Low profile built-in multi-band antenna

A built-in, low-profile antenna having an inverted planar inverted F-type (PIFA) antenna and a meandering parasitic element having a wide bandwidth to facilitate communications within a plurality of frequency bands is disclosed. The main element is placed at a predetermined height above a substrate of a communication device and the parasitic element is placed on the same substrate as the main antenna element and is grounded at one end. The feeding pin of the PIFA is proximal to the ground pin of the parasitic element. The coupling of the meandering, parasitic element to the main antenna results in two resonances. These two resonances are adjusted to be adjacent to each other in order to realize a broader resonance encompassing the DCS, PCS and UMTS frequency ranges.

BACKGROUND
 The present invention relates generally to radio communication systems and,
 in particular, to built-in antennas incorporated into portable terminals
 and having a wide bandwidth to facilitate operation of the portable
 terminals within different frequency bands.
 The cellular telephone industry has made phenomenal strides in commercial
 operations in the United States as well as the rest of the world. Growth
 in major metropolitan areas has far exceeded expectations and is rapidly
 outstripping system capacity. Innovative solutions are required to meet
 these increasing capacity needs as well as maintain high quality service
 and avoid rising prices.
 Throughout the world, one important step in the advancement of radio
 communication systems is the change from analog to digital transmission.
 Equally significant is the choice of an effective digital transmission
 scheme for implementing the next generation technology, e.g., time
 division multiple access (TDMA) or code division multiple access (CDMA).
 Furthermore, it is widely believed that the first generation of Personal
 Communication Networks (PCNs), employing low cost, pocket-sized, cordless
 telephones that can be carried comfortably and used to make or receive
 calls in the home, office, street, car, etc., will be provided by, for
 example, cellular carriers using the next generation digital cellular
 system infrastructure.
 To provide an acceptable level of equipment compatibility, standards have
 been created in various regions of the world. For example, analog
 standards such as AMPS (Advanced Mobile Phone System), NMT (Nordic Mobile
 Telephone) and ETACS and digital standards such as D-AMPS (e.g., as
 specified in EIA/TIA-IS-54-B and IS-136) and GSM (Global System for Mobile
 Communications adopted by ETSI) have been promulgated to standardize
 design criteria for radio communication systems. Once created, these
 standards tend to be reused in the same or similar form, to specify
 additional systems. For example, in addition to the original GSM system,
 there exists the DCS1800 (specified by ETSI) and PCS1900 (specified by JTC
 in J-STD-007), both of which are based on GSM. A recent evolution in
 cellular communication services involves the adoption of additional
 frequency bands for use in handling mobile communications, e.g., for
 Personal Communication Services (PCS) services. Taking the U.S. as an
 example, the Cellular hyperband is assigned two frequency bands (commonly
 referred to as the A frequency band and the B frequency band) for carrying
 and controlling communications in the 800 MHZ region. The PCS hyperband,
 on the other hand, is specified in the United States to include six
 different frequency bands (A, B, C, D, E and F) in the 1900 MHZ region.
 Thus, eight frequency bands are now available in any given service area of
 the U.S. to facilitate communication services. Certain standards have been
 approved for the PCS hyperband (e.g., PCS1900 (J-STD-007)), while others
 have been approved for the Cellular hyperband (e.g., D-AMPS (IS-136)).
 Other frequency bands in which these devices will be operating include GPS
 (operating in the 1.5 GHz range) and UMTS (operating in the 2.0 GHz
 range).
 Each one of the frequency bands specified for the Cellular and PCS
 hyperbands is allocated a plurality of traffic channels and at least one
 access or control channel. The control channel is used to control or
 supervise the operation of mobile stations by means of information
 transmitted to and received from the mobile stations. Such information may
 include incoming call signals, outgoing call signals, page signals, page
 response signals, location registration signals, voice channel
 assignments, maintenance instructions, hand-off, and cell selection or
 reselection instructions as a mobile station travels out of the radio
 coverage of one cell and into the radio coverage of another cell. The
 control and voice channels may operate using either analog modulation or
 digital modulation.
