Antenna arrangement having magnetic field reduction in near-field by high impedance element

An apparatus and a method for reducing magnetic field in near-field by presenting high impedance at an operating frequency band are disclosed. A conducting element (112), which is suspended substantially parallel to a first side (104) of a printed circuit board (102) of a wireless portable communication device (100) over an electrically grounded conductor (116), forms a reactive element. The reactive element provides high impedance at operating frequencies of the wireless portable communication device and diverts radio frequency currents from the first side of the printed circuit board to the second side of the printed circuit board such that a magnetic field produced by the radio frequency currents on the first side of the printed circuit board is reduced in the near-field.

FIELD OF THE INVENTION

The present invention generally relates to a method and an apparatus for an antenna arrangement, and more specifically to a method and an apparatus for an antenna arrangement reducing undesired magnetic field in a near-field.

BACKGROUND OF THE INVENTION

As wireless portable communication devices, such as cellular telephones, are made smaller, corresponding components including antennas used for those devices are also made smaller, and/or are sometimes at least partially integrated with other components. As a result, a printed circuit board (“PCB”), which is populated with electronic and mechanical components, is effectively used as part of radiating antenna elements of the wireless portable communication device. Radio frequency (“RF”) current flows on the PCB and the PCB acts as an antenna. For cellular telephones operating in lower frequency band such as Global System for Mobile Communications (“GSM”), which covers the frequency band from about 880 MHz to 960 MHz, and Advanced Mobile Phone System (“AMPS”), which covers the frequency band from about 824 MHz to 894 MHz, the effect of the PCB radiation is more apparent compared to cellular telephones operating in higher frequency band such as Personal Communications Services (“PCS”), which covers the frequency band from about 1850 MHz to 1990 MHz. Because the size of an antenna is typically made to have an electrical length corresponding to the wavelength of the frequency used, the size of the antenna is generally larger for a lower frequency application.

By using a PCB of a cellular telephone as a radiating element, the radiating efficiency of the cellular telephone may be easily degraded due to the PCB being in close proximity to a user's body. For example, when a cellular telephone is used, it is typically held in a user's hand, which essentially covers one side of the PCB, and the other side of the PCB is held against the user's face. When the cellular telephone is carried in a user's pocket or is carried by a belt-clip, one side of the PCB faces the user's body. This presence of the user's body in close proximity to the PCB, which is being used as a radiating element, may significantly affect the radiation efficiency of the cellular telephone.

A cellular telephone may use a variety of types of antennas, such as a helical antenna and an internal antenna. The helical antenna may be viewed as a dipole-like structure comprising the antenna as a quarter-wave radiator and the PCB as another quarter-wave radiator. The internal antenna may be viewed as a matching network to the PCB for the 824-960 MHz bands of operation. Compared to the PCB, the antenna itself is generally much smaller in volume, and therefore contains more concentrated radiation energy than the PCB. To reduce the degradation in efficiency due to the presence of the user's body in close proximity, the antenna is typically located in the cellular telephone where it is kept away from the body of the user. However, the PCB, having RF currents flowing and emitting radiation, is still kept next the user's body, and the radiation efficiency of the cellular telephone is still considerably susceptible to the proximity of the user's body to the PCB.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a method and an apparatus for an antenna arrangement suitable for a wireless portable communication device, such as a cellular telephone, that reduces undesired magnetic field in a near-field. The near-field is generally defined as an area or a volume defined by a distance of a few wavelengths of operating radio frequency from an origin of radio frequency radiation, such as an antenna of the cellular telephone. In a typical cellular telephone usage, a user may place the cellular telephone in a pocket, may clip it to his belt, or may hold in his hand and hold it against his face. Therefore, when the cellular telephone transmits a signal, part of the user's body is placed in the near-field created by the transmitted signal, and attenuates the transmitted signal, which results in reducing a usable transmitted power of the cellular telephone. It is generally desirable to direct the transmitted power away from the user. By reducing the transmitted power towards the user while maintaining the total transmitted power, the usable transmitted power is effectively increased. The transmitted power from the cellular telephone at radio frequency can be considered as a product of an electric field and a magnetic field produced by a transmitter of the cellular telephone and radiated from the antenna and a printed circuit board of the cellular telephone. Because the magnetic field is proportional to a current flow, the magnetic field may be reduced by reducing the current flow. Therefore, by reducing the current flow, which reduces the magnetic field, the transmitted power, as the product of the electric field and now the reduced-magnetic field, can be reduced. By reducing the current flow in an appropriate place, the resulting transmitted power towards the user can be reduced, without reducing the total transmitted power, effectively improving the usable transmitted power.

