Patent Publication Number: US-9407006-B1

Title: Choke for antenna

Description:
PRIORITY CLAIM 
     This application is a continuation under 35 U.S.C. §120 of U.S. application Ser. No. 13/842,674, and claims priority thereto and the benefit thereof. 
    
    
     TECHNICAL FIELD 
     The invention relates generally to the field of electromagnetic propagation, and more particularly to antennas. 
     BACKGROUND 
     Antennas are used in a variety of applications for transmission and receipt of information via electromagnetic waves. The direction at which an antenna radiates or receives power can be optimized by the shape and structure of the antenna, as well as the method of driving it. In some applications, a highly directional antenna is desired, while in others an omnidirectional antenna is desired. In the transmission mode, an input signal connects to a feed on the antenna and drives a radiator. The electrical signal of the input is converted to electromagnetic radiation that propagates from the radiator in accordance with its directivity. The process basically works in reverse when an antenna is receiving a signal. 
     In addition, for maximum efficiency, the load presented by the antenna itself, or more specifically, by the radiator of the antenna, should be matched to the input impedance of the feed. This minimizes loss due to reflections and standing waves created by impedance mismatching. 
     Space considerations also play a role in antenna design. For example, an elongated antenna (such as a traditional dipole) may provide an ideal power distribution pattern for a given application; however, the device or product of which the antenna is a part, or the application in which the antenna is used, may not permit the use of a long, somewhat fragile antenna such as a traditional dipole. 
     For terrestrially based applications, in which the device receiving signals from or transmitting to an antenna is positioned away from the antenna at relatively small angle from horizontal, it is desirable that the antenna&#39;s power distribution be directed primarily outward (or horizontally), rather than vertically. A traditional dipole antenna provides such a radiation pattern but often proves too large or fragile for a given application. One use of antennas includes transmitting from a location located at or near ground level to receivers located on power or telephone poles, or buildings, which may be located in any direction from the antenna. In such locations, the size of the antenna is a key consideration, as well as the likelihood that the antenna will inevitably come into contact with persons or objects. 
     When a dipole, ring, yagi, or similar type antenna is fed with a coax connection, the coaxial cable may act as a radiator, in addition to the radiator of the antenna itself. To isolate the antenna radiator from the coax feed cable, and prevent coax cable from radiating, a choke balun may be added between the antenna and the feed line. This is prior art. These types of antennas, however, do not have a ground plane. Some circular antennas include a ground plane having concentric circular grooves formed in it, effectively leaving a series of concentric circular walls. In these devices, the choke is “above” the ground plane, with respect to the feed line. 
     For antennas with a radiator positioned over a ground plane, such as a patch antenna, prior art designs assume that that the ground plane isolates the radiator from the feed line (which is connected from below the ground plane), such that the feed line does not affect or interfere with the radiation pattern of the antenna. It has been discovered, however, that the ground plane does not provide adequate isolation and a coaxial feed cable can interfere with radiation patterns of antenna, even where the antenna radiator is separated from the coaxial cable by the ground plane. 
     Thus, there is a need for a relatively compact antenna that provides a substantially omnidirectional power distribution oriented primarily horizontally, rather than vertically. There is also a need for an antenna that is structurally resistant to bumps and knocks that may be experienced in a terrestrial installation. There is also a need for further isolating the radiation patterns of an antenna in which the radiator is separated from a feed, such as coaxial feed line, by a ground plane. 