 The signals transmitted by a base station in the downlink over the traffic
 and control channels are received by mobile or portable terminals, each of
 which have at least one antenna. Historically, portable terminals have
 employed a number of different types of antennas to receive and transmit
 signals over the air interface. For example, monopole antennas mounted
 perpendicularly to a conducting surface have been found to provide good
 radiation characteristics, desirable drive point impedances and relatively
 simple construction. Monopole antennas can be created in various physical
 forms. For example, rod or whip antennas have frequently been used in
 conjunction with portable terminals. For high frequency applications where
 an antenna's length is to be minimized, another choice is the helical
 antenna.
 In addition, mobile terminal manufacturers encounter a constant demand for
 smaller and smaller terminals. This demand for miniaturization is combined
 with desire for additional functionality such as having the ability to use
 the terminal at different frequency bands and different cellular systems.
 It is commercially desirable to offer portable terminals which are capable
 of operating in widely different frequency bands, e.g., bands located in
 the 1500 MHZ, 1800 MHZ, 1900 MHZ, 2.0 GHz and 2.45 GHz regions.
 Accordingly, antennas which provide adequate gain and bandwidth in a
 plurality of these frequency bands will need to be employed in portable
 terminals. Several attempts have been made to create such antennas.
 Japanese patent no. 6-37531 discloses a helix which contains an inner
 parasitic metal rod. In this patent, the antenna can be tuned to dual
 resonant frequencies by adjusting the position of the metal rod.
 Unfortunately, the bandwidth for this design is too narrow for use in
 cellular communications.
 Dual-band, printed, monopole antennas are known in which dual resonance is
 achieve by the addition of a parasitic strip in close proximity to a
 printed monopole antenna. While such an antenna has enough bandwidth for
 cellular communications, it requires the addition of a parasitic strip.
 Moteco AB in Sweden has designed a coil matching dual-band whip antenna
 and coil antenna, in which dual resonance is achieved by adjusting the
 coil matching component (1/4.lambda. for 900 MHZ and 1/2.lambda. for 1800
 MHZ). This antenna has relatively good bandwidth and radiation
 performances and a length in the order of 40 mm. A non-uniform helical
 dual-band antenna which is relatively small in size is disclosed in
 copending, commonly assigned U.S. patent application Ser. No. 08/725,507,
 entitled "Multiple Band Non-Uniform Helical Antennas."
 Conventional built-in antennas currently in use in mobile phones include
 microstrip antennas and planar inverted-F antennas. Microstrip antennas
 are small in size and light in weight. The planar inverted-F antenna
 (PIFA) has already been implemented in a mobile phone handset, as
 described by K. Qassim, "Inverted-F Antenna for Portable Handsets", IEE
 Colloqium on Microwave Filters and Antennas for Personal Communication
 Systems, pp.3/1-3/6, February 1994, London, UK. More recently, Lai et al.
 have published a description of a meandering inverted-F antenna (WO
 96/27219). This antenna has a size which is about 40% of that of the
 conventional PIFA antenna.
 However, as mobile phones become smaller and smaller, both conventional
 microstrip patch and PIFA antennas are still too large to fit future phone
 chassis. In copending, commonly assigned U.S. patent application No.
 09/112,366, entitled "Miniature Printed Spiral Antenna for Mobile
 Terminals", a printed spiral built-in antenna with a matching post was
 proposed. The size of the antenna was reduced to 20-30% of the
 conventional PIFA antenna, which is less than 1/10.sup.th of a wavelength,
 in order to make it suitable for future mobile phones.
