Patent Publication Number: US-6339405-B1

Title: Dual band dipole antenna structure

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to dipole antenna structures and more particulary to a dual band dipole antenna structure operative to efficiently transmit radio frequency (RF) energy at two different frequencies. 
     BACKGROUND OF THE INVENTION 
     In order to efficiently operate, the length of a dipole antenna is typically related to the operating frequency thereof. The length of the dipole element is a multiple of the frequency to be transmitted or received. For example, the dipole element may have a length that is ¼, ½, or ¾ the wavelength of transmission. As will be recognized, a single dipole element cannot efficiently operate for multiple operating frequencies because the length thereof must change. 
     For instance, in wireless technology, the device may need to operate on two different frequency bands. The device may have an operating frequency of either 800 MHZ or 1900 MHZ depending upon the type of service the wireless device is accessing. As such, the antenna structure must be capable of efficient transmission and reception of RF energy at both of those bands. 
     Printed antenna structures are widely used to provide compact antennas for portable devices. The printed antenna structures are typically formed on a substrate such as a PCB by forming conductive traces on the PCB. In this regard, the printed antenna structure can be integrated with other electronic devices on the substrate. Typically, the antenna structure is designed on a rigid PCB having a thickness of about 3-5 mm. Therefore, the size and thickness of the PCB restrict the size of the device that the antenna can be placed within. Typically, in portable wireless devices (i.e., cellular telephones), the housing for the device is designed around the size of the antenna structure. 
     In order to efficiently transmit over both frequency bands, printed antenna structures have been designed with complicated wire patterns in order to provide the correct dipole length. For instance, in U.S. Pat. No. 5,949,383 to Hayes et al. entitled “Compact Antenna Structures Including Baluns”, the printed antenna structure includes multiple radiating sections and a balun in order to tune the antenna for two operating frequencies. The printed antenna structure further includes a tunning shunt across the balun in order to provide dual band operation. In this sense, the printed antenna structure includes a complicated trace structure and tunning mechanism to provide dual band operation. 
     The present invention addresses the above-mentioned deficiencies in the prior art antenna structures by providing a dipole antenna structure that is compact in size and easily formed. More specifically, the present invention provides an antenna structure that is formed on a thin film PCB and comprises two dipole elements and corresponding dipole grounds. In this sense, the design of the antenna structure for the present invention provides for dual band operation with a compact and easily fabricated structure. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a dual band antenna structure having a substrate with first and second sides. The first side includes a first dipole element, and a second dipole element formed in substantially parallel relation to the first dipole element and electrically connected thereto. The first side of the antenna further includes a generally wedged shaped transformer electrically connected to the first and second dipole elements. The second side of the antenna structure includes a first dipole ground disposed in generally opposite relation to the first dipole element and a second dipole ground disposed in generally opposite relation to the second dipole element. The first and second dipole grounds are electrically connected together via a ground line. Accordingly, RF energy fed into the transformer can be transmitted at a first frequency by the first dipole element and can be transmitted at a second frequency by the second dipole element. 
     In accordance with the present invention, the first dipole element has a length equal to about ¼ the wavelength of the first frequency and the second dipole element has a length equal to about ¼ the length of the second frequency. The first dipole ground has a length equal to about ¼ the wavelength of the first frequency, while the second dipole ground has a length equal to about ¼ the length of the second frequency. Both the first and second dipole elements are disposed in substantially parallel relation to the transformer element. 
     In the preferred embodiment, the shape of the first dipole ground is substantially similar to the shape of the first dipole element, while the shape of the second dipole ground is substantially similar to the shape of the second dipole element. In this respect, both the first dipole element and the second dipole radiating element are substantially rectangular. The first and second dipole grounds are disposed in opposite relation on the second side of the substrate in substantially mirror-image relation to respective first and second dipole elements. 
     In accordance with the present invention, the substrate is a thin film such as a thin film PCB. The thin film may additionally be flexible. The first and second dipole elements are formed as conductive tracings on the PCB through conventional techniques. A microstrip is formed as the ground line connecting the first and second dipole grounds, as well as to connect the first dipole element, the second dipole element and the transformer. 