     SUMMARY 
     Embodiments of the present invention satisfy these needs. One embodiment is an antenna comprising an annular radiator, a ground plane, a feed located in the center of said radiator, a plurality of radial microstrips, each microstrip having an inner end and an outer end, each outer end coupled to the radiator, each inner end coupled to the ground plane, where each microstrip is coupled to the feed between its inner and outer ends. The antenna has a resonant frequency defining a wavelength, and, in one embodiment, the outer end of each of the plurality of microstrips is coupled to the radiator within about one-fourth wavelength of the outer end of an adjacent one of the microstrips. The radiator has a load impedance and the feed has an input impedance, and, in another embodiment of the antenna, the ratio of the input impedance to the load impedance is a function of the ratio of the length of each microstrip from its first end to the feed, to the length of each microstrip from its first end to its second end. Another embodiment of the invention comprises an antenna having a radiator over a ground plane fed by a coaxial feed, in which a cylindrical choke approximately one-quarter wavelength in length is placed around the feed and connected to the underside of the ground plane. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be explained, by way of example only, with reference to certain embodiments and the attached Figures, in which: 
         FIG. 1  is a top view of one embodiment of the present invention; 
         FIG. 2  is a side view of the embodiment of  FIG. 1 ; 
         FIG. 3  is a perspective view of the embodiment of  FIG. 1 ; 
         FIG. 4A  is a perspective view of the power distribution of the radiation pattern of the embodiment of  FIG. 1 ; 
         FIG. 4B  is a top view of the power distribution of the radiation pattern of the embodiment of  FIG. 1 ; 
         FIG. 5  is a side view of another embodiment of the present invention, comprising a quarter-wave choke around a coaxial feed line beneath the ground plane; 
         FIG. 6  is a top view of the embodiment of  FIG. 5 ; 
         FIG. 7  is a perspective view of the embodiment of  FIG. 6 ; 
         FIG. 8A-B  are exemplary charts for an antenna showing VWSR amplitude versus frequency from coax feeds of one to four inches with ( FIG. 8B ) and without ( FIG. 8A ) an embodiment of the antenna choke of the present invention; and 
         FIG. 9A-B  are exemplary charts for the antenna of  FIG. 8  showing radiation patterns with ( FIG. 9B ) and without ( FIG. 9A ) an embodiment of the antenna choke of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     As shown in  FIGS. 1-3 , one embodiment of the present invention is an antenna  10 , comprising a radiator  20 , a ground plane  30 , a feed  40 , and a plurality of microstrips  50  extending from the ground plane  30  to the radiator  20 , with the feed  40  coupled to the microstrips  50  at feed point  55  between the ground plane  30  and radiator  20 . As shown more clearly in  FIGS. 2-3 , the radiator  20  and the ground plane  30  lie in parallel planes separated by a gap  15 . The gap  15  may be filled with air or with a solid or semi-solid dielectric material. In a preferred embodiment, the radiator  20  is annular and the ground plane  30  is circular. The radiator  20  may be other regular or irregular shapes, but for improved omnidirectional performance, it should be a symmetrical shape, such as circular or polygonal, with a circular shape providing optimal performance. The shape of the ground plane  30  preferably corresponds to that of the radiator  20 . The perimeter of the ground plane  30  preferably extends beyond that of the radiator  20  by a distance that is equal to or greater than the width of the gap  15 . 
     The outer ends  52  of the microstrips  50  are coupled to the radiator  20  at drive points  25 . The microstrips  50  are coupled to the ground plane  30  at their inner end  54 . The microstrips  50  are preferably coplanar with the radiator  20  through a substantial portion of their length, from the outer end  52  to a bend  53 , where the microstrip turns downward across the gap  15  to meet the ground plane  30  at proximal end  54 . As shown, the microstrips  50  may be tapered such that they become progressively narrower from the area near the coupling with the feed  40  to outer end  52 . As discussed below, in a preferred embodiment, the number of microstrips is determined according to the dimensions of the radiator  20  and resonant wavelength of the antenna in order to drive the radiator  20  substantially in phase. In one embodiment, the radiator  20  and microstrips  50  are stamped from a single sheet of metal, and the bend  53  is formed simply by bending or crimping the microstrip  50  a distance from its inner end  54  that corresponds to the desired width of the gap  15  separating the ground plane  30  from the radiator  20 . 
     The feed  40  is preferably a standard connector allowing coupling of the antenna  10  to a standard coaxial cable. That is, the feed  40  comprises a central conductor  42  carrying the input signal, which is coupled to the microstrips  50  at feed point  55 , and an outer sheath of conductors  44  for the return signal path coupled to the ground plane  30 . The central and outer conductors are separated by an insulator and constructed as is known by those of ordinary skill in the art. While the feed  40  is shown as being a standard coaxial feed, any other connector suitable for carrying a signal from an input source to the antenna  10  may be used, including hard wired connections directly to the feed point  40  and ground plane  30 . 