 In addition to a reduced antenna size, next generation mobile phones will
 require the capability to tune to more than one frequency band for
 cellular, wireless local area network, GPS and diversity. In copending,
 commonly assigned U.S. patent application Ser. No. 09/112,152, entitled
 "Twin Spiral Dual Band Antenna", a multiple band, built-in antenna was
 proposed which is suitable for future mobile phones. The built-in antenna
 comprises two spiral conductor arms which are of different lengths and
 capable of being tuned to different frequency bands. In this design, the
 bandwidth of the antenna is smaller because thin strip lines are used as
 radiators. In order to increase bandwidth of the antenna, a compensation
 method is used by introducing a resistor loading technique on the matching
 bridge. While this approach leads to a wider bandwidth, it also results in
 a loss of gain. This antenna is designed for use in two frequency bands.
 In copending, commonly assigned U.S. patent application Ser. No.
 09/212,259, entitled "Printed Multi-Banded Patch Antenna", another new
 type of dual band patch antenna is disclosed. In contrast to the twin
 spiral dual band antenna which uses thin strip lines as radiators, the
 multi-band patch antenna uses patches with slot cutting. The patches are
 used as radiators and facilitate a wider bandwidth. The multi-band patch
 antenna is also designed for two frequency bands.
 FIG. 1 illustrates the geometry of a conventional PIFA antenna 100. The
 PIFA antenna includes a radiating element 110, a feeding pin 120 for the
 radiating element, a ground pin 130 for the radiating element and a
 printer circuit band (PCB) ground 140. The radiating element 110 is
 suspended above the PCB ground 140 in such a manner that the PCB ground
 140 covers the area under the radiating element 110. This type of antenna,
 however, has a small bandwidth in the order of 100 MHZ. In order to
 increase the bandwidth for an antenna of this design, the vertical
 distance between the radiating element and the PCB ground has to be
 increased (that is, the height at which the radiating element 110 is
 placed above the PCB 140 is increased). This, however, is an undesirable
 modification as the height increase makes the antenna unattractive for
 small communication devices.
 An alternative method for obtaining a greater bandwidth is illustrated by
 the antenna 200 of FIG. 2 which corresponds to the antenna design of U.S.
 patent application No. 09/507,673 referred to above. The PCB board 240 of
 the antenna 200 does not cover the entire area under the radiating element
 210. This increases the distance between the radiating element 210 and the
 PCB ground 240. That is, the radiating element 210 extends out from the
 edge of the PCB 240. While the design of antenna 200 leads to a greater
 bandwidth than antenna 100 of FIG. 1, it is not adequate for covering the
 frequency bands corresponding to DCS, PCS and UMTS.
 The antennas described above lack adequate bandwidth to cover, for example,
 all of the DCS, PCS and UMTS frequency bands. Therefore, there exists a
 need for a lowprofile, built-in antenna which can be incorporated into
 portable terminals and which allow the portable terminals to communicate
 within the different frequency bands.
 SUMMARY
 The present invention overcomes the above-identified deficiencies in the
 art by providing a low-profile, built-in antenna with a wide bandwidth
 which enables the antenna to be operable at a plurality of frequency bands
 corresponding to the DCS, PCS and UMTS frequency ranges.
 This is accomplished by a built-in planar inverted F-type antenna (PIFA)
 having a main radiating element located at a first predetermined height
 above a substrate within said communication device and tuned to a first
 frequency range, and a parasitic element located at a second predetermined
 height above said substrate and tuned to a second frequency range that is
 different from said first frequency range.
 In another exemplary embodiments, the antenna comprises a built-in planar
 inverted F-type antenna (PIFA) having a main radiating element located at
 a first predetermined height above a substrate and at a first
 predetermined distance from an edge of said substrate within said
 communication device and tuned to a first frequency range, and a parasitic
 element located at a second predetermined distance from the edge of said
 substrate and tuned to a second frequency range that is different from
 said frequency range

DETAILED DESCRIPTION
 In the following description, for purposes of explanation and not
 limitation, specific details are set forth, such as particular circuit
 components, antenna elements, techniques, etc. in order to provide a
 thorough understanding of the present invention. However, it will be
 apparent to one skilled in the art that the present invention may be
 practiced in other embodiments that depart from these specific details. In
 other instances, detailed descriptions of well-known methods and elements
 are omitted so as not to obscure the description of the present invention.
