Patent Publication Number: US-9419336-B2

Title: Compact broadband antenna

Description:
REFERENCE TO RELATED APPLICATIONS 
     This is a continuation of application Ser. No. 13/978,092, having a 371(c) date of Aug. 8, 2013, which is a U.S. National Stage Entry of PCT/IL2012/000001, filed on Jan. 3, 2012, which claims the benefit of priority to U.S. Provisional Patent Application 61/429,240 entitled SLIT-FEED MULTIBAND ANTENNA, filed Jan. 3, 2011, all of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to antennas and more particularly to antennas for use in wireless communication devices. 
     BACKGROUND OF THE INVENTION 
     The following publications are believed to represent the current state of the art: 
     U.S. Pat. Nos. 7,843,390 and 7,825,863. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide a novel compact broadband antenna, for use wireless communication devices. 
     There is thus provided in accordance with a preferred embodiment of the present invention an antenna including a substrate formed of a non-conductive material, a ground plane disposed on the substrate, a wideband radiating element having one end connected to an edge of the ground plane and an elongate feed arm feeding the wideband radiating element and having a Maximum width of 1/100 of a predetermined wavelength, the predetermined wavelength being defined by 
               λ   p     =     1     f   ⁢       μ   ⁡     [       (         ɛ     r   r       +   1     2     )     +         (         ɛ     r   r       -   1     2     )     ⁡     [     1   +     12   ⁢     (     H   W     )         ]         -   0.5         ]                   
wherein λ p  is the predetermined wavelength, f is a lowest operating frequency of the wideband radiating element, μ is a permeability of the substrate, ∈ r  is a relative bulk permittivity of the substrate, W is a width of a conductive, trace disposed above the substrate and H is a thickness of the substrate, wherein
 
     
       
         
           
             
               W 
               H 
             
             ≥ 
             1. 
           
         
       
