Patent Publication Number: US-2015084819-A1

Title: Antenna System Providing High Isolation between Antennas on Electronics Device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of and claims priority to U.S. patent application Ser. No. 12/899,900, filed Oct. 7, 2010, which is a claims priority from (1) U.S. Provisional Patent Application Ser. No. 61/250,344 filed on Oct. 9, 2009 and (2) U.S. Provisional Patent Application Ser. No. 61/363,085 filed on Jul. 9, 2010. The contents of each of the foregoing is/are hereby incorporated by reference into this application as if set forth herein in full. 
    
    
     BACKGROUND 
     The present application relates generally to antenna systems in portable electronics devices having two or more antennas operating simultaneously. 
     Portable electronics devices (e.g., USB Dongles and other wireless routers, cellular handsets, personal digital assistants, smart phones, and portable personal computers) typically include electronics components on a printed circuit board (PCB) assembly. Antennas for radio communications to and from such a device may be attached to the PCB assembly. For example, single-ended antennas may be fed directly from the PCB assembly, which then serves as a counterpoise for the antennas, allowing the antennas to be much smaller than otherwise possible. When the counterpoise is small (e.g., with dimensions on the order of the operating wavelength of the antennas or less), feeding two or more antennas from the same counterpoise can have the disadvantage of introducing too much coupling from one antenna to another. This is an example of a coexistence problem where more than one radio must operate at the same time from the same device. 
     One example of a device having two or more antennas fed from the same counterpoise is a portable wireless router device using a first radio for communication with a wide area network (WAN) using WiMAX in the 2500 to 2700 MHz band, and a second radio for local area network (LAN) communication using 802.11 (WiFi) protocols in the 2400 to 2500 MHz band. It is desirable to obtain as much isolation as possible between the antenna(s) connected to the WiMAX radio and the antenna(s) connected to the WiFi radio because the adjacent operating bands make the radios particularly vulnerable to interfering with each other. 
     Additionally, industrial design trends for portable electronics devices are driving slimmer form factors. At the same time, advanced communications systems using multiple-input, multiple-output (MIMO) signal processing techniques are driving multiple radio transmitters onto these platforms. The combination of two or more radios and a slim form factor creates significant difficulties in meeting Specific Absorption Rate (SAR) regulatory requirements. 
     BRIEF SUMMARY OF EMBODIMENTS OF THE INVENTION 
     In accordance with one or more embodiments, an antenna system is provided in a portable electronics device. The antenna system includes a first antenna and a second balanced antenna provided on the printed circuit board assembly of the portable electronics device. The first antenna is fed from a portion of the printed circuit board assembly such that a ground plane of the printed circuit board assembly serves as a counterpoise for the first antenna. The second balanced antenna has dipole ends configured and oriented to generally minimize coupling to the ground plane of the printed circuit board assembly to increase isolation between the first antenna and the second balanced antenna. 
     Various embodiments of the invention are provided in the following detailed description. As will be realized, the invention is capable of other and different embodiments, and its several details may be capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not in a restrictive or limiting sense. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view of an exemplary antenna system in accordance with one or more embodiments. 
         FIG. 1B  is a cross section view of the antenna system of  FIG. 1A . 
         FIG. 1C  is an enlarged perspective view of the balanced antenna shown in  FIG. 1A . 
         FIG. 1D  is an enlarged perspective view of the balanced antenna of  FIG. 1C  with the carrier removed for purposes of illustration. 
         FIGS. 2A-2C  are graphs illustrating return loss and coupling measured between the test ports of the antenna system of  FIG. 1A . 
         FIGS. 3A-3C  illustrate measured radiation patterns for the balanced antenna of the antenna system of  FIG. 1A . 
         FIG. 4  is a perspective view of an alternative antenna system in accordance with one or more embodiments. 
         FIGS. 5A-5D  are graphs illustrating return loss and coupling measured between the test ports of the antenna system of  FIG. 4 . 
