Abstract:
The invention is a low profile antenna array, particularly beneficial for AM broadcasting stations, which dramatically reduces the height of the antenna. The antenna array is constructed of at least two conductors oriented horizontally or sloping slightly up and extending away from a center location where they have upright sections extending above the ground plane by as little as 0.01 wavelength. The individual conductors are each approximately one-quarter wavelength long. The multiple conductors of the antenna array are each fed by impedance matching sections to achieve high individual impedances, which, when connected in parallel can be made to match any desired impedance with dimensional changes.

Description:
BACKGROUND OF THE INVENTION 
     This invention deals generally with antennas and more specifically with an antenna with significantly lower height above ground than those usually available. 
     A single quarter wavelength vertical monopole antenna may be the classic of all antenna designs. We see it for virtually every AM broadcasting station, and we see approximations of it on almost every automobile in the rod attached to an upper surface of the automobile. Such an antenna is an unbalanced antenna, and for transmitting it must be fed against a ground plane, a relatively flat conductive surface. The impedance of such an antenna is approximately 36 ohms at resonance. 
     For use in AM medium wave broadcasting the quarter wavelength antenna must be 447 feet (136 meters) high at 550 kHz, and at 1600 kHz it is still 154 feet (47 meters) high. Such heights not only require towers demanding considerable design and expense, but often such high towers are restricted by local building codes or pose a danger to aircraft. 
     One solution to the problem of high antennas was proposed as early as 1909 by Simon Eisenstein of Kiev, Russia. This was an antenna with a single vertical conductor and top loading. For the top loading, a horizontal plane formed of multiple radial conductors is added atop the vertical section that has a height much less than a quarter wavelength. Unfortunately, however, this configuration produces an antenna with a very low impedance if the height is in the desirable range of less than 0.1 wavelength. 
     It would be very beneficial, particularly to broadcasters and for vehicle antenna users, to have an antenna design which has both a very low height, that is, less than 0.1 wavelength, and also could be matched to a typical 50 ohm impedance system. 
     SUMMARY OF THE INVENTION 
     The present invention is a low profile antenna configuration constructed of multiple top loaded approximately vertical upright conductors clustered together. In the preferred embodiment, in order to yield an omnidirectional pattern, the individual conductors are spaced along a small circular pattern, with each conductor being excited by only an appropriate fraction of the total signal power which would be applied to a single monopole antenna for the same application. Thus a four conductor pattern has the upright conductors spaced apart at 90 degree separation points, and for a transmitting antenna configuration, each conductor is fed one-quarter of the total power. 
     Another feature of the invention is that the top loading on each upright conductor is only a single conductor extending away from the cluster of upright conductors, and the length of the top loading conductors and the heights of the their ends above the ground plane can be adjusted to achieve resonance and to provide the desired impedance. For ease of construction, the upright sections and the non-vertical extension can be a continuous conductor bent to form both the upright section and the non-vertical extension. With the further use of impedance matching sections on each feed to an upright conductor, a high enough impedance can be achieved on each individual conductor feed to result in an impedance which matches the commonly used 50 ohm impedance systems when all the individual antennas are fed in parallel. 
     The present invention thereby furnishes low profile, very low height, antenna configurations which can match the typical signal feed cables and yield nearly uniform omnidirectional radiation fields substantially identical to the fields provided by quarter wavelength monopole antennas. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view schematic diagram of an antenna configuration with two sloped conductors. 
     FIG. 2 is a,top view schematic diagram of an antenna configuration with four conductors. 
     FIG. 3 is a top view schematic diagram of an antenna configuration with eight conductors. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a side view schematic diagram of antenna assembly  10  with two top loading sloped conductors  12  located on a diameter of a circle. The typical spacing A between upright conductors  18  is 0.20 wavelength, and dimension A must be below ½ wavelength at which distance the signals on the upright conductors would interfere. Ground plane  14  for such an antenna is also shown schematically. As is well understood by those skilled in the art, the ground plane is a conductive surface or network of conductors and can be the earth itself. It typically extends to approximately the same outer diameter as the antenna assembly, and for a ground mounted antenna for AM broadcast the diameter typically can vary between 0.50 and 1.0 wavelengths. 
     Each antenna conductor  12  is independently excited and connected to a common radio frequency signal device  16  through signal converter  17 . In the case when radio frequency signal device  16  is a transmitter, signal converter  17  is selected to be a splitter which divides the total signal, power generated by radio frequency signal device  16  equally between all the conductors  18  with the total signal power being the equivalent of the power that would be applied to a single conventional vertical quarter wavelength monopole antenna. 
     In the case when radio frequency signal device  16  is a receiver, signal converter  17  is selected to be a combiner which combines the signals from all the conductors  18  and sends the combined signal to radio frequency signal device  16 . That combined signal is also the equivalent of the signal received by a single conventional vertical quarter wavelength monopole antenna. 
