Patent Application
20060055605
The present invention relates generally to antennas, and specifically to devices and methods for controlling the Specific Absorption Rate (SAR) of radiation from the antenna of a mobile communication device in the tissues of a user of the device.
Concern has been growing over the radiation hazard involved in use of cellular telephones. Complaints of headaches, dizziness and fatigue are common among heavy users of cellular phones. Recent studies have indicated that long exposure to radio frequency (RF) radiation emitted by cellular phone antennas could cause serious medical problems due to the interference with brain cell activity, possibly leading to brain cancer. Some governments have already started warning users in regard to risks associated with use of cell phones. Recently, the British government has issued a recommendation to parents to limit the time their children use mobile phones. In the United States and in other countries, cellular and other wireless handsets must meet regulatory requirements for maximum specific absorption rate (SAR) levels in body tissues.
The concerns about the adverse health effects of cellular phone use arise from the fact that their antennas can deliver large amounts of RF energy to very small areas of the user's brain. In many cases, over 70% of the electromagnetic power emitted by the antenna in the 800-900 MHz band is absorbed in the human head. Although the RF emissions of wireless handsets are classified as non-ionizing, they are able to transfer energy in the form of heat to any absorptive material. The antenna location, near field emission characteristics, radio frequency power, and frequency establish the basis for conformance to SAR limits. Energy absorption in the head also introduces extra loss into the power budget of the cellular phone itself, causing increased power consumption and reduced battery life for a given levee of antenna emission.
Some attempts to reduce the health hazards of radio telephone antennas use RF-absorbing materials to shield the head. For example, U.S. Pat. Nos. 5,666,125 and 5,777,586, whose disclosures are incorporated herein by reference, describe an antenna assembly that includes a radiation absorber defining an open curved shape. At least some of the radiation emitted from the antenna in directions toward the user is blocked by the radiation absorber. Similarly, U.S. Pat. No. 5,694,137, whose disclosure is incorporated herein by reference, describes an arc-shaped shield, made of material impervious to radiation, which is positionable along an exterior of an antenna. While such absorbing shields may reduce the SAR in the head, however, they only aggravate the power loss problem. Therefore, an optimal antenna design should be based on improving efficiency of the radiation pattern as the key means for reducing SAR in body tissues.
As an alternative to absorbing materials, manufacturers often use electrically-conducting (grounded) surfaces to shield the user from the antenna. For example, U.S. Pat. No. 6,088,579 describes a radio communication device that has a conductive shielding layer between the antenna and the user. The shielding layer may be movable away from the antenna when not in use. Similarly, U.S. Pat. No. 5,613,221 describes a radiation shield for a hand-held cellular telephone made of a metal strip placed between the antenna rod of the telephone and the user. U.S. Pat. No. 6,075,977 describes a dual-purpose flip shield for retrofit to an existing hand-held cellular telephone. The shield, made of a polished material, preferably aluminum, is flipped up to a position between the telephone antenna and the user's head when the telephone is in use so as to provide high reflectance of electromagnetic waves away from the user. Other conductive antenna shielding devices are described in U.S. Pat. Nos. 6,088,603, 6,137,998, 6,097,340, 5,999,142 and 5,335,366. The disclosures of all the patents mentioned in this paragraph are incorporated herein by reference.
Conductive shields of the types described in these patents are not very effective in redirecting antenna energy, however, particularly when monopole antennas are involved. The problems with conductive shields stem from the fact that the boundary condition of the electromagnetic fields on a conductive surface requires the total electric field tangential to the surface to be zero. Therefore, the conductive surface necessarily has a reflection coefficient with a phase shift of 180° in the electric field. For the direct and reflected fields to be in phase, so that the antenna field is not canceled (shorted out) by destructive interference, the distance between the antenna and the reflector must be one quarter wave, which is about 8 cm in the 800-900 MHz band. To implement this solution with a monopole antenna is cumbersome, since the reflecting element must be located between the user and the antenna, meaning that the antenna itself must be at least 8 cm from the user's head.
