Abstract:
An antenna structure including a ground plane is capable of producing an EM interference pattern to produce signal gains at a zenith of the antenna. The slots in the ground plane are positioned at a predetermined distance from a radiator and extend a predetermined distance from the axis of the antenna. The slots generate an interference pattern relative to the radiated signal to create a circular polarization thereby producing a signal gain at the zenith of the antenna. The slots may be provided during the manufacturing process of the ground plane or added later to existing antennas.

Description:
BACKGROUND 
     Monopole antennas are a common and inexpensive way to radiate an omnidirectional signal. The vertical radiator of a basic monopole antenna may include a ground to plane redirect a portion of the radiated electromagnetic energy over the surface of the earth. Otherwise a portion of the radiated energy may be lost, constructively cancelled, or dissipated. The ground plane may comprise the earth&#39;s surface or an artificial metallic or conductive plate that serves as an electromagnetic field reflector. The present disclosure, however, concerns an improvement to artificial ground planes. A monopole antenna having an artificial conductive plate ground plane, for example, simulates the function of a dipole antenna. Furthermore, the functionality of a monopole antenna with a sufficiently large ground plane approaches that of a dipole antenna. 
     Standard monopole antennas are vertically polarized elements that may produce an electromagnetic field over a ground plane. Monopole antennas produce null emissions at their zeniths, which make them ill suited for short range communications at high incident angles. The null at zenith also prevents full hemispherical coverage which is more important in airborne links from a ground station. 
     Previous techniques to fill null emission patterns included bending the monopole element. Unfortunately, this technique requires extraordinary precision and advanced manufacturing methods, especially at high frequencies. Bending of the monopole element also distorts the azimuthal symmetry of the pattern. 
     It is therefore an object of the present invention to provide a new antenna with a structure to partially obviate null emission patterns at its zenith. 
     It is further an object of the present invention to provide an existing antenna with a ground plane structure to at least partially fill otherwise null emission patterns at its zenith. 
     It is another object of the present invention to provide an antenna with an inexpensive mechanism to reduce null emission patterns at its zenith without bending the radiator element. 
     SUMMARY OF THE INVENTION 
     The present invention provides an antenna with the capability of filling a null at zenith. The invention takes advantage of slots in the ground plane which produce a measurable amount of circularly polarized radiation toward the zenith. The slots interrupt the currents and redirect energy into space. 
     The angle and the distance of the slots from the center of the monopole play a critical role in the amount of radial current redirected and the phase of the redirected energy respectively. A radial slot will not interrupt the radial current and will therefore not radiate; whereas slots perpendicular to the radius interrupt the most current and radiate the strongest. One embodiment utilizes an angle of forty-five degrees for the slots. In addition to the angle of the slots, this embodiment utilizes two slots that radiate vertically and two slots that radiate horizontally. The slots are spaced to cause a ninety degree phase difference. The ninety degree phase difference radiates circular polarization toward the zenith. 
     As there is no preferred azimuthal orientation, the circular polarization fills the null for all azimuthal orientations. The pattern at zenith corresponds to shorter ranges; therefore, a minimal gain of between −10 to −15 dB is useful at the zenith. In addition to reducing null depth by approximately 20 dB, the pattern of the monopole near the horizon is relatively unaffected. The present invention may be utilized while constructing a new antenna or in retrofitting or modifying an existing antenna. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a standard monopole antenna including a ground plane comprising a conductive material; 
         FIG. 2  is a perspective view of a monopole antenna with a slotted ground plane; 
         FIG. 3  is a comparison of radiation patterns of a standard monopole antenna and a monopole antenna with a slotted ground plane; 
         FIG. 4  is a top view of a monopole antenna with a slotted ground plane; 
         FIGS. 5-9  are top views of several embodiments of a slotted ground plane monopole antennas; 
     
    
    
