Abstract:
A compact lightweight antenna for receiving microwave direct line of sight wireless data signals used in services such as Local Multipoint Distribution Services (LMDS). The antenna provides for precise control over isolation of polarized signals. The antenna consists of an external parabolically shaped dome formed of a suitably resilient material such as thermoplastic. A polarizing conductive grating is formed on the interior surface of the dome and serves as a transreflector for initially passing received radiation having a vertical polarization. A twist reflector disposed at a point along an axis defined by the conductive grating reflects the received radiation, back in the direction of the transreflector with a different polarization. The now differently polarized energy is reflected by the parabolically shaped conductive grating at a feed point located in the center of the twist plate. The transreflecting element may be manufactured by providing a substrate that has been printed and etched and/or a film nonconductive substrate which has been silk screened with a conductive ink. In each of these cases in a preferred embodiment, the substrate or carrier film becomes an integral part of the mold in the resulting article.

Description:
RELATED APPLICATION(S)  
       [0001]    This application is a continuation-in-part of a prior U.S. patent application Ser. No. 09/317,767, filed May 24, 1999, entitled “Transreflector Antenna For Wireless Communication System.” The entire teachings of the above application(s) are incorporated herein by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    There continues to be ever increasing demand for distributed high speed access to computer networks such as the Internet and private networks. Competition is fierce among various schemes which rely upon wires for physical layer connectivity, such as T1 carrier, Digital Subscriber Line (xDSL), cable modem, fiber optic distributed data interface (FDDI), and the like. However, it is readily apparent that wireless access systems continue to hold the promise of reducing network buildout costs, especially in areas where telephone, cable and/or fiber optic lines are not yet installed. Wireless systems almost always promise the most rapid and flexible deployment of access services and a quicker return on investment.  
           [0003]    Certain radio frequency bands have been allocated in the United States and in other countries to provide so-called Local Multipoint Distribution Service (LMDS). LMDS uses super high frequency microwave signals in the 28 or 40 gigahertz (GHz) band to send and receive broadband data signals within a given area, or cell, approximately up to six miles in diameter. On the surface, LMDS systems work in a manner analogous to that of narrow band cellular telephone systems. In the typical LMDS system, a hub transceiver services several different subscriber locations. The antenna at the hub has a wide viewing angle to allow access by multiple subscribers that use individual narrowly focused subscriber antennas. A high speed data communication service is provided by deploying appropriate modem equipment at both the hub and subscriber locations. Depending upon the particular modems used, the services provided to each subscriber can be, for example, a point-to-point dedicated service.  
           [0004]    This type of service can compete directly with wired services available from telephone companies and cable company networks. However, the designers of LMDS systems are faced with several challenges at the present time. Because such systems send very high frequency radio signals over short line-of-sight distances, cell layout has proven to be a complex issue. Some factors that must be considered in cell site design are line of sight, analog versus digital modulation, overlapping cells versus single is transmitter cells, transmit and receive antenna height, foliage density, and expected rainfall. The configuration of antennas and transceivers at a hub site determines the specific coverage of the different sectors within a cell. Antennas with wide viewing angles result in fewer sectors at each cell site. Narrow sectors can be established, but narrower sectors require more hub equipment to cover the same field of view. Also, narrow sectors using the same polarization increase the amount of interference from one hub to the other. Wireless communication system designers can overcome this limitation by using polarization diversity at a cell site. In one approach, narrow sectors using orthogonal polarizations (i.e., the signals radiated from two hubs are 90 degrees to one another) are interleaved to reduce the interference. This polarization diversity can be achieved using orthogonally polarized antennas with very low cross-polarization levels. However, the design of antennas with low cross-polarization levels throughout the sector remains a challenge.  
           [0005]    Another challenge is in the electronics technology needed to implement the service. For example, transmitter amplifiers for such high frequency systems require sophisticated semiconductor technology such as using monolithic millimeter-wave integrated circuits (MMICs) based on gallium arsenide technologies. These MMICs generate considerable heat in the transceiver unit and the heat needs to be dissipated by careful design of the heat sink of the transceiver. Furthermore, transceiver systems must provide precise control over signal levels in order to affect the maximum possible link margin at the receiver.  
           [0006]    One overriding concern with LMDS services is that they are fixed services and therefore have certain properties that are dramatically different than for mobile services. One difference in particular is that LMDS service is completely line of sight, meaning that a clear path for signal propagation between the hub and subscriber is an absolute requirement. Locations without direct line of sight access typically require auxiliary reflectors and/or amplifiers, if they can be made to work at all.  