In view of the known drawbacks of conductive reflectors, there have been attempts to improve their performance by addition of other electrical elements. For example, U.S. Pat. No. 6,114,999, whose disclosure is incorporated herein by reference, describes an antenna device for a mobile phone, wherein a distance between a miniaturized radiator and a miniaturized reflector is shortened by means of an introduced dielectric material. As an additional means for reducing the field directed toward the user, at least two thin isolated metal strips run parallel to the edges of the reflector element to form chokes at the rear of the reflector, so as to concentrate the near-field to an area between the chokes. European Patent Application EP 0 588 271 A1, whose disclosure is likewise incorporated herein by reference, describes an antenna for a portable transceiver having an asymmetric radiation pattern. At least one reflector can be placed in a rear zone of the antenna radiator. It is suggested that the reflector can be made of tuned dipoles operating in a passive manner, or by a vertical reflecting screen composed of densely-spaced horizontal turns.
Other antenna designs, such as patch antennas and variants on the loop antenna, permit more design flexibility without resorting to cumbersome reflector elements. These designs, however, have not shown the necessary near-field behavior to reduce SAR in the head. Another practice known in the art is to generate a quasi-directional far-field free-space pattern, rather than an omni-directional pattern. For example, U.S. Pat. No. 6,031,495, whose disclosure is incorporated herein by reference, describes an antenna system for reducing SAR that uses a pair of phased radiating elements to create a bi-directional radiation pattern with high attenuation perpendicular to the user's head. In the near field, however, the RF power density toward the user is not necessarily reduced by such an approach.
It is an object of the present invention to provide improved structures and methods for creating antennas having asymmetrical magnetic and/or electric near field distributions.
It is a further object of some aspects of the present invention to provide antenna assemblies with enhanced near-field directional characteristics.
It is yet a further object of some aspects of the present invention to provide apparatus and methods for reducing the SAR of RF radiation emitted by a personal communication device, such as a cellular telephone, in the head of a user of the device.
It is still a further object of some aspects of the present invention to provide antenna assemblies for use with personal communication devices that reduce the overall device power budget.
In preferred embodiments of the present invention, an antenna for a personal communication device comprises a feed structure, which is driven by the device to radiate an electromagnetic field in the operating frequency band of the device. A reactive surface is positioned adjacent to the rear surface of the feed structure, between the feed structure and the user's head. An electrically-asymmetrical cavity is thus defined between the rear surface of the feed structure, which is typically conductive, and the reactive surface adjacent to it.
The asymmetrical cavity supports two parallel current distributions in the conductive surface and the reactive surface, running in opposite directions (i.e., out of phase) on the two surfaces. On the front side of the feed structure, only the current on the conductive surface has an effect, thereby creating a strong field on the front side of the assembly, away from the user's head. The effect of the other current, running on the reactive surface, is shielded by the conductive surface. On the rear side of the feed structure, however, a null field is created in the cavity, since the individual effects of the currents on the conductive and reactive surfaces cancel one another.
Preferably, the feed structure is designed so as to minimize its size relative to the operating frequency. In some preferred embodiments of the present invention, the feed structure comprises a miniature cavity or, preferably, an array of such cavities, having a resonant frequency in the operating frequency band of the device. In other preferred embodiments of the present invention, the feed structure comprises a reduced-height monopole feed or an inverted-F feed, preferably with a meandered structure. Alternative feed structures will be apparent to those skilled in the art.
The novel combination of the feed structure with the asymmetrical cavity provides strong asymmetry of the near-field distribution of the electromagnetic energy radiated by the antenna assembly. Therefore, absorption of radiation from the antenna in the user's head is reduced. The electrical and mechanical characteristics of these feed structure and reactive surface allow the antenna assembly to be made small in size, with minimal impact on the mechanical design of the communication device. Furthermore, because both the feed structure and the reactive surface are substantially non-absorbing of radiation, the antenna structure radiates energy efficiently. By “reclaiming” energy that would otherwise be absorbed in the user's head, the antenna assembly improves the overall power budget of the communication device.
