Abstract:
A continuous transverse stub element array structure which forms a coaxial geometry formed from a plurality of cylindrical segments, wherein each of the cylindrical segments has a rim at a top end and a bottom end, wherein each rim extends transversely away from the cylindrical segment relative to a longitudinal axis thereof to thereby form a stub element, wherein the individual cylindrical transverse stub elements are aligned end-to-end to thereby form a coaxial cable structure which surrounds a central core material. The series of these stubs form reactive or radiating elements for microwave, millimeter-wave, and quasi-optical filters and antennas. Purely reactive elements are formed by leaving the conductive coating on the terminus of the stub elements, whereas radiating elements are formed when stub elements of moderate radius are opened to free space. For tunability, each of the plurality of cylindrical segments and the central axis materials are coated with a conductive material which is ferro-electrical or liquid crystal in nature. The individual stub elements are separated from each other by air gaps or an appropriate material.

Description:
BACKGROUND 
     1. The Field of the Invention 
     This invention relates generally to antennas, filters and transmission lines. More specifically, the invention relates to a new configuration of a continuous transverse stub element array structure which is formed in a coaxial arrangement to provide superior omnidirectional performance at microwave, millimeter, and quasi-optical wavelengths. 
     2. The State of the Art 
     The state of the art of antennas, antenna arrays, parallel plate waveguides, filters and couplers beginning at microwave frequencies is characterized, for example, by several types of more conventional designs of mono-poles or dipoles, slotted waveguide arrays, printed patch arrays, and reflector and lens systems as mobile or base station terminal antennas. These designs are used to build high gain antenna arrays, adaptive arrays, phased arrays and smart antennas. Furthermore, when moving into frequencies which are greater than 20 GHz and commonly known as millimeter wave and quasi-optical frequencies, antennas and filters suffer from relatively low Q factors due to high dissipative conductor and dielectric losses, interconnect losses, and from relatively difficult fabrication due to dimensional tolerances. Accordingly, antenna and filter designs have moved to more advantageous configurations which are known as continuous transverse stub element array structures which can be radiating (antenna) or reactive (filter). 
     The known configurations and methods of fabricating these continuous transverse stub element array structures are generally taught, for example, in U.S. Pat. Nos. 5,266,961, 5,412,394, 5,583,524, 5,604,505 and 5,771,567 to list but a few. In essence, these patents teach that a continuous transverse stub element residing in one or both conductive plates of a parallel plate waveguide is employed as a coupling, reactive, or radiating element in coupler, filter and antenna designs. These patents contributed to the parallel-plate continuous transverse array designs that are characterized by pencil beam patterns. 
     In the planar-type prior art, a typical continuous transverse stub element array structure will include a dielectric element forming a plane which has a second dielectric element which extends transversely to the plane to form a stub. A first conductive element is disposed coextensive with the dielectric element forming the plane, and a second conductive element is disposed along a surface of the second dielectric stub element. 
     FIG. 1 is provided as a close-up illustration of the prior art which shows a typical dielectric plane  10  and stub element  12 . FIG. 2 is provided to show a perspective view of an array of stub elements  14  in a planar array  16 . 
     It is taught in the prior art that purely-reactive stub elements are realized through conductively terminating (a short circuit) or by narrowing (an open circuit) the terminus of the stub element. Radiating elements are formed when stub elements of moderate height are opened to free space. Precise control of stub element coupling or excitation by way of coupling of parallel plate waveguide modes is accomplished through variation of longitudinal stub element length, stub element height, parallel plate separation, and the properties of the parallel plate and the stub element media. 
     The prior art is also characterized by teaching that continuous transverse stub elements can be arrayed to form planar apertures and structures of arbitrary area which are comprised of a linear array of continuous transverse stub elements fed by a conventional line-source or sources. 
     The prior art teaches that the transverse stub elements are varied by modifying their height, width, length, and cross section. Other cited variations include changing the number of stub elements, and adding additional structures to the basic stub element. However, these arrays structure fail to provide a coaxial arrangement for a filter or antenna array system, or any means of conveniently providing a signal feed thereto. 