           [0007]    Another consideration in an LMDS system is that connection is expected to be full duplex, in the sense that the transmitter is expected to operate at the same time as the receiver, with minimal interference being generated between them. Thus, broadband communication systems such as LMDS require a highly directional (i.e., narrowly focused) antenna that has very low cross-polarization levels throughout the viewing area. Also, since these transceiver equipments are used for subscriber units, these need to be small, compact and should fit in with the decor of the subscriber dwellings. An additional advantage would be provided if some type of heat dissipation capability was also provisioned for the unit.  
           [0008]    Certain compact microwave and millimeter-wave radars operating at extremely high frequencies have been developed using a folded folding optics design. Such a design uses an external lens for focusing electromagnetic radiation to define an antenna axis. A separate transreflector placed in a plane orthogonal to the axis of the lens and a separate twist reflector assembly is also placed in the same plane. Such assemblies typically require fabrication of multiple individual components. See, for example, the antennas described in U.S. Pat. No. 5,455,589 issued to Huguenin, G. R. and Moore, E. L. on Oct. 3, 1995 and assigned to the Assignee of the present application, as well as U.S. Pat. No. 5,680,139 issued on Oct. 21, 1997 to the same inventors, and also to the same Assignee.  
         SUMMARY OF THE INVENTION  
         [0009]    Briefly, the present invention is a compact, lightweight, inexpensive antenna for use with wireless communication services including, but not limited to, line of sight microwave frequency services such as Local Multipoint Distribution Services (LMDS).  
           [0010]    The antenna provides for transmission and reception on a vertical and/or horizontal plane as well as isolation for cross-polarized components. The design provides for precise control over isolation and polarization characteristics.  
           [0011]    More particularly, the antenna consists of an exterior shaped housing, or dome, formed of a suitable inexpensive resilient material such as plastic. A polarizing conducting grating is formed on an interior facing surface of the dome.  
           [0012]    The dome is spaced apart from a twist reflector formed of a metal plate in one embodiment. Grooves are cut in the surface of the twist plate facing the polarizing grid.  
           [0013]    In another embodiment, the twist reflector is made of a metal backed dielectric layer of a thickness approximately equal to one-quarter wavelength at the frequency of operation, in the dielectric medium. The conductive grating is formed on the dielectric layer, facing the dome surface of the transreflector. Thus, in general, twist reflectors can be constructed in many different ways, the intent in all cases being to achieve a 90 degree rotation of polarization between incident and reflected signals.  
           [0014]    A waveguide feed is placed preferably in the center of the twist reflector in either embodiment to provide for bidirectional signal coupling between the antenna and transceiving equipment.  
           [0015]    In operation, in the receive direction, microwave line of sight signals are received at the dome and only those with a desired polarization pass through the grating.  
           [0016]    Signals of an orthogonal polarization are reflected away from the dome, thereby providing very low cross-polarization levels. The twist reflector then reflects such signals back towards the dome and the grating. In this instance, the twist reflector imparts a rotation, such as 90 degrees, to this reflected energy. When the reflected energy reaches the conductive grating a second time, it is reflected. Since the dome and hence the conductive grating are of a shape which focuses reflected energy, such as parabolic or spherical, the energy reflected by the grating is focused at a point in the center of the twist reflector at which the waveguide feed is placed.  
           [0017]    The transreflector arrangement operates analogously in the transmit direction.  
           [0018]    That is, transmit signal energy in all directions exiting the waveguide is directed to the polarizing grating. The grating in turn reflects such energy along its parabolic shape back to the twist plate, essentially with all rays in parallel. The twist plate imparts a 90 degree rotation to this energy and reflects it back to the grating. Now having the opposite polarization, the transmit energy passes through the grating and out along a line of sight defined by the axis.  
           [0019]    The exterior dome serves not only as a support base for the polarizing grating, but also as a casement for the components contained within the antenna.  
           [0020]    The transreflecting element may be manufactured by providing a substrate that has been printed and etched and/or a film nonconductive substrate which has been silk screened with a conductive ink. In each of these cases in a preferred embodiment, the substrate or carrier film becomes an integral part of the resulting molded article.  
           [0021]    The transreflector may be manufactured by providing a series of spaced parallel stripes of a conductive material upon the surface of a substrate. The substrate may be a synthetic resin carrier film on which the parallel stripes are deposited. However, alternatively, the substrate may itself be a conductive substrate such as may be provided by a conductive ink which has been etched. In either event, the film can be placed against the surface of a mold defining a desired concave curvature for the transreflector. A second mold half defining the desired convex external curve is then placed in a spaced relationship with the first mold. Synthetic resin may then be introduced in the mold cavity to produce the desired transreflector element. The spaced parallel stripes will thus be disposed on an internal or external concave surface thereof. The conductive&#39;s carrier film may then possibly be removed. Alternatively, the conductive film may remain within the completed transreflector element, depending upon various considerations.  