FIG. 1is an exemplary diagram of a first embodiment of an antenna arrangement for a wireless portable communication device100in accordance with the present invention. The wireless portable communication device100comprises a printed circuit board102, which has a first side104and a second side106, a transceiver108, which includes a transmitter, a receiver, and a controller, disposed on the second side of the printed circuit board102, an antenna110coupled to the transceiver108, and a conducting element112suspended parallel to the printed circuit board102over the first side104. The conducting element112is supported by a post114. An area on the first side104of the printed circuit board102under the conducting element112is substantially covered by a conductor116, which is electrically grounded, and the conducting element112and the conductor116form a capacitor. A capacitance of the capacitor formed is proportional to an area of the conducting element112and is also inversely proportional to the distance between the conducting element112and the conductor116. Mathematically, the capacitance, C in Farads (“F”), can be expressed as:C=Aɛh,where A, in square meters (“m2”), is the area of the conducting element112,ε, in Farads per meter (“F/m”), is the dielectric constant of a material between the conducting element112and the conductor116, andh, in meters (“m”), is the distance between the conducting element112and the conductor116.

The post114, which supports and electrically connects to the conducting element112, is also electrically grounded through the conductor116, and behaves as an inductor. An inductance of the inductor, L in Henries (“H”), is proportional to the length of the post114, and can be expressed mathematically as:
L=αh,where α in Henries per meter (“H/m”) is a constant based upon a thickness of the post114, andh, in meters, again is the distance between the conducting element112and the conductor116, which is the length of the post114.

A resonant frequency of the combination of the capacitor and the inductor, at which the impedance of the combination of the capacitor and the inductor approaches infinity, is inversely proportional to a square root of the product of the capacitance and the inductance. Mathematically, the resonant frequency, f, can be expressed as:f=12⁢π⁢LC=12⁢π⁢α⁢⁢h⁢Aɛh=12⁢π⁢α⁢⁢Aɛ.
Therefore, the resonant frequency of the conducting element112can be adjusted to be equal to a desired frequency by varying any one or more of the terms present in the above equation.

When the wireless portable communication device100transmits a signal at a desired transmit frequency, radio frequency power associated with the signal is radiated from the antenna110as well as the printed circuit board102due to radio frequency currents generated by the transmitter of the transceiver108flowing on the printed circuit board102. However, by adjusting the resonant frequency of the conducting element112to be the desired transmit frequency, the radio frequency currents flowing on the first side104of the printed circuit board102encounters high impedance at the conducting element112, and a portion of the radio frequency currents is diverted to the second side106of the printed circuit board102. Because the flow of the radio frequency currents on the first side104is reduced by diverting the radio frequency currents to the second side106, the magnetic field in the near-field produced by the radio frequency currents on the first side104is reduced, thereby reducing the radiated radio frequency power from the first side104of the printed circuit board102. By reducing the radiated radio frequency power from the first side104of the printed circuit board102without changing the total power transmitted by the transmitter of the transceiver108, the radiated radio frequency power from the second side106of the printed circuit board102is effectively increased, thereby increasing an effective, or usable, radiated radio frequency power of the transceiver108.

