Abstract:
This invention provides low profile distributed antenna which comprises a first and second elongated continuous conductors being kept parallel to each other and forming a transmission line, a plurality of perturbation radiators on the first elongated continuous conductor, wherein a substantial amount of radio frequency energy transmitted by the transmission line radiates from the plurality of perturbation radiators, therefore, the transmission line serves as a low profile distributed antenna.

Description:
CROSS REFERENCE 
       [0001]    The present application claims the benefits of U.S. Provisional Application Ser. No. 60/808,444, which was filed on May 25, 2006. 
     
     BACKGROUND 
       [0002]    The present invention relates generally to radio energy transmission, and more specifically related to a distributed antenna for transporting radio energy through a defined medium. 
         [0003]    The performance of indoor wireless communication systems, such as a radio frequency identification (RFID) system or wireless local area networks (WLANs), depends on the signal strength available at the receiving antenna, or more specifically, the signal-to-noise ratio (SNR) that the systems can obtain at the receiving end. Most systems use a single base station antenna that broadcasts enough power to sufficiently cover a given area. However, the signal strength may have a very significant variation, which is determined by the distance from the base station antenna to the receiver, signal attenuation caused by intervening structures between the base station and the receiver and the multi-path caused by scattering from nearby structures. Hence, the coverage is always limited, and an improvement is implemented to use higher transmitting power and/or multiple base stations to provide proper coverage for larger areas. 
         [0004]    An example of a problematic indoor wireless environment is a room or enclosure that is long and narrow, such as a hallway, a long warehouse or factory, an aircraft cabin or a passenger car on a train. A single base station antenna in such an environment will not provide uniform coverage because the signal will be attenuated along the length of the enclosure. Therefore, multiple base stations or multiple antennas would need to be deployed in a distributed fashion in such a way that the coverage is uniform along the whole enclosure. Such a system would be complex, expensive, and invasive using existing technologies. 
         [0005]    Another example of a communication system is the RFID system using RF transmission to identify, categorize, locate and track objects. The system is made up of two primary components: a transponder or the RFID tag and a reader. The tag is a device that generates electrical signals or pulses interpreted by the reader. The reader is a transmitter/receiver combination (transceiver) that activates and reads the identification signals from the transponder. 
         [0006]    RFID tags are considered to be intelligent bar codes that can communicate with a networked system to track every object associated with a designated tag. RFID tags will communicate with an electronic reader that will detect the “tagged” object and further connects to a large network that will send information on the objects to interested parties such as retailers and product manufacturers. For example, the tag can be programmed to broadcast a specific stream of data denoting identity such as serial and model numbers, price, inventory code and date. Therefore, the RFID tags are expected to be widely used in the wholesale, distribution and retail businesses. 
         [0007]    A reader also contains an RF antenna, transceiver and a micro-processor. The transceiver sends activation signals to and receives identification data from the tag. The antenna may be enclosed within the reader or located outside the reader as a separate piece. The reader may be either a hand-held or a stationary component that checks and decodes the data it receives. 
         [0008]    It is of interest to communicate with RFID tags attached to merchandise (or containers) stored on shelves in a warehouse or retail establishment. With existing technology, this may be achieved in one of two ways: (1) a mobile RFID scanner that moves along the shelves, possibly hand-held, or (2) by mounting a large number of fixed scanners to cover all the shelves. The former approach is very time consuming and labor-intensive, while the latter approach is very complex and expensive. Furthermore, in the case of having multiple fixed scanners or base station antennas, it is difficult to conceal these devices in an aesthetically pleasing manner. 
         [0009]    In view of the above applications, there is clearly a need to develop a system of improved wireless coverage without greatly increasing the level of complexity and cost for a wireless system such as the RFID system. 
