Abstract:
An omni-directional loop antenna for radiating an electromagnetic signal from a signal source includes a differential feed and at least six radiating elements. The differential feed generates a first signal feed and a second signal feed. The radiating elements include at least three evenly-numbered radiating elements and at least three oddly-numbered elements. Each of the evenly-numbered radiating elements is coupled to the first signal feed and each of the oddly-numbered radiating elements is coupled to the second signal feed. Each of the oddly-numbered radiating elements is reactively coupled to two different ones of the evenly-numbered radiating elements. No two of the first radiating elements are reactively coupled a same pair of second radiating elements.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to antennas and, more particularly, to omni-directional antennas.  
         [0003]     2. Background of the Invention  
         [0004]     An Alford loop antenna is typically used in radio navigation systems, such as a VOR system, and in instrument landing systems. An Alford Loop Antenna includes several elements, each of which is driven with a correct ratio of power and at a right phase difference with respect to the other elements of the Array, so that the radiated signal pattern will consist of a RF Carrier, a Sideband Carrier modulated at 90 Hz and the other Sideband Carrier modulated at a selected frequency in space by a process known as space modulation.  
         [0005]     The problem with existing four segment (2 dipole) Alford Loop antennas is that their physical size becomes impractically small at the higher frequencies (e.g., greater than 2 GHz). At and above the PCS cellular band the diameter of a practical four segment Alford Loop is about 38 mm. The result is an antenna with segment lengths and segment coupling components that are too small to be tuned practically or adjusted by a human operator.  
         [0006]     U.S. Pat. Nos. 2,283,897 and 2,372,651 (issued to Alford) disclose general information about omni-directional antennas and are incorporated herein by reference. U.S. Pat. No. 5,751,252 (issued to Phillips) discloses an omni-directional antenna of reduced size and is incorporated herein by reference.  
         [0007]     Therefore, there is a need for an omni-directional loop-type antenna that produces a substantially circular radiation pattern, while having a physical geometry that can be more readily adjusted.  
       SUMMARY OF THE INVENTION  
       [0008]     In one aspect, the present invention is an omni-directional loop antenna for radiating an electromagnetic signal from a signal source. The antenna includes a differential feed and at least six radiating elements. The differential feed generates a first signal feed and a second signal feed, each corresponding to the electromagnetic signal. The radiating elements each include a first end and a spaced-apart second end. The radiating elements also include at least three evenly-numbered radiating elements and at least three oddly-numbered elements. Each of the oddly-numbered radiating elements is coupled to the first signal feed and each of the evenly-numbered radiating elements is coupled to the second signal feed. Each of the oddly-numbered radiating elements is reactively coupled to two different ones of the evenly-numbered radiating elements. No two of the first radiating elements are reactively coupled to a same pair of second radiating elements.  
         [0009]     In another aspect, the invention is an antenna for radiating an electromagnetic signal from a balanced feed signal source that generates a first signal feed and a second signal feed, each corresponding to the electromagnetic signal. The first signal feed is approximately one half wavelength out of phase with the second signal feed. The antenna includes a substantially planar dielectric disc having a first side and a second side. A first radiating member is disposed on the first side and a second radiating member is disposed on the second side. The first radiating member includes a first centrally-located conductive disc and at least three first conductive spokes extending radially from the centrally-located conductive disc. Each first conductive spoke includes a proximal end and a distal end. The proximal end is coupled to the first centrally-located conductive disc. At least three first curvilinear radiating elements, each including a first end and a second end, extend circumferentially from, but are electrically isolated from, a different one of the first conductive spokes. The second radiating member includes a second centrally-located conductive disc and at least three second conductive spokes extending radially from the centrally-located conductive disc. Each second conductive spoke includes a proximal end and an opposite distal end, in which the proximal end is coupled to the first centrally-located conductive disc. At least three second curvilinear radiating elements, each including a first end and an opposite second end, extend circumferentially from, but are electrically isolated from, a different one of the second conductive spokes. Each of the second curvilinear radiating elements is capacitively coupled to two different ones of the first curvilinear radiating elements. No two of the second curvilinear radiating elements is capacitively coupled to a same pair of first curvilinear radiating elements.  
