Abstract:
A quadrifilar helix antenna used to transmit and receive wireless signals may include a number of inductively loaded antenna elements. The antenna elements may be helically wound around a cylindrical structure.

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 based on U.S. Provisional Application No. 60/807,110 filed Jul. 12, 2006, U.S. Provisional Application Ser. No. 60/807,112 filed Jul. 12, 2006, and U.S. Provisional Application Ser. No. 60/807,107 filed Jul. 12, 2006, the disclosures of which are all hereby incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to miniaturization of resonant antennas and, more particularly, to miniaturization of resonant antennas for hand-held terminals used for mobile wireless communications. 
     BACKGROUND 
     In wireless communications involving artificial satellites, a circular polarized antenna with near omni-directional coverage is generally required such that the link is not interrupted by mobility at either end of the communications link. For simultaneous two-way communications, two separate communication channels or frequency bands are used. The electromagnetic frequency spectrum used for this purpose generally falls within the range of 700 megahertz (MHz) through 7500 MHz. In one such application for commercial purposes, the transmit band for terrestrial terminals is 1626.5 MHz through 1660.5 MHz and the receive band is 1525 MHz through 1559 MHz. 
     A quadrifilar helix antenna (QHA) is capable of performing these communication functions for hand-held terminals. In order for optimum power transfer to take place through the antenna, such antennas operate at resonance for both the channels (i.e., receive and transmit). 
     The QHA typically includes an array of monopoles twisted into a helical structure and connected by a non-radiating feed structure to excite the desired sense of circular polarization of the desired sense (right handed or left handed). The antenna is constricted with less than a full turn such that the radiation pattern of the antenna becomes substantially omni-directional without any null occurring in the communication space. This condition is generally known as the normal mode of operation of the helical antenna. The antenna elements are generally constructed as printed conducting strips on a thin dielectric substrate. The frequency bands for communication determines the resonant length of the antenna elements and hence the antenna height, whereas the radiation pattern or antenna gain requirements determine the diameter to height ratio in terms of the operating central frequency of the band. 
     Convenience and ease of operation creates the need to miniaturize the antenna such that the individual consumer can easily carry the hand-held terminal. Wireless signals transmitted directly from a satellite may not be strong enough to penetrate walls to reach a person inside a building or operating in a city area with high rise building. However, a hybrid system generally known in the industry as Mobile Satellite Services (MSS) with Ancillary Terrestrial Components (ATC) overcomes this shortcoming by using satellites integrally with terrestrial cellular wireless communication networks. This advancement in mobile wireless communication technology has created a consumer demand to have hand-held satellite-cellular terminals that are thinner and smaller. 
     For the frequency bands of mobile wireless communications in the frequency range of 1000 MHz through 3000 MHz, the regular QHAs for the transmit and the receive bands are about the size of a cigar. For example, the QHA may have a length of approximately six to seven inches and diameter of about 0.75 inches. 
     SUMMARY 
     Aspects described herein provide a QHA having a very small size. For example, aspects described herein provide a QHA that is approximately 2.75 inches to 3 inches in length or less and approximately 0.3 to 0.35 inch in diameter or less. This enables the hand-held terminal in which the QHA is implemented to have a reduced size. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference is made to the attached drawings, wherein elements having the same identifier/name may represent like elements throughout. 
         FIG. 1  illustrates an exemplary network in which devices, systems and methods described herein may be implemented. 
         FIG. 2  is an exemplary block diagram of the mobile terminal of  FIG. 1 . 
         FIG. 3  illustrates a schematic diagram of an exemplary QHA. 
         FIG. 4  is an exemplary diagram of unfurled printed conducting strip antenna elements of a QHA. 
         FIG. 5  is an exemplary diagram of an unfurled printed conducting strip antenna element. 
         FIG. 6  illustrates an exemplary inductively loaded printed conducting strip antenna element. 
         FIG. 7  illustrates an exemplary capacitively loaded printed conducting strip antenna element. 
         FIG. 8  illustrates an exemplary reactively loaded printed conducting strip antenna element. 
         FIG. 9  illustrates an exemplary printed conducting strip antenna element having a constant pitch angle. 
         FIG. 10  illustrates an exemplary inductively loaded printed conducting strip antenna element with a discretely variable pitch angle. 
         FIG. 11  illustrates exemplary inductively loaded printed conducting strip antenna elements with a discretely variable pitch angle. 
         FIG. 12  illustrates an exemplary conducting strip antenna element in which a portion of the antenna element has a constant pitch angle and another portion of the antenna element has a discretely variable pitch angle. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description of the invention refers to the accompanying drawings. The same reference numbers in different drawings may identify the same or similar elements. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims and equivalents. 
