Patent Publication Number: US-6218992-B1

Title: Compact, broadband inverted-F antennas with conductive elements and wireless communicators incorporating same

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to antennas, and more particularly to antennas used with wireless communications devices. 
     BACKGROUND OF THE INVENTION 
     Radiotelephones generally refer to communications terminals which provide a wireless communications link to one or more other communications terminals. Radiotelephones may be used in a variety of different applications, including cellular telephone, land-mobile (e.g., police and fire departments), and satellite communications systems. Radiotelephones typically include an antenna for transmitting and/or receiving wireless communications signals. Historically, monopole and dipole antennas have been employed in various radiotelephone applications, due to their simplicity, wideband response, broad radiation pattern, and low cost. 
     However, radiotelephones and other wireless communications devices are undergoing miniaturization. Indeed, many contemporary radiotelephones are less than 11 centimeters in length. As a result, there is increasing interest in small antennas that can be utilized as internally-mounted antennas for radiotelephones. 
     In addition, it is becoming desirable for radiotelephones to be able to operate within multiple frequency bands in order to utilize more than one communications system. For example, GSM (Global System for Mobile) is a digital mobile telephone system that operates from 880 MHz to 960 MHz. DCS (Digital Communications System) is a digital mobile telephone system that operates from 1710 MHz to 1880 MHz. The frequency bands allocated for cellular AMPS (Advanced Mobile Phone Service) and D-AMPS (Digital Advanced Mobile Phone Service) in North America are 824-894 MHz and 1850-1990 MHz, respectively. Since there are two different frequency bands for these systems, radiotelephone service subscribers who travel over service areas employing different frequency bands may need two separate antennas unless a dual-frequency antenna is used. 
     Inverted-F antennas are designed to fit within the confines of radiotelephones, particularly radiotelephones undergoing miniaturization. As is well known to those having skill in the art, inverted-F antennas typically include a linear (i.e., straight) conductive element that is maintained in spaced apart relationship with a ground plane. Examples of inverted-F antennas are described in U.S. Pat. Nos. 5,684,492 and 5,434,579 which are incorporated herein by reference in their entirety. 
     Conventional inverted-F antennas, by design, resonate within a narrow frequency band, as compared with other types of antennas, such as helices, monopoles and dipoles. In addition, conventional inverted-F antennas are typically large. Lumped elements can be used to match a smaller non-resonant antenna to an RF circuit. Unfortunately, such an antenna would be narrow band and the lumped elements would introduce additional losses in the overall transmitted/received signal, would take up circuit board space, and add to manufacturing costs. 
     High dielectric substrates are commonly used to decrease the physical size of an antenna. Unfortunately, the incorporation of higher dielectrics can reduce antenna bandwidth and may introduce additional signal losses. As such, a need exists for small, internal radiotelephone antennas that can operate within multiple frequency bands, including low frequency bands. 
     SUMMARY OF THE INVENTION 
     In view of the above discussion, the present invention can provide various configurations of compact, broadband inverted-F antennas for use within communications devices, such as radiotelephones. According to one embodiment, an inverted-F antenna has an elongated, meandering conductive element maintained in adjacent, spaced-apart relationship with a first ground plane, such as a printed circuit board. An elongated, meandering conductive element according to this embodiment, includes a set of spaced-apart, U-shaped undulations that extend towards the first ground plane. The U-shaped undulations capacitively couple to the first ground plane and allow the antenna to resonate at lower frequencies than a conventional inverted-F antenna. 
     According to another embodiment of the present invention, a second ground plane may be oriented in a direction transverse to the first ground plane so as to be positioned in adjacent, spaced-apart relationship with one or more of the U-shaped undulations. The one or more U-shaped undulations are capacitively coupled to the second ground plane, as well as to the first ground plane. 
     According to another embodiment of the present invention, one or more raised portions extend outwardly from a ground plane and capacitively couple to portions of an elongated conductive antenna element. 
     According to another embodiment of the present invention, one or more inductive elements may be electrically connected to an elongated conductive element. An inductive element may comprise helical turns formed in an elongated conductive element or one or more electronic components that serve an inductive function. 
     Antennas according to the present invention may be particularly well suited for use within a variety of communications systems utilizing different frequency bands. Furthermore, because of their small size, antennas according to the present invention may be easily incorporated within small communications devices. In addition, antenna structures according to the present invention may not require additional impedance matching networks, which may save internal radiotelephone space and which may lead to manufacturing cost savings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an exemplary radiotelephone within which an antenna according to the present invention may be incorporated. 
