Abstract:
A multi-layered slotted antenna with reduced susceptance and increased conductance with a minimal effect on antenna radiation in the azimuth. Separate conductors are connected across the individual slots to reduce the slot susceptance with a minimal effect on slot radiation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an antenna for the broadcast of electromagnetic wave energy and, more particularly, to class of antennas known as slotted antennas. 
     2. Description of the Prior Art 
     Slotted antennas generally have a conductive mast that has a plurality of layers with one or more slots in each layer that are positioned along the axial direction of the mast. Slotted antennas have been used to radiate horizontally polarized waves for television applications. To impart elliptical or circular polarization, separate dipoles have been placed in juxtaposition to the slots. The dipoles are operable to provide a vertical component of radiation to the horizontal component provided by the slots. Examples of slotted antennas with elliptical polarization are disclosed in U.S. Pat. Nos. 4,129,871 and 4,899,165. 
     Some slotted antennas require a standing wave design. A standing wave design requires the admittance of each layer to add in parallel such that the resulting input admittance achieves a desired bandwidth at the antenna input. In order to achieve the best overall bandwidth at the antenna input, it is desirable to add up admittances of the layers along the real axis of the Smith chart. The slots of current standing wave antennas tend to have a high enough susceptance that it is difficult to achieve the best overall bandwidth at the antenna input. 
     An object of the present invention is to provide an antenna with reduced slot susceptance and improve antenna bandwidth. 
     SUMMARY OF THE INVENTION 
     An antenna according to the present invention includes a conductive mast having one or more slots that extend axially of the mast. A coupler is disposed within the mast to provide energy for exciting the slots to radiate waves of energy. Conductors are individually connected across the slots and positioned at a location of the mast that reduces the antenna susceptance and increases the antenna conductance with a minimal effect on the antenna radiation in the azimuth. 
     The conductors may extend across the slots perpendicularly to the axial direction of the mast or at an angle thereto. The location of the conductor relative to the slot is selected to give minimum susceptance and cause minimal effect to the radiation in the azimuth. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other and further objects, advantages and features of the present invention will be understood by reference to the following specification in conjunction with the accompanying drawings, in which like reference characters denote like elements of structure and: 
     FIG. 1 is a schematic diagram in elevation of an antenna according to the invention; 
     FIG. 2 is a schematic circuit diagram of the FIG. 1 antenna; 
     FIG. 3 is a perspective view of one layer of another antenna embodiment according to the invention; 
     FIG. 4 is a side view of the FIG. 3 antenna; 
     FIG. 5 is a top view of the FIG. 3 antenna; 
     FIG. 6 is a perspective view of one layer of a further antenna embodiment according to the invention; 
     FIG. 7 is a side view of the FIG. 6 antenna; 
     FIG. 8 is top view of the FIG. 6 antenna; and 
     FIG. 9 is a portion of a Smith chart demonstrating the slot admittance with and without a conductor connected across the slot. 
    
    
     DESCRIPTION OF THE INVENTION 
     With reference to FIG. 1, there is provided an antenna  20  according to the present invention. Antenna  20  includes a hollow conductive mast  22  that has a plurality of layers  24 - 1 ,  24 - 1  through  24 -N−1 and  24 -N. Each layer  24 - 1  through  24 -N includes one or more slots that are located at the periphery of mast  22  and with their elongated dimension along the axial direction of mast  22 . The number of slots per layer is a matter of choice of design and generally is dependent on the desired radiation pattern in the azimuth. 
     Mast  22  may have either a cylindrical construction for a coaxial mast or a non-cylindrical construction for a wave guide mast. For the purpose of description, mast  22  will be described for the coaxial construction. Mast  22  has an inner conductor  28  that is concentric with mast  22  and extends along the axis of mast  22 . Antenna  20  receives a signal to be transmitted between inner conductor  28  and mast  22  at the bottom end of antenna  20 . Antenna  20  is terminated at its top end with an impedance (not shown) connected between mast  22  and inner conductor  28 . For a standing wave antenna, the terminating impedance is zero. That is, antenna  20  at its top end is shorted across mast  22  and inner conductor  28 . 
     Associated with each slot  26  is a coupler  30 , shown only in FIGS. 4 and 7. Coupler  30  serves to provide excitation energy to slot  26  in the manner known in the art. Though not shown in the drawing figures, separate dipoles may be separately associated with slots  26 . This would add a vertical radiation component to the horizontal radiation component provided by slots  26  so that antenna  20  will have elliptical or circular polarization. 
     Referring to the schematic circuit diagram of FIG. 2, admittances Y 1 , Y 2  through Yn−1 and Yn of slots  26  of layers  24 - 1 ,  24 - 2  through  24 -N−1, respectively, are connected in parallel so as to be additive for an input admittance Yin. 
     According to the present invention, separate conductors  32  that are connected across slots  26  reduce the susceptance of slots  26 . Conductors  32  are located at the periphery of mast  22  so as to provide a conductive connection across slots  26  without adding any radiation, particularly in the azimuth. That is, conductors  32  have minimal effect on the radiation produced by their respective slots. By reducing the susceptance, the admittance Yin is mostly a function of the slot conductance and, therefore achieves an optimum overall bandwidth for antenna  20 . 
     With reference to FIGS. 3 through 8, conductor  32  is shown as being a loop. In FIGS. 3 through 5, the loop is arcuate, while in FIGS. 6 through 8, the loop is non-linear. Although conductor  32  is shown as located at substantially the midpoint of the associated slot  26 , its position is optimally determined by a position that yields the lowest susceptance and the minimum radiation effect on the radiation produced by the associated slot. Conductors  32  may extend across slots  26  substantially perpendicular to an axial direction  34  of mast  22  as shown in FIGS. 1 and 3 through  5 , or at an angle to axial direction  34  as shown in FIGS. 6 through 8. 
     The admittances, both conductance and susceptance were measured for a set of single layers without conductors  32  a set of single layers with conductors  32  at a frequency of 755 MHz. The layers of each set consisted of a mast having an inner conductor diameter of 6.125 inches, an outer mast diameter of 16 inches, a 2.25 inches thick wall and eight slots. Each set included separate layers with slot lengths of 9.5, 10 and 10.5 inches. 
     Referring to FIG. 9, curve  40  represents a plot of the admittance for the set of layers without conductors  32  and curve  42  represents a plot of admittance for the set of layers with conductors  32 . By inspection, curve  42  for the set of layers with conductors  32  is closer to the real axis. Therefore, its slots have a lower susceptance. For example, the 10.5 inch slot without conductor  32  has a susceptance value of about 0.345 compared to a value of 0.05 with conductor  32 . Also, the conductance is higher. Thus, the 10.5 inch slot without conductor  32  has a conductance value of 0.11 versus a value of about 0.23 with conductor  32 . 
     The present invention having been thus described with particular reference to the preferred forms thereof, it will be obvious that various changes and modifications may be made therein without departing from the spirit and scope of the present invention as defined in the appended claims.