Abstract:
The present invention antenna preserves the general size and form factor of the prior art loop antennas while providing the benefits of multiple point feeds at less than one wavelength in separation of feed points. The invention antenna obtains omnidirectional radiation and improved efficiency over the prior art by way of dual slotted, open ended cylindrical or rectangular box structures fed with high impedance feed lines.

Description:
FIELD OF THE INVENTION 
     The present invention relates to loop or folded dipole antennas. More particularly, the invention relates to such antennas 
     BACKGROUND OF THE INVENTION 
     Loop or folded dipole antennas include simple circular or square loops, whose impedance is readily calculated. An example of a prior art circular loop antenna  100  is shown in  FIG. 1  comprising a loop portion  101 , gap  102 , and input/output connections  103 . 
     Antennas of resonant size, where the size is in the vicinity of a half wavelength, have been typically fed at one feed point with series or shunt structures. In series fed structures, a signal is fed to one side of a gap formed in the conductor loop. Signal feeding to a series structure is considered to be a voltage generator. In a shunt fed structure, a signal is fed to conductor loop at two points without creating a gap. Signal feeding to a shunt structure is considered to be a current generator. 
       FIG. 3  shows a well known toroidal radiation pattern for a dipole antenna. It is readily appreciated that an electrical component of the electromagnetic wave forms a vertical radiation portion and the magnetic component forms a horizontal plane portion. 
     The design of non-traditional shaped antennas is not easy to analyze and are approximated by various design parameters. Variations away from traditional antenna structures have resulted in low efficiency antennas. In the past, non-traditional designs have been used in low frequency applications. As a result, necessarily poor performance of these antennas was adequate and was acceptable. 
       FIG. 2  shows cylindrical antenna  104  comprising a cylindrical band  105  with a gap  106  and feed points  107 . The circular or tubular radiator of  FIG. 1  is replaced by a larger surface area cylinder band  105 . The radiation surface of band  105  is clearly increased over that of loop  101  of  FIG. 1 . For the antenna of  FIG. 2 , current distribution becomes concentrated opposite the input/output terminals  107 , creating an undesirable cardioid pattern, i.e., providing negligible radiation to a substantial portion of an outward radiation plane leading to an undesirable asymmetric antenna radiation pattern. The prior art teaches that multiple feeds should not be made to an antenna where such feed points are less than a wavelength apart as coupling between/among the feed points is strong and in essence eliminates the benefits of multiple feeds, i.e., that the radiation from such an antenna would be the same as if it were made by one feed. 
     SUMMARY OF THE INVENTION 
     The invention antenna departs from the prior art loop dipole devices of  FIG. 1  and  FIG. 2  and the radiation components of  FIG. 3 , in that the radiation patterns of the electrical and magnetic fields of the invention antenna are switched so that an electrical component of the electromagnetic wave forms a horizontal plane radiation and the magnetic component forms a vertical plane radiation. The invention antenna preserves the general size and form factor of the prior art loop antennas while providing the benefits of multiple point feeds at less than one wavelength in separation of feed points. The invention antenna obtains omnidirectional radiation and improved efficiency over the prior art. 
     The invention antenna provides for two gaps in a cylindrical or rectangular conductor wall structure, where, in top view, the two gaps separate two mirror image structures. This symmetrical arrangement allows for even current distribution across the entire device and that overall efficiency is increased when feed points are placed a distance away from the edges defining the gaps. It is preferred that a high-impedance network of balanced transmission lines be used to feed the invention antenna. 
     In a specific example, a circumferential distance along a continuous conductor surface between feed points in the invention antenna is one quarter wavelength or a total circumference distance around the cylinder or reactance of about 0.6 wavelength. A distance defined by the gaps and a section of the conductor wall from the edge of the gaps to the feed points equals about 0.1 wavelength. The shape and size of the invention antenna causes it to operate as two back-to-back folded dipole antennas that are a quarter wavelength long. The combination of folded dipoles in the invention antenna provides an omnidirectional radiation pattern, eliminating the cardioid pattern deficiencies in prior art dipole antennas. The invention antenna radiates essentially as a magnetic dipole device. 
     It is an object of the present invention to provide structures and methods whereby efficiency of increases over a similarly formed loop antenna by at least five percent. 
     It is a further object of the invention to provide a small loop or folded dipole antenna fed at multiple points with high-impedance, balanced transmission lines to achieve an efficient radiator where the gain almost equals the directivity. 
     It is a further object of the invention to feed a loop antenna at multiple points by way of a spaced apart transmission line to provide a more efficient radiator with gain almost equal to the directivity of the beam patterns. 