     It would be an advantage over the prior art to provide a different continuous transverse stub element array design which can provide a high Q-factor filter and a low cost antenna array with two-dimensional beam steering capability. It would be another advantage to provide omnidirectional capabilities at microwave, millimeter wave and quasi-optical wavelengths which would permit construction of a small and mobile array design. It would be a further advantage to provide a large beam scan width in a new continuous transverse stub element array design which provided low loss and had improved impedance matching. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a new and low cost configuration for a continuous transverse stub element array structure for both antenna array and filter designs. 
     It is another object to provide the new continuous transverse stub element antenna array which is omnidirectional at millimeter frequencies. 
     It is another object to provide the new continuous transverse stub element array which can accomplish two-dimensional steering using ferro-electric or liquid crystal materials. 
     It is another object to provide the new continuous transverse stub element array which utilizes thin film technology for the ferro-electric or liquid crystal materials to obtain uniform distribution for improved steering characteristics. 
     It is another object to provide the new continuous transverse stub element array structure which can operate over a broad frequency band. 
     It is another object to provide the new continuous transverse stub element array structure which can be used for base stations or mobile units. 
     It is another object to provide the new continuous transverse stub element array structure which is characterized by low losses and superior impedance matching. 
     It is another object to provide the new continuous transverse stub element array structure in the form of a coaxial design which provides improved tolerance for manufacturing errors. 
     The preferred embodiment of the present invention is a continuous transverse stub element array structure which forms a coaxial geometry formed from a plurality of cylindrical segments, wherein each of the cylindrical segments has a rim at a top end and a bottom end, wherein each rim extends transversely away from the cylindrical segment relative to a longitudinal axis thereof to thereby form a stub element, wherein the individual cylindrical transverse stub elements are aligned end-to-end to thereby form a cable structure which surrounds a central axis material. A linked series of these stubs form reactive or radiating elements for microwave, millimeter-wave, and quasi-optical wavelength filters and antennas. Purely reactive elements are formed by leaving the conductive coating on the terminus of the stub elements, whereas radiating elements are formed when stub elements of moderate radius are opened to free space. Where tunability of the array structure is desired, each of the plurality of cylindrical segments and the central axis material are coated with a conductive material which is ferro-electric or liquid crystal in nature. The individual stub elements are separated from each other by air gaps or an appropriate material. 
     In a first aspect of the invention, the ferro-electric or liquid crystal material is applied to the dielectric material of the individual stub elements using thin-film coating technologies. 
     In a second aspect of the invention, the thickness of the ferro-electric or the liquid crystal material is modified depending upon the desired characteristics of the array which are a function of the thickness of the ferro-electric or liquid crystal material, and the applied voltages. 
     In a third aspect of the invention, precise control of element coupling or excitation (amplitude and phase) via coupling of coaxial waveguide modes is accomplished through variation of longitudinal stub length, dish stub radius, interior and exterior radius of the coaxial structure, and the properties of the coaxial structure and stub filling media. 
     In a fourth aspect of the invention, coaxial continuous transverse stub elements are arrayed to form a linear array of continuous transverse stub elements fed by a conventional coaxial source. 
     In a fifth aspect of the invention, conventional methods of filter or antenna array synthesis and analysis may be employed in either the frequency or spatial domains to thereby construct stub elements and arrays to meet substantially any application. 
     In a sixth aspect of the invention, the principles of the present invention are applicable to all horizontal omnidirectional arrays, phased arrays, adaptive arrays and smart antenna applications at microwave, millimeter-wave and quasi-optical wavelength frequencies. 
     In a seventh aspect of the invention, advantages in millimeter-wave and quasi-optical wavelength filter designs are realized due to the enhanced reproducibility and relative low-loss (high Q) of the continuous transverse stub elements as compared to stripline, microstrip elements. 
    
    
     These and other objects, features, advantages and alternative aspects of the present invention will become apparent to those skilled in the art from a consideration of the following detailed description taken in combination with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a close-up cross-sectional view of a single transverse stub element as taught in the prior art. 
     FIG. 2 is a perspective view of a continuous transverse stub element planar array as taught in the prior art. 
     FIG. 3 is a perspective view of the presently preferred embodiment which is made in accordance with the principles of the present invention. 