     Exemplary Network 
       FIG. 1  is a diagram of an exemplary network  100  in which devices, systems and methods described herein may be implemented. Referring to  FIG. 1 , network  100  may include mobile terminal  110  (also referred to as a hand-held terminal), communication device  120 , terrestrial network  130  and satellite  140 . The number of devices shown in network  100  is provided for simplicity. It should be understood that network  100  may include additional devices that aid in the transmission and reception of information, as well as additional mobile terminals and communication devices. 
     Mobile terminal  110  may include components for transmitting and receiving radio frequency (RF) signals via terrestrial network  130  and satellite  140 . In an exemplary implementation, mobile terminal  110  may include a cellular radiotelephone, a Personal Communications System (PCS) terminal that may combine a cellular radiotelephone with other data processing/communications capabilities; a personal digital assistant (PDA), a conventional laptop and/or palmtop receiver or other appliance that includes a radiotelephone transceiver. In an exemplary implementation, mobile terminal  110  may be configured to communicate with other devices/systems, such as communication device  120 , via terrestrial network  130  and/or via satellite  140 . In an exemplary implementation, mobile terminal  110  may communicate with terrestrial network  130  using, for example, the L-band, the S-band, or another RF band. 
     Communication device  120  may include any type of device that is capable of transmitting and receiving voice signals and/or data signals to/from a network. For example, communication device  120  may include any conventional telephone that interfaces with, for example, the public switched telephone network (PSTN) or a wireless network to place and receive telephone calls. Communication device  120  may be a standard PSTN-based telephone, a cordless telephone, a cellular telephone, a PDA, a mobile device similar to mobile terminal  110  or another type of conventional telephone. 
     Communication device  120  may also include any client, such as a computer device, web-based appliance, etc., that is configured to provide telephone functions. For example, communication device  120  may be a session initiation protocol (SIP)-based telephone. 
     Terrestrial network  130  may include one or more wired and/or wireless networks that are capable of receiving and transmitting data and voice signals. For example, terrestrial network  130  may include one or more PSTNs or other type of switched network. Terrestrial network  130  may also include packet switched networks, such as the Internet, an intranet, a wide area network (WAN), a metropolitan area network (MAN) or another type of network capable of transmitting data from a source device to a destination device. 
     Terrestrial network  130  may also include one or more earth-based cellular networks that include components for transmitting and receiving data and voice signals using RF communications. Such components may include base station antennas and transmission towers (not shown) that transmit and receive data from mobile terminals within their vicinity. Such components may also include base stations (not shown) that connect to the base station antennas and communicate with other devices, such as switches and routers (not shown) in accordance with known techniques. 
     Satellite  140  may represent one or more space-based components that are included in a satellite-based network. Satellite  140  may communicate with mobile terminal  110  and other devices in system  100 , such as various gateways, routers, etc., that interface with other networks, such as terrestrial network  130 . Satellite  140  may communicate with mobile terminal  110  using, for example, the L-band, the S-band, or another RF band. 
       FIG. 2  is a block diagram illustrating an exemplary configuration of mobile terminal  110 . Mobile terminal  110  may include bus  210 , processing logic  220 , memory  230 , input device  240 , output device  250 , communication interface  260  and antenna  270 . Bus  210  permits communication among the components of mobile terminal  110 . One skilled in the art would recognize that mobile terminal  110  may be configured in a number of other ways and may include other or different elements. For example, mobile terminal  110  may include one or more power supplies (not shown). Mobile terminal  110  may also include a modulator, a demodulator, an encoder, a decoder, etc., for processing data. 
     Processing logic  220  may include a processor, microprocessor, an application specific integrated circuit (ASIC), field programmable gate array (FPGA) or the like. Processing logic  220  may, in some implementations, execute software instructions/programs or data structures to control operation of mobile terminal  110 . 
     Memory  230  may include a random access memory (RAM) or another type of dynamic storage device that stores information and instructions for execution by processing logic  220 ; a read only memory (ROM) or another type of static storage device that stores static information and instructions for use by processing logic  220 ; a flash memory (e.g., an electrically erasable programmable read only memory (EEPROM)) device for storing information and instructions; and/or some other type of magnetic or optical recording medium and its corresponding drive. Memory  230  may also be used to store temporary variables or other intermediate information during execution of instructions by processing logic  220 . Instructions used by processing logic  220  may also, or alternatively, be stored in another type of computer-readable medium accessible by processing logic  220 . 