     It is a further object of the invention to provide a loop antenna fed with relatively large cross-section transmission lines spaced apart to provide additional signal radiation surface area or a feed antenna structure to the loop antenna. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a prior art loop antenna. 
         FIG. 2  is a perspective view of a cylinder form of the antenna of  FIG. 1 . 
         FIG. 3  is a perspective view of a toroid radiation of a dipole antenna, where an electrical component is shown vertically oriented and a magnetic component is shown horizontally oriented. 
         FIG. 4  is a perspective view of a toroid radiation pattern for the invention antenna, where an electrical component is shown horizontally oriented and a magnetic component is shown vertically oriented. 
         FIG. 5  is a perspective view of the invention antenna in a cylindrical form. 
         FIG. 6  is a perspective view of the invention antenna in a rectangular form. 
         FIG. 7  is the invention antenna of  FIG. 6  incorporating capacitance across the two gaps. 
         FIG. 8  shows a tail fin location on a commercial airplane as a preferred location for the invention antenna as a receiver. 
         FIG. 9  shows a sloped form of the invention antenna of  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is now discussed with reference to the figures. 
       FIG. 4  is a perspective view of a toroid radiation pattern for the invention antenna, where an electrical component current I is shown horizontally oriented and a magnetic component H is shown vertically oriented. This is in contrast to the opposite orientations of components H and I shown in  FIG. 3  for a dipole antenna. Although similar in form factor to the prior art loop antennas, the invention antenna is fundamentally operating to radiate is electromagnetic waves in a very different manner. Operating by way of multiple feeds to the radiating surfaces at feed points less than one wavelength apart has resulted in this stark change in pattern of radiation of components of electromagnetic waves in the invention antenna. 
       FIG. 5  is a perspective view of the invention antenna  110  comprising two generally cylindrical half sections  111  and  119  separated by gaps  117  and each having a conductor wall height  112 . Height  112  is preferably greater than one centimeter. Diameter  113  is a consequence of design choices with respect to arc separation  119 A separating feed points  118 A and  114 A for feed conductors  118  and  114  respectively to half section  118 . Identical orientation for feed points is made upon an inside surface of half section  111  for feed conductors  118  and  114 . Separation  119 A is preferably about one fourth wavelength for the desired antenna operational range of frequencies. Arc distance  119 B is a distance between feed point  114 A and an edge of gap  117 . Said arc distance is identical for all the feed points from a gap edge on the inside surfaces of half sections  111  and  118 . Feed conductors  114  and  118  are respectively fed by way of feed lines  116  and  115 . Providing signal feed to one of feed lines  115  or  116  necessarily drives the operation of the invention antenna where the other feed line is connected to ground. Four arc distances (of which arc distance  119 B is one) and a distance across gaps  117  equal about 0.1 to 0.25 wavelength for the desired antenna operational range of frequencies for the invention antenna. Gap distances  117  are preferably about ten percent or less of the total of four arc distances (of which arc distance  119 B is one) and a distance across gaps  117 . In a specific example, diameter  113  is 20 inches and wall height  112  is 7 inches. 
       FIG. 6  is a perspective view of the invention antenna  120  comprising two generally U-shaped half sections  121  and  124  to form a rectangular shape overall but separated by gaps  131 . Each of half sections  121  and  124  have a conductor wall height  125  and a width  126  of their end walls. Height  125  is preferably greater than one centimeter. An overall length of antenna  120  includes gaps  131 , wall distances  132  between gap edges and feed points  133 , and wall distances  134  between feed points  133  and said end walls. Feed conductors  127  and  128  respectively connect feed lines  130  and  129  to said feed points  133 . Width  126  plus twice distance  134  is preferably about one fourth wavelength for the desired antenna operational range of frequencies. Providing signal feed to one of feed lines  129  or  130  necessarily drives the operation of the invention antenna where the other feed line is connected to ground. Four distances  132  and a distance across gaps  131  equal about 0.1 to 0.25 wavelength for the desired antenna operational range of frequencies for the invention antenna. Gap distances  131  are preferably about ten percent or less of the total of four distances  132  plus two gap distances  131 . 
       FIG. 7  is the antenna  120  of  FIG. 6  incorporating capacitors across the two gaps  131 , where conductive plates  135  are arranged so that each is identical in size and orientation as to gaps  131 . Plates  135  are secured to an inside surface of half sections  121  and  124  near gaps  131  and separated from them by capacitive distance  136 , which is preferably filled with a dielectric substance for support and capacitive effect. Overall capacitance of each plate  135  is preferably with the range of 10 to 20 picofarads, more preferably at about 14 picofarads in a specific embodiment. Said capacitance in the entire circuit structure (1) allows for tuning an operating center frequency for the invention antenna and (2) lowers the operating center frequency of the antenna. As a specific example of this embodiment of the invention VOR antenna, overall length  140  of the rectangular structure is about 26 inches, width  126  is about 5 inches, and height  125  is about 7 inches. 