           [0022]    Advantageously, the twist plate may be integrally formed on the outer surface of a metal enclosure within which are placed the transceiver circuits, modem interface circuits, and the like. In this instance, the metallic twist plate may also serve as a heat sink, dissipating the heat generated by the operating transceiver electronic modules.  
           [0023]    This arrangement provides a low cost, minimum part count, low profile, easy to manufacture antenna for use in line of sight, full duplex microwave signaling applications. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]    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.  
         [0025]    [0025]FIG. 1 is a block diagram of a Local Multipoint Distribution Service (LMDS) system which uses a compact antenna assembly according to the invention.  
         [0026]    [0026]FIG. 2 illustrates a typical installation of the antenna assembly at a subscriber location such as on the roof of a building.  
         [0027]    [0027]FIG. 3 is a more detailed view of the antenna assembly as mounted to a mast.  
         [0028]    [0028]FIG. 4 is an exploded view of the various components of the antenna assembly.  
         [0029]    [0029]FIG. 5 is a cross-sectional view of the assembled antenna useful for understanding of how the antenna works.  
         [0030]    [0030]FIG. 6 is a cross-sectional view of another embodiment.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]    [0031]FIG. 1 is a block diagram of a system  10  for providing a high speed direct line of sight wireless data service such as Local Multipoint Distribution Service (LMDS) using millimeter-wave frequency radio signals for a physical layer medium. The system  10  consists of equipment at a hub location  12  as well as equipment at multiple subscriber locations  14 . It should be understood that the subscriber units  14  may individually be located in a particular sector of a cell to provide support for a greater number of subscribers within a given cell using a limited number of carrier frequencies. In the illustrated system  10 , multiple subscribers are provided with a high speed data service to provide access to the Internet.  
         [0032]    The equipment at the hub  12  consists of a connection to a point-of-presence (POP) into the network or other Internet access device  20 , and multiple modems  22 - 1 ,  22 - 2 ,  22 -n. In the transmit (e.g., forward link) direction, the modems  22  convert baseband digital signals to modulated radio frequency signals using digitization and  15  modulation schemes appropriate for line of sight microwave transmission. For example, the point-to-point (PTP) class of modems available for purchase from Integrity Communications, Inc. of Richmond, Va. provide data links that operate at full duplex speeds up to 10 megabits per second (Mbps).  
         [0033]    Continuing in the transmit direction, the modulated signals representing multiple transmit signals provided by the modems  22  are fed through an RF combiner  24  to a microwave frequency transmitter  26 . The microwave signals produced by the transmitter are then fed to a hub antenna  28  which then forwards them over multiple forward radio links  30  to subscriber locations  14 .  
         [0034]    At the subscriber locations  14 , a subscriber antenna  32  receives the line of sight microwave signals. The subscriber antenna  32  is the particular focus of the present invention and will be described in greater detail below. The subscriber antenna  32  receives the microwave frequency signals and forwards them to a subscriber transceiver  34 . A power supply  35  feeds power to the subscriber transceiver  34 , modem  36 , and local area network (LAN)  38 . A modem  36  converts the signals back to an appropriate digital form suitable for transmission over a local area network (LAN)  38  to which computing equipment may be connected in a well known manner.  
         [0035]    Operation in the reverse link direction is analogous. Signals originating at the subscriber site  14  are received over the LAN  38  by the modems  36  and fed to the transceivers  34 . The subscriber antenna  32  in turn couples these over the radio links  30  to the hub location  12 , at which point the receiver  27  and splitter  23  provide multiple signals to the receiver portions of the modems  22 .  
         [0036]    Of particular interest to the present invention are the antenna  32  and transceiving equipment  34  used at the subscriber location  14 . As shown in FIG. 2, such an antenna  32  is typically arranged at a building site  50 . The antenna  32  may be mounted to a mast  52  located on the roof of the building  50 , and a transceiver  34  maybe located within the equipment mounted on the mast  50 . In this instance, a single coaxial cable  56  may be run from the transceiver  34  down the mast  52  to provide radio frequency and power connections to the multiple modems  36  distributed throughout the building  50 . Care is taken to keep the radio frequency link power budget for the multiple modems within the overall power and modulation budgets of the transceiver pairs  34  and  26 .  