     Input device  240  may include one or more mechanisms that permits an operator to input information to mobile terminal  110 . For example, input device may include a microphone, a keyboard, a keypad, a mouse, a pen, voice recognition and/or biometric mechanisms, etc. Input device  240  may be used to facilitate placing telephone calls to other devices, carrying on a conversation, etc. 
     Output device  250  may include one or more mechanisms that output information to the user, including a display, one or more speakers, a printer, etc. Output device  250  may be used to facilitate receiving telephone calls from other devices, carrying on a conversation, etc. 
     Communication interface  260  may include any transceiver-like mechanism that enables mobile terminal  110  to communicate with other devices and/or systems. For example, communication interface  260  may include mechanisms for communicating via a network, such as a wireless network. For example, communication interface  260  may include one or more radio frequency (RF) transmitters and receivers and/or transceivers, used to transmit and receive RF signals via antenna  270 . 
     Antenna  270  may include one or more antennas, transmitters and receivers that enable mobile terminal  110  to communicate with terrestrial network  130  using, for example, L band, S band or another RF band. In an exemplary implementation, antenna  270  may include one or more QHAs, as described in detail below. 
     The size of the antenna, such as a QHA  270 , may be represented by the length and diameter of a cylindrical frame on which antenna elements reside and may be dictated by the frequency bands of the transmit and receive signals and the requirements of near omni-directional coverage. Wireless communications with mobile terminal  110  take place when QHA  270  is in resonance. During communications, wireless signals being processed through QHA  270  experience an impedance which has a real component and an imaginary component. The imaginary component of the impedance (i.e., reactance) at a particular frequency component of the communicating signal is either positive (inductive) or negative (capacitive). QHA  270  is said to be in resonance when this imaginary part of the impedance is zero at the center frequency of the signal band. 
     The resonance condition of QHA  270  depends on various structural parameters of the antenna elements and various physical parameters that define the surrounding electrical environment. The resonant frequency of each antenna element and hence, QHA  270  itself, can be lowered by increasing the reactive component of the antenna element impedance. Aspects described herein provide for optimal modification of each of the antenna elements to increase its reactance within the available real estate for the antenna element, so as to reduce the size of the antenna by lowering the resonant frequency. 
       FIG. 3  is a schematic diagram of antenna  270  according to an exemplary implementation. In this implementation, antenna  270  is a QHA that includes cylindrical structure  310  having a height H and diameter D. In accordance with one implementation, H may be approximately 2.75 inches and D may be approximately 0.35 inches. Other heights and diameters may be possible in other implementations. For example, H may range from about 2.75 inches to about 3 inches, or less, and D may range from about 0.3 inches to 0.35 inches or less. Cylindrical structure  310  may be coupled to feed structure  320 . 
     As illustrated in  FIG. 3 , structure  310  includes multiple conductive strips  312  (e.g., four conductive strips  312  that form a four element array) of conducting monopoles helically wound around cylindrical structure  312  in the longitudinal direction. In the implementation illustrated in  FIG. 3 , each of the conductive strips  312  has a square wave-like pattern. 
     The four conductive strips  312  (i.e., antenna elements) may each be connected to feed structure  320  to provide circular polarization capability of the desired sense to the wireless signals processed by antenna  270  for transmission and reception. Feed structure  320  may include an impedance matching structure  322  and a power divider  324  to allow for maximum power transfer within the operating bands. However, in an exemplary implementation, feed structure  320  and the antenna elements may be designed to be impedance matched to each other without the need for a separate impedance matching structure  322 . 
     Each of the conductive strips  312  (i.e., antenna elements), and antenna  270  as a whole, is a resonant device with multiple resonances determined by the electrical lengths of the antenna elements. Each of the discrete resonances can be expressed by an equivalent inductance L eq  and a capacitance C eq  in the following known expression. 
               f   reso     =     1     2   ⁢   π   ⁢         L   eq     ⁢     C   eq                   
where f reso  is a resonant frequency. If the transmit and receive frequency bands are located close enough to each other, a single resonance can perform both the transmit and receive functions by accommodating the two bands and providing necessary isolation between the separate transmit and receive signals outside of antenna  270 . Otherwise, two separate resonant antennas may be used to provide the transmit and the receive functions.
 
     The electrical length L of antenna element  312  that corresponds to a resonance can be reduced by: 1) increasing the equivalent inductance L eq ; 2) increasing the equivalent capacitance C eq ; or 3) reducing both the equivalent inductance L eq  and the equivalent capacitance C eq . In this manner, the size of antenna elements  312  (e.g., the length of conductive strips  312 ) in the QHA may be reduced, thereby reducing the overall size of the QHA. 