Although preferred embodiments described herein are directed to personal communication devices, and particularly to protecting users of such devices from RF radiation emitted by device antennas, the usefulness of the present invention is by no means limited to such applications. Rather, the principles and techniques of the present invention may be applied to produce near-field directional antenna assemblies for other uses, as well.
There is therefore provided, in accordance with a preferred embodiment of the present invention, an antenna assembly for a communication device, the assembly including:
Preferably, the feed structure and reactive surface are adapted to be mounted on the communication device so that the reactive surface intervenes between the feed structure and a head of a user of the device and shields the head from the radiated field.
Typically, the reactive surface includes an array of reactive circuit elements, including inductors and/or capacitors. In preferred embodiments, the reactive surface includes a printed circuit board having a plurality of faces in one or more layers, and the reactive circuit elements include traces printed on at least two of the faces of the printed circuit board. Preferably, the traces are printed so as to define inductive coils or, alternatively or additionally, so as to define parallel-plate or interdigitated capacitors. The reactive circuit elements may be mutually connected in series or in parallel.
Preferably, the reactive surface has a resonant response in the given frequency band. Further preferably, the rear side of the feed structure is substantially planar, and the reactive surface is positioned substantially parallel to the rear side of the feed structure. In a preferred embodiment, the feed structure further has an upper surface, and the reactive surface is configured and positioned so as to substantially cover the upper surface of the feed structure.
In some preferred embodiments, the front and rear sides of the feed structure define at least one resonant cavity therebetween having a resonance in the given frequency band and opening through at least one aperture in the front side of the feed structure, through which aperture the electromagnetic field radiates when the feed structure is driven by the device. Preferably, the at least one resonant cavity includes an array of cavities.
Preferably, the feed structure includes at least one transmission line, which is configured to form the at least one resonant cavity between the front and rear sides. Most preferably, the at least one transmission line defines a waveguide that forms the resonant cavity. Typically, the at least one transmission line is configured to form a spiral shape or is meandered. In a preferred embodiment, the transmission line is configured so that the at least one resonant cavity has corners, and including corner elements in the corners of the resonant cavity, which are arranged to inhibit reflection of the electromagnetic radiation at the corners of the at least one cavity. Preferably, the at least one transmission line is configured so that the resonant cavity has an electrical length approximately equal to one quarter wave in the given frequency band.
In a preferred embodiment, the at least one aperture includes a plurality of apertures. In a further preferred embodiment, the feed structure further includes one or more lumped circuit elements coupled across the at least one aperture. Additionally or alternatively, the feed structure includes one or more fins, positioned in the at least one resonant cavity so as to enhance a capacitance of the cavity. Further additionally or alternatively, the feed structure includes at least one of a dielectric material and a magnetic material, which is contained in the at least one resonant cavity.
In another preferred embodiment, the feed structure includes top and side surfaces, and further includes an awning protruding over at least one of the top and side surfaces so as inhibit leakage of the electromagnetic radiation toward the rear side of the structure. Preferably, the feed structure includes a capacitor positioned adjacent to the awning so as to enhance inhibition of the leakage of the electromagnetic radiation toward the rear side.
In still another preferred embodiment, the feed structure includes a monopole feed structure. In yet another preferred embodiment, the feed structure includes an inverted-F feed structure, wherein the front side of the feed structure includes a meandered electrical conductor.
Preferably, the rear side of the feed structure is electrically conductive.
There is also provided, in accordance with a preferred embodiment of the present invention, a method for wireless communication using a communication device operating in a given frequency band, the method including:
The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which:
The combination of the asymmetrical cavity and the feed structure causes the near field of the antenna assembly to be strongly asymmetrical, with a sharp drop of the magnetic and/or electric field between high values at the front of the assembly and very low values at the rear. The design of the antenna assembly not only reduces absorption of radiation in the head, but also redirects the energy supplied to the feed structure into the communication channel, thereby improving the overall power budget of telephone 20.