     As with any antenna, the antenna  10  according to embodiments of the present invention has a resonant frequency f r  that is a function of the materials and structure of the device. Certain dimensions of antennas are often expressed in terms of wavelength λ at the resonant frequency; for example, a quarter-wave dipole antenna refers to a dipole antenna with a length that is one-fourth as long as the wavelength λ of the signal propagated at the resonant frequency f r . In a preferred embodiment, the length of each microstrip (from the inner end  54  to the bend  53  to the feed point  55 , and on to the outer end  52 ) is approximately ¼ λ. The design of the antenna  10  allows for the microstrips  50  to extend through the feed point  55  at the center of the radiator  20  and then down to the ground plane  20 . As a result, the distance from feed point  55  (at the center of the radiator  20 , in a preferred embodiment) to the outer end  52  of the microstrip is less than ¼ λ, and thus the radiator has a radius less than ¼ λ while achieving the performance of a full ¼λ antenna. The size of the antenna is effectively reduced by the length of that portion of the microstrips  50  from the feed point  55  to the bend  53 . Satisfactory performance characteristics are achieved with the gap  15  between the ground plane  30  and the radiator  20  being approximately 1/10 λ. Embodiments of the present invention provide the performance of a half-wave dipole at one-fifth the height. 
     According to one embodiment of the present invention, the placement of the feed point  55  relative to the length of microstrips  50  allows a lower input impedance of the feed  40  to be leveraged to match a higher load impedance of the radiator  20 . Specifically, the ratio of the length of the microstrip  50  (from outer end  52  to the bend  53  and down to inner end  54 , defined as L 1 ) to the distance from inner end  54  up to the bend  53  and to feed point  55  (defined as L 2 ) is directly proportional to the ratio of the load impedance of the radiator  20  (R L ) to the input impedance at the feed point (R I ): 
                 L   1       L   2       ∝       R   L       R   I             
Thus, if the feed point  55  is placed 1/10 of the length L 1  from the inner end  54 , then a 10Ω input impedance at feed  40  will be leveraged to match the impedance of a radiator  20  having a 100Ω load impedance. If, using a more typical example, the radiator has a load impedance of 250Ω and the input impedance is 50Ω (typical of co-ax connection), then the ratio of L 1  to L 2  should be 5:1. The tapering of the microstrips  50 , discussed above, aids in matching the impedance of the feed  40  to the radiator  20 .
 
     As shown in  FIGS. 1-3 , the antenna  10  comprises a plurality of microstrips  50  connecting the radiator  20  to the ground plane  30 , with each coupled to the single feed  40 . Thus, a simple single ended drive may be used to drive the antenna  10  from feed  40  through each of the microstrips  50  simultaneously. It is desirable that the signal driven on radiator  20  be substantially in phase along the entirety of the radiator  20 . To do so, the drive points  25  at which the microstrips  50  connect to the radiator  20  should be close enough together such that substantial phase variances do not develop between drive points. In a preferred embodiment, with each microstrip being about ¼ λ in length, the distance between adjacent drive points  25  should be within about ¼ λ, in order to drive the radiator  20  substantially in phase throughout its circumference. Thus, in such a design, six microstrips will provide optimum transmission characteristics. 
     With the entirety of the radiator driven substantially in phase, an electromagnetic signal propagates uniformly from the radiator, with its power oriented primarily radially, rather than axially, with respect to the radiator, as shown in  FIG. 4A . The power distribution of the signal is approximately toroidal in shape, with its peak power found at a distance D and at an elevation angle (ID from horizontal. This profile is suitable for transmitting to terrestrially based receivers, such as those located on telephone or power poles, buildings, and the like. Power is not wasted in such applications by being transmitted axially, or vertically, from the radiator  20 . 
     Embodiments of the present invention therefore find application in antennas in which size and footprint are important, and in which the targeted receivers of the antenna&#39;s signal are displaced substantially horizontally, rather than vertically, from the antenna. The antenna is flat (about 1/10 λ thick) and less than ½ λ in diameter. One exemplary application is its use as a pit antenna in an automated water metering system. Water meters are often located in a small depression, or pit, in the yard of the premises. The meter may be equipped with a meter interface unit (MIU) that automatically records the meter readings and transmits them to a collecting device located on a telephone or power pole in the vicinity. One such collector may service thousands of MIUs. Because the MIUs are located at or near ground level, and the collector is located at a relatively low angle Φ relative to horizontal from the MIUs, and antenna having the power distribution characteristics of antenna  10 , as shown in  FIG. 4A-B , is advantageous. Further, the compact size and flat shape of the antenna  10  allows it to be integrated into the lid of the meter or otherwise fitted safely and securely into the meter pit. 
