Abstract:
A dual band directional antenna with low frequency band reflectors that form desired antenna patterns in a low frequency band while remaining transparent to a higher frequency band. As a result of such frequency transparency, pattern changes in the lower frequency bands do not affect patterns in the higher band frequencies.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of U.S. provisional application No. 61/800,854 filed Mar. 15, 2013. The disclosures of the aforementioned application is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention generally relates to dual band directional antennas. The present invention more specifically relates to reflector switching between high-band and low-band patterns. 
         [0004]    2. Description of the Related Art 
         [0005]    Antennas that provide dual band coverage (for example, 2.4 GHz and 5.0 GHz) with a single feed are common. Attempting to form a directional pattern in one of the frequency bands using commonly available antennas with reflecting parasitic elements, however, will often cause unwanted changes in the patterns of the other band. Such changes complicate simultaneous operation in both frequency bands. 
         [0006]    More specifically, changes in lower frequency band reflectors are prone to affect patterns in the higher frequency band patterns. Changes in the high frequency band reflectors typically will not affect low frequency band patterns because high frequency band reflectors are shorter with respect to the low-band wavelength. As a result, the band patterns of the lower frequencies are not affected. This is true, however, only when the frequency ratio between the high frequency band and low frequency band is sufficiently large (e.g., a frequency ratio of 2:1 or greater). When the frequency ratio between the high frequency band and low frequency band is not large enough (e.g., less than 2:1), the high frequency band may interfere with low frequency band operations. 
         [0007]    There is a need in the art for dual band directional antennas that allow for simultaneous operation in high and low frequency bands. More specifically, there is a need for dual band directional antennas with low frequency band reflectors that form desired patterns in low frequency while remaining transparent to high frequency bands such that patterns in the high frequency are not otherwise adversely affected. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  illustrates an exemplary meander line reflector; 
           [0009]      FIG. 2  illustrates an exemplary meander line reflector including a plurality of stacked horizontal transmission lines; and 
           [0010]      FIG. 3  illustrates an exemplary equivalent circuit as used in a meander line reflector. 
       
    
    
     SUMMARY OF THE INVENTION 
       [0011]    Embodiments of the present invention provide for a dual band directional antenna with low frequency band reflectors that form desired antenna patterns in a low frequency band while remaining transparent to a higher frequency band. As a result of such frequency transparency, pattern changes in the lower frequency bands do not affect patterns in the higher band frequencies. As used herein, transparency, with respect to a reflector, refers to a reflector in one band (e.g., the low-band) that is invisible to or will not otherwise affect the pattern of another frequency band (e.g., the high-band). 
         [0012]    Embodiments of the present invention use low frequency reflectors rather than ground plane slots or otherwise inefficient reflectors such as inductively tuned short reflectors. Embodiments of the presently disclosed antenna system allow for two-band independent pattern steering with minimized hardware costs and without sacrificing peak gain, front-to-back ratio, or pattern bandwidth in either band. The use of a dual band array, as opposed to two separate smart antenna systems, may result in reduced size and hardware costs. Additional radio chains may also be supported in a given radio frequency (RF) environment. 
       DETAILED DESCRIPTION 
       [0013]    Embodiments of the present invention involve the use of reflectors for dual band directional antennas in the low frequency band such that the reflectors form desired patterns yet remain transparent in the high frequency band thereby avoiding unwanted or otherwise undesirable changes to patterns in that band. 
         [0014]    While reference is made to operation in the 2.4 GHz and 5.0 GHz range, these references are exemplary with respect to the operation of a dual band antenna. It will be understood that the dual band directional antennas described herein may operate in any suitable frequency bands, which may include the 2.4 GHz or 5.0 GHz frequency bands or any other suitable frequency bands. Embodiments of the present invention allow for a dual-band directional antenna with a dual-band driven element and switched high-band and low-band reflectors to be switched on or off as to the low-band reflectors without disturbing the high-band patterns. 
         [0015]    In some embodiments, a directional antenna system includes a dual band driven element, a high-band reflector positioned relative the dual band driven element, and a low-band reflector element positioned relative the dual band driven element. The low-band reflector element may include a meander line, for example, meander line  100  of  FIG. 1  or meander line  200  of  FIG. 2 , as described below. 
         [0016]      FIG. 1  illustrates an exemplary meander line  100 . In some embodiments, meander line  100  may be implemented as a trace on a dielectric substrate, on a printed circuit board (PCB), as a sheet metal part, or can be constructed from wires or bent tubing such as a copper conductor. Meander line  100  includes meander feed  105 , transmission lines  160  connected by vertical sections  165  of height hvert  150 , and ground plane  110 . In some embodiments, meander line  100  may be implemented in a low-band reflector element of a directional antenna system. 
         [0017]    Reflectors for directional antennas over a ground plane (i.e., ground plane  110 ) are usually in the order of λ/4 in height, where λ denotes wavelength. In some embodiments, meander line  100  (i.e., low-band reflector with meander line  100 ) is implemented when there are restrictions on reflector height. For example, the available height h, shown as  135 , may be less than λ/4. Thus, a meander line may allow for implementation of the dual band directional antenna in space-constrictive form factors, especially with regard to restrictions on height h  135 . In some embodiments, a specifically configured meander line reflector  100  may be specifically configured so that it may be used to shorten the low-band reflector while simultaneously making it transparent to high-band frequencies. 
         [0018]      FIG. 2  illustrates meander line  200 . In some embodiments, meander line  200  is similar to meander line  100  of  FIG. 1 . Meander line  200  includes meander feed  210 . Meander line  200  includes horizontally stacked, short circuited transmission lines  280 , which are connected by short vertical sections  220 , each having a vertical height denoted hvert, shown, for example, in  FIG. 1  as  150 . The reactance seen between points “a” and “b” and then “c” and “d,” shown as  230 ,  240 ,  250 , and  260  in  FIG. 2  (also shown as  115 ,  120 ,  125 , and  130  in  FIG. 1 ) is given by Equation 1: 
         [0000]        X   n   =Z 0·tan(2πltr/λ),  (1)
 