       SUMMARY 
       [0010]    This invention provides a low profile distributed antenna (LPDA) which comprises a first and second elongated continuous conductors being kept parallel to each other and forming a transmission line, a plurality of perturbations on the first elongated continuous conductor, wherein a substantial amount of radio frequency energy transmitted by the transmission line radiates from the plurality of perturbations, therefore, the transmission line serves as a low profile distributed antenna. The LPDA may be mounted along a wall, ceiling, or along shelves, and may have wide wireless applications. 
         [0011]    The simplicity of this antenna system is that each radiator is fed in series; thus, one can have many radiators but only one feed point. In addition, the transmission line used to feed this structure is a very simple parallel-plate structure as opposed to more complex rectangular waveguides or coax cables. To illustrate this point, one can think of the parallel-plate structure as being made of a thin foam spacer that is used to separate two conductors, which can be conducting tape, conducting thin films, etc. Obviously, this type of parallel-plate structure is much simpler to build but it does not seem to be very precise or structurally sound. That is not the case in that the foam spacer can be manufactured today to very fine tolerances (a few thousands of an inch tolerance is achievable today in mass production). Also, this antenna can be encapsulated in a conduit that is used to precisely align the parallel-plate structure along its length and to protect it from a hostile outside environment. Since the conduit structure can be easily made using mass production techniques, this whole new antenna concept lends itself to precise, low cost, high volume antenna applications. 
         [0012]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a perspective view of a V-shaped notch serving as a perturbation or radiator in a transmission line which functions as a low profile distributed antenna (LPDA) according to one embodiment of the present invention. 
           [0014]      FIG. 2  is a cross-sectional view of the LPDA transmission line embedded in a foam conduit and encased in a plastic outer shell. 
           [0015]      FIG. 3  is a top-view of the LPDA transmission line with the multiple perturbations represented by the V-shaped notches cut in its top plate. 
           [0016]      FIG. 4  is schematic diagram illustrating a feed circuit for the LPDA. 
           [0017]      FIGS. 5A and 5B  are cross-sectional views of splicing structures for joining two LPDA transmission lines. 
       
    
    
     DESCRIPTION 
       [0018]    The present invention provides an external radio energy propagation channel through the introduction of a low-profile linear distributed antenna (LPDA). The disclosed LPDA provides controlled radiation customized for each environment of interest. Further, it is a very cost-effective solution even though it is applied in terms of long lengths to provide the desired coverage within enclosed areas. 
         [0019]      FIG. 1  is a perspective view of a V-shaped notch serving as a perturbation or radiator in a transmission line which functions as a low profile distributed antenna (LPDA) according to one embodiment of the present invention. The notched transmission line  100  comprises two thin-line parallel conductive plates  110  and  120  with bi-lateral V-shaped notches  113  and  115  cut at the top plate  110 . As a result of the V-shaped notches  113  and  115 , the notched transmission line  100  is “pinched” at the narrowed location, which causes radio energy to “leak out” or radiate. Therefore the notched transmission line  100  can serve as one example of a perturbation radiator. Although V-shaped notches  113  and  115  are used to create the perturbation, one having skills in the art would appreciate various other perturbation structures, either by cutting out notches of shapes other than the V-shape or by adding protruding objects on the transmission line, may be applied to turn the transmission line into a radiator. 
         [0020]      FIG. 2  is a cross-sectional view of the LPDA transmission line  100  embedded in a foam conduit  230  and encased in a predetermined outer shell  240  which can be made by various suitable materials such as plastic or rubber. The foam conduit  230  is a spacer and has very low-loss for RF energies. In a field application, the LPDA transmission line  100  may be placed on a mounting surface  250  on a bottom side closer to the bottom plate  120 . Although the bottom plate  120  as shown in  FIG. 2  is wider than the top plate  110  for practical purposes, one having skills in the art would recognize that a bottom plate can be the same or even smaller width than the top plate  110 , though it may not perform as well. The V-shaped notches  113  and  115  shown in  FIG. 1  are cut on the top plate  110 . One having skills in the art may also recognize that the V-shaped notches  113  and  115  or any other perturbations may be added to either the top plate  110  or the bottom plate  120  or even on both plates  110  and  120 . The shape, size and orientation of the perturbations may be used to control the radiation bandwidth, level, polarization, etc. 