     FIG. 2 is a schematic illustration of a conventional arrangement of electronic components for enabling a radiotelephone to transmit and receive telecommunications signals. 
     FIG. 3A is a perspective view of a conventional planar inverted-F antenna. 
     FIG. 3B is a graph of the VSWR performance of the antenna of FIG.  3 A. 
     FIG. 4A is a side elevation view of an inverted-F antenna having an elongated, meandering conductive element with a plurality of U-shaped undulations in spaced-apart, adjacent relationship with a ground plane according to an embodiment of the present invention. 
     FIG. 4B is a side elevation view of the inverted-F antenna of FIG. 4A disposed on a dielectric material. 
     FIG. 4C is a side elevation view of the inverted-F antenna of FIG. 4A disposed within a dielectric material. 
     FIG. 5 is a side elevation view of an inverted-F antenna having an elongated, meandering conductive element in spaced-apart, adjacent relationship with a first ground plane and a second ground plane oriented transverse to the first ground plane, according to an embodiment of the present invention. 
     FIG. 6A is a side elevation view of an inverted-F antenna having an elongated conductive element in spaced-apart, adjacent relationship with a ground plane, and wherein the ground plane has a plurality of raised portions extending towards the elongated, conductive element, according to an embodiment of the present invention. 
     FIG. 6B is a side elevation view of the inverted-F antenna of FIG. 6A disposed within a dielectric material. 
     FIG. 6C is a side elevation view of the inverted-F antenna of FIG. 6A disposed on a dielectric material. 
     FIGS. 7A and 7B are side elevation views of an inverted-F antenna having an inductive element electrically connected to an elongated conductive element on respective sides of an RF signal feed, according to respective embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions may be exaggerated for clarity. Like numbers refer to like elements throughout the description of the drawings. It will be understood that when an element such as a layer, region or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. Moreover, each embodiment described and illustrated herein includes its complementary conductivity type embodiment as well. 
     Referring now to FIG. 1, a radiotelephone  10 , within which antennas according to various embodiments of the present invention may be incorporated, is illustrated. The housing  12  of the illustrated radiotelephone  10  includes a top portion  13  and a bottom portion  14  connected thereto to form a cavity therein. Top and bottom housing portions  13 ,  14  house a keypad  15  including a plurality of keys  16 , a display  17 , and electronic components (not shown) that enable the radiotelephone  10  to transmit and receive radiotelephone communications signals. 
     A conventional arrangement of electronic components that enable a radiotelephone to transmit and receive radiotelephone communication signals is shown schematically in FIG. 2, and is understood by those skilled in the art of radiotelephone communications. An antenna  22  for receiving and transmitting radiotelephone communication signals is electrically connected to a radio-frequency transceiver  24  that is further electrically connected to a controller  25 , such as a microprocessor. The controller  25  is electrically connected to a speaker  26  that transmits a remote signal from the controller  25  to a user of a radiotelephone. The controller  25  is also electrically connected to a microphone  27  that receives a voice signal from a user and transmits the voice signal through the controller  25  and transceiver  24  to a remote device. The controller  25  is electrically connected to a keypad  15  and display  17  that facilitate radiotelephone operation. 
     As is known to those skilled in the art of communications devices, an antenna is a device for transmitting and/or receiving electrical signals. A transmitting antenna typically includes a feed assembly that induces or illuminates an aperture or reflecting surface to radiate an electromagnetic field. A receiving antenna typically includes an aperture or surface focusing an incident radiation field to a collecting feed, producing an electronic signal proportional to the incident radiation. The amount of power radiated from or received by an antenna depends on its aperture area and is described in terms of gain. 
     Radiation patterns for antennas are often plotted using polar coordinates. Voltage Standing Wave Ratio (VSWR) relates to the impedance match of an antenna feed point with a feed line or transmission line of a communications device, such as a radiotelephone. To radiate radio frequency (RF) energy with minimum loss, or to pass along received RF energy to a radiotelephone receiver with minimum loss, the impedance of a radiotelephone antenna is conventionally matched to impedance of a transmission line or feed point. 