FIG. 2is an exemplary block diagram of a second embodiment of an antenna arrangement200for the wireless portable communication device100in accordance with the present invention.FIG. 2illustrates a side view of the conducting element112and the conductor116, which is electrically grounded, disposed on the first side104of the printed circuit board102. Instead of the post114, which provides a fixed inductance at a fixed location, in the second embodiment of the antenna arrangement200provides a plurality of switches (only four switches,202,204,206, and208are shown) configured to electrically couple from a plurality of element locations (only four element locations,210,212,214, and216are shown) of the conducting element112to a plurality of conductor locations (only four conductor locations,218,220,222, and224are shown) of the conductor114. By varying the location where the conducting element112is coupled to the conductor114, by varying the number of locations where the conducting element112is coupled to the conductor114, or by varying the locations and the number of locations where the conducting element112is coupled to the conductor114, a resulting reactance can be varied to achieve a desired effect of producing high impedance for the given desired transmit frequency to reduce radio frequency current flow on the first side104of the printed circuit board102. For multiple frequency band operations, different sets of switches may be activated with each set of switches corresponding to a specific band. For example, a dual band cellular telephone having a first band at GSM 900 MHz band and a second band at GSM 1900 MHz, the switches202,204, and206may be activated to achieve a desired effect of producing high impedance at the 900 MHz band while only the switch208may be activated to achieve a desired effect of producing high impedance at the 1900 MHz band.

FIG. 3is an exemplary block diagram300of the switches202,204,206, and208. Each switch configured to couple the conducting element112to the conductor114is a PIN diode configured to be activated by a selector302.FIG. 4is an exemplary block diagram400of the PIN diode switches202,204,206, and208, each of which further comprising a corresponding reactive element to increase or decrease reactance when it is activated to produce desired high impedance at a desired frequency band and is activated by the selector302. In this example, inductors402,404,406, and408as the reactive elements are shown to be connected in series with corresponding PIN diode410,412,414, and416, respectively, to form switches,202,204,206, and208, respectively.FIG. 5is an exemplary block diagram500of the switches202,204,206, and208. Each switch is a varactor diode configured to provide variable capacitance set by a controller502.

FIG. 6is an exemplary illustration of a wireless portable communication device600utilizing an antenna arrangement in accordance with the present invention. In this example, the conducting element112is a metallic bezel, which holds in place a display602of the wireless portable communication device600to the printed circuit board102. The conducting element112, the metallic bezel, is coupled to the conductor116, which is electrically grounded, by four switches202,204,206, and208. As previously described, the switches202,204,206, and208may be activated individually or as a combination of any of the switches, and may comprise reactive elements or varactors.

FIG. 7is an exemplary block diagram of a third embodiment of the antenna arrangement700having a plurality of conducting elements, of which only four conducting elements,702,704,706, and708are shown, in accordance with the present invention. The individual conducting elements702,704,706, and708are configured to couple to the conductor116, which is electrically grounded, by their corresponding switches,710,712,714, and716, respectively, at a plurality of conductor locations,718,720,722, and724. By coupling different conducting element or elements to the conductor116, a resulting reactance can be varied to achieve a desired effect of producing high impedance for a given desired transmit frequency to reduce radio frequency current flow on the first side104of the printed circuit board102. For multiple frequency band operations, different sets of switches may be activated to couple different set of conducting elements with each set of conducting elements corresponding to a specific band. For example, a dual band cellular telephone having a first band at GSM 900 MHz band and a second band at GSM 1900 MHz, the switches710,712, and714may be activated to couple the conducting elements702,704, and706achieving a desired effect of producing high impedance at the 900 MHz band while only the switch716may be activated to couple the conducting element708achieving a desired effect of producing high impedance at the 1900 MHz band.

FIG. 8is an exemplary block diagram800of the switches710,712,714, and716where each switch is a PIN diode configured to couple the corresponding conducting element702,704,706, or708to the conductor114by a selector802.FIG. 9is an exemplary block diagram900of the PIN diode switches710,712,714, and716, each of which further comprising a corresponding reactive element to increase or decrease reactance when it is activated to produce desired high impedance at a desired frequency band and is activated by the selector802. In this example, inductors902,904,906, and908as the reactive elements are shown to be connected in series with corresponding PIN diode910,912,914, and916, respectively, to form switches,710,712,714, and716, respectively.FIG. 10is an exemplary block diagram1000of the switches710,712,714, and716. Each switch is a varactor diode configured to provide variable capacitance set by a controller1002.

While the preferred embodiments of the invention have been illustrated and described, it is to be understood that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.