         FIG. 6A  is a perspective view of an alternate antenna system in accordance with one or more embodiments. 
         FIG. 6B  is a perspective view of the antenna system of  FIG. 6A  showing the balanced antenna separated from the printed circuit board assembly for purposes of illustration. 
         FIG. 6C  is a cross-section view of the antenna system of  FIG. 6A  showing the balanced antenna separated from the printed circuit board assembly for purposes of illustration. 
         FIGS. 7A-7D  are graphs illustrating various antenna performance parameters for the antenna system of  FIG. 6A . 
         FIG. 8  is a perspective view of an antenna system in accordance with one or more alternate embodiments. 
         FIG. 9  is a perspective view of an antenna system in accordance with one or more alternate embodiments. 
     
    
    
     Like reference numerals generally represent like parts in the drawings. 
     DETAILED DESCRIPTION 
     Various embodiments disclosed herein are directed to antenna systems for electronic communications devices having two or more antennas operating simultaneously. As discussed in greater detail below, the antenna system includes a printed circuit board assembly having a ground plane and a first antenna and a second balanced antenna provided on the printed circuit board assembly. The first antenna is fed from a portion of the printed circuit board assembly such that the ground plane of the printed circuit board assembly serves as a counterpoise for the first antenna. The second balanced antenna has dipole ends configured and oriented to generally minimize coupling to the ground plane of the printed circuit board to increase isolation between the first antenna and the second balanced antenna. In one or more embodiments, the peak near fields created by each antenna do not substantially overlap, thereby reducing the increase in SAR that may otherwise occur when both antennas are used to transmit simultaneously. 
       FIGS. 1A-1D  illustrate an antenna system assembly  100  in accordance with one or more embodiments. In this example, the assembly comprises a 60×100 mm PCB  102  and three antennas. The PCB  102  is representative of a PCB that may be used to hold the electronics of a portable WiMAX/WiFi device. Two WiMAX antennas  104  are attached to one end of the PCB  102 . The WiMAX antennas  104  are fed from the edge of the PCB  102  (at feed points  110 ) such that the ground plane  108  of the PCB  102  serves as the counterpoise for both antennas  104 . 
     A third balanced antenna  112 , generally optimized for operation in the WiFi frequency band, is located at the opposite end of the PCB  102 . The antenna  112 , shown in the side cross-section view of  FIG. 1B  and isometric view of  FIG. 1C , is formed using a copper foil pattern  114  applied to a plastic supporting piece or carrier  116 . Connection to the feed point can be made with a  1 . 1  mm diameter coaxial cable  118 . A feed terminal  120  is connected to the shield of the coaxial cable  118 , and a feed terminal  122  is connected to the center conductor of the coaxial cable  118 . The balanced antenna  112  is oriented to produce far E-field polarization normal to the ground plane  108 . Referring to  FIG. 1A , the PCB  102  and associated ground plane  108  lie in the X-Y plane, and the balanced antenna  112  is oriented to produce far E-field polarization aligned with the Z-axis. 
       FIG. 1D  shows the WiFi antenna  112  with the carrier  116  removed for purposes of illustration. The antenna  112  comprises a center-fed dipole with capacitive end plates  124  and an inductive connection  126  between ends. The end plates  124  serve to lower the resonant frequency of the antenna  112  so that the antenna  112  can be much shorter than the nominal half-wavelength dipole. A short dipole has lower input impedance than a half-wave dipole and the inductive connection serves to increase the real input impedance of the antenna  112  to match to 50-ohms. In this example, the antenna height, or z-axis dimension, is 10 mm or 1/12 wavelength at 2500 MHz, making it amenable to embedding within a low-profile product. 
