Abstract:
An oblique angle defining the slot face opposing a coupler in a slotted coaxial antenna increases the apparent slot length and therewith the capacitance of the driven element. The altered slot angle, in concert with a flattened facing surface on the associated coupler, increases the radiating efficiency of the antenna.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to antennas. More particularly, the present invention relates to resonant-slotted coaxial antennas.  
       BACKGROUND OF THE INVENTION  
       [0002]     Coaxial transmission lines carrying radio frequency (RF) energy can be used as antennas provided the center conductor of the coaxial line couples RF energy to an aperture in the outer conductor, termed a slot, with sufficient efficiency to cause emission of a significant proportion of the energy applied to the slotted coax/antenna. A typical slotted coaxial line antenna designed for ultra-high frequency (UHF) broadcast, for example, may have from about four to several dozen slots in line, typically occurring at one-wavelength intervals. Such an antenna may also have more than one radially-disposed set of slots.  
         [0003]     In order for the slotted coax to radiate efficiently, the two sides of the slot should have a differential distance to the center conductor. This is commonly realized by affixing a conductive rod parallel to the center conductor near or adjacent to one edge of the slot. The impedance mismatch induced by the rod tends to promote radiation out the slot.  
         [0004]     Various modifications of the basic concept of the slotted coax antenna and the tradeoffs associated therewith have been attempted by many practitioners of slotted coax design. For instance, it is known that increasing the length of each slot to a full wavelength can be electrically beneficial—but produces a structure with one or more continuous slots, which compromises the mechanical integrity of the antenna. Enlarging the size of each slot near the ends to form a shape known in the art as a “dog bone” can increase the perimeter length while preserving the capacitance at the center of the slot, improving radiation performance but incurring other drawbacks. Inserting a block of higher-dielectric-constant material, such as polytetrafluoroethylene (PTFE, sold for example under the trade name Teflon®) in the slot can reduce the slot&#39;s electrical width, but can promote contamination and arcing during extended use. Increasing antenna outer conductor diameter allows the slots to be shorter, but may increase weight and wind drag.  
         [0005]     Accordingly, it is desirable to provide an apparatus and method for a slotted coax antenna that increases overall performance with minimal deleterious effects.  
       SUMMARY OF THE INVENTION  
       [0006]     The foregoing needs are met, to a great extent, by the present invention, wherein in one aspect a slotted coax antenna is provided that in some embodiments incorporates a parallel-sided slot, each of whose sides lies in a plane parallel to the coax longitudinal axis, but whose slot center plane does not include the coax longitudinal axis. That is, the parallel-sided slot is tilted rather than radially-oriented with respect to the coax longitudinal axis. The number of slots may be any number, limited by structural considerations and performance needs. The effect of this invention is to increase the capacitance of the slot compared to previous designs while reducing penalties of decreased voltage capability, bandwidth, and structural strength.  
         [0007]     In accordance with one embodiment of the present invention, a slotted coaxial antenna comprises a section of coaxial signal line capable of conducting radio-frequency electromagnetic signals, an outer conductor of the coaxial signal line section, an inner conductor of the coaxial signal line section, a longitudinal axis of the coaxial signal line, a first slot in the coaxial signal line wherein a first planar region of the first slot is situated obliquely to a radial projection from the longitudinal axis of the coaxial signal line projected through the center of the slot, and a first coupler with a long axis thereof extending parallel to the longitudinal axis of the coaxial signal line, wherein the first coupler is positioned proximally to the first slot.  
         [0008]     In accordance with another embodiment of the present invention, a slotted coaxial antenna comprises conducting means for conducting a radio frequency signal, confining means for confining the conducted radio frequency signal within a closed, electrically conductive boundary, allowing means for allowing a portion of the radio frequency signal to be emitted from within the closed, electrically conductive boundary, coupling means for coupling the allowed portion of the radio frequency signal into a condition for emission; and tilting means for tilting and extending the effective physical dimensions of the allowing means.  
         [0009]     In accordance with yet another embodiment of the present invention, a process for emitting radio frequency signals comprises the steps of conducting a radio frequency signal, confining the conducted radio frequency signal within a closed, electrically conductive boundary, allowing a portion of the radio frequency signal to be emitted from within the closed, electrically conductive boundary, coupling the allowed portion of the radio frequency signal into a condition for emission; and tilting and extending the effective physical dimensions of the allowing step.  