An asymmetrical cavity 35 is defined by reactive surface 28, positioned adjacent to a rear surface 27 of feed structure 25. Preferably, the reactive surface bends over the top of feed structure 25, as well, to further reduce radiation that may reach the user's head from the top of the feed structure. As noted above, cavity 35 supports two parallel current distributions in conductive rear surface 27 and in reactive surface 28. The current distributions run in opposite directions (i.e., out of phase) on the two surfaces. At front surface 26 of feed structure 25, only the current at conductive surface 27 has an effect, thereby creating a strong field on the front side of antenna assembly 30, away from the user's head. On the front side of the antenna assembly, the effect of the current on reactive surface 28 is shielded by conductive surface 27. On the rear side of feed structure 25, however, a null field is created in cavity 35, since the individual effects of the currents on conductive surface 27 and reactive surface 28 cancel one another.
The inductance values of inductors 38 and their positions are chosen so that the field emitted by feed structure 25 excites cavity 35, causing an electric current to flow at reactive surface 28 in opposite phase to the electric current on conductive surface 27. The current flowing at the reactive surface thus nulls the electromagnetic field at the rear of feed structure 25. In this sense, reactive surface 28 acts as a virtual magnetic wall (VMW), as described in the above-mentioned PCT patent application entitled, “Antenna with Virtual Magnetic Wall.” Alternative VMW structures, as described in that application, may be used in reactive surface 28 in place of the inductor array shown in
The values of inductors 38 and their spacing are chosen to give a resonant response in the operating frequency band (or bands) of telephone 20. Typically, for operation in the 800-900 MHz cellular band, the unit cell of the inductor array on surface 28 is 4 mm×4 mm, having lumped inductors of L=8.3 nH. In another example, the unit cell is 3.5 mm×3.5 mm, and the value of the inductors is L=10 nH. The depth of cavity 35 is preferably 2 mm. In other embodiments, capacitors may be used, as well as inductors. The addition of capacitors is particularly useful when the antenna assembly must be designed for dual-band operation.
Feed line 34 preferably comprises a coaxial cable, which is connected to the bottom cavity 41. Alternatively, the feed line may protrude through the bottom cavity and connect to some or all of the upper cavities. The cavities that are not directly connected to the feed line are excited by coupling through apertures 37 and lumped elements 44. Alternatively or additionally, the walls separating cavities 41 may be replaced by a combination of perforation and wires, in a manner similar to that shown in
Although feed line 34 is shown in
The cavities formed by transmission lines 62 are preferably filled with a dielectric or magnetic material 66, in order to reduce the physical length of the transmission lines needed to provide the proper quarter-wave electrical length. Feed structure 25 can thus be made still more compact. Dielectric or magnetic materials may similarly be used in the cavities of feed structure 25 in other preferred embodiments of the present invention.
The values of D and L, as well as the dimensions of cavity 82, depend on the desired center resonant frequency and bandwidth of the feed structure. For operation in the 800-900 MHz cellular band, for example, typical dimensions of cavity 82 are 42 mm wide×20 mm high×8 mm deep. The cavity can be filled with dielectric or magnetic materials with relative permittivity or permeability, respectively, of 1 to 20 or higher. Coaxial cable 86 protrudes 25-35 mm into the cavity, and pin 88 is 2-5 mm long.
Other feed structures and associated cavity configurations will be apparent to those skilled in the art and are considered to be within the scope of the present invention.
Furthermore, although preferred embodiments are described herein with specific reference to cellular telephones, the principles of the present invention are similar applicable to the construction of elements for shielding and redirection of radiation from devices of other types. It will thus be appreciated that the preferred embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.
This application claims the benefit of U.S. Provisional Patent Application No. 60/255,570, filed Dec. 14, 2000, and U.S. Provisional Patent Application No. 60/303,923, filed Jul. 6, 2001. It is related to a PCT patent application entitled, “Antenna with Virtual Magnetic Wall,” filed Dec. 6, 2001. All of these related applications are assigned to the assignee of the present patent application, and their disclosures are incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IL01/01152 | 12/12/2001 | WO | 9/22/2003 |
| Number | Date | Country | |
|---|---|---|---|
| 60255570 | Dec 2000 | US | |
| 60303923 | Jul 2001 | US |