 The above mentioned limitations of conventional antennas are overcome by
 exemplary embodiments of the present invention which provide a greater
 bandwidth thus facilitating operation of the communication device in the
 DCS, PCS and UMTS frequency ranges. This embodiment is illustrated in FIG.
 3. The dimensions of the antenna 200 of FIG. 2 remain constant. The wider
 bandwidth is realized by providing a parasitic, meandering radiating
 element 350 in addition to the main radiating element 310.
 According to an exemplary embodiment of the present invention which
 facilitates an increased bandwidth, the antenna 300 comprises a main
 radiating element 310 (in the form of a PIFA), a feeding pin 320 for the
 main radiating element 310, and a ground pin 330 for connecting the main
 radiating element 310 to the PCB ground 340. The main radiating element
 310 (with the feeding pin 320 and ground pin 330) is placed at a
 predetermined height with respect to the PCB ground 340. The antenna 300
 is similar in structure to antenna 200 of FIG. 2. However, an additional
 element in the form of a meandering, parasitic element 350 is included
 which is in the same plane as the PCB ground 340; that is, the parasitic
 element is at the same height as the PCB ground. The parasitic element 350
 is connected at one end to the PCB ground 340.
 The parasitic element 350 creates an additional resonance. This additional
 resonance can be adjusted so that it occurs near or adjacent the higher
 resonance frequency of the main antenna element 310. As a result, the two
 resonances merge into a broader resonance. According to exemplary
 embodiments of Applicants' invention, there are additional tuning
 parameters for the antenna 300 beside the thickness of the antenna
 substrate, positions of the feeding pin 320 and ground pin 330. These
 additional parameters are the distance between the PCB ground 340 and main
 radiating element 310, distance between the main element 310 and parasitic
 element 350 as well as the length of each of the main element 310 and the
 parasitic element 350. In particular, to achieve a greater bandwidth, the
 distance between the feeding pin 320 of the main radiating element 310 and
 the parasitic element 350 is minimized. This distance may, for example, be
 approximately 0.5 mm. The radiating element 310 and the parasitic element
 350 also have a low-profile in order to enable the placement of the
 antenna on a circuit board of a cellular telephone, for example.
 The bandwidth of antenna 300 of FIG. 3 is limited by the thickness of the
 antenna substrate. If this thickness (i.e., of the substrate) is
 increased, the bandwidth of the antenna increases. In the alternative, a
 parasitic element, such as element 350, can be used to obtain a resonance
 that is distinct and separate (i.e., not adjacent) from the resonance of
 the main element if a particular application requires such an arrangement
 (i.e., two distinct resonances that do not merge into one resonance).
 The dimensions of the antenna 300 are similar to that of antenna 200. The
 presence of the parasitic element 350 results in a much wider bandwidth.
 The voltage standing wave ratio (VSWR) for the antenna arrangement of FIG.
 3 is illustrated in FIG. 5. As shown, for a VSWR of less than 2.5:1, the
 bandwidth is approximately 600 MHZ.
 VSWR values can range from 1 to infinity and indicate the amount of
 interference between two waves traveling in opposite direction in a
 transmission line feeding the antenna ad thus describes the rate of the
 matching of the antenna to the desired impedance (usually about 50 ohms).
 One of the waves is the source feeding the antenna while the other is the
 reflection from the antenna back to the transmission line. The objective
 in designing an antenna is to minimize this reflection. The maximum VSWR
 value of infinity occurs when the reflected wave has the same intensity as
 the incident one. That is, the whole signal is reflected and no power is
 provided to the radiating element. The minimum VSWR of 1 occurs when the
 antenna is perfectly matched; that is, no power is reflected. An antenna
 may operate efficiently when the VSWR value is approximately less than 2.5
 at the frequencies of operation.