         [0010]     There have thus been outlined, rather broadly, certain embodiments of the invention in order that the detailed description thereof herein may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional embodiments of the invention that will be described below and which will form the subject matter of the claims appended hereto.  
         [0011]     In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting.  
         [0012]     As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a view illustrating a slotted coaxial line according to the prior art.  
         [0014]      FIG. 2  is a chart showing the relationship between coax diameter and slot length.  
         [0015]      FIG. 3  is a section view illustrating a representative prior-art slotted coaxial line antenna.  
         [0016]      FIG. 4  is a section view of a prior-art slotted coaxial line antenna with a slot extender.  
         [0017]      FIG. 5  is a section view of a prior-art slotted coaxial line antenna with a dielectric insert.  
         [0018]      FIG. 6  is a section view of an exemplary slotted coaxial line antenna incorporating the preferred embodiment of the invention.  
         [0019]      FIG. 7  is a section view of an exemplary slotted coaxial line antenna incorporating a multiplicity of radially distributed slots according to a preferred embodiment of the invention.  
         [0020]      FIG. 8  is a side view of an exemplary slotted coaxial line antenna incorporating a multiplicity of linearly distributed radial arrays of slots according to a preferred embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0021]     The invention will now be described with reference to the drawing figures, in which like reference numerals refer to like parts throughout. An embodiment in accordance with the present invention provides increased slot capacitance while largely preserving other performance aspects in a slotted-coax antenna.  
         [0022]      FIG. 1  illustrates a section of a prior-art coax  10  with a generic resonant slot  12  in the outer conductor, used as a radiating aperture. The long axis of the slot  12  is oriented parallel to the axis of the coax  10  of the ends  14  of the slot  12  are rounded to minimize voltage gradients.  
         [0023]     Referring to  FIG. 1 , the tradeoffs in providing an antenna with larger outer and inner coax diameters correlate to a great extent with the penalties in weight and wind drag. If the outer conductor  20  inner diameter is doubled, for example, while the outer conductor  20  thickness is maintained constant, then the weight of the outer conductor  20  approximately doubles, while the wind loading due to conductor size likewise roughly doubles, both of which factors can increase the structural loading on a broadcast tower. To maintain constant impedance, both outer conductor  20  inner diameter and inner conductor  22  outer diameter are required to increase at the same rate, assuming an unchanged dielectric constant. Increasing the inner conductor  22  outside surface area decreases RF current density, allowing a higher-power signal to be transmitted through the coax without overloading the power capability thereof, and allows an increased number of slots  12  to be used, since there is more non-slot structure to support an increased number of circumferentially disposed slots  12 .  
         [0024]      FIG. 2  is a graph illustrating the inverse relationship  17  between coax diameter  16  in wavelengths and minimum effective slot length  18  in wavelengths. As demonstrated in  FIG. 2 , a larger-diameter coax outer conductor  16  radiates more efficiently with a shorter slot  18 . However, a small diameter for the coax is desirable in order to minimize material and fabrication cost, weight, and wind drag.  
         [0025]      FIG. 3  shows a cross section view of a prior-art single-slot antenna  28 . An outer conductor  30  and an inner conductor  32  comprise the coaxial line. A conductive-surfaced block functions as a coupler  34  to establish the conditions for radiation from a slot  36 . The coupler  34  is attached to the outer conductor  30  at a first wall  38  of the slot  36 , opposite a second wall  39 , using mounting hardware  40  that maintains electrical continuity between the outer conductor  30  and the coupler  34 . The mounting hardware  40  typically comprises a bolt-and-nut fastening  42  and a spacer  44 . The coupler  34  may be mounted at any distance from the outer conductor  30  that can be shown to promote efficient RF emission at a selected frequency. In general, the closer the coupler  34  is positioned to the inner conductor  32 , the higher the signal level and the greater the emission will be at that slot  36 . This can result in an association between the total number of slots  36  in the antenna  28  and the closeness of the coupler  34  to the inner conductor  32 .  