       FIG. 7  shows a separation distance  141  of feed lines  129  and  130  and feed conductors  127  and  128  (of  FIG. 6 ), which provides a substantial separation (i.e., about 0.5 inches to 1.0 inches) of these conductors in contradiction to the teaching of the prior art. The &#39;897 patent teaches a currently well-known design paradigm of using “closely spaced conductors” for feed lines and feed conductors in an attempt to neutralize radiation from them. The invention antenna maintains a substantial separation of those conductors to improve overall performance, in combination with using high impedance, spaced-apart feed lines and feed conductors. Separation distance  141  of feed lines and feed conductors results in increasing the effective radiating surface of the antennas of the invention, thereby reducing overall size thereof over the prior art antennas. 
     A further advantage of the in invention antenna as shown in  FIG. 7  is use of a high Q air capacitor in the antenna loop, resulting in uniform current distribution across the radiator surfaces. Arranging feed points  133  (as in  FIG. 6 ) to be separated from each other at one quarter wavelength apart effectively reduces the resonant frequency of the antenna. This, in turn, results in significant size reduction of the overall antenna as to a peripheral size. 
     The &#39;897 patent teaches that each feed line should be connected to a two dimensional radiator at it&#39;s end point and that each feed line should be connected to one or two radiators which share no connection with radiators of the other feed line. In the present invention, two back-to-back, concave and symmetrical three-dimensional radiators are fed with inputs from both feed lines at a mid-point of a height of the antenna. With such dual feeds to a single radiating element, antenna efficiency is improved over the prior art. 
     In a further distinguishing feature of the invention, the periphery of the new antenna is less than half wavelength. The antenna of the &#39;897 patent is a full wavelength in effective length. The present invention antenna provides an efficient omnidirectional loop type radiation from a dipole like type radiator by careful selection of the capacitance linking the three dimensional dipole elements. 
       FIG. 8  shows a tail fin location  138  on a commercial airplane  137  as a preferred location for the invention antenna as a portion of a receiving device for radio communications. 
       FIG. 9  shows a sloped form of the invention antenna  120  of  FIG. 7 , where height  125  is reduced starting at point  144  on side walls of half section  121 , continuing down across gap  131 , plate  135  at down side walls of half section  124  so its endwall has a height  139 . This sloped form of the invention antenna somewhat reduces efficiency while providing for incorporation into a most distal end of the tail fin location  138  of  FIG. 8 . Height  139  is about 4 inches in a specific example continued from above. 
     It is a further object of the invention that the invention antenna operates as transmission antennas and reception antennas for modern jetliner navigation. VOR stands for VHF Omni-directional Radio Range. It is a jetliner radio navigation system. These systems broadcast a VHF radio composite signal including the station&#39;s morse code identifier and data that allows the airborne receiving equipment to derive the magnetic bearing from the station to the aircraft. An intersection of two radials from different VOR stations on a chart allows for a determination of a specific position of the aircraft. 
     A preferred embodiment of the current invention is as a VOR receiving antenna in the fin cap of an aircraft as in  FIG. 9 . It is sloped to conform to the shape of the top of the fin. VOR signals are horizontally polarized at a center frequency of 112.975 MHz. A frequency of 112.975 MHz corresponds to a wavelength of 2.66 m. A traditional dipole with horizontal polarization would yield nulls in the fore and aft directions of the aircraft. Nulls in this direction are unacceptable. However, the current invention yields a “loop”-like pattern where the nulls are directly overhead and directly below the aircraft. Nulls in this direction are acceptable because of the “cone of uncertainty” associated with VOR ground stations. 
     The size of the fin cap for any particular aircraft is fixed. The preferred embodiment allows the design of an antenna that occupies the budgeted volume of a VOR antenna. By occupying the budgeted volume, the antenna is optimally efficient. The resonant frequency of the antenna is decreased to coincide with the center frequency (112.975 MHz) of the VOR band by the use of high-Q dielectric capacitors. The radiators are excited by a feed network of balanced, high-impedance transmission lines. 
     The above design options will sometimes present the skilled designer with considerable and wide ranges from which to choose appropriate apparatus and method modifications for the above examples. However, the objects of the present invention will still be obtained by that skilled designer applying such design options in an appropriate manner.