         [0021]    Referring to  FIG. 2 , the conduit is shown as a solid structure surrounding the LPDA transmission line  100 . This conduit can also be designed in terms of two pieces that can be taken apart to adjust the radiators or to add splice sections as discussed later. To allow for easy access these two conduit pieces can be snapped together, for example. That being the case, one can change the LPDA antenna in the field. This may be very useful in complex application environments. 
         [0022]      FIG. 3  is a top-view of the LPDA transmission line  300  with multiple V-shaped notches  313 [0:n] and  315 [0:n] cut in its top plate  310  to serve as perturbation radiators. In this embodiment, a bottom plate  320  has no notch, and is wider than the top plate  310 . The V-shaped notches  313 [0:n] and  315 [0:n] are cut symmetrically on both edges of the top plate  310 , i.e.,  313 [0] and  315 [0],  313 [1] and  315 [1], etc., are symmetrical. The V-shaped notches  313 [0:n] and  315 [0:n] may be cut at a regular interval L or at an irregular length across the length of the notched transmission line  300 . The interval is determined by signal strengths of the radiations from the notches  313 [0:n] and  315 [0:n] to make sure that the areas in between the perturbation radiator locations are covered by the radiations therefrom. When radio frequency (RF) signal is fed at a left end  332  from a base station  340 , the LPDA transmission line  300  will function as a low profile distributed antenna (LPDA) for the RF signal. To terminate the RF energy transmission at a right end  336 , a termination  350  is connected thereto. This specific termination  350  is designed to provide better illumination of the enclosed environment. 
         [0023]    Referring to  FIG. 3 , a depth of the notch, D 0  for notch  313 [0], D 1  for notch  313 [1] or Dn for notch  313 [n], determines the amount of radiation from the notch. The deeper the notch is, the higher the radiation comes from the notch. At the same time the farther the RF signal propagates along the LPDA transmission line  300 , the more it is attenuated. In order to keep the relative radiation from each notch uniform along the length of the transmission line  300 , the depth of the notches  313 [0:n] and  315 [0:n] varies from small to large from the feed end  332  to the termination end  336 . For example, D 1  is designed to be larger than D 0 , and Dn is the largest among all the notches as it is at the right end  336  of the notched transmission line  300 . It is this controlled radiation that provides a uniform coverage for the wireless system. Also by controlling the width of the notches one can also ensure that very little energy is lost due to reflections back into the feed or absorption at the termination. 
         [0024]    As the LPDA transmission line  300  may be constructed by other perturbation structures, one having skills in the art would employ different mechanisms for controlling the radiations that are appropriate for the respective perturbation structures, yet still produce similar uniform radiation patterns as described above, or a prescribed radiation pattern for a specific application. 
         [0025]    In an alternative embodiment, the notched transmission line  300  may be divided into multiple sections of various lengths. Notches within a section may have the same or different depths, while notches in sections farther away from the feed end  332  become deeper as the distances grow. This allows the radiation to become more uniform along the full length of the LPDA. 
         [0026]    Although, as shown in  FIG. 3 , the perturbations or notches,  313 [0:n] on one edge and  315 [0:n] on the other of the top plate  310 , are symmetrical, one having skills in the art would realize that the perturbations can be unsymmetrical or even just on one edge to emphasize radiation on that edge of the LPDA transmission line  300 . 