         [0026]      FIG. 4  shows a cross section view of another prior art antenna  46 . This is substantially identical to the design of  FIG. 3 , with the addition of a device known in the art as a slot extender  48 , attached at the second wall  39 , opposite the coupler  34 . The extended lip  49  of the slot extender  48  reduces the effective width of the slot  36 , increasing capacitance and thus radiation efficiency. The slot extender  48  adds an additional unit of hardware that requires adjustment and can exhibit a tendency to lose electrical continuity, and thus effectiveness, over time, when subjected to climate variations, for example. However, the reduced interelectrode spacing caused by the slot extender  48  increases the possibility of arcing, and thus can limit allowable peak transmitted power.  
         [0027]      FIG. 5  shows yet another cross section view of a prior art antenna  50 . This is also substantially identical to the design of  FIG. 3 , with the addition of a dielectric insert  52 . The insert  52  increases capacitance, which can be beneficial, but provides a surface connecting the walls of the slot  36  that can accumulate dirt, moisture, and other contaminants. This accumulation of contaminants can, over time, establish a conductive path across the slot gap, and can lead to gradual performance deterioration.  
         [0028]      FIG. 6  shows a cross section view of an exemplary single-slot antenna  54  according to this invention. The exemplary single-slot antenna  54  comprises an outer conductor  56  and an inner conductor  58 , a coupler  60 , and a non-radially-edged slot  62 . A conductive-surfaced coupler  60  helps to establish the conditions for radiation from a slot  62 . The coupler  60  is attached to the outer conductor  56  near a first wall  64  of the slot  62  using mounting hardware  66  that maintains electrical continuity between the outer conductor  56  and the coupler  60 . The mounting hardware  66  is shown as comprising a bolt-washer-and-nut fastening  68  and a spacer  70 . However, other forms of mounting may be used, as desired. The coupler  60  may be mounted at any location between the inner  56  and outer  58  conductors; experimentation may identify an optimum position for efficient RF emission at a selected frequency. Where preferred performance so dictates, the spacer  70  may not be required.  
         [0029]     The second wall  72  of the slot  62  may be spaced away from the first wall  64  by a distance determined by the voltage level of the broadcast signal and as a function of the transmitting frequency. The second wall  72  in the exemplary embodiment  54  of  FIG. 6  is oriented at an angle to a radial projection  74  from the centerline  76  of the coax through the center of the slot, rather than parallel to that projection  74 . The exemplary coupler  60  may not be cylindrical as in some prior art designs, but may have, for example, a flat surface  78  parallel to the second wall  72 , establishing thereby an effective slot width W.  
         [0030]     The presence of parallel surfaces  72  and  78  of the slot  62  increases the surface area and accordingly the effective width of the slot  62 , and thus the capacitance and radiating efficiency of the slot  62 . The positioning of the walls of the slot  62  at an angle θ, by causing the outer conductor  56  material to be cut obliquely, can similarly increase the slot surface area without intruding additional material into the coax, adding external flanges, or thickening the material from which the outer conductor  56  is formed.  
         [0031]     The magnitude of the angle θ may vary according to the dimensions of the elements making up the antenna  54 , and of the frequency and bandwidth characterizing the signal to be radiated by the antenna  54 . For an exemplary low-UHF implementation, an angle θ of 20 degrees has been demonstrated to improve performance of a single-slot 3.5 inch diameter antenna compared to an all-orthogonal configuration in an otherwise similar antenna.  
         [0032]     As in all high-voltage RF apparatus, individual elements of the exemplary embodiment  54  can preferably be rounded and free of burrs and rough surfaces, particularly on exposed edges, to avoid voltage gradients that could promote arcing.  
         [0033]      FIG. 7  shows a cross section view of an exemplary slot antenna  80  having four slot radiators  82  uniformly distributed around a coax  84 . Slot antennas shown thus far depict a single slot piercing the outer conductor of a coax, with the slot antenna formed thereby radiating a single lobe in the direction in which the slot opens and has a low-level signal in all other directions, which is known in the art as a skull radiation pattern. Two slots on opposite sides of the coax can produce opposed twin lobes in a so-called peanut radiation pattern, while three equally spaced slots will typically produce a three-lobed radiation pattern.  
         [0034]     With four or more slots  82  having angled wall surfaces and placed at uniform intervals around the coax  84 , a substantially uniform circular pattern can be achieved. The features of this invention can be used to produce each of the above-described patterns, generally with measurably greater efficiency than in prior-art slotted coax antennas, as described, for example, in  FIGS. 3, 4 , and  5 . Additionally, the exemplary embodiments of this invention do not suffer from deterioration over time as in prior-art slotted coax designs that rely on slot lengtheners  48  or dielectric inserts  52  to enhance performance.  