     FIG. 4 is a cross-sectional profile view of the coaxial continuous transverse stub element array structure embodiment shown in FIG.  3 . 
     FIG. 5 is an end view of the coaxial continuous transverse stub element array structure shown in FIG. 4 along line A—A. 
     FIG. 6 is a graph of the radiation pattern from a single coaxial continuous transverse stub element array structure. 
     FIG. 7 is a graph of the radiation patterns created by 18 elements of a coaxial continuous transverse stub element antenna array structure. 
     FIG. 8 is a perspective view of a coaxial continuous transverse stub element array structure which has a ferro-electrical material applied for steering purposes. 
     FIG. 9 is a graph of a radiation pattern for a 50 element coaxial continuous transverse stub element array structure which is unbiased. 
     FIG. 10 is a graph of a radiation pattern for a 50 element coaxial continuous transverse stub element array structure which is biased. 
     FIG. 11 is a graph of a voltage distribution across the 50 stub elements for the (biased?/unbiased?) configuration. 
     FIG. 12 is a graph of ohmic loss as a function of dielectric loading. 
     FIG. 13 is a graph of ohmic loss as a function of dielectric loading. 
     FIG. 14 is a cross-sectional profile view of a stub element with a corrugated core element. 
     FIG. 15 is a cross-sectional profile view of a stub element with a core element made from two conductive materials. 
     FIG. 16 is a cross-sectional profile view of a stub element with a step-wise segmented cylindrical segment and rims. 
     FIG. 17 is a cross-sectional profile view of a stub element with step-wise segmented rims and a step-wise segmented core element. 
     FIG. 18 is a cross-sectional profile view of two stub elements, where one element is radiating and the other is reactive. 
     FIG. 19 is a cross-sectional profile view of a stub element with tapering rims. 
     FIG. 20 is a cross-sectional profile view of a stub element with an inclined radiation surface. 
     FIG. 21 is a cross-sectional profile view of a stub element with an inclined radiation surface and tapering rims. 
     FIG. 22 is an end view of an oval rim. 
     FIG. 23 is an end view of a sectored rim. 
     FIG. 24 is an end view of a sectored rim. 
     FIG. 25 is an end view of a sectored rim. 
     FIG. 26 is an end view of a sectored rim. 
     FIG. 27 is a profile view of a coaxial continuous transverse stub element array which has a helical structure. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made to the drawings in which the various elements of the present invention will be given numerical designations and in which the invention will be discussed so as to enable one skilled in the art to make and use the invention. It is to be understood that the following description is only exemplary of the principles of the present invention, and should not be viewed as narrowing the claims which follow. 
     The present invention has application to both radiating and non-radiating (reactive) array structures. Accordingly, the array structures to be described hereinafter are capable of finding application in both antenna and filter technologies, respectively. The significant differences in structure of an antenna array and a filter array arise in the structure of the transverse stub elements. 
     The geometry of the presently preferred embodiment of the present invention is illustrated in FIG.  3 . FIG. 3 is a perspective view of two continuous transverse stub elements  12 ,  14 , and a core element  30  with which the stub elements are coaxial. This basic geometry, the coaxial relationship of the stub elements  12 ,  14 , gives rise to one of the most significant advantages of the preferred embodiment. Specifically, the array structure  10  of FIG. 3 is omnidirectional. The planar array structures described in the prior art all fail to achieve this function. 
     FIG. 4 is provided as an elevational cut-way view of the continuous transverse stub element array of FIG.  3 . Specifically, FIG. 4 illustrates that a single transverse stub element  20  is formed at the junction of the rims  22  of two cylindrical segments  24 . The middle cylindrical segment is therefore an integral component of a first transverse stub element  12  and a second transverse stub element  14 . Each cylindrical segment  24  is generally in the form of a spool, having a rim  22  at each end which is generally transverse to a longitudinal axis  28  of the cylindrical segment. 
     To be in a true coaxial relationship as that term should be understood in this document, a cylindrical segment  24  does not only share the longitudinal axis  28  with a core element  30 , but it is also electrically isolated therefrom. 