 The position of the feeding pin 320 and ground pin 330 as well as the
 lengths of the main radiating element 310 and parasitic element 350 are
 used for matching and tuning the antenna 300. The dimensions of the
 antenna 300 are approximately 39 mm length, 14 mm width and 4 mm height.
 The length of the main radiating element 310 is approximately 24 mm and
 that of the parasitic element 350 is approximately 40 mm. These particular
 dimensions enable this antenna to be placed in a communication device such
 as a cellular phone circuit board, for example. The antenna substrate 340
 is made of porous material which has a dielectric permitivity (.di-elect
 cons..sub.r) of 1 and a loss tangent (tan .delta.) of almost zero. These
 dimensions yield a bandwidth of over 600 MHZ in the 1600 MHZ to 2200 MHZ
 frequency range.
 A second exemplary embodiment of the present invention is illustrated in
 FIG. 4. The antenna 400 is similar in structure to antenna 300 of FIG. 3.
 However, the parasitic element 450 is not at the same plane as the PCB
 ground 440. In addition, the PCB ground 440 is below the antenna 400. The
 length of the main radiating element 410 is approximately 20 mm and that
 of the parasitic element 450 is approximately 45 mm. While this particular
 design results in smaller bandwidth than that of antenna 300, the
 bandwidth realized is much greater than the PIFA antenna 200, for example.
 The VSWR of antenna 300 of FIG. 3 according to the dimensions specified
 above is illustrated in FIG. 5. As shown, for a ratio of less than 2.5:1,
 the bandwidth is approximately 600 MHZ which is more than adequate for the
 desired DCS/PCS/UMTS application.
 In order to illustrate the effectiveness of the present invention, FIG. 5
 sets forth results of a measurement for the exemplary antenna illustrated
 in FIG. 3. As seen in FIG. 5, for a VSWR of approximately 2.5:1, the
 bandwidth ranges from approximately 1.675 GHz to 2.34 Ghz resulting in a
 bandwidth of approximately 650 MHZ. Purely for purposes of illustrating
 the present invention, the following values for the various parameters
 enumerated above for an antenna may be used. The substrate may be porous
 material.
 The type of material used for the substrate affects the antenna
 performance. Therefore, if the substrate material is altered (for example,
 from porous to some other material), the antenna may have to be re-tuned.
 If the dielectric constant (i.e., the permitivity constant) of the
 material is increased, the bandwidth decreases. The present invention,
 however, is not limited to porous material. Therefore, other materials
 with reasonable electric parameters will also provide an adequate
 bandwidth for the antenna of the present invention.
 FIG. 6 illustrates an exemplary communication device, such as a cellular
 telephone 600 that can operate in any of the DCS, PCS and UMTS frequency
 ranges. Communication device 600 includes a chassis 610 having a
 microphone opening 620 and speaker opening 630 located approximately next
 to the position of the mouth and ear, respectively, of a user. A keypad
 640 allows the user to interact with the communication device, e.g., by
 inputting a telephone number to be dialed. The communication device 600
 also includes a PIFA antenna with a meandering, parasitic element 650.
 The foregoing has described the principles, preferred embodiments and modes
 of operation of the present invention. However, the invention should not
 be construed as being limited to the particular embodiments discussed
 above. For example, while the antenna of the present invention has been
 discussed primarily as being a radiator, one skilled in the art will
 appreciate that the antenna of the present invention would also be used as
 a sensor for receiving information at specific frequencies. Similarly, the
 dimensions of the various elements (such as, the substrate) may vary based
 on the specific application. Thus, the above-described embodiments should
 be regarded as illustrative rather than restrictive, and it should be
 appreciated that variations may be made in those embodiments by workers
 skilled in the art without departing from the scope of the present
 invention as defined by the following claims.