         [0010]     These and other aspects of the invention will become apparent from the following description of the preferred embodiments taken in conjunction with the following drawings. As would be obvious to one skilled in the art, many variations and modifications of the invention may be effected without departing from the spirit and scope of the novel concepts of the disclosure. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWINGS  
       [0011]      FIG. 1  is a top plan view of one illustrative embodiment of an omni-directional antenna according to one embodiment of the invention.  
         [0012]      FIG. 2  is a cross-sectional view of the antenna shown in  FIG. 1 , taken along line  2 - 2 .  
         [0013]      FIG. 3  is an exploded view of a portion of the antenna shown in  FIG. 1 .  
         [0014]      FIG. 4  is a schematic diagram of the antenna shown in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]     A preferred embodiment of the invention is now described in detail. Referring to the drawings, like numbers indicate like parts throughout the views. As used in the description herein and throughout the claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise: the meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.” Also, as used herein, “spoke” means elongated element that extends radially from a central location and is not intended necessarily to imply any additional meaning involving mechanical behavior.  
         [0016]     As shown in  FIGS. 1-3 , one illustrative embodiment of the invention is an omni-directional antenna  100  that radiates an electromagnetic signal from a differential feed signal source  166 , which is coupled to the antenna (for example, a balun fed from a coaxial cable  164 ) and that generates a first signal feed  168  and a second signal feed  170  corresponding to the electromagnetic signal. In at least one embodiment, the differential feed corresponds to a balanced feed produced by a balun, which receives a source signal from a typically unbalanced coaxial feed line. The first signal feed  168  is generally out of phase with the second signal feed  170  by one-half of a wavelength.  
         [0017]     The antenna  100  includes a substantially planar dielectric disc  110  that has a first side  112  and an opposite second side  114 . A first conductive member  120  is disposed on the first side  112  and a second conductive member  140  is disposed on the second side  114 . The first conductive member  120  includes a first centrally-located conductive disc  122  and at least three first conductive spokes  124 , each having a proximal end and a distal end relative to the centrally-located conductive disc conductive  122 , such that the proximal end of each first conductive spoke  124  is electrically coupled to the conductive disc  122  and each first conductive spoke  124  extends radially from the centrally-located conductive disc  122 . A first curvilinear radiating element  126 , including a first end and an opposite second end, extends circumferentially from, but is electrically isolated from, each first conductive spoke  124 .  
         [0018]     Similarly, the second conductive member  140  includes a second centrally-located conductive disc  142  and at least three second conductive spokes  144 , each having a proximal end and an opposite distal end. The proximal end of each second conductive spoke  144  is electrically coupled to the conductive disc  142  and each second conductive spoke  144  extends radially from the centrally-located conductive disc  142 . A second curvilinear radiating element  146 , including a first end and a second end, extends circumferentially from, but is electrically isolated from, each second conductive spoke  144 .  
         [0019]     Each of the first curvilinear radiating elements  126  is capacitively coupled to a different one of the second conductive spokes  144  and each of the second curvilinear radiating elements  146  is capacitively coupled to a different one of the first conductive spokes  124 . In the embodiment shown, the curvilinear radiating elements  126  and  146  are capacitively coupled; however, it is conceivable that they could be inductively coupled. As shown with respect to the first radiating member  120 , the each spoke end  128  includes a first sub-region  131  that is in electrical communication with the distal end  125  of a conductive spoke  124  a second sub-region  129  that is in electrical communication with the first end  127  of a curvilinear radiating element  126 . The first sub-region  131  is electrically isolated the second sub-region  129  by a non-conductive region  130  (typically an air gap) that isolates the spoke  124  from the curvilinear radiating element  126 . The first sub-region  131  may also define a partial gap  132  that facilitates tuning of the antenna. The second radiating member  140  includes a capacitive coupling  148  similar to the one described with respect to the first radiating member  120 . The first sub-region  131  coupled to a first spoke  124  (i.e., on the first side  112  of the dielectric disc  110 ) is capacitively coupled to the corresponding second sub-region  129  coupled to a second curvilinear radiating element  146  (i.e., on the second side  114  of the dielectric disc  110 ) with the dielectric disc  110  acting as the dielectric of the capacitance. However, because of the non-conductive region  130 , there is substantially little or no coupling between the first sub-region  131  and the second sub-region  129  on the same side (e.g.,  112  or  114 ) of the dielectric disc  110 .  