     The most important characteristic of antenna assembly  10  is its low profile, its very low height above the ground plane. Thus, for the two conductor embodiment shown in FIG. 1, when matching a 50 ohm impedance the heights of upright conductors  18  are only 0.01 wavelengths, and the heights B of the ends of conductors  12  above ground plane  14  are only 0.065 wavelengths. That means that for the two conductor sloping, version of FIG. 1 the antenna with a 50 ohm impedance for a broadcast station in the middle of the AM band, at 1000 kHz, is only 64 feet high at its outer perimeter and 10 feet high at the center. Each sloped antenna conductor  12  is approximately one quarter wavelength (246 feet) long, so that the entire antenna assembly at 1000 kHz is approximately 500 feet in diameter. 
     However, this compares to the conventional antenna tower which is 246 feet tall and has the same ground plane diameter of approximately 500 feet diameter. 
     It is important to understand that the length of sloped conductors  12  and the heights B of their ends above the ground plane can be adjusted to achieve resonance and adjust impedance. Thus, antenna assembly  10  can have conductors  12  with lengths in the range between 0.15 wavelengths and 0.30 wavelengths, upright conductors  18  with lengths in the range of between 0.001 and 0.25 wavelengths, and distances B of the remote ends above ground plane  14  can be in the range of between 0.001 and 0.25 wavelengths. 
     Moreover, with the further use of impedance matching sections  19  between each antenna conductor  12  and its signal source  16 , a high enough impedance can be achieved on each individual antenna feed to result in the commonly used 50 ohm impedance when all the individual antennas are fed in parallel. Such matching sections can be applied both to the sloping configuration shown in FIG.  1  and to an antenna assembly with the conductors  12  oriented in a single plane. 
     FIG. 2 is a top view schematic diagram of antenna configuration  20  with four top loading conductors  22  extending away from each other, with the signal fed into each of the conductors  22  from an upright segment located at their inner ends  24  adjacent to the other conductors of the configuration. It should be appreciated that the benefit of the symmetry and equal spacing between conductors  22  is that they yield an omnidirectional radiation pattern, and it is not a requirement of the invention itself. As with the two conductor configuration shown in FIG. 1, each antenna conductor  22  in FIG. 2 can be connected to a radio frequency signal device (not shown) through a signal converter (not shown) and an impedance matching section (also not shown in FIG.  2 ). Thus, the total radio frequency power excitation of all the conductors of any of the antenna configurations of the invention can be split in the transmitting mode or combined in the receiving mode so that the total signal power exciting an antenna configuration of the invention is the equivalent of the power that would be applied to a single conventional vertical quarter wavelength monopole antenna. However, for the antenna configurations of the invention, this power can be broadcast or received by an antenna of significantly lower height. 
     It is important to appreciate that the slopes of the extending conductors  22 , their lengths, and their heights above the ground plane are variable and can be used effectively to tune the antenna assembly. Thus, for the previously discussed station in the middle of the AM band, at 1000 kHz, the four conductor antenna of FIG. 2 when designed to match a 50 ohm impedance is only 46 feet high at its outer perimeter, and it is 10 feet high at the center. Furthermore, each sloped antenna conductor  22  is still one quarter wavelength (246 feet) long, so that the entire four conductor antenna assembly at 1000 kHz is still approximately 500 feet in diameter but only 46 feet at its maximum height. 
     FIG. 3 is a top view schematic diagram of an antenna configuration  30  with eight top loading conductors  32 . The operation and dimensions for the eight conductor version with sloping antenna conductors  32  are the same as the  4  conductor version of FIG. 2 except for the obvious difference of splitting the power into or combining it from a greater number of paths. 
     However, when all the conductors  12  of the two conductor system of FIG. 1, conductors  22  of the four conductor embodiment of FIG. 2, or conductors  32  of the eight conductor embodiment of FIG. 3 are oriented horizontally, the dimensions change somewhat for the 50 ohm impedance match. 
     For the two, four, and eight conductor 1000 kHz antenna configurations the height of all the conductors above the ground plane is the same, at 43 feet. However, the length of each conductor of the two conductor horizontal system is 207 feet, and the length of each conductor for both the four and eight conductor systems is 185 feet. 
     It is also interesting to note that for a cellular antenna at 850 MHz, the four and eight conductor antennas of FIG.  2  and FIG. 3 become almost flush with the ground plane, with a center height of 0.14 inch, an outside height of 0.65 inch, and a total diameter of only approximately 7 inches. 
     The antenna design of the invention thus yields a very practical antenna, not only for low frequency AM broadcasting transmitters, but also for high frequency wireless communication. 
     It is to be understood that the form of this invention as shown is merely a preferred embodiment. Various changes may be made in the function and arrangement of parts; equivalent means may be substituted for those illustrated and described; and certain features may be used independently from others without departing from the spirit and scope of the invention as defined in the following claims. 
     For example, the antenna configurations of the invention can be used for any frequency, since the dimensions of the design are specified in wavelength, and the antennas of the invention can also be used in systems with any impedance with suitable adjustment of dimensions. Furthermore the conductors need not be located on exact radii of circles, as is shown in the FIGURES for convenience of drawing.