     
     In accordance with a preferred embodiment of the present invention, a feed point is located on the feed arm. 
     Preferably, the antenna also includes a second radiating element galvanically connected to and fed by the feed point. 
     Preferably, the feed arm is disposed in proximity to but offset from the wideband radiating element and the edge of the ground plane. 
     In accordance with another preferred embodiment of the present invention, the wideband radiating element includes a first portion and a second portion. 
     Preferably, the first and second portions are generally parallel to each other and to the edge of the ground plane. 
     Preferably, the first portion is separated from the edge of the ground plane by a distance of less than 1/80 of the predetermined wavelength. 
     In accordance with a further preferred embodiment of the present invention, the substrate has at least an upper surface and a lower surface. 
     Preferably, at least the ground plane and the wideband radiating element are located on one of the upper and lower surfaces. 
     Preferably, at least the feed arm is located on the other one of the upper and lower surfaces. 
     Alternatively, at least the ground plane, the wideband radiating element and the feed arm are located on a common surface of the substrate. 
     In accordance with yet another preferred embodiment of the present invention, the wideband radiating element radiates in a low-frequency band. 
     Preferably, the low-frequency band includes at least one of LTE 700, LTE 750, GSM 850, GSM 900 and 700-960 MHz. 
     Preferably, a length of the wideband radiating element is generally equal to a quarter of a wavelength corresponding to the low-frequency band. 
     Preferably, the second radiating element radiates in a high-frequency band. 
     Preferably, a frequency of radiation of the wideband radiating element exhibits negligible dependency upon a frequency of radiation of the second radiating element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which: 
         FIGS. 1A and 1B  are simplified respective top and underside view illustrations of an antenna, constructed and operative in accordance with a preferred embodiment of the present invention; 
         FIG. 2  is a simplified graph showing the return loss of an antenna of the type illustrated in  FIGS. 1A and 1B ; 
         FIGS. 3A, 3B and 3C  are simplified respective top, underside and side view illustrations of an antenna, constructed and operative in accordance with another preferred embodiment of the present invention; and 
         FIG. 4  is a simplified graph showing the return loss of an antenna of the type illustrated in  FIGS. 3A, 3B and 3C . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is now made to  FIGS. 1A and 1B , which are simplified respective top and underside view illustrations of an antenna, constructed and operative in accordance with a preferred embodiment of the present invention. 
     As seen in  FIGS. 1A and 1B , there is provided an antenna  100 , including a ground plane  102  and a radiating element  104 , an end  106  of which radiating element  104  is preferably connected to an edge  108  of the ground plane  102 . Preferably, radiating element  104  is galvanically connected to the edge  108  of the ground plane  102 . Alternatively, radiating element  104  may be non-galvanically connected to the edge  108  of the ground plane  102 . 
     As seen most clearly in  FIG. 1A , radiating element  104  preferably has a compact folded configuration including a first portion  110  and a second portion  112 , which first and second portions  110  and  112  preferably extend generally parallel to each other and to the edge  108  of ground plane  102 . It is appreciated, however, that other configurations of radiating element  104  are also possible and are included within the scope of the present invention. 
     Radiating element  104  is fed by an elongate feed arm  114 , which feed arm  114  is preferably disposed in proximity to but offset from both the first portion  110  of radiating element  104  and from the edge  108  of the ground plane  102 . As seen most clearly in section A-A of  FIG. 1A , in accordance with a particularly preferred embodiment of the present invention, feed arm  114  is disposed in a plane offset from the plane in which the radiating element  104  and ground plane  102  are disposed. Feed arm  114  receives a radio-frequency (RF) input signal by way of a feed point  116  preferably located thereon. Preferably, feed arm  114  has an open-ended structure. Alternatively, feed arm  114  may terminate in other configurations, including a galvanic connection to the ground plane  102 . 
     As best seen at section A-A of  FIG. 1A , feed arm  114  is very narrow. The extremely narrow width of feed arm  114  is a particular feature of a preferred embodiment of the present invention and confers significant operational advantages on antenna  100 . The narrow width of feed arm  114  serves, among other features, to distinguish the antenna of the present invention over conventional, seemingly comparable antennas that typically utilize significantly wider feeding elements. 