     As a practical matter, some material  36  must fill the space  32  between an inner surface  34  of the cylindrical segment  24 . Such a material  36  is known to those skilled in the art of continuous transverse stub elements. For example, the filler material  36  can be selected from such materials as TEFLON™, low density foam, rexolite, polyethylene, stycast, lexan and other materials which have similar electrical and mechanical properties. The filler material  36  provides basic structural integrity to the array structure  10 . 
     The array structures  10  of FIGS. 3 and 4 are shown in this embodiment as not having a filler material directly between the rims  22  of the cylindrical segments  24 . This is only one configuration of the array structure  10 . A filler material can be provided to fill the gap  38  between the rims  22 . This filler material is typically comprised of the same materials which are used as the filler material  36 , with the exception of air. 
     A final step in obtaining the desired array structure  10  is the application of a uniformly conductive coating on the exterior surfaces (exactly which surfaces? Please point to all surfaces in FIG. 4) of the cylindrical segment  24 . The uniformly conductive coating is typically a thin film of an appropriately conducting material, as is known to those skilled in the art of continuous transverse stub elements. 
     FIG. 5 is provided as an end view of the array structure  10  shown in FIG. 4 along points A—A. Note that in the presently preferred embodiment, the rim  22  is generally circular, as is the core element  30 . 
     Fabrication of the coaxial array structure shown in FIG. 4 is possible through machining, extruding or molding. It should also be remembered that the material used for the cylindrical segments  24  is a dielectric material, as is known to those skilled in the art. After the shape of the dielectric cylindrical segments  24  is completed, the segments are coated or plated with the uniform conductive coating. 
     The coaxial structure of the array is advantageously forgiving. In other words, the design has good tolerance for manufacturing errors. 
     Finally, if the coaxial array structure  10  is to be used in an antenna array, the ends  40  of the transverse stub elements  20  need to be ground or machined in order to create radiating elements when there is a filler material  36  in the gap  38 , because the conductive coating will otherwise electrically connect the rims  22 . 
     FIG. 3 also includes a number of dimensions which are provided for illustration purposes only and must not be considered limiting. These dimensions can be varied to suit the needs to the application to which the present invention is applied. Nevertheless, dimension  60  is 0.25 mm, dimension  62  is 0.575 mm, dimension  64  is 6 mm, dimension  66  is 3.5 mm, and dimension  68  is 5 mm. 
     FIG. 6 is provided to illustrate the radiation pattern of a single transverse stub element of the coaxial array structure shown in FIGS. 3,  4  and  5  when used in a radiating application. 
     FIG. 7 is provided to illustrate the radiation pattern of 18 stub elements of a continuous transverse stub element coaxial array. 
     Omnidirectionality of the preferred embodiment shown in FIGS. 3,  4  and  5  is only one of the advantages of the present invention over the prior art. There are other advantageous features of the coaxial geometry, and of materials applied to the array structure to obtain improved performance, as will be explained. 
     It is first observed that without any modifications, the preferred embodiment shown in FIG. 3 is capable of achieving improved base station and mobile performance. The array structure is advantageously utilized as a base station antenna array because it is omnidirectional. Furthermore, the small size of the coaxial antenna array structure  10  makes it easy to move. The design is not only low cost, but it also operates in the microwave, millimeter and quasi-optical frequency ranges. In other words, the antenna array structure at least operates in frequency ranges of 20 Ghz to 60 Ghz. Another advantage of the coaxial antenna array structure is the fact of good impedance matching because of the coaxial design. Typically, an impedance of 50 ohms can be achieved, making it simple to provide a coaxial feed for signal transmission and reception when the array structure is used as an antenna. 
     The advantages of the coaxial array structure describe above are significant. However, there are other advantageous structures which can be easily obtained from the basic design. Specifically, it was discovered that the present invention is also capable of two dimensional steering. This makes the coaxial array structure ideal for such applications as satellite communications which typically operate in millimeter wave frequencies. 
     The present invention has achieved a beam scan capability of −60° to +60° with low loss (a high Q factor). The significant two dimensional scanning abilities of the present invention were obtained in an alternative embodiment shown in FIG.  8 . FIG. 8 is identical to FIG. 3, with the addition of a ferro-electrical material  50  to the core element  30 . 
