Patent Publication Number: US-6337666-B1

Title: Planar sleeve dipole antenna

Description:
FIELD OF THE INVENTION 
     The present invention is directed to an antenna unit for a wireless communications device, and more particularly to a compact antenna which is fabricated by disposing a conductive pattern on a substrate. 
     BACKGROUND OF THE INVENTION 
     A conventional sleeve antenna comprises a radiation element having an electrical length of one quarter wavelength, a sleeve having an electrical length of one quarter wavelength, and a coaxial cable for feeding a radiation element, wherein an outer conductor of the cable is connected to the sleeve, while an inner conductor of the coaxial cable is extended through the sleeve to be connected to the radiation element. 
     A conventional inverted type coaxial dipole antenna is constructed such that a central conductor of a coaxial cable is connected via a feeding line to a sleeve, wherein the feeding line is extended through a slot which is formed through an outer tube. 
     A conventional flat antenna comprises a flat substrate, on a first surface of which a microstrip of a thin conductive film is formed, and on a second surface of which a dipole antenna element and a feeding slot are formed. 
     The conventional sleeve antenna and inverted type coaxial dipole antenna involve complicated fabrication and adjustment because the feeding coaxial cable is connected to the sleeve. 
     U.S. Pat. No. 5,387,919 discloses a printed circuit antenna comprising an electrically insulating substrate on opposite sides of which are oppositely directed U-shaped, quarter wave, metallic radiators disposed symmetrically about a common longitudinal axis. The bases of the U-shaped radiators overlie each other and are respectively coupled to balanced transmission line conductors to one end of which a coaxial cable is connected, the other end being connected to a balun. By arranging the balun, coaxial cable and the balance conductors along the axis of the radiators, they do not interfere with the radiation pattern from the radiators. The requirement to use a balun limits the usage of the printed antenna because the antenna itself cannot be coupled directly to an input circuit of a receiver and/or output circuit of a transmitter. 
     U.S. Pat. No. 5,754,145 discloses a printed circuit antenna comprising an end fed elongate first dipole element provided on one side of a dielectric substrate. A second dipole element is provided on the opposite side of the dielectric substrate. The second dipole comprises first and second elongate elements disposed one on each side of the longitudinal axis of the first dipole element as viewed through the substrate. A ground plane on the second side of the substrate is connected to the first and second elements at a distance from a free end of the first dipole element corresponding substantially to a quarter wavelength of the frequency of interest. 
     SUMMARY OF THE INVENTION 
     In view of the above-mentioned limitations of the prior art antennas, it is an object of the present invention to provide an antenna for use with a portable wireless communications device. 
     It is another object of the invention to provide an antenna unit which is lightweight, compact, highly reliable, and efficiently produced. 
     According to one aspect of the present invention there is provided a printed antenna comprising an end fed elongate first dipole half element provided on one side of a dielectric substrate, a second dipole half element provided on a second side of the dielectric substrate, the second dipole comprising first and second elongate elements disposed one on each side of the longitudinal axis of the first dipole half element as viewed through the substrate and a ground plane coextensive with a feed portion of the first dipole half element, said ground plane being connected to the first and second elements. The first and second elements may extend parallel to the longitudinal axis of the first dipole half element as viewed perpendicular to the plane of the substrate. 
     In preferred embodiments of the present invention, an antenna which couples to a transmitter/receiver, includes a printed circuit board (PCB) substrate. The antenna unit may be mass produced using printed circuit board (PCB) technology, where a dielectric material is selectively configured with a conductive material. The PCB antenna unit can be encapsulated in plastic or other material to create a solid, robust package which is durable and resistant to damage and deterioration. 
     The antenna unit can be used as part of a wireless voice or data link, or as part of an RF modem. The antenna unit is particularly suitable for use in compact, wireless communication devices such as portable computers, PDA&#39;s, palm sized computers or information devices, or as an RF modem for desktop and mainframe computer systems. 
     Additionally, the antenna unit can be configured to be connected to the device through PCMCIA or Universal Serial Bus (USB) or other types of plug-in ports used in computers and PDA type devices. The antenna can be implemented to transmit and receive on desired frequencies of the device users, including analog or digital U.S. or European cell phone bands, PCS cell phone bands, 2.4 GHZ Bluetooth bands, or other frequency bands as would be obvious to one skilled in the art. 
     An antenna unit according to the present invention features broad VSWR and gain bandwidth greater than 15%. The invention is an omnidirectional antenna, having efficiency of 90% or greater. The invention can be encapsulated in plastic to produce a mechanically rugged device that is not easily damaged as with common whip dipole antennas. 
     Yet another aspect of the present invention is an antenna assembly having a selectively movable portion for adjusting the spatial orientation of the antenna, and hence, the polarization characteristics of the antenna. Such a selectively movable portion may include a hinged element having an interiorly disposed antenna displaying vertical, horizontal, or combined polarization characteristics as the hinged movable portion is biased into different positions. 