         [0037]    As shown in FIG. 3, the antenna assembly  32  may be mounted to the mast  52  by suitable mounting bracket  58 . The antenna assembly  32  is carefully aimed at the time of installation to provide the required line of sight to the antenna  28  associated with the hub  12 .  
         [0038]    [0038]FIG. 4 is a more detailed view of certain portions of the antenna assembly  32 . In particular, the antenna assembly  32  consists of a housing  60  formed of an appropriate suitable material such as an ABS thermoplastic. The housing  60  has an outer portion thereof shaped as a thin plastic dome  62  having an approximately parabolic shape in the preferred embodiment. An alternate shape for the outer portion is spherical. As will be described in more detail later on, the dome  62  has formed, on an interior surface thereof, a parallel conductive grating or grid  63 . In a preferred embodiment, the thickness of the dome is approximately one-half the wavelength of the frequency of operation within the dielectric material of the dome  62 .  
         [0039]    A second component of the antenna  32  is a twist reflector or plate  64 . The twist plate imparts a 90 degree rotation in the polarization of the incident and reflected signals, and can be designed in many ways. In the present embodiment, the metal twist plate  64  has formed therein a grooved conductive surface  65  facing the interior of the housing  60 . In particular, the groove surface  65  faces the parallel conductive grating  63  formed on the interior of the parabolic surface  62 . A circular waveguide feed  66  is placed in preferably the center of the twist plate  64 . The waveguide feed  66  serves as a focal point for received radiated energy and as a feed point for transmitted radiated energy.  
         [0040]    In another embodiment, the twist plate is made of a metal backed dielectric layer of a thickness approximately equal to one-quarter wavelength at the frequency of operation, in the dielectric medium. A thin metal grating is formed on the dielectric layer, facing the dome surface of the transreflector. Thus, in general, twist reflectors can be constructed in many different ways, the intent in all cases being to achieve a 90 degree rotation of polarization between incident and reflected signals.  
         [0041]    The twist reflector  64  with waveguide feed  66  typically has mounted on the rear surface thereof a printed wiring board  68  on which are placed the components of the transceiver  34 . A rear cover  70  serves as both a conductive shield against interfering electromagnetic radiation and as a shield against the weather and other physical elements.  
         [0042]    The dome  62  and more specifically the grid  63  define a center axis  72  of the antenna. The twist plate  64  is arranged so that its center point is located along the same axis  72 . The axis  72  defines the direction in which the antenna  32  transmits and from which it receives electromagnetic radiation.  
         [0043]    [0043]FIG. 5 is a cross sectional view of the antenna  32  which will be used in describing the operation of the antenna  32  in greater detail. As previously mentioned, the parabolic surface  62  and in particular the parallel strip conductive grating  63  serve not only a transreflector but also as a type of lens or focusing element. For example, in a receive mode, as energy arrives at the antenna assembly  32 , it first passes directly through the plastic dome  62 , reaching the conductive grating  63 . The dashed line labeled “A” serves to indicate generally the direction of received radiation. If the individual parallel metallic conductor  71  of the grating  63  are oriented in a horizontal direction, as shown in the sketch, the only energy proceeding to point B along the axis  72  will be vertically polarized energy.  
         [0044]    This vertically polarized energy then reaches the twist plate  64  and, in particular, the parallel slot pattern  65  formed thereon. The twist plate  64  is positioned with respect to the dome  62  so that the slot pattern  65  is oriented with a 45 degree angle with respect to the grating  63 . This 45 degree offset to the incoming vertically polarized radiation not only reflects the incident radiation in the general direction of the arrows C, but also imparts a 90 degree rotation to its polarization. The reflected energy is now horizontally polarized.  
         [0045]    When the now horizontally polarized energy reaches the surface of the grating  63  a second time, the energy is reflected since it is of the same orientation as the grating is  63 . Since the grating  63  is shaped in a parabolic form, assuming rays entering the antenna  32  are in parallel, the resulting reflected energy generally travels in the direction of arrows D, and is focused at the waveguide feed  66  placed in the center of the twist plate  64 .  
         [0046]    The transreflector  68  and in particular the curvature of the grating  63  is preferably parabolic as previously mentioned. The parabola has a normal equation which may be represented as  
           Y   2 =4 fx    
         [0047]    where f is the desired focal length of the antenna, and x is the direction normal to the transreflector plane. That is, x is the distance in the direction of the horizontal line  72  formed between the center line of the twist plate  64  and transreflector  68 , and measured from the center of the transreflector  68 . The distance between the transreflector  68  and twist plate  64  may be fairly small or up to the focal length of the parabola of the dome  62 .  
