Patent Publication Number: US-6339404-B1

Title: Diversity antenna system for lan communication system

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of priority pursuant to 35 USC §119(e)(1) from the provisional patent application filed pursuant to 35 USC §111(b): as Ser. No. 60/148,909 on Aug. 13, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an antenna system for wireless communication devices, and more particularly to a simplified, low cost antenna system providing spatial diversity to combat multipath effects in communication systems. 
     2. Description of Related Art 
     Local area networks (LAN) are used in the wireless transmission and reception of digitally-formatted data between sites within a building, between buildings, or between outdoor sites, using transceivers operating at frequencies in the range 2.4-2.5 GHz., 5.2-5.8 GHz., and others. Antennas operating over these frequency bands are required for the transceivers in LAN devices. A LAN structure permits many devices, such as computers, to communicate with each other or with other devices such as servers or printers. The individual stations in a LAN may be randomly positioned relative the other stations in the LAN, therefore an omnidirectional antenna is often required for the LAN&#39;s transceivers. One drawback of an omnidirectional antenna is its susceptibility to multipath interference which can reduce signal strength by phase cancellation. This may result in unacceptable error rates for the digital information being transferred over a LAN. 
     In many wireless systems it is necessary to employ some form of antenna diversity to combat multipath effects in the communication system. The antenna diversity can be accomplished in the form of frequency diversity, time diversity, or spatial diversity. In frequency diversity, the system switches between frequencies to combat multipath interference. In time diversity systems, the signal is transmitted or received at two different times. In spatial diversity systems, two or more antennas are placed at physically different locations to combat multipath interference. 
     Many prior art systems use a pair of ceramic patch antennas to form a spatially diverse antenna configuration. A ceramic patch antenna typically includes a ceramic substrate, a metalized patch formed on one surface of the substrate, and a ground plane disposed on the opposite surface of the substrate. A feed hole couples the metallized patch to the receiver/transmitter. The use of high dielectric constant materials for the ceramic substrate results in an antenna which is physically small. However, ceramic patch antennas tend to be relatively expensive. Furthermore, connecting the antenna to a low cost circuit board often requires special connectors and cabling, which add cost to the system. 
     SUMMARY OF THE INVENTION 
     A compact diversity antenna system for use with a communication system such as a LAN (local area network) is described. The antenna system consists of two moderately directional arrays disposed back-to-back, with separate rf feed ports for each array. The construction of the arrays is unique in the use of a common reflector element with two driven elements. Further, the driven elements are compact, and provide electrical performance nearly equal to full-size elements. The antenna volume has been minimized, making the antenna suitable for internal or external mounting on LAN devices. The antennas are formed by conductive traces on a first major surface of a dielectric substrate, such as a printed wiring board. Balun/feed networks are provided on a second, parallel major surface of the substrate. The balun traces are microstrip transmission lines using the wide reflector element trace on the first surface as a ground plane. 
     The antenna of the present invention provides two rf ports, each connected to a moderately directional antenna. The two patterns of the antennas effectively isolate azimuth sectors of 180 degrees, with maximum isolation to the rear of an array and maximum gain to the front of an array. In this way appropriate circuitry in a LAN device&#39;s transceiver can switch between antenna ports and select the antenna with maximum signal strength. Multipath signals coming from directions other than that of the strongest signal will be attenuated. 
     Additional objects of the antenna system according to the present invention include the provision of a compact, low cost antenna fabricated on a printed circuit board. 