     Due to its narrow elongate structure, feed arm  114  has a high series inductance. Furthermore, the close proximity of feed arm  114  to the edge  108  of ground plane  102  confers a significant shunt capacitance on the ground plane  102 . The compensatory interaction of these two reactances, namely the series inductance and shunt capacitance, leads to improved impedance Matching between radiating element  104  and feed point  116 . This improved impedance matching allows radiating element  104  to operate as a wideband radiating element, capable of radiating efficiently over a broad range of frequencies despite its compact folded structure. The mechanism via which the elongate narrow feed arm  114  contributes to the wideband operation Of radiating element  104  will be further detailed henceforth. 
     Antenna  100  is preferably supported by a non-conductive substrate  118 . Substrate  118  is preferably a printed circuit board (PCB) substrate and may be formed of any suitable non-conductive material, including, by way of example, FR-4. 
     As seen most clearly in sections A-A and B-B of  FIGS. 1A and 1B  respectively, ground plane  102  and radiating element  104  are preferably disposed on an upper surface  120  of substrate  118  and feed area  114  is preferably disposed on an opposite lower surface  122  of substrate  118 . However, it is appreciated that the reference to upper and lower surfaces  120  and  122  is exemplary only and that feed arm  114  may alternatively be located on upper surface  120  of substrate  118  and ground plane  102  and radiating element  104  located on lower surface  122  of substrate  118 . It is further appreciated that, depending on design requirements, feed arm  114  may optionally be disposed on the same surface of substrate  118  as that of ground plane  102  and radiating element  104 , provided that feed arm  114  remains offset from both the edge  108  of ground plane  102  and radiating element  104 . 
     In operation of antenna  100 , feed arm  114  receives an RF input signal by way of feed point  116 . Consequently, near field coupling occurs between feed arm  114 , the adjacent edge  108  of ground plane  102  and the adjacent first portion  110  of the radiating element  104 . This near field coupling is both capacitive and inductive in its nature, its inductive component arising due to the narrow elongate structure of feed arm  114 . The near field inductive and capacitive coupling controls the impedance match of radiating element  104  to feed point  116 . 
     In effect, feed arm  114 , the edge  108  of ground plane  102  and the lower portion  110  of radiating element  104  function in combination as a loosely coupled transmission line terminated in a short circuit by end  106 , which loosely coupled transmission line feeds the upper portion  112  of the radiating element  104 . The loosely coupled nature of the transmission line is attributable to the feed arm  114  being disposed in proximity to but offset from the radiating element  104  and ground plane  102 . The loosely coupled nature of the transmission line is further enhanced by the gap between the lower portion  110  of radiating element  104  and the edge  108  of the ground plane, which gap is preferably conductor-free, save for the connection of the lower portion  110  at end  106  to the edge  108 . 
     The loosely coupled transmission line thus formed acts as a distributed matching circuit, leading to improved impedance matching over the frequency band of radiation of radiating element  104  and hence endowing radiating element  104  with wideband performance. 
     It is appreciated that the improved impedance matching between radiating element  104  and feed point  116  is due in large part to the compensatory interaction of the significant series inductive coupling component arising from the narrow elongate structure of the feed arm  114  and the shunt capacitive coupling component arising from the close proximity of feed arm  114  to the ground plane edge  108 . In the absence of the series inductive coupling component, near field capacitive coupling alone would provide a poorer impedance match and hence narrower bandwidth of performance of radiating element  104 . 
     Feed arm  114  preferably has a maximum width of 1/100 of a predetermined wavelength λ p , which predetermined wavelength λ p  is preferably defined by: 
               λ   p     =     1     f   ⁢       μ   ⁡     [       (         ɛ     r   r       +   1     2     )     +         (         ɛ     r   r       -   1     2     )     ⁡     [     1   +     12   ⁢     (     H   W     )         ]         -   0.5         ]                   
wherein f is a lowest operating frequency of radiating element  104 , μ is the permeability of substrate  118 , ∈ r  is the relative bulk permittivity of substrate  118 , W is the width of a conductive trace disposed above substrate  118 , forming a microstrip transmission line bounded by air, and H is the thickness of substrate  118 . The expression
 