         [0027]      FIG. 4  is schematic diagram illustrating a feed circuit  400  specially designed for the LPDA formed by the LPDA transmission line  300  as shown in  FIG. 3 . The feed circuit  400  matches the parallel plates  310  and  320  of the notched transmission line  300  to a standard coaxial cable  420  from a RF signal transmitter (not shown), so that reflections back into the transmitter are minimized. In one case, the feed circuit  400  comprises a 180 degree hybrid  410  with input difference terminal  0  connected to the coax cable  420 . A 0 degree hybrid output  2  is connected to the top plate  310  or bottom plate  320  of the LPDA transmission line  300 , while a 180 degree output  3  is connected to the other plate. The feed circuit  400  may also be used for the termination of the LPDA transmission line  300  in the matched load  350  as shown in  FIG. 3 . The feed can also be done by using a standard cable connection in which the outer conductor is connected to one of the plates and the coaxial center conductor to the other plate. 
         [0028]    In addition, one wants to match the impedance of the LPDA transmission line with the feed impedance. This may be done having the LPDA parallel-plate spacing transition from its normal dimension to one that provides the desired impedance. At the same time, the conductor widths can be changed if needed to provide the desired impedance level to match that of the feed network as described previously. 
         [0029]    With the parallel plate structure, two pieces of the transmission line  300  can be easily joined together or even spliced in the field when greater length of coverage by the LPDA is needed. 
         [0030]      FIGS. 5A and 5B  are cross-sectional views of splicing structures for joining two LPDA transmission lines. The cross-sections are made along lengths of the LPDA transmission lines. Referring to  FIG. 5A , a left-hand-side transmission line has a top plate  510  and a bottom plate  515 . A right-hand-side transmission line of the same dimension as the left-hand-side transmission line has a top plate  520  and a bottom plate  525 . In order to join the left-hand-side and right-hand-side transmission lines, a splice conductor having a top plate  530  and a bottom plate  535  is used. One can think of the LPDA transmission lines being mounted in the conduit  230  as shown in  FIG. 2 . The conduit  230  is made in two parts that separate and can be snapped, bonded or held together. That being the case, the splice piece  535  can be placed in the bottom section of the conduit  230 . The two LPDA transmission lines are laid on top of this splice piece  535 . After that the top splice piece  530  is added and the top of the conduit  230  is used to hold everything together. With continued reference to  FIG. 5A , one can see that a connection between the top plates  510  and  520  is made through the top plates  530 , and a connection between the bottom plates  515  and  525  is made through the bottom plate  535 . 
         [0031]    Referring to  FIG. 5B , the left-hand-side transmission line remains the same as shown in  FIG. 5A , however, the right-hand-side transmission line has a wider space between a top end-plate  570  and a bottom end-plate  575 . The rest of the top plate  560  and bottom plate  565  of the right-hand-side transmission line have the same spacing as the top and bottom plates  510  and  515  of the left-hand-side transmission line. The wider space between a top end-plate  570  and a bottom end-plate  575  just allows the top and bottom plates  510  and  515  to slide in and maintain tight contacts between the top end-plate  570  and the top plate  510  and between the bottom end plate  575  and the bottom plate  515 . In such a way, the left-hand-side transmission line and the right-hand-side transmission line are spliced. However, splice connectors for joining the left-hand-side and the right-hand-side transmission lines need to be carefully designed to minimize reflections from the junctions. Note that the conduit structure can be used to hold the LPDA transmission in alignment even though the LPDA transmission line itself can be rather flimsy. 
         [0032]    Designing of the LPDA can be assisted by electromagnetic (EM) modeling software to determine the radiator size, shape, orientation, etc. A properly designed LPDA may be used to cover indoor wireless bands from 800 MHz up to 6 GHz and even beyond. 
         [0033]    Although the present disclosure uses notches to illustrate the inventive LPDA structure, one having skills in the art would appreciate that the essence of the present invention lies in the fact that one can use any perturbation along the length of this parallel-plate transmission line to cause radiation. The size, shape and orientation of these radiators can be used to control the radiation bandwidth, radiation level, radiated polarization, etc. Therefore, other kinds of radiators may also be used to form the LPDA, as long as at least one conductor of the transmission line has a plurality of radiators from each of them a substantial amount of transmitted RF energy can be radiated from the transmission line. 
         [0034]    The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims. 
         [0035]    Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.