Abstract:
A miniaturized directional antenna for use with system to provide data communication over wireless radio channels. The unit supports multiple antenna elements with a known orientation with respect to an earth ground plane reference. This greatly provides predictability in the steerability and other directional attributes of the antenna array using miniaturized chip multilayer or helical antenna elements, the unit may be constructed in a case or other form factor of approximately 3×3×1 inches for operation within the frequency bands around 1900 MHz.

Description:
RELATED APPLICATION(S) 
     This application claims priority to U.S. Provisional Application No. 60/100,995 filed Sep. 18, 1998, the entire teachings of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The users of computers, Personal Digital Assistants (PDAs), and other data processing equipment increasingly rely upon various types of network connections in order to obtain access to data in various forms. For example, sophisticated business users now desire high speed Internet access whether on the road or in their home location. Corporate information technology departments often need to rapidly set up and tear down access for their users as locations change and temporary visitors need to be accommodated. In addition, organizations in the appliance repair, package delivery, and other service industries also require data access. 
     Although present wireless communication infrastructure such as provided by the cellular telephone network is in widespread use for voice traffic, its use has not spread in particular for data applications. This is due in part perhaps to the relatively slow available speeds for sending data over cellular connections, which supports rates of only 9600 or 14400 baud. Another consideration is convenience. For example, in order to use the cellular system, one must not only carry around a cellular telephone, but also specialized modem equipment in addition to a laptop computer or other personal computing equipment. 
     Digital cellular equipment typically makes use of handsets that have the traditional single dipole antenna. Unfortunately, such antenna units are not optimized for maximizing data speeds. For example, in networks that make use of Code Division Multiple Access (CDMA) signaling, power levels must be carefully controlled, especially for transmission from the subscriber back to the base station (reverse link). By optimizing the effective radiated power, data rates can be maximized. 
     Unfortunately, known dipole antenna arrangements, or even known combinations of dipole arrangements, do not provide adequate control over effective radiated power. This is due in part to a number of causes. Dipole antennas alone do not provide directional antenna patterns that allow the power to be more effectively directed to the base station. Moreover, implementing such devices within handset form factors, or within other form factors such as integral to the case of the computer equipment, makes it difficult to ensure that the antenna elements are properly oriented with respect to the earth. 
     What is needed is a small and convenient unit that can be used to provide wireless data access such as over existing cellular telephone networks. The device should have a convenient form factor such as will fit in a shirt pocket or purse. 
     SUMMARY OF THE INVENTION 
     The present invention is a miniaturized directional antenna array that can be used to provide directional gain to optimize digital data communications. The antenna array is packaged in a palm sized case which may be placed on a table or other approximately horizontal surface convenient to the portable computing equipment. The arrangement of the array elements within the case automatically provides a proper orientation of the antenna elements with respect to the earth. 
     In the preferred embodiment, the array is a five element array having a center element and four outlying or comer elements. The outlying elements are spaced at approximately one-quarter of a wave length radial distance from the center element. The antenna elements are fastened to an appropriate support structure disposed within the case which is formed of a convenient material such as plastic which is transparent to radio wave propagation. Other electrical elements such as strip line power dividers, phase shifter components, and power routing components are placed on a multilayer printed circuit card disposed beneath the antenna array support structure. 
     In the preferred embodiment, the radiating elements themselves are a type of miniaturized antenna element known as a multilayer chip antenna. Such chip antennas are extremely small in size and may be conveniently mounted within the support structures in accordance with well known manufacturing techniques. 
     Alternatively, the radiating elements may be helical antennas that are also mounted within the support structure with the proper vertical orientation. 
     The overall result is an antenna package that does not exceed approximately one (1) inch in height and three (3) inches in width and depth, which can be used to greatly enhance the radio link signaling characteristics for data signals. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     FIG. 1 is an external view of an antenna unit and a computer interface card according to the invention. 
     FIG. 2 is a more detailed view of the interior of the antenna unit. 
     FIG. 3 is a more detailed view of a chip multi-layer antenna element. 
     FIG. 4 is a more detailed view of a helical antenna element that may be used in the array. 
     FIGS. 5,  6  and  7  are antenna patterns resulting from a simulation of an antenna array structure according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning attention now to the drawings, FIG. 1 is an isometric view of an antenna unit  10  according to the invention. The antenna unit  10  is a generally rectangular case formed of material such as plastic that is transparent to radio waves. The antenna unit  10  is connected via a bi-directional control cable  11  over a suitable computer interface such as a PCMCIA interface card  12 . 
     The exterior of the antenna unit  10  is typically labeled with an indicator such as an arrow  14  to instruct the user on the proper orientation of the unit. During operation, the unit  10  is, for example, placed on a table or desk or other convenient horizontal surface and connected to the computing equipment such as a laptop portable computer, personal digital assistant (PDA), or other computing device via the PCMCIA card  12 . The user ensures that the arrow is pointing in the upward direction. 
     The antenna unit  10  encloses not only radiating antenna elements but also circuitry including radio frequency (RF), intermediate frequency (IF), and digital circuitry on one or more layers  16  of a printed circuit board. The circuit layers  16  are generally indicated in FIG. 1, with the understanding that they may also be implemented on more than one printed circuit board. 