     In accordance with the present invention, there is provided a dual band antenna structure having a substrate, a first antenna array, a second antenna array, and a transformer. The first antenna array has a first dipole element disposed on a first side of the substrate. Furthermore, the first antenna array has a first dipole ground disposed on a second side of the substrate. The first dipole ground is disposed in substantially mirror-image relationship to the first dipole element. The second antenna array has a second dipole element disposed on the first side of the substrate and a second dipole ground disposed on the second side of the substrate. The second dipole ground is disposed in substantially mirror-image relationship to the first dipole element. The transformer is formed on the first side of the substrate and electrically connects the first and second dipole elements. In this respect, the first array is operative to transmit electromagnetic energy at a first frequency and the second array is operative to transmit electromagnetic energy at a second frequency when the electromagnetic energy is fed to the transformer. The length of the first dipole element is chosen to transmit the first frequency and the length of the second dipole element is chosen to transmit the second frequency. 
     In accordance with the present invention, there is provided a method of forming a dual band antenna structure for transmitting a first and a second frequency. The method comprises providing a thin film substrate having a first side and a second side. Next a first dipole element is formed on the first side of the substrate. A first dipole ground is formed on the second side of the substrate in substantially mirror-image relation to the first dipole element. A second dipole element is formed on the first side of the substrate and a second dipole ground is formed on the second side of the substrate in substantially mirror-image relation to the second dipole element. Finally a transformer is formed on the first side of the substrate. The transformer is electrically connected to the first dipole element and the second dipole radiating element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These, as well as other features of the present invention, will become more apparent upon reference to the drawings wherein: 
     FIG. 1 is a plan view of a first side of a dual band antenna structure constructed in accordance with the present invention; and 
     FIG. 2 is a plan view of a second side of the antenna structure shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings wherein the showings are for purposes of illustrating a preferred embodiment of the present invention only, and not for purposes of limiting the same, FIG. 1 is a plan view of an antenna structure  10 . Specifically, the antenna structure  10  has a non-conductive substrate  12  with conductive tracings formed thereon. The substrate  12  has a first side  14  as seen in FIG. 1, and a second side  16  as seen in FIG.  2 . In the preferred embodiment of the present invention, the substrate  12  is a thin film, flexible printed circuit board (PCB) with a cross-sectional thickness of about 0.5 mm. The conductive tracings are formed on the PCB substrate  12  through conventional techniques such as photo-etching. 
     Referring to FIG. 1, the substrate  12  has a first dipole element  18  formed on the first side  14  thereof. The first dipole element  18  is formed from a conductive material such as copper on the first side  14  of the substrate  12 . The first dipole element  18  is generally rectangular and has a length l 1  equal to about ¼ the wavelength of the lowest frequency that the antenna structure  10  is designed for. Similarly, the antenna structure  10  includes a second dipole element  20  formed on the first side  14  of the substrate  12 . The second dipole element  20  is generally rectangular and has a length l 2  that is equal to about ¼ the wavelength of the highest frequency that the antenna structure is designed for. Accordingly, the first dipole element  18  is designed to transmit and receive electromagnetic radiation in a first frequency bandwidth, while the second dipole element is designed to transmit and receive electromagnetic radiation in a second frequency bandwidth. For the antenna structure  10  depicted in FIGS. 1 and 2, the first dipole element  18  is designed to transmit frequencies in a band that is lower than the second dipole element  20  thereby providing for dual band operation. 