         [0000]    where ltr denotes electrical length of the transmission line  290 , λ denotes wavelength, and X n , denotes the reactance of the nth transmission line at the frequency, F. The frequency F is given by F=c/λ, wherein c denotes velocity of propagation in the transmission media. 
         [0019]    The wavelength λ varies as a function of the frequency F, as illustrated in Equations  2   a  and  2   b:   
         [0000]      λhigh= c/F   high   (2a)
 
         [0000]      λlow= c/F   low   (2b)
 
         [0000]    As used herein, Z0 denotes the characteristic impedance of the transmission line. Z0 is a function of the parameters w, shown as  155  in  FIG. 1 , and sptr, shown as  145  in  FIGS. 1 and 270  in  FIG. 2 , and the dielectric constant of the material in which the low-band reflector element including meander line  200  is immersed. 
         [0020]      FIG. 3  illustrates an exemplary equivalent circuit  300  for use in a meander line. In some embodiments, equivalent circuit  300  may be implemented with the meander line  100  of  FIG. 1  or the meander line  200  of  FIG. 2 . Equivalent circuit  300  includes feed  310  and ground plane  320 . Equivalent circuit  300  is illustrated as including resistor  360  and any number of inductors, with exemplary inductors “x1,” “x2,” and “x3” respectively shown as  330 ,  340 , and  350 . The value of the reactance of the nth transmission line X n , may differ at high-band and low-band frequencies. In order to make the reflector transparent at the high-band, the electrical length of the transmission line, ltr, (e.g., ltr  290  of  FIG. 2  and ltr  140  of  FIG. 1 ) may be adjusted according to Equation 3: 
         [0000]      2πltr/πhigh=90  (3)
 
         [0021]    Adjusting the length of the transmission line according to Equation 3 results in a very large reactance X n , if not theoretically infinite. No current flows in the reflector, and as a result, the reflector is transparent to high-band radiation. At the low-band, X n  is given by Equation 1 with λ=λlow, as defined in Equation 2b. By adjusting the number of sections and the parameter hvert, shown s  150  in  FIG. 1 , the reflector can be tuned to resonance in the low-band. 
         [0022]    While the foregoing reflector implementation is described as a single instance, multiple reflectors may be implemented to create an array of the same. For example, a dual band driven element may be positioned relative a 2 GHz and a 5 GHz reflector implementation. Further instances of that reflector implementation may be disposed around the dual band driven element to allow for the formation of multiple beams in different directions, for example, a 2 GHz beam in one direction and a 5 GHz beam in a different direction. 
         [0023]    The foregoing detailed description has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the embodiments of the present invention as modifications and variations are possible and envisioned in light of the above teachings. The described embodiments were chosen in order to best explain the principles of the technology and its practical application to thereby allow one of skill in the art to understand how to implement the same.