         [0035]     Experiments have shown that a slot antenna having two or three slots incorporating the features of this invention will generate patterns with prominent lobes. Analysis suggests that by increasing the number of slots, an effectively omnidirectional radiation pattern can be generated. In building a directional antenna from slotted coax, minimizing coax diameter may be a preferable strategy, while in building an omni antenna, it may be preferable in at least some instances to use a larger diameter coax with multiple slots as in  FIG. 7  rather than a multiplicity of smaller, radially positioned single-slot antennas.  
         [0036]      FIG. 8  illustrates a side view of an exemplary slot coaxial line antenna  86  featuring a vertical array of slot radiators  88 . A single slot  62  in  FIG. 6  or a single radial array of slots  82  in  FIG. 7  distributed around a vertically oriented coax  90  can produce a propagating wave that is horizontally polarized and whose pattern is distributed above and below the horizontal much as a dipole in free space is distributed. That is, the signal strength is greatest at the horizon and decreases with angle toward the zenith and nadir. Since a typical broadcast application may have little need for signal strength significantly above and below the horizon, it can be advantageous to position a multiplicity of elements, where each element is a slot or radial array of slots  82 , in a vertical array  88 . The signals emitted from a vertical array  88  so configured can constructively interfere in azimuth with respect to the coax&#39;s longitudinal axis  92 , and destructively interfere in elevation. An exemplary slot antenna  88  can typically have from about four elements to some forty or more depending on the directivity required.  
         [0037]     Spacing the elements of a slot coaxial line antenna  86  uniformly at approximately one wavelength intervals  94  along the coaxial line  90  produces a beam arraying effect which reinforces the signals to be vertically centered near the midpoint of the slot array  88 , if fed from one end  96  and terminated at the other end  98 . Such an antenna  86  may depend for its performance on matching between the slot-to-slot spacing  94  and the center frequency of the signal for which the antenna  86  is to be used. By spacing the elements uniformly closer, for a bottom-fed antenna  86 , the antenna pattern can be tilted downward. Similarly, spacing the elements uniformly further apart, for a bottom fed antenna  86 , produces an antenna pattern tilted upward. Of course, upward and downward are relative terms, depending on the positioning of the feed in the antenna  86 .  
         [0038]     For broadcasting purposes, due to propagation delay from the first slot to the last slot, an end-fed slot coaxial line antenna  86  may to some extent reduce the time precision with which a signal can be detected by a receiver, but nonetheless produce acceptable results for such applications as high-quality video and audio reception. For higher data density communications or other applications where increased time precision of a received broadcast signal is desirable, an array design that reduces the time delay from the first to the last radiator may be preferable. Such a design may comprise a multiplicity of slotted shorter coaxial sections driven in parallel from a splitter or from a center-driven coax, or may be of another style according to design preference.  
         [0039]     The exemplary antennas described herein may require a matched end load termination  96  on each coaxial line  90 , such as that shown in  FIG. 8 . Since the exemplary couplers  60  shown in  FIG. 6  represent a number of impedance lumps when used, for example, in the exemplary embodiment of  FIG. 8 , it may be necessary to adjust the characteristics of the load termination  96  to prevent reflections. A shorted or open termination may be possible for some designs.  
         [0040]     Although the exemplary embodiments are shown using a planar, oblique slot  62  and a noncylindrical coupler  60  with at least one planar face  78 , such as shown in  FIG. 6 , it will be appreciated that other implementation strategies can be used for the various exemplary embodiments described herein. Also, although the exemplary embodiment increases coupling efficiency when used in a slotted-coax low UHF television broadcast antenna, it can also be used in other frequency bands and for other communications and radiative purposes. For example, smaller coaxes, supporting higher frequency bands at reduced power levels, can be used for business communications. Devices including the features of the invention can likewise be used for heating in industrial processes, for RF excitation of particles, and for other non-communications-oriented purposes.  
         [0041]     It should be appreciated that, while the various exemplary embodiments describe an oblique slot design for use with coaxial line, it is evident that the concept can be applied to waveguide systems or non-coaxial line systems as well.  
         [0042]     The many features and advantages of the invention are apparent from the detailed specification, and, thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, that fall within the scope of the invention.