     Other aspects and advantages of the invention are disclosed upon review of the figures, the detailed description, and the claims which follow. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form a part of this specification, illustrate preferred embodiments of the invention. In the drawings: 
     FIG. 1 illustrates a perspective view of a wireless communication device utilizing an antenna assembly according to the present invention; 
     FIG. 2 bottom plan view of the antenna assembly of FIG. 1; 
     FIG. 3 is a top plan view of the antenna assembly of FIG. 1; 
     FIG. 4 is a side elevational view of the antenna assembly of FIG. 1; 
     FIG. 5 shows the return loss vs. frequency plot for each antenna of the preferred configuration from FIG. 1; and 
     FIG. 6 shows the free-space azimuth pattern, gain, and front to back ratio of the preferred configuration from FIG.  1 . 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION 
     Referring to FIG. 1, a wireless communication device  10  utilizing an antenna assembly  12  according to the present invention is illustrated. Wireless communication device  10  may include a computer, printer device, or other LAN functional devices. FIGS. 2-4 further illustrate the antenna assembly  12  of FIG.  1 . Antenna assembly  12  includes a substrate  14  upon which one or more small substantially flat antennas may be positioned. The substrate is preferably substantially planar, though alternative configurations may be practicable. The substrate  14  may be a printed circuit board manufactured of epoxy resin/glass cloth laminate, but other compounds may also be used. The substrate  14  has a relative dielectric constant of 1-10 with a preferred value of 4.5. The substrate preferably has a thickness of between 0.010-0.25 inches. The substrate  14  defines first and second substantially parallel major surfaces  16 , 18  upon which conductive structures  20  of the antenna assembly  12  are disposed. Conductive structures  20 , including radiator elements  22 , transmission line traces  24 , 26 , reflector element  28 , and impedance matching tabs  30 , 32 , have preferred thickness of 0.001-0.002 inches. Although in the preferred embodiment, the conductive structures  20  are shown etched upon the substrate  14 , it will be recognized by those skilled in the art that ordinary wire conductors may also be used and disposed on the substrate  14 . 
     Referring particularly to FIG. 2, the conductive structure  20  of the first major surface  16  of the dielectric substrate  14  includes a plurality of fed radiating elements  22  in relation to a common reflector element  28 . Fed elements  22  consist of generally J-shaped traces whose serpentine shape form a radiator as the monopole antenna. Alternative shapes or forms for the radiator segments may be practicable. In the preferred embodiment, four fed elements  22  are defined by serpentine segments and are disposed in symmetric and reflective relation to the common reflector element  28 . The four fed elements  22  are symmetrically disposed relative to both longitudinal and transverse centerlines of the dielectric substrate  14 . The common reflector element  28  includes a base portion  40  for coupling the reflector element  28  to the shield conductors  42  of the coax feedlines  44 , as will be described hereinafter. 
     Referring to FIG. 3, the second major surface  18  of the dielectric substrate  14  has conductive structures  20  including two microstrip transmission lines  24 , 26 , impedance matching tabs  30 , 32 , and baluns  46 . The microstrip transmisson lines  24 , 26  utilize the common reflector element  28  on the reverse major surface  16  as a ground plane. The microstrip transmission lines  24 , 26  are coupled at a first end to a pair of center conductors  48  of the coax feedlines  44  at a substrate edge  50 , and at a second end to the pair of balun structures  46 . Baluns  46  or matching networks are configured as serpentine conductive traces and provide a means for coupling rf power to the driven radiator elements  22 . In a preferred embodiment, the baluns  46  are symmetrically disposed relative to a longitudinal center line of the dielectric substrate  14 . Conductive structures  20  of the second major surface  18  further include a pair of impedance matching tabs  30 , 32 , each associated with a transmission line  24 , 26  and a balun  46 . 
     Referring to FIG. 4, a pair of 50 ohm coax signal lines  44  from the wireless communications device  10  may be coupled between the conductive structures  20  of the first and second major surfaces  16 , 18  of the dielectric substrate  14 . In a preferred embodiment, the edge  50  of the substrate  14  may be contiguous with a portion of the printed circuit substrate of a communications device  10 , and microstrip lines  24 , 26  may be connected to corresponding microstrip lines of the device  10  which corresponds to VSWR of less than 1.5:1. 
     Referring to FIG. 5, markers  1  &amp;  2  are at frequencies 2.40 and 2.45 GHz., respectively. Minimum return loss at the feed locations is seen to be 17 dB, assuring efficient power transfer. 
     Referring to FIG. 6, the peak gain over the frequency range 2.4-2.45 GHz is +5 dBi, and the front-to-back ration is 7.5 dB. 
     Although particular embodiments of the invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited only to the embodiments disclosed, but is intended to embrace any alternatives, equivalents, or modifications falling within the scope of the invention as defined by the following claims.