project power from the surface. As described in the parent application, the array of in-phase current loops that are disposed adjacent to one another to define a surface and to define a virtual current loop at a periphery of the surface that produces a same direction virtual current while current in adjacent portions of adjacent current loops flows in opposite directions, can provide acceptable power to RFID tags, while reducing the risk of violating regulatory constraints.
It will be understood that, as used herein, the terms “in-phase”, “same direction”, “opposite direction” and “out-of-phase” refer to relationships at a given point in time. In particular, since alternating currents are used, that vary over time, the terms “in-phase”, “same direction”, “opposite direction” and “out-of-phase” refer to instantaneous
db Tag, Inc.
Le Hoang-anh
Myers Bigel & Sibley & Sajovec
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