       FIG. 4  illustrates the unfurled surface area of the QHA with a cylindrical helical structure containing four printed antenna elements  312 . Each antenna element  312  is illustrated as a linear conducting strip  312 . It should be understood that more or fewer conducting strips  312  may be used in other implementations and the size and/or shape of the conducting strips may be different in other implementations. 
       FIG. 5  illustrates a printed antenna element  500  with distributive inductive loading that may be used for antenna  270 . Referring to  FIG. 5 , antenna element  500  may include alternately opposing or inverted omega shaped portions  510  connected to each other via straight line portions  512 . In this implementation, a number of the antenna elements (e.g., four antenna elements  500 ) may be wound on cylindrical structure  310  with a constant pitch angle. That is, the angle connecting elements  510  and  512  is constant throughout each of the antenna elements  500 , as illustrated in  FIG. 5 . 
       FIG. 6  illustrates another implementation of a printed antenna element that may be used for antenna  270 . Referring to  FIG. 6 , antenna element  600  includes groupings of square wave-like portions  610  connected to each other by straight line portions  612 . In this implementation, a number of the antenna elements (e.g., four antenna elements  600 ) may be wound on cylindrical structure  310  with a constant pitch angle. In this manner, the QHA  270  may have distributed inductive loading to allow for the QHA to have a reduced height with respect to conventional QHAs. 
       FIG. 7  illustrates another implementation of antenna  270 . Referring to  FIG. 7 , antenna  270  includes a number (e.g., four) antenna elements  700  having distributed capacitive loading, thereby enabling the cylindrical structure  310  of antenna  270  to have a reduced height as compared to conventional QHAs. As illustrated, each antenna element  700  has segments  710 ,  712  and  714 . The gap, illustrated at reference numeral  720  in  FIG. 7 , between each of the adjacent segments may be designed to be less than the width of each of the segments. For example, gap  720  separating element  710  and  712  may be set to be less than the width of each of elements  710  and  712 . Similarly, the gap separating elements  712  and  714  may be set to be less than the width of each of elements  712  and  714 . The size of gap  720  and the width of the segments  710 - 714  of antenna elements  700  may be adjusted to control the capacitive loading, thereby controlling the desired resonant frequency. In this implementation, each of antenna elements  700  may be wound into a helical structure with a constant pitch angle on cylindrical structure  310 . 
       FIG. 8  illustrates another implementation of a printed antenna element  800 . Referring to  FIG. 8 , antenna element  800  includes alternately opposing/inverted segments  810  that have a mushroom-like shape. Each segment  810  may be separated from adjacent segments  810  by a gap, labeled as reference number  820  in  FIG. 8 . In this implementation, antenna element  800  has a distributed reactive loading. That is, antenna element  800  has simultaneous inductive and capacitive loading. In this implementation, a number of antenna elements  800  (e.g., four or more) may be wound into a helical structure with a constant pitch angle on cylindrical structure  310 . For example, pitch angles  812  between adjacent segments  810  of antenna element  800  are the same for each of the segments  810 . The distributed reactive loading enables QHA antenna  270  to have reduced height as compared to conventional QHAs. 
       FIG. 9  illustrates still another implementation of a printed circuit element that may be used in antenna  270 . Referring to  FIG. 9 , antenna element  900  may include a number of segments  910  and  912 . Each of the segments  910  and  912  may be oriented with the same pitch angle with respect to an adjacent segment. In this implementation, each segment  900  may be connected to a portion of segment  912 . In addition, portions of segments  912  may be separated from one another by gaps, labeled as numeral  920  in  FIG. 9 . In this manner, antenna element  900  has a distributed reactive loading (i.e., simultaneous inductive and capacitive loading). In addition, a number of antenna elements  900  (e.g., four or more) may be wound into a helical structure with a constant pitch angle on cylindrical structure  310 . The distributed reactive loading enables QHA antenna  270  to have reduced height as compared to conventional QHAs. 
       FIG. 10  illustrates still another implementation of a printed circuit element that may be used in antenna  270 . Referring to  FIG. 10 , antenna element  1000  may include a number of alternately opposing/inverted segments  1010 - 1040 . Each of segments  1010 - 1040  may be oriented with a different pitch angle with respect to another segment. For example, segment  1010  may be oriented to segment  1020  with a first angle, labeled  1012 , and segment  1020  may be oriented with respect to segment  1030  with a second angle, labeled  1022 , that is different from the first angle. Further, segment  1030  may be oriented with respect to segment  1040  with a third angle, labeled  1032 , that is different from the first and second angles, as illustrated in  FIG. 10 . In addition, as illustrated, each of segments  1010 - 1040  may include gaps between adjacent segments. In this implementation, antenna element  1000  has a distributed reactive loading (i.e., simultaneous inductive and capacitive loading). In addition, a number of antenna elements  1000  (e.g., four or more) may be wound into a helical structure with a discretely variable pitch angle on cylindrical structure  310 . The distributed reactive loading enables QHA antenna  270  to have reduced height as compared to conventional QHAs. 