     The above and other objects and advantageous features of the present invention will be made apparent from the following description with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention will be described in detail hereinafter with reference to the accompanying drawings wherein: 
     FIG. 1 is a perspective view of a conventional sleeve antenna; 
     FIG. 2 is a cross sectional view of a conventional inverted type coaxial dipole antenna; 
     FIG. 3 is a plan view of a conventional flat antenna; 
     FIG. 4 is perspective view of a wireless communications device and an antenna unit according to the present invention; 
     FIG. 5 is a perspective view of another wireless communications device an antenna unit according to the present invention; 
     FIG. 6 a detailed perspective view of the antenna unit of FIG. 4; 
     FIG. 7 is a top plan view of a portion of the antenna unit of FIG. 4; 
     FIG. 8 is a top plan view of a detailed portion of the antenna unit of FIG. 7; 
     FIG. 9 is a bottom plan view of a portion of the antenna unit of FIG. 4; 
     FIG. 10 is a graph illustrating gain characteristic of the antenna unit of FIG. 4; and 
     FIG. 11 is a graph showing directional characteristic of the antenna unit of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE PRESENT INVENTION 
     Prior to explaining an antenna in a preferred embodiment according to the present invention, the aforementioned conventional antennas will be explained in more detail. FIG. 1 illustrates a conventional sleeve antenna. Numeral  110  designates a radiating element having an electrical length of one quarter wavelength, numeral  112  a sleeve (a cylindrical tube) having an electrical length of one quarter wavelength, and numeral  114  a feeding coaxial cable. The outer conductor of the coaxial cable  114  is connected to the sleeve  112 , while a central conductor of the coaxial cable  114  is connected to the radiating element  110 . This sleeve antenna has operating performance equal to a dipole antenna comprising the radiation element  110  and the sleeve  112 , good efficiency, good directivity, and stable impedance. 
     FIG. 2 illustrates a cross-sectional view of a conventional inverted type coaxial dipole antenna where a central conductor  210  and an outer tube  212  are replaced with each other. The central conductor  210  is connected to the sleeve  214  via a feeding line  216  passing through a slot  218  of the outer tube  212 . This inverted type coaxial dipole antenna has operation performance equivalent to the above sleeve antenna, good efficiency, good directivity, and stable impedance. Further, a plurality of this type of antennas may be arranged to form an array antenna. 
     FIG. 3 illustrates a conventional flat antenna comprising a conductor provided on a substrate. In the drawing, numeral  310  designates a dielectric substrate, numeral  312  a microstrip line of a thin-film conductor, numeral  314  a dipole antenna element of a conductor provided on the side of the substrate  310  opposite to the micro-strip line  312 , numeral  316  a feeding slot, and numeral  318  a notch having an electrical length of one quarter wavelength. This antenna has operation performance equivalent to the above sleeve antenna, good efficiency, good directivity, and stable impedance. 
     Next, an antenna in a preferred embodiment according to the present invention will be explained. 
     FIGS. 4 and 5 illustrate a selectively attachable antenna assembly  12  having disposed therewithin an antenna unit or device  14  according to the present invention. FIG. 4 illustrates a wireless communications device  10 , such as a cellular telephone or PDA device. FIG. 5 illustrates a portable computer. The antenna assembly  12  may be coupled directly to the wireless communications device  10 , as shown in FIGS. 4 and 5, or may be remotely disposed, such as wall-mounted (not shown), and coupled to the device  10  via a signal cable, etc. The antenna assembly  12  can be used as part of a wireless voice or data link, or as part of an RF modem. The antenna assembly  12  can be coupled to the wireless device  10  through its PCMCIA or Universal Serial Bus (USB)  16  or other plug-in port. In preferred embodiments, the antenna assembly  12  can be implemented to transmit and receive on desired frequencies of the device users, including analog or digital U.S. or European cell phone bands, PCS cell phone bands, 2.4 GHZ Bluetooth bands, or other frequency bands as would be obvious to one skilled in the art. 
     Referring particularly to FIG. 6, the antenna device  14  may be disposed within a portion of the selectively attachable antenna assembly  12  designed to be coupled to a plug-in port  16  of the wireless communications device  10 . The antenna assembly  12  may include a digital signal line  18 , an RF modem board  20  coupled to the digital signal line  18 , and a coax signal line  22  for coupling to the antenna  14 . The antenna assembly  12  of FIGS. 4-6, includes a selectively movable portion  24  within which the antenna device  14  is disposed. The selectively movable portion  24  is coupled to the remaining portion of the antenna assembly  12  via a hinge apparatus  26 , though alternative coupling approaches would also be practicable. The hinged movable portion  24  may be biased by the user to provide a particular spatial orientation of the antenna device  14 . For example, a preferred orientation of 90° (vertical) is shown in FIGS. 4 and 5. Additional polarizations may be accommodated by adjusting the movable portion  24  to 180° for vertical polarization or to 135° for equal horizontal and vertical antenna polarization characteristics. 
     The printed antenna  14  includes a substrate  40  of, for example Duroid or glass fiber, or known dielectric printed circuit board material. The substrate element  40  may be a dielectric PC board having a thickness between 0.005″ to 0.125″ thick. A flexible PCB substrate may also be practicable. Apertures  42  are included in the substrate  40  to facilitate plastic encapsulation of the antenna  14 . The details of such encapsulation processes would be appreciated by those skilled in the relevant arts. 