     DETAILED DESCRIPTION 
     Now referring to the drawings,  FIG. 1  shows a standard monopole antenna  100  comprising a vertical radiator  102  and a ground plane  104 . The monopole antenna  100  radiates an RF signal  106  radially outward from a vertical axis thereof. The radiator  102  further comprises a first end  108  and a second end  110 . Above the second end  110 , a zenith  112  is devoid of any signal  106 . It should be recognized that the invention can be applied to an antenna with a radiator and ground pole. 
       FIG. 2  shows a monopole antenna  200  comprising a radiator  202  and a ground plane  204 . The monopole antenna  200  radiates a signal  206  radially outward from the vertical axis of the radiating element  202 . The radiator  202  further comprises a first end  208  and a second end  210 . A zenith  213  is located above the second end  210 . The ground plane  204  further comprises a first metallic or conductive surface  212 , a second metallic or conductive surface  214 . The ground plane has a given thickness  216  between the first surface  212  and the second surface  214 . The thickness  216  may be of the same conductive material or metal of the first surface  212  and second surface  214  or be made of an insulating material. The ground plane  204  contains a series of slots  218 . 
     The slots  218  in the ground plane redirect induced currents in the ground plane which, in turn, stabilize an interference pattern in the signal  206  to redirect energy towards the zenith. The slots  218  may traverse a portion of the insulator thickness  216  or the entire thickness  216 . For thin ground planes, the slot preferably runs the entire thickness of the ground plane. For thicker ground planes, it may be sufficient for the slots to only span a portion of the thickness to obtain a desired signal pattern. The angle of the slots  218  relative to a radius of the ground plane determines the extent of induced current flow in the ground plane and the amount of energy redirected. Radial slots do not interrupt radial EM emissions; while slots perpendicular to the radius interrupt a greater amount of EM emissions. Depending on the orientation of the slot angle, the EM emission may radiate horizontally or vertically. The number of slots  218  also may be varied to increase symmetry of the radiated signal. 
     In addition to the angle of the slots relative to a radius of the ground plane, the distance of the slots  218  from the radiator  202  determine the phase of the redirected EM emissions. Again referring to  FIG. 2 , a series of slots  218  are shown at a first distance  220  and a second distance  222  from the radiator  202 . The first distance  220  and the second distance  222  are such that they cause the signal  206  to be out of phase. The phase difference caused by the slots  218  is preferably ninety degrees. The ninety degree phase difference along with the horizontal and vertical radiation produces a circular polarization  230  towards the zenith  213 . 
     The circular polarization  230 ; therefore, fills the null at the zenith  213 . As there is no preferred azimuthal orientation at the zenith  213 , the circular polarization is effective for filling the signal null at all azimuthal orientations. The gain needed at the zenith  213  corresponds to shorter ranges; hence only a minimal gain is needed at the zenith  213 . A gain of −10 to −15 dB can provide sufficient gain while the pattern of the monopole antenna  200  is relatively unaffected. Any fill in the null is detrimental to side radial radiation and can be thought of a zero sum. Thus, any amount gained at the null is lost radially. 
     Now referring to  FIG. 3 , the radiation pattern a standard monopole antenna compared to a monopole antenna containing slots in the ground plane in terms of the EM radiation pattern. The standard monopole antenna pattern is depicted by dashed line  302 , while the monopole antenna containing slots pattern is depicted by the solid line  304 . The x-axis is in degrees while the y-axis is in decibels. The zenith of both antennas is depicted at zero degrees. The monopole antenna containing slots has a gain at zenith of about twenty decibels. 
     As detailed above, the number of slots, the angle of the slots relative to the ground plane radius, the distance of the slots, as well as the size and shape of the slots has an effect on the signal radiated by the slots. Each of the variables can be adjusted to develop a preferred particular embodiment for each antenna. Now referring to  FIG. 4 , one embodiment of the invention is detailed. Monopole antenna  400  comprises a radiator  402  and a ground plane  404  that is preferably disc shaped and planar having a radius and a diameter, and made of metal or some other conductive material. The radiator  402  is attached to the ground plane  404  at a center  406  of the ground plane  404 . The radiator  402  further comprises a free end  405 . The radiator  402  and the ground plane  404  are capable of radiating a signal. The ground plane  404  contains ground currents  408 . 
     The ground plane  404  further comprises a first slot  410 , a second slot  412 , a third slot  414  and a fourth slot  416 , each having a long dimension and a short dimension. The slots  410 ,  412 ,  414  and  416  are positioned such that the slots make a forty five degree angle relative to the ground currents  408 . The slots  410 ,  412 ,  414  and  416  interrupt the ground current  408  and produce a radiated polarization  420  perpendicular to the long dimension of the slots  410 ,  412 ,  414  and  416 . The slots  410 ,  412 ,  414 , and  416  are oriented such that two of the slots  410 ,  412 ,  414  and  416  radiate vertically and two of the slots  410 ,  412 ,  414 , and  416  radiate horizontally. Slots  410  and  416  are further oriented such that they are perpendicular to slots  412  and  414 . 
     Again referring to  FIG. 4 , slots  410  and  414  are a first distance  422  from the center  406  and the radiator  402 . Slots  412  and  416  are a second distance  424  from the center  406  and the radiator  402 . Additionally, slots  412  and  416  are one hundred eighty degrees from one another as measured from the center  406 , and slots  410  and  414  are one hundred eighty degrees from one another. The distance of the slots from the radiator  402  determines the phase of the radiated polarization  420 . This embodiment has the first distance  422  and the second distance  424  positioned relative to one another that they cause a ninety degree phase shift. The combination of horizontal radiation, vertical radiation and the ninety degree phase shift produce circular polarization towards the free end  405  of the radiator  402 . The circular polarization produces a gain at the free end  405  and fills the signal null at zenith. 
     Now referring to  FIGS. 5 ,  6 ,  7 ,  8 , and  9 , several alternate embodiments are shown. The monopole antennas  500 ,  600 ,  700 ,  800  and  900  each comprise a radiator  502  and a ground plane  504 . The ground plane  504  has a diameter and a radius. Each ground plane  504  contains a series of slots  510 . The slots  510  may be of differing sizes, shapes, numbers, and distances from the radiator  502 , as well as have varying angles relative to the radius. Additionally, the slots  510  may be contained within the ground plane  504  or continue to an edge  512  of the ground plane  504  such as in monopole antennas  600  and  700 . The possible embodiments are limitless; however, the slots  510  must be able to produce a polarization that fills a signal null at zenith. Furthermore, the principles are written in terms of monopole antennas; however, those principles are also applicable to other antennas. 
     The slots in the embodiments may be made during the initial construction of the monopole antenna or by retrofitting or modifying existing monopole antennas. The slots may be made separately or with the manufacture of the ground plane, by stamping, chemical material removal, cutting, laser cutting, plasma cutting, water-jet cutting, a circuit board manufacturing process, or any other similar or known processes. Furthermore, the ground plane may also be a layer on a substrate. 
     Having thus described the invention in connection with the several embodiments thereof, it will be evident to those skilled in the art that various revisions can be made to the several embodiments described herein with out departing from the spirit and scope of the invention. It is my intention, however, that all such revisions and modifications that are evident to those skilled in the art will be included with in the scope of the following claims. Any elements of any embodiments disclosed herein can be used in combination with any elements of other embodiments disclosed herein in any manner to create different embodiments.