     Because the antenna  112  is balanced, it does not require connection to a counterpoise. Nonetheless even if the antenna  112  is not intentionally connected to the PCB ground  108 , it will readily couple to the PCB ground  108  through near field interaction without specific arrangement avoid this effect. To reduce coupling, the antenna  112  is placed generally symmetrically about the PCB ground  108  in the z-axis, as can be seen from the side view of the assembly of  FIG. 1B . In this way, the dipole ends, which are at electric potentials of equal magnitude but opposite sign, are equidistant from the ground plane  108  and result in neutral potential at the ground plane  108 , and consequentially the net coupling to the ground plane is zero. If the dipole is offset in the z-axis, then a net potential can be imparted to the end of the ground plane  108 . This could undesirably couple to horizontal resonance modes of the ground plane  108  and hence to the antennas  104  for which the ground plane  108  is serving as counterpoise, and thereby couple the antennas  104 ,  112 . 
     The design and arrangement of the balanced antenna  112  to avoid coupling to the PCB ground  108  has several advantages, including, as stated above, that the coupling to other antennas  104  that already interact with the PCB ground  108  is reduced. In addition, the pickup of noise or other unwanted conducted signals from the PCB ground  108  is also reduced. Furthermore, scattering by the PCB ground  108  is reduced, such that the embedded dipole maintains the omni-directional azimuth pattern of a free-space dipole. Refer to the theta=90 degrees plot of the measured radiation patterns for the balanced antenna  112  provided in  FIG. 3C . 
     Plots of measured S parameters for a prototype of the assembly of  FIG. 1A  are shown in  FIGS. 2A-2C . For these plots, the Port 1 is connected to the balanced antenna  112  and Ports 2 and 3 are connected to the WiMAX antennas  104 . Coupling between the balanced antenna  112  and WiMAX antennas  104  (S12 and S13) is between −28 and −40 dB. By contrast, coupling between the two WiMAX antennas  104  on the PCB is about −15 dB. 
       FIG. 4  illustrates an antenna system  400  in accordance with one or more alternate embodiments, which uses the same two WiMAX antennas  104  but with a two-port balanced WiFi antenna  402 . The two-port balanced antenna  402  is a two-port antenna designed to provide generally optimal isolation between the two WiFi ports and is similar to antennas described in U.S. Pat. Nos. 7,688,273 and 7,688,275, the contents of which are hereby incorporated by reference herein. In general, the second balanced antenna  402  includes two antenna elements  404 ,  406 , each operatively coupled to a respective antenna port  408 ,  410 . A connecting element  412  electrically connects the antenna elements  404 ,  406  such that electrical currents on one antenna element flow to the other antenna element and generally bypass the antenna port coupled to the other antenna element. The electrical currents flowing through each antenna element are generally equal in magnitude, such that an antenna mode excited by one antenna port is generally electrically isolated from a mode excited by the other antenna port at a given desired signal frequency range. 
     In the  FIG. 4  example, the balanced antenna  402  is designed to produce z-axis polarization, with low profile (10 mm height) and symmetry about the plane of the PCB ground. 
     Plots of simulated S parameters for a model of the assembly of  FIG. 4  are included as  FIGS. 5A-5D . For these plots, the Ports 1 and 2 are connected to the WiMAX antennas, and Ports 3 and 4 are connected to the balanced two-port antenna. Coupling between the WiMAX antennas (S12) is about −15 dB as before ( FIG. 5A ). For the two-port antenna, both ports are well matched and have enhanced isolation over the WiFi band (2400 to 2500 MHz). The coupling between WiMAX and either WiFi antenna port is less than 35 dB ( FIGS. 5C and 5D ). This antenna configuration therefore provides adequate isolation for co-existence between WiFi and WiMAX radios, while allowing full MIMO or diversity operation within the 802.11n or 802.11b protocols. 