     Referring particularly to FIGS. 6,  7 , and  9 , the substrate element  40  includes a first major surface  44  and an opposed second major surface  46 . Disposed upon the first major surface  44  of the substrate  40  are: an RF coupling structure  50  for coupling the antenna  14  to the telecommunications device  10  (via digital signal line  18 , RF modem  20 , and coax signal line  22 ); a microstrip transmission line  52 , and an end-fed quarter wavelength dipole half element  54 . A feed point  56  is defined proximate the junction between the microstrip transmission line  52  and the radiating half element  54 . In use it is intended that the dipole half  54  be arranged vertically such that the effective part of the dipole  54  is the upper section having an electrical length corresponding substantially to a quarter wavelength of the frequency (or center frequency) of interest. 
     Referring now to FIG. 8, a detailed illustration of the RF coupling structure  50  is disclosed. RF coupling structure  50  includes a signal coupling point  60  at the free end of the microstrip transmission line  52 , to which the center conductor  62  of the coax signal line  22  is coupled. RF coupling structure  50  further includes a shield coupling structure  64 , including opposed shield conductor pad portions  66  disposed on either side of the signal coupling point  60  and connected via an intermediate conductor portion  68 . The shield conductor  63  of the coax signal line  22  is directly coupled to one or both of the shield conductor pad portions  66 . Each shield pad portion  66  includes a plated through-hole  70  for coupling to the opposite major surface  46  of the substrate  40  as further described herein. 
     Referring particularly to FIG. 9, disposed upon the second major surface  46  is a ground plane conductor  72  and a second half dipole  74  comprising first and second elements  76 , 78  which are connected to the ground plane  72  at a distance corresponding to substantially a quarter wavelength from the free end of the first dipole half element  54  and extending away therefrom. The ground plane  72  is coupled proximate one end to the shield conductor  63  of the coax signal line  22  via the plated through-holes  70  in the substrate  40  at the shield conductor portion  64  of the RF coupling structure  50 . 
     Each of the first and second elements  76 , 78  has a length corresponding to a quarter wavelength of the frequency (or center frequency) of interest. The first and second elements  76 , 78  are parallel to the longitudinal axis of the first dipole half element  54 . From an RF point of view the first dipole half element  54  and the first and second elements  76 , 78  form a half wave antenna with the electrical junction between the two half dipoles  54 , 74  being at a low impedance, typically 50 ohms. The central feed point  56  is proximate the point of convergence of the first and second elements  76 , 78 . The lateral spacing of the lower radiating arms  76 , 78  from the central microstrip transmission line ground plane  72  is optimized to reduce currents on the connecting feed cable  22 . 
     Each conductor element  52 ,  54 ,  72 ,  76 ,  78  on the substrate  40  may be produced by printed board fabrication processes. Alternatively, the conductor elements  52 ,  54 ,  72 ,  76 ,  78  may be prepared by applying a conductive foil, for example, a copper foil. In the antenna  14  shown in FIGS. 4-8, the conductor elements  52 ,  54 ,  72 ,  76 ,  78  are provided on a planar substrate, realizing a thin, lightweight antenna  14 . Further, since the antenna  14  may be prepared by printed board fabrication processes, the dimensional accuracy is very good. Since the substrate  40  and the conductors  52 ,  54 ,  72 ,  76 ,  78  are integral with each other, there is no need for extensive assembly. 
     Those skilled in the relevant arts may appreciate that the conductor elements  52 ,  54 ,  72 ,  76 ,  78  could be implemented as meandered conductor lines to reduce the overall antenna  14  package length. 
     The operation of the antenna  14  will be explained. A feed signal applied to the microstrip transmission line  52  via the RF coupling structure  50  passes to the first dipole half element  54 . This permits a radio wave to be radiated from the radiation element  54 . Impedance matching between the first dipole half element  54  and the microstrip transmission  52  may be performed by regulating the position, in the longitudinal direction of the dipole radiating element  54 , at which the feed point  56  is coupled to the radiating element  54 . 
     FIG. 10 is a plot of an VSWR measurement of the present antenna  14 , taken at the output/input coupling structure using a network analyzer. Markers  1 ,  2  and  3  on the plot correspond to measurement frequencies of 2.400, 2.440, and 2.485 GHz, yielding corresponding to VSWR measurements of 1.3195, 1.0961, and 1.1140, respectively. The measurements confirm an effective operating bandwidth of 85 MHz for the disclosed antenna  14 . FIG. 11 is an elevational pattern of the present antenna  14 , taken with an automated antenna measurement system. FIG. 11 reveals that the antenna configuration yields a gain greater than 0 dBi over 75° in elevation (from +45° degrees to −30°). An azimuth pattern yields an omnidirectional pattern at horizon with a variation of less than 1 dB. 
     While the foregoing description represents preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made, without departing from the spirit and scope of the invention as defined by the following claims.