             [       (         ɛ     r   r       +   1     2     )     +         (         ɛ     r   r       -   1     2     )     ⁡     [     1   +     12   ⁢     (     H   W     )         ]         -   0.5         ]         
corresponds to the effective dielectric constant for the substrate system. This definition of λ p  assumes that
 
               W   H     ≥   1         
and is based upon equations derived by I. J. Bahl and D. K. Trivedi in “A Designer&#39;s Guide to Microstrip Line”, Microwaves, May 1977, pp. 174-182.
 
     It is appreciated that the conductive trace referenced in the above equation is simply an entity of computational convenience, used in order to define the substrate-specific wavelength corresponding the lowest operating frequency of radiating element  104  and hence the preferable maximum width of feed arm  114 . It is understood that such a conductive trace is not necessarily actually formed in a preferred embodiment of substrate  118 . 
     Wideband radiating element  104  preferably operates as a low-band radiating element, preferably capable of radiating in at least one of the LTE 700, LIE 750, GSM 850, GSM 900 and 700-960 MHz frequency bands. Thus, by way of example, when wideband radiating element  104  Operates at a lowest frequency of 700 MHz, the predetermined wavelength λ p  to 700 MHz and defined with respect to a 50 Ohm microstrip transmission line formed of a limn thick FR-4 PCB substrate  118  is approximately 230 mm. The maximum width of feed arm  114  according to this exemplary embodiment is approximately 2.3 mm. 
     Radiating element  104  preferably has a total physical length approximately equal to a quarter of its operating wavelength. It is appreciated that the first portion  110  of radiating element  104  thus has a dual function, in that it both contributes to the near field coupling between the feed arm  114  and the radiating element  104 , as described above, and constitutes a portion of the total length of radiating element  104 . A second end  124  of radiating element  104 , distal from its first end  106  connected to ground plane  102 , is preferably bent in a direction towards edge  108  of ground plane  102 , whereby radiating element  104  is arranged in a compact fashion. 
     Antenna  100  operates optimally when radiating element  104  is located in close proximity to the edge  108  of ground plane  102 , due to the contribution of the edge  108  of the ground plane  102  to the above-described effective matching circuit. Particularly preferably, first portion  110  of radiating element  104  is separated from the edge  108  of the ground plane  102  by a distance of less than 1/80 of the above-defined predetermined wavelength λ p . Thus, by way of example, when wideband radiating element  104  operates at a lowest frequency of 700 MHz, the predetermined wavelength λ p  corresponding to 700 MHz and defined with respect to a 50 Ohm microstrip transmission line formed of a 1 mm thick FR-4 PCB substrate  118  is approximately 230 mm. The separation of first portion  110  of radiating element  104  from the edge  108  of the ground plane, according to this exemplary embodiment, is less than approximately 2.8 mm. 
     The close proximity of radiating element  104  to the ground plane  102  is a highly unusual feature of antenna  100  in comparison to conventional antennas that typically require the radiating element to be at a greater distance from the ground plane, in order to prevent degradation of the operating bandwidth and radiating efficiency of the antenna. The location of the radiating element  104  in such close proximity to the ground plane  102  in antenna  100  allows antenna  100  to be advantageously compact. 
     The extent of the coupling between feed arm  114 , the edge  108  of the ground plane  102  and the first portion  110  of the radiating element  104  is influenced by various geometric parameters of antenna  100 , including the length and width of the feed arm  114 , the configuration of the first and second portions  110  and  112  of radiating element  104  and the respective separations of first portion  110  and second end  124  of radiating element  104  from the edge  108  of the ground plane  102 . 
     Feed arm  114  and radiating element  104  may be embodied as three-dimensional conductive traces bonded to substrate  118 , or as two-dimensional conductive structures printed on the surfaces  120  and  122  of substrate  118 . A discrete passive component matching circuit, such as a matching circuit  126 , may optionally be included within the RF feedline driving antenna  100 , prior to the feed point  116 . 
     Reference is now made to  FIG. 2 , which is a simplified graph showing the return loss of an antenna of the type illustrated in  FIGS. 1A and 1B . 
     First local minima A of the graph generally corresponds to the frequency response of antenna  100  provided by radiating element  104 . As is evident from consideration of the width of region A, the response of antenna  100  is wideband and spans, by way of example, a range of 700-960 MHz with a return loss of better than −5 dB. As described above with reference to  FIGS. 1A and 1B , the wideband low-frequency response of antenna  100  is due to the improved impedance match of radiating element  104  to feed point  116 , as a result of the narrow elongate structure of feed arm  114 . 
     As is evident from consideration of region B of the graph, antenna  100  does not exhibit a significant high-band response. This is because feed arm  114  does not have a significant high-frequency resonant response associated with it, due to its narrow structure and very close proximity to the ground plane  102 . The poor radiating performance of feed arm  114  is an advantageous feature of antenna  100 , since it allows the addition of a separate high-band radiating element, capable of operating with negligible dependence on low-band radiating element  104 , as will be detailed below with reference to  FIGS. 3A-3C . 
     Reference is now made to  FIGS. 3A, 3B and 3C  which are simplified respective top, underside and side view illustrations of an antenna, constructed and operative in accordance with another preferred embodiment of the present invention. 
     As seen in  FIGS. 3A-3C , there is provided an antenna  300 , including a ground plane  302  and a first wideband radiating element  304 , connected at one end  306  thereof with an edge  308  of the ground plane  302  and including a first portion  310  and a second portion  312 . First wideband radiating element  304  is fed by a narrow feed arm  314  preferably having a feed point  316  located thereon. As seen most clearly in sections A-A and B-B of  FIGS. 3A and 3B  respectively, feed arm  314  is preferably disposed in proximity to but offset from ground plane  302  and first portion  310  of radiating element  304 . Particularly preferably, feed arm  314  is disposed in a plane offset from the plane in which radiating element  304  and ground plane  302  are disposed. 
     Antenna  300  is preferably supported by a non-conductive substrate  318  having respective upper and lower surfaces  320  and  322 , on which upper surface  320  ground plane  302  and radiating element  304  are preferably located and on which lower surface  322  feed arm  314  is preferably located. 
     Feed arm  314  preferably has a maximum width of 1/100 of a predetermined wavelength λ p , which predetermined wavelength λ p  is preferably defined by: 
               λ   p     =     1     f   ⁢       μ   ⁡     [       (         ɛ     r   r       +   1     2     )     +         (         ɛ     r   r       -   1     2     )     ⁡     [     1   +     12   ⁢     (     H   W     )         ]         -   0.5         ]                   
wherein f is a lowest operating frequency of radiating element  304 , μ is the permeability of substrate  318 , ∈ r  is the relative bulk permittivity of substrate  318 , W is the width of a conductive trace disposed above the substrate  318 , forming a microstrip transmission line bounded by air, and H is the thickness of substrate  318 . The expression
 
             [       (         ɛ     r   r       +   1     2     )     +         (         ɛ     r   r       -   1     2     )     ⁡     [     1   +     12   ⁢     (     H   W     )         ]         -   0.5         ]         
corresponds to the effective dielectric constant for the substrate system. This definition of λ p  assumes that
 
               W   H     ≥   1         
and is based upon equations derived by I. J. Bahl and D. K. Trivedi in “A Designer&#39;s Guide to Microstrip Line”, Microwaves, May 1977, pp. 174-182.
 