     Referring to FIG. 1, the antenna structure  10  further includes a microstrip  22  electrically connecting the first dipole element  18  to the second dipole element  20 . Specifically, the microstrip  22  is a conductive material such as copper formed on the first side  14  of the substrate  12  and connecting the same ends of respective first and second dipole elements  12 ,  14 . The microstrip  22  functions to end feed the first and second dipole elements  18 ,  20 , as will be further explained below. The microstrip  22  is electrically connected to a generally wedged-shaped transformer  24  formed on the first side  14  of the substrate  12 . The transformer  24  is formed from a conductive material such as copper and has a connecting portion  26  wherein a conductor from a transceiver is connected. Specifically, the connecting portion  26  is adapted to be electrically attached to the transceiver such that electromagnetic energy to be transmitted by the antenna structure  10  is fed to the transformer  24  and electromagnetic energy received by the antenna structure  10  is fed from the transformer  24  at the connecting portion  26  to the transceiver. The connecting portion  26  has four outer apertures  27  for soldering a wire thereto. The outer circumference of each of the apertures  27  is in contact with the transformer  24  at the connecting portion  26 . In this respect, a conductor soldered into each of the outer apertures  27  would be electrically connected to the transformer  24 . 
     As seen in FIG. 1, the transformer  24  tapers from the connecting portion  26  to the microstrip  22 . In this regard, the taper of the transformer  24  is operative to provide impedance matching as is currently known in the art between the transceiver and the first and second dipole elements  18 ,  20  attached to the transformer  24  via microstrip  22 . The transformer  24  and microstrip  22  provide a method of end feeding electromagnetic energy to the first and second dipole elements  18 ,  20 . 
     Referring to FIG. 2, the antenna structure  10  further includes a first dipole ground  28  disposed on the second side  16  of the substrate  12 . Specifically, the first dipole ground  28  is formed from a conductive material such as copper on the second side  16  of the substrate  12 . The shape of the first dipole ground  28  is substantially similar as the first dipole element  18 . In this respect, the first dipole ground  28  is generally rectangular and has length l 1 . Furthermore, as seen in FIGS. 1 and 2, the first dipole ground  28  is disposed in a generally mirror-image relationship to the first dipole element  18 . Specifically, the first dipole ground  28  is in mirror-image relation to the first dipole element  18  about axis “A”. In this regard, the first dipole ground  28  is formed as if the first dipole element were rotated about axis “A” and placed on the second side  16  of substrate  12 . 
     Referring to FIG. 2, the antenna structure  10  further includes a second dipole ground  30  formed on the second side  16  of the substrate  12 . The second dipole ground  30  is formed as a mirror-image of the second dipole element  20  rotated around axis “A”. The shape of the second dipole ground  30  is substantially similar to the shape of the second dipole element  20 . In this respect, the second dipole ground  30  has a length of l 2  and is generally rectangularly shaped. 
     The antenna structure  10  further includes a generally T-shaped ground line  32  electrically connected to the ends of both of the first and second dipole grounds  28 ,  30 . As seen in FIG. 2, the ground line  32  extends from the ends of each of the dipole grounds  28 ,  30  to a “T” junction and then extends to the connecting portion  26 . Specifically, the ground line  32  extends to an inner aperture  36  of the connecting portion  26 . The outer circumference of the inner aperture  36  is in electrical contact with the ground line  32  such that a conductor soldered into the inner aperture  36  will be electrically connected to the ground line  32  and hence first and second dipole grounds  28 ,  30 . Typically, a ground of the transceiver is attached to the inner aperture  36 . 
     In accordance with the present invention, the combination of the first dipole element  18  and the first dipole ground  28  define a first antenna array  38 . Similarly, the second dipole element  20  and second dipole ground  30  define a second antenna array  40 . The first antenna array  38  is operative to transmit and receive signals in a first frequency bandwidth corresponding to the length of the first dipole element  18 . The second antenna array  40  is operative to transmit and receive signals in a second frequency bandwidth corresponding to the length of the second dipole element  28 . In this respect, the combination of the first and second antenna arrays  38 ,  40  are operative to transmit and receive electromagnetic energy within two distinct bandwidths. 
     Additional modifications and improvements of the present invention may also be apparent to those of ordinary skill in the art. Thus, the particular combination of parts described and illustrated herein is intended to represent only a certain embodiment of the present invention only, and is not intended to serve as a limitation of alternative devices within the spirit and scope of the invention.