         [0020]     The second end of each of the first curvilinear radiating elements  126  and of each of the second curvilinear radiating elements  146  terminates in an inwardly-directed extension  136  and  156 . The inwardly-directed extension  136  of each of the first curvilinear radiating elements  126  is capacitively coupled to a different inwardly-directed extension  156  of one of the second curvilinear radiating elements  146  to the extent that they overlap on opposite sides of the dielectric substrate  110 . In some instances, one or more of the inwardly-directed extensions  136  or  156  may have a portion  138  or  158  removed therefrom, which can effect the corresponding capacitance, which in turn, facilitates tuning of the antenna. As can be seen, each of the first curvilinear radiating elements  126  is paired with a corresponding second curvilinear radiating element  146  at the overlap of the respective inwardly-directed extensions  136  and  156 , thereby forming a dipole. Thus, when six curvilinear radiating elements  126  and  146  are used in an antenna  100 , the antenna  100  effectively embodies three dipoles.  
         [0021]     The electrical relationships between the elements are shown in  FIG. 4 . Each first spoke  124  exhibits a first transmission line impedance  412  with respect to each of the second radiating elements  146  and each second spoke  144  exhibits a second transmission line impedance  414  to each of the first radiating elements  126 . As can be seen, an effective capacitance C exists between each first radiating element  126  and each second radiating element  146  at their respective second ends  136  and  156  in view of the portions that overlap. Also, a capacitance c a  exists between the first signal feed  168  and each corresponding second radiating element  146 . Similarly, a capacitance c c  exists between the second signal feed  170  and each corresponding first radiating element  126 . Also, a capacitance c b  exists between the distal end of each first conductive spoke  124  and the corresponding second conductive spoke  144 .  
         [0022]     While the embodiment shown illustrates the use of six radiating elements  126  and  146 , the diameter of the antenna  100  may be made greater for a given transmission frequency by adding still further radiating elements. A greater number of radiating elements would result in the field being more circular. However, as the number of elements increases, the task of tuning the antenna  100  will sometimes become a little more complex. Also, as the number of elements increases, a number of other parameters in the antenna feed structure must change as well. For example, the impedance of the feed lines going to the individual segments must go up accordingly (say from 100 ohms to 150 ohms). Such changes may put a practical upper limit on the number of segments employed as some of the physical dimensions of high impedance transmission lines can become unmanageably small.  
         [0023]     Larger embodiments could employ a dielectric disc  110  made of a printed circuit board-like material: smaller embodiments could be made using integrated circuit material.  
         [0024]     The embodiments disclosed above use an impedance matching transmission line and a capacitive transformer with or without shunt input capacitor. The equation for matching transmission line is: Z(x-line)=F(Zo, N, radius, Lsegment, Lx-line, Z′ant), where Zo is the output impedance, N is the number of segments employed, Lsegment is the length of each segment, Lx-line is the transmission line inductance and Z′ant is the impedance of the antenna.  
         [0025]     The embodiments disclosed above could be especially useful in test labs for mobile devices and antennas. They are also useful in WIFI distribution systems that require omni-directional loop antennas that operate at the higher frequencies (e.g., around 5.2 GHz)  
         [0026]     The above described embodiments, while including the preferred embodiment and the best mode of the invention known to the inventor at the time of filing, are given as illustrative examples only. It will be readily appreciated that many deviations may be made from the specific embodiments disclosed in this specification without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is to be determined by the claims below rather than being limited to the specifically described embodiments above.