     FIG. 2 is a more detailed exploded view of the unit  10 . The unit  10  includes within the interior thereof an antenna array  20  and multiple circuit board layers  16 - 1 ,  16 - 2 ,  16 - 3 , and  16 - 4  as previously mentioned. The antenna array  20  in the preferred embodiment consists of five antenna elements  22 - 1 ,  22 - 2 ,  22 - 3 ,  22 - 4 , and  22 - 5  arranged as shown. In particular, a center element  22 - 1  is arranged with four outlying elements  22 - 2 ,  22 - 3 ,  22 - 4 , and  22 - 5  placed on the outer corners of a generally rectangular frame used as a support structure  24 . 
     The support structure  24  consists of a number of vertically oriented surfaces including a back wall  25 - 1 , a front wall  25 - 2 , a right side wall  25 - 3 , a left side wall  25 - 4 , and a center wall  25 - 6 . The center wall  25 - 6  supports the center element  22 - 1 . The right hand wall  25 - 3  supports the rear right clement  22 - 3  and forward right element  22 - 4 . The left wall  25 - 4  supports a rear left element  22 - 2  and a front left element  22 - 5 . 
     In this embodiment, the elements  22  are chip multilayer antennas such as the model LDA36D1920 antenna available from Murata Manufacturing Company Ltd. This type of element is described in further detail in connection with FIG.  3 . 
     The spacing between the elements  22  is critical to proper performance of the array  20 . In the preferred embodiment, the spacing of the array elements  22  depends in particular upon the wavelength, λ, of the intended center frequency of operation. In the preferred embodiment of operation within the Personal Communication System (PCS) frequency bands of approximately 1850 to 1990 MHz, the wavelength λ is approximately 6.215 inches. 
     In general, however, the element spacing is such that the center points of the outlying elements  22 - 2 , . . . ,  22 - 5  are set at a radial distance of approximately 0.26 times λ from the center element  22 - 1 . It should be understood that this spacing can be varied somewhat in order to obtain desired effects. The array should be a square array such that the spacing should be the same among all adjacent outer elements. For example, the best spacing between front elements  22 - 5  and  22 - 4  is approximately the square root of 0.26 times λ which is the same as the spacing between the elements along the side elements  22 - 2  and  22 - 5 . For operation at approximately 1900 MHz, the entire unit  10  is only about 3 inches by 3 inches by 1 inch high. 
     The support structure  24  can also be formed of any convenient material transparent to the transmission of radio waves such as plastic, ceramic, or other materials. What is important is that the support structure  24  orient the antenna elements in a predictable way with respect to the earth. Thus, when the user places the antenna unit  10  with the correct orientation as indicated by the arrow  14 , the elements  22  will have a known orientation with respect to the earth, and more predictable operation results. 
     The array  20  also requires other components in order to properly operate. For example, the array  20  is a directional array which can be steered in a number of different directions by changing the phase of the electrical signals applied to the individual elements  22 . Thus, additional components such as power dividers, phase shifters, and signal routing traces are also placed and formed within the antenna unit  10 . Preferably these components are placed within one of the circuit board layers  16  as previously described. For example, an upper layer  16 - 1  may be a ground plane layer, and a second layer  16 - 2  may accommodate strip line power dividers to provide five way splitting of electrical signal energy applied to the antenna array  20 . A third layer  16 - 3  may provide another ground plane and fourth layer  16 - 4  may provide a surface for mounting and interconnecting phase shifter components, additional power dividing components, and signal and power wiring. 
     Conductors  26 - 1 , . . . ,  26 - 5  are extended from a feed point of each of the elements  22 - 1 , . . . ,  22 - 5  to provide a connection to the electrical components such as the strip line power divider components on layer  16 - 2 . The circuit boards  16  and/or circuit layer may be solid ground planes or have interruptions at various places to accommodate wiring. 
     The arrangement in FIG. 2 thus provides a structure for miniaturized antenna elements forming a steerable array which, in a relatively small package, provides a known orientation of antenna elements in order to optimize operation such as, for example, in wireless digital data networks. 
     FIG. 3 is a more detailed view of one of the miniature antenna elements  22 . This particular element, as obtained from Murata Manufacturing Company Ltd., is a miniaturized type of antenna known as the LDA36D series. The element  22  is of the top capacitive loading type has a substrate  30  on which are formed a laser trim line  30  and internal top loading structure  34 . A feed end point  36  provides a point at which a connection to a feed line can be made. The element  22  may be fabricated on a convenient material such as a ceramic substrate. The antenna element acts as a one-quarter wave length type radiating element. 
     In an alternative embodiment, the antenna elements  22  may be implemented as miniaturized helical antennas such as available from Toko America, Inc. Elements such as the model HEAW-T01-002 have an overall height H  3  of approximately 1.32 inches. In the case of the instance of the use of helical antennas  40 , they may be mounted directly to the underlying circuit layers  16 - 1 , and therefore do not need as elaborate a support structure  24  as in the case of the chip antennas  28 . However, the structure  24  must provide a proper orientation of such helical coil antennas with respect to the earth so they will always be placed in a known orientation by the user. 
     Samples of the types of antenna patterns which appear to be achievable with the antenna unit  10  are shown in FIGS. 5,  6  and  7 . FIG. 5 is an antenna pattern developed from a simulation of the structure with the antenna phases set to optimize a directional orientation with respect to zero degrees. It illustrates that the geometry can be used to obtain an acceptable beamwidth of approximately 30 degrees. 
     FIGS. 6 and 7 show the result when the phase element weights are optimized for 22 degrees and 45 degrees steering respectively. The relative magnitude of the results of the simulation indicated an expected directional gain of approximately 9 decibels with respect to isotropic (dBi). 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.