         [0048]    The amount of isolation provided by the grating  63  with respect to other polarizations is a function of the spacing of the grating  63  and the density of the individual grid wires  71 . The grating  63  must have sufficient density in that the number of wires  71  for a given unit wavelength are needed to provide a certain desired amount of isolation. One rule of thumb which has been found to be particularly useful in practice is that at least five grid wires  71  and the associated five spacings should be provided along a distance equivalent to the operating wavelength. Providing fewer grid lines per unit spacing makes the antenna  32  easier to manufacture; however, having more grid lines per unit spacing provides higher isolation. The grid spacing  71  in the typical embodiment for use at LMDS frequencies would be approximately 0.5 to 1 millimeters (mm).  
         [0049]    The precise dimensions of the grooves  65  in the twist plate  64  also depend upon the precise frequency of operation. The depth of the individual slots is typically selected to be approximately one-quarter of the operating wavelength. The width of each slot, and correspondingly the number of the resulting ridges  74  per unit spacing is a practical consideration depending upon fabrication requirements. For operation at LMDS frequencies, it is preferable to try to keep approximately three slots per operating wavelength. With the indicated dimensions and numbers of slots, it is possible to obtain 40 decibels (dB) of isolation or more.  
         [0050]    The twist plate  64  is preferably also integrally formed with a rearward facing rim  78  such that an enclosure  80  is provided for placement of the printed wiring board  68  (not shown in FIG. 5). This permits the twist reflector  64  to be integrally molded into the same casting which is used to house the electronics. This design approach further minimizes the number of individual component parts of the antenna assembly  32 .  
         [0051]    Because the antenna is sensitive to polarized energy, it may be conveniently used in an environment where the forward and reverse link signals for different subscribers  14  have different polarizations. For example, transceivers operating in adjacent sectors from the same hub may have different polarizations. Subscribers  14  located close enough to one another to be in the same line of sight with the cell site having hub antennas with orthogonal polarizations may orient their subscriber antenna assembly  32  differently, to effect greater isolation between them, or even to permit two subscribers  14  to use identical carrier frequencies.  
         [0052]    A transreflector element according to the present invention may be produced in a preferred embodiment by providing a conductor substrate that has been printed and etched or a carrier film substrate which has been silk screened with a conductive ink or pad printed. In each of these implementations, the substrate or film typically would become an integral part of the molded transreflector article.  
         [0053]    By using either of these techniques for defining and providing the conductive grating, the conductive stripes can be formed with a high degree of precision.  
         [0054]    Registration of the patterns on the transfer film and/or substrate relative to the a mold cavity can be, for example, readily affected by providing formations such as perforated openings on the edges of the film or marks which can be readily detected by an electronic sensor. A line width and spacing can range from as little as 0.001 inch to greater than 1 inch with variable tolerances.  
         [0055]    The curvature of the transreflector body can range from ½ to several inches in depth to obtain good registration and avoid defamation of the pattern of conductive lines on the substrate. However, the diameter is limited only by the capacity of an injection molding machine which may be used to form the substrate.  
         [0056]    The twist plate  64  may also be implemented in other ways to achieve the desired phase rotation of the incident and reflected signals. One such embodiment is shown in FIG. 6. Here, the twist plate  64  is formed from a grooved dielectric layer  82  having a metal backing  83 . Radiation arriving at the twist plate will be subjected to two different propogation delays as presented by the different thicknesses of dielectric layer  82 . In other words, radiation that passes through the tops or peaks of the dielectric layer  82  will be delayed by a longer amount than the radiation which passes through the thinner “valley” sections in the dielectric formed by the grooves  65 .  
         [0057]    The dielectric layer  82  may be formed from any suitable rigid, thermoset plastic having good dielectric properties at microwave radio frequencies. One such plastic that is known to provide predictable dielectric constants up to 500 GHz is the polystyrene and divinylbenzene translucent plastic sold under the tradename Rexoliteg by C-Lec Plastics, Inc. of Beverly, N.J. However, it is possible to use other dielectric materials as well.  
         [0058]    The grooves  65  are again formed and spaced as for the previous embodiments already described above. Being a relatively dimensionally stable plastic, Rexolite sheets are readily machined or laser cut to form the desired grooves. The grooves  65  may typically be cut to a depth of ¼ wavelength of the expected operating frequency. A spacing between grooves is selected based upon the desired operating frequency and bandwidth for the twist plate  64 .  
         [0059]    The metallic backing  83  may be implemented by screening an appropriate metallic layer onto the rear of the dielectric layer  82 . Alternatively, the twist plate may be formed in other ways such as by adhereing a separate metallic layer to the back of the dielectric layer  82 .  
         [0060]    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.