       FIGS. 1-3  illustrate a feature of an alternative embodiment of the present invention. The alternative embodiment includes an annular collar  35  around the periphery of the ground plane  30 . The collar  35  is optional and may be added to increase the structural integrity of the antenna  10 . In a preferred embodiment, the collar  35  is wedge-shaped in cross section, as shown in  FIG. 2 , and extends at least as high as gap  15 , such that the top of the collar  35  is coplanar with the radiator  20  or higher. The distance  37  from the outer edge of the radiator  20  to the inner edge of the collar  20  is preferably greater than the height of the gap  15 , to prevent the collar  35  from degrading the performance characteristics of the antenna  10 . The collar  20  serves to make the antenna  10  rugged and structurally resistant to side forces, as well as forces from above that are delivered by an object larger than the diameter of the antenna. The collar  20  is rigid and preferably made of a solid material. This structure may protect the antenna  100  from being bent or broken if stepped on by a person, or even if run over by a vehicle. As long as the person&#39;s shoe or the vehicle tire spans the collar  20  from one side to the other, the antenna  100  is protected as the collar supports the person or vehicle&#39;s weight, rather than the radiator  20 . 
     In a preferred embodiment, the antenna  10  was designed to resonate at 460 Mhz. Four microstrips  50  were used, as shown in  FIGS. 1-3 . A small amount of ripple in the voltage driven signal was measured from point to point along the radiator  20 , but at approximately one meter away the power of the propagated signal was substantially in phase in all directions from the radiator  20 , such that the ripple was immaterial. The uniformity of phase could be improved at the radiator  20  by using six microstrips; however, given the uniformity measured just one meter out from the radiator using four microstrips, it was determined that using six microstrips was not necessary. 
     Another embodiment of the present invention comprises a cylindrical choke approximately one-quarter wavelength in length, placed under the ground plane of an antenna having a radiator over a ground plane.  FIGS. 5-7  illustrate an exemplary embodiment of a center driven circular plate antenna  100  having a circular radiator  110 , approximately one-fourth wavelength in diameter in one embodiment, over a ground plane  120 . The radiator  110  is resonated by two to four or more inductive pins  130  with a diameter and location chosen to achieve resonance at a predetermined frequency and drive impedance, as is known in the art. In one embodiment, the ground plane  120  is substantially larger than the radiator  110 , which substantially reduces the effects of objects near the antenna  110 . The antenna  110  may be fed by a coax feed  140 . In this embodiment, the central conductor of the feed  140  connects to the radiator  110 , and the outer sheath connects to the ground plane  120 . A cylindrical choke  150  surrounds the feed  140  just below the ground plane  120 . The choke  150  comprises a thin metal cylinder and is connected to the ground plane  120 . The choke  150  may or may not be filled with a high dielectric constant material for size reduction. The choke  150  is approximately one-quarter wavelength (¼ λ) of the resonant frequency of the antenna  100  in length. 
       FIG. 8A-B  are exemplary charts for an antenna showing VWSR amplitude versus frequency from coax feeds of one to four inches with ( FIG. 8B ) and without ( FIG. 8A ) an embodiment of the choke  150 . As shown in  FIGS. 8A-B , the VWSR is much more consistent when the choke  150  is used and nearly eliminates the effects of cable dress on antenna performance.  FIG. 9A-B  are exemplary charts for the antenna of  FIG. 8  showing radiation patterns with ( FIG. 9B ) and without ( FIG. 9A ) an embodiment of the antenna choke of the present invention. Likewise, the choke  150  substantially increased the available energy above the ground (where the antenna is mounted in at ground level, or slightly underground such as in a pit of a water meter) and substantially eliminates the effects of cable dress on variation in radiation pattern. 
     A quarter-wavelength choke of this embodiment of the present invention may be used with any antenna having a radiator over a ground plane, fed by a coax feed line, including the antenna  10  of  FIG. 1 . 
     Although the present invention has been described and shown with reference to certain preferred embodiments thereof, other embodiments are possible. The foregoing description is therefore considered in all respects to be illustrative and not restrictive. Therefore, the present invention should be defined with reference to the claims and their equivalents, and the spirit and scope of the claims should not be limited to the description of the preferred embodiments contained herein.