     Conventional radiotelephones typically employ an antenna which is electrically connected to a transceiver operably associated with a signal processing circuit positioned on an internally disposed printed circuit board. In order to maximize power transfer between an antenna and a transceiver, the transceiver and the antenna are preferably interconnected such that their respective impedances are substantially “matched,” i.e., electrically tuned to filter out or compensate for undesired antenna impedance components to provide a 50 Ohm (Ω) (or desired) impedance value at the feed point. 
     Referring now to FIG. 3A, a conventional inverted-F antenna is illustrated. The illustrated antenna  30  includes a linear conductive element  32  maintained in spaced apart relationship with a ground plane  34 . Conventional inverted-F antennas, such as that illustrated in FIG. 3A, derive their name from a resemblance to the letter “F.” The conductive element  32  is grounded to the ground plane  34  as indicated by  36 . A hot RF connection  37  extends from underlying RF circuitry through the ground plane  34  to the conductive element  32 . FIG. 3B is a graph of the VSWR performance of the inverted-F antenna  30  of FIG.  3 A. As can be seen, the antenna  30  was designed to radiate at about 2375 Megahertz (MHz). 
     Referring now to FIG. 4A, an inverted-F antenna  40  having an elongated, meandering conductive element  42 , according to an embodiment of the present invention, is illustrated in an installed position within a wireless communications device, such as a radiotelephone. The elongated, meandering conductive element  42  is maintained in adjacent, spaced-apart relationship with a ground plane  44  (e.g., a printed circuit board). A signal feed  45  electrically connects the conductive element  42  to an RF transceiver  24  within a wireless communications device. A ground feed  47  grounds the conductive element  42  to the ground plane  44 . 
     In the illustrated embodiment, the elongated, meandering conductive element  42  includes a first plurality of segments  48  that are spaced apart from the first ground plane by a first distance H 1 . A second plurality of segments  49  are spaced apart from the first ground plane by a second distance H 2  which is greater than the first distance H 1 . The distance H 1 , between the conductive element segments  48  and the ground plane  44  is preferably maintained at between about 1 mm and about 5 mm. The distance H 2  between the conductive element segments  49  and the ground plane  44  is preferably maintained at between about 5 mm and about 15 mm. 
     In the illustrated embodiment, the elongated, meandering conductive element  42  includes a plurality of spaced-apart undulations  50 . Each undulation  50  has a U-shaped configuration that extends towards the ground plane  44 . Each U-shaped undulation  50  in the illustrated embodiment includes a pair of spaced-apart side segments  51  that extend towards the ground plane  44 . Each U-shaped undulation  50  also includes a base segment  48  that connects a respective pair of spaced-apart side segments  51  together. Each base segment  48  is capacitively coupled with the ground plane  44 . 
     In the illustrated embodiment, the base segment of each U-shaped undulation  50  is substantially orthogonal to the respective pair of spaced-apart side segments  51  (and substantially parallel with the ground plane  44 ). It is understood, however, that an elongated, meandering conductive element according to the present invention can have undulations with various shapes and configurations and is not limited to the illustrated U-shaped undulations  50 . 
     Referring now to FIGS. 4B and 4C, alternative embodiments of the present invention are illustrated. In FIG. 4B, an inverted-F antenna  40 ′ has an elongated, meandering conductive element  42  disposed (i.e., formed) on dielectric material  60 . The elongated, meandering conductive element  42  may be formed by etching a conductive layer formed on the dielectric material  60 . In FIG. 4C, an inverted-F antenna  40 ″ has an elongated, meandering conductive element  42  disposed within dielectric material  60 ′ (e.g., a dielectric substrate). 
     Referring to FIG. 5, the embodiment of FIG. 4A has been modified to include a second ground plane  70  that is oriented in a direction transverse to the first ground plane  44 . The illustrated second ground plane  70  is in adjacent, spaced-apart relationship with the U-shaped undulations  50 . Preferably, the second ground plane  70  is spaced apart from the U-shaped undulations  50  by a distance of less than or equal to 10 mm. 
     In the illustrated embodiment of FIG. 5, the U-shaped undulations  50  are capacitively coupled to the second ground plane  70 , as well as to the first ground plane  44 . The second ground plane  70  is not limited to the illustrated embodiment. The second ground plane  70  may be configured to be in adjacent, spaced apart relationship with one or more portions of the elongated, meandering conductive element  42 . For example, the second ground plane  70  may be in adjacent, spaced apart relationship with a single U-shaped undulation  50 . Alternatively, the second ground plane  70  may be in adjacent, spaced apart relationship with selected U-shaped undulations  50 . Multiple second ground planes also may be provided. 