     First portion  310  of radiating element  304  is preferably separated from the edge  308  of the ground plane  302  by a distance of less than 1/80 the above-defined predetermined wavelength λ p . 
     It is appreciated that antenna  300  may resemble antenna  100  in every relevant respect, with the exception of the inclusion of a second radiating element  330  in antenna  300 . Second radiating element  330  shares feed point  316  with feed arm  314  and is preferably galvanically connected to feed point  316 , as seen most clearly in  FIG. 3B . 
     As seen most clearly in  FIG. 3C , second radiating element  330  is preferably disposed in a plane offset from the plane defined by substrate  318 . In accordance with a particularly preferred embodiment of the present invention, second radiating element  330  is disposed in a plane offset from the plane defined by substrate  318  by a distance of 4 mm. In accordance with another particularly preferred embodiment of the present invention, second radiating element  330  is disposed in a plane offset from the plane defined by substrate  318  by a distance of 7 mm. 
     In operation of antenna  300 , first radiating element  304  preferably operates as a wideband low-frequency radiating element, generally in accordance with the mechanism described above in reference to low-frequency wideband radiating element  104  of antenna  100 . Additionally, second radiating element  330  preferably operates as a high-frequency radiating element fed by feed point  316 . Antenna  300  thus operates as a multiband antenna capable of radiating in low- and high-frequency bands, respectively provided by first and second radiating elements  304  and  330 . 
     It is a particular feature of a preferred embodiment of the present invention that respective first and second radiating elements  304  and  330  operate with an exceptionally low degree of mutual interdependence, despite being fed by way of a common feed point  316 . The low and high operating frequencies of antenna  300  thus may be adjusted freely, due to the almost complete absence of the strong low-band and high-hand tuning interdependencies exhibited by conventional multi-band antennas. 
     As described above with reference to  FIG. 2 , the comparatively independent operation of the low- and high-frequency radiating elements  304  and  330  of antenna  300  is attributable to the narrow elongate structure of feed arm  314  and its location in close proximity to the ground plane  302 , which features prevent feed arm  314  from acting as a high-band radiating element in its own right and therefore from interfering With the operation of high-band radiating element  330 . 
     Second high-band radiating element  330  may have an inverted L-shaped configuration, as seen most clearly in  FIGS. 3A and 3B . It is appreciated, however, that the illustrated configuration of second radiating element  330  is exemplary only and that other compact configurations are also possible. 
     Other features and advantages of antenna  300 , including its wideband response due to the improved impedance matching provided by elongate narrow feed arm  314 , are generally as described above in reference to antenna  100 . 
     Reference is now made to  FIG. 4 , which is a simplified graph showing the return loss of an antenna of the type illustrated in  FIGS. 3A-3C . 
     First local minima A of the graph generally corresponds to the wideband low-frequency band of radiation provided by first radiating element  304  and second local minima B generally corresponds to the high-frequency band of radiation preferably provided by second radiating element  330 . 
     As is evident from comparison of region A of  FIG. 4  to region A of  FIG. 2 , which regions respectively correspond to the frequency responses of low-band radiating element  104  in antenna  100  and low-band radiating element  304  in antenna  300 , the addition of high-band radiating element  330  in antenna  300  does not detract from the wideband response of the low-band radiating element. 
     As shown in  FIG. 4 , by way of example, the operating frequencies of second radiating element  330  may be centered around 1800 MHz. However, it is appreciated that the operating frequencies of second radiating element  330  may be adjusted by way of modifications to various geometric parameters of radiating element  330 , including, but not limited to, its total length and separation from the ground plane  302 . 
     It will be appreciated by persons skilled in the art that the present invention is not limited by what has been particularly claimed hereinbelow. Rather, the scope of the invention includes various combinations and subcombinations of the features described hereinabove as well as modifications and variations thereof as would occur to persons skilled in the art upon reading the forgoing description with reference to the drawings and which are not in the prior art. In particular, it will be appreciated that although embodiments including only single ones of the antennas of the present invention have been described herein, the inclusion of multiple ones of the antennas of the present invention on a single antenna substrate is also possible.