       FIG. 11  illustrates another implementation of printed circuit elements that may be used in antenna  270 . Referring to  FIG. 11 , antenna  270  may include a number of antenna elements  1110  shown unfurled on the surface of cylindrical structure  310 . Each of printed circuit elements  1110  may include a square wave-like pattern in which the height of the square waves varies over the length of structure  310 . For example, the height of the square wave like pattern may be shorter on the left side of the structure  310  illustrated in  FIG. 11  than on the right side of structure  310 . The heights may also rise in a continuous or variable manner over the length of structure  310 . Antenna elements  1110  may be wound into a helical structure with a discretely variable pitch angle on cylindrical structure  310 . That is, the pitch angle associated with the square wave-like pattern of antenna elements  1110  may vary over the length of structure  310 , as illustrated in  FIG. 11 . For example, the angle of one square wave with respect to an adjacent square wave of an antenna element  1110  may be different than the angles associated with other adjacent square waves located in other portions of the antenna element  1110 . In this implementation, antenna  270  has a distributed inductive loading with a discretely variable pitch angle. The distributed inductive loading enables QHA antenna  270  to have reduced height as compared to conventional QHAs. 
     In the implementations described above, antenna  270  may be a QHA with a number of conducting strip antenna elements. The QHA may operate in a normal mode to provide near omni-directional coverage. In this normal mode of operation, the QHA  270  may be of a fractional turn (e.g., 0.75 turn) so as to avoid formation of nulls in the radiation pattern in elevation. In addition, the diameter to height ratio of the cylindrical structure  310  controls the radiation pattern along the elevation. Therefore reduction in height results in simultaneous reduction in the diameter of the antenna structure. The electrical length of the diameter in relation to the height may also determine the quality of circular polarization (of the desired sense with low cross polarization) along the elevation. 
     CONCLUSION 
     Systems and methods described herein provide for reduced size QHAs. This may allow the size of the mobile terminal incorporating the QHA to be reduced. 
     The foregoing description of preferred embodiments of the invention provides illustration and description, but is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. 
     For example, implementations consistent with the invention have been described above with respect to use of a mobile terminal that may be used in a hybrid network that utilizes a terrestrial network and a satellite/space-based network. It should be understood, however, that implementations consistent with the invention may be used in other types of networks and are not limited to any particular type of network. Implementations have also been described as being used in a mobile terminal  110  for placing and receiving telephone calls. In other instances, other types of signals, such as global positioning system (GPS) signals, video signals, multi-media signals, or any other type of signals may be received and/or transmitted using QHAs described above. 
     It will also be apparent to one of ordinary skill in the art that aspects of the invention have been described above with reference to various shaped and sized conductive strip antenna elements. Details associated with forming these elements has not been described in order to not unduly obscure the thrust of the invention—it being understood that one of ordinary skill in the art would be able to fabricate such elements based on the description herein. In addition, it should be understood that conductive strip elements having shapes and/or sizes other than those described above may be used in other implementations. 
     For example, lumped element type antenna elements may be used in some implementations, as opposed to distributed antenna elements. In this case, the lumped type antenna element may include one or more printed circuit elements located on structure  310  that are formed in a more concentrated manner with respect to portions of structure  310 , as opposed to being uniformly distributed over the length of structure  310 . 
     In addition, in some implementations, combinations of constant pitch angle and discretely variable pitch angle antenna elements may be used. For example, one conductive strip antenna element that forms part of a QHA may have a number of segments with a constant pitch angle and another conductive strip element on the same QHA may have a number of segments with a discretely variable pitch angle, as illustrated in  FIG. 12 . For example, QHA  1200  illustrated in  FIG. 12  may include one conductive strip element  800  having a number of segments  810  with a constant pitch angle  812  between adjacent segments  810 , and another conductive strip element  1000  with segments  1010 - 1040  having different, discretely variable pitch angles  1012 ,  1022  and  1032 . 
     No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items. Where only one item is intended, the term “one” or similar language is used. Further, the phrase “based on” is intended to mean “based, at least in part, on” unless explicitly stated otherwise.