     Referring now to FIGS. 6A-6C, additional embodiments of the present invention are illustrated. In FIG. 6A, an inverted-F antenna  140  having an elongated conductive element  142 , according to an embodiment of the present invention, is illustrated in an installed position within a wireless communications device, such as a radiotelephone. The elongated conductive element  142  is maintained in adjacent, spaced-apart relationship with a ground plane  44 . A signal feed  45  electrically connects the conductive element  142  to an RF transceiver  24  within a wireless communications device. A ground feed  47  grounds the conductive element  142  to the ground plane  44 . 
     In the illustrated embodiment, a plurality of raised portions  80  extend outwardly from the ground plane  44 . The illustrated grounded portions  80  may be extensions formed within a printed circuit board. The illustrated elongated conductive element  142  is spaced apart from the ground plane by a distance H 2 , and from each of the raised portions  80  by a distance H 1  that is less than the distance H 2 . The elongated conductive element  142  is capacitively coupled to the raised portions  80  of the ground plane  44 . 
     The distance H 1  between the conductive element  142  and the ground plane  44  is preferably maintained at between about 1 mm and about 5 mm. The distance H 2  between the conductive element  142  and the raised portions  80  extending from the ground plane  44  is preferably maintained at between about 5 mm and about 15 mm. 
     A ground plane incorporating raised portions  80  can be thought of as a meandering ground plane. The raised portions  80  can be thought of as spaced-apart undulations. An inverted-F antenna incorporating a meandering ground plane can resonate similarly to an inverted-F antenna having a meandering conductive element. The antenna of FIG. 4A is equivalent to the antenna of FIG.  6 A. 
     Referring now to FIGS. 6B and 6C, alternative embodiments of the antenna of FIG. 6A are illustrated. In FIG. 6B, an inverted-F antenna  140 ′ has an elongated conductive element  142  disposed within dielectric material  60  (e.g., a dielectric substrate). In FIG. 6C, an inverted-F antenna  140 ″ has an elongated conductive element  142  formed on a dielectric material  60 ′ (e.g., a dielectric substrate). 
     Referring now to FIGS. 7A and 7B, inverted-F antennas according to the present invention may include one or more inductive elements  90 . One or more inductive elements  90  may be electrically connected to the elongated conductive element  142  between the RF signal feed  45  and the ground feed  47 , as illustrated in FIG.  7 A. Alternatively, one or more inductive elements  90  may be electrically connected to the elongated conductive element  142  adjacent the RF signal feed  45  as illustrated in FIG.  7 B. An inductive element  90  may comprise helical turns formed in the elongated conductive element  142 . Alternatively, various electronic components that can serve an inductive function may be electrically connected to the elongated conductive element  142 . 
     In each of the above-illustrated embodiments, a preferred conductive material out of which an elongated conductive element ( 42  of FIGS. 4A-4C and FIG. 5;  142  of FIGS. 6A-6C and FIGS. 7A-7B) may be formed is copper. For example, the conductive elements  42 ,  142  may be formed from copper wire. Alternatively, the conductive elements  42 ,  142  may be a copper trace disposed on or within a substrate, as illustrated in FIGS. 4B,  4 C,  6 B,  6 C. However, an elongated conductive element according to the present invention may be formed from various conductive materials and is not limited to copper. 
     The elongated conductive element  42 ,  142  is typically 0.5 ounce (14 grams) copper. However, conductive elements  42 ,  142  according to the present invention may have various thicknesses. The width of an elongated conductive element according to the present invention may vary (either widened or narrowed), and need not remain constant. 
     Antennas according to the present invention may also be used with wireless communications devices which only transmit or receive radio frequency signals. Such devices which only receive signals may include conventional AM/FM radios or any receiver utilizing an antenna. Devices which only transmit signals may include remote data input devices. 
     The foregoing is illustrative of the present invention and is not to be construed as limiting thereof. Although a few exemplary embodiments of this invention have been described, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the claims. Therefore, it is to be understood that the foregoing is illustrative of the present invention and is not to be construed as limited to the specific embodiments disclosed, and that modifications to the disclosed embodiments, as well as other embodiments, are intended to be included within the scope of the appended claims. The invention is defined by the following claims, with equivalents of the claims to be included therein.