       FIGS. 6A-6C  illustrate an antenna system  600  in accordance with one or more further embodiments. The antenna system can be used, e.g., in a USB dongle assembly for communication over WiMAX. The antenna system  600  includes a printed antenna  602  (that uses the ground plane  604  of a printed circuit board assembly as a counterpoise) and a balanced antenna  606 . Both antennas are located at the same end of the PCB assembly as shown in  FIG. 6B . The balanced antenna  606  in this example is formed by wrapping a flexible printed circuit (FPC) onto a plastic carrier  608 . The plastic carrier  608  can be slid onto the end of the PCB. Spring contacts  610  on the top and bottom side of the PCB provide connection to the feed and ground terminals of the balanced antenna, respectively, as depicted in  FIG. 6C . 
     Plots of antenna performance parameters VSWR, S12, efficiency, and antenna cross-correlation are provided as  FIGS. 7A-7D . These plots demonstrate good performance across the entire band from 2500 to 2700 MHz. 
       FIG. 8  is a perspective view of an alternate antenna system  800  in accordance with one or more embodiments. The antenna system  800  includes a balanced antenna  802  that is formed from a single piece of stamped metal. The balanced antenna can be attached to the PCB, e.g., by sliding it onto the PCB. For simplicity, antennas coupled to the ground plane of the printed circuit board are not shown in  FIG. 8 . 
       FIG. 9  is a perspective view of another alternative antenna system  900  in accordance with one or more embodiments. The antenna system includes a balanced antenna that is formed from two pieces of stamped metal  902 , each forming a half of the balanced antenna. A balanced antenna is completed by attaching the two pieces  902  (e.g., by soldering) to the top and bottom sides of a PCB  904 . Each antenna piece has two legs. The legs on one side of the stamped pieces are soldered to pads on the PCB  904  that are connected together. The connected pads thereby complete the inductive connection between the top and bottom halves  902  of the balanced antenna. The ends of the legs on the other side of the pieces serve as the antenna feed terminals. One terminal is attached to the top side of the PCB  904  and the opposite terminal is attached to the bottom side of the PCB  904 . For simplicity, antennas coupled to the ground plane of the printed circuit board are not shown in  FIG. 9 . 
     Another advantage of antenna systems in accordance with various embodiments is that they produce reduced SAR values for devices that simultaneously transmit from two antennas, thereby facilitating compliance with SAR regulations. 
     It is common for two or more antennas in portable electronics devices to use the PCB ground plane as a counterpoise. Since the PCB ground plane is typically the largest conductor in the device, it tends to dominate the radiation environment. The near field distribution is also dominated by this feature. If two antennas are coupled to the same ground plane and are in close proximity to each other (i.e., less than a quarter of a wavelength apart), their near-field distributions will be largely overlapping. Connecting two transmitters, one to each antenna, will effectively double the resultant near-field (as compared to a single transmitter). In turn, the SAR values will also double. 
     This problem is mitigated by antenna systems in accordance with various embodiments because they provide increased isolation between antennas (one coupled to the main PCB ground as a counterpoise and a separate antenna that is balanced on and is not coupled into the PCB ground). The antenna system is configured such that the resultant near field distribution created by each antenna does not substantially overlap. As mentioned above, SAR values can double for overlapping near fields. However, SAR values are reduced in exemplary embodiments, e.g., to 1.5 times that of a single transmitter, which is preferable and is achieved from an antenna configuration that reduces the overlapping region of the near-field from each antenna. 
     By way of example, in the antenna system of  FIG. 6A , the peak SAR locations of the printed antenna and those for the balanced antenna are generally not coincident. In particular, for the printed antenna, the peak SAR is found around a circumference about the PCB assembly near the location between the antenna and the grounded PCB assembly. On the other hand, the peak SAR location for the balanced antenna is off the end of PCB assembly. 
     It is to be understood that although the invention has been described above in terms of particular embodiments, the foregoing embodiments are provided as illustrative only, and do not limit or define the scope of the invention. Various other embodiments, including but not limited to the following, are also within the scope of the claims. For example, elements and components described herein may be further divided into additional components or joined together to form fewer components for performing the same functions. 
     Having described preferred embodiments of the present invention, it should be apparent that modifications can be made without departing from the spirit and scope of the invention.