     As a practical matter, the ferro-electrical material  50  is applied as a thin film using an appropriate thin film application technology, such as spraying. It is envisioned that more than one layer of the ferro-electric material can be applied to create multiple layers. 
     As known to those skilled in the art, steering is achieved by applying a voltage to the ferro-electric material  50 . However, the amount of voltage to apply to achieve the desired scanning and steering capabilities varies greatly depending upon the geometry and the nature of the selected ferro-electric material. 
     In the present invention, one such ferro-electric material which demonstrates promising results is Barium Strontium Titanium Oxide (BSTO). BSTO advantageously changes its dielectric properties with an applied bias voltage, thereby achieving the desired tunability and scanning capabilities. It is also noted that BSTO can also function as a filler material in a waveguide feed section of the coaxial array structure. 
     The thickness of BSTO results in changes in the required bias voltage. This can affect the tunability of the array structure, where tunability is a function of an unbiased dielectric constant and the biased dielectric constant as shown in Equation 1.        Equation    1:                                                                                                             Tunability   =         ɛ   1     -     ɛ   2         ɛ   1                              
     It has been observed that practical values of applied biasing voltage have been between 0.7 to        3.0                     V   μm     .                            
     Furthermore, another desired characteristic of dielectric loading when using BSTO is that it has a low loss tangent. 
     The selection of BSTO as a useful ferro-electric material for scanning has also required experimentation in order to obtain the desired tunability characteristics. Table 1 is provided as a brief example of some of the experimental results which show the advantageous characteristics of BSTO having 40% oxide. 
     Table 2 is provided as a means of illustrating dielectric and conductor losses. The table is only illustrative of some of the results that were obtained using BSTO 40% oxide, unbiased and biased. 
     An important consideration when designing a coaxial array structure of the present invention is the tradeoff between thickness of the ferro-electric material and conductor losses in decibels. Experimentation has shown that the present invention provides good performance at lowε and relatively large thickness of the substrate, but a drop off in performance, as expected, with highε and a thin substrate which is better for two dimensional beam steering. 
     FIGS. 9,  10  and  11  are provided as illustrations of a radiation pattern for a 50 element coaxial continuous transverse stub element array using BSTO 40% oxide III as the ferro-electric material, where FIG. 9 is unbiased, and FIG. 10 is biased. FIG. 11 illustrates the voltage distribution across the stubs for the (biased?/unbiased?) scenario. 
     Some of the tradeoffs to consider when designing a coaxial continuous transverse stub element array antenna include the increased losses at lower substrate (BSTO) thicknesses, the higher biasing voltage required for thicker substrates, and the concern for multimode propagation at large heights. These considerations are illustrated in FIGS. 12 and 13. 
     With the change in dielectric properties of the ferro-electric material, waveguide impedance also changes. With the contribution of reflected voltage, the phase difference between radiating elements changes in an inconsistent manner. The result is a deterioration in a radiation pattern. 
     It has been discovered that one method for overcoming this difficulty is to change the waveguide dimensions to thereby achieve continued impedance matching, and therefore controlled distance between radiating elements. This method requires that the biasing voltage be variable. 
     If an average value of the impedance is implemented in the design, deterioration of the radiation pattern may be overcome by using larger waveguide impedance values. As a practical matter, a greater number of elements may be required to obtain practical values of gain. 
     In an alternative embodiment of the present invention, the ferro-electrical material used for scanning purposes can be replaced by a liquid crystal material. The liquid crystal advantageously is less lossy than the ferro-electric materials described above. In addition, the liquid crystal material is less expensive. As a tradeoff, tunability or steering of a beam is not as good as with BSTO 40% oxide III. 
     The preferred and alternate embodiments of the present invention described above have uniform circular rims on a cylindrical segment body. However, there are variations to the geometry of the stub elements which can affect the performance of the present invention. The figures which follow are provide so as to illustrate some of the various geometries. In essence, the geometry of both the core element and the cylindrical segments of the stub elements can be modified. For example, the modifications include, but should not be considered limited to, changes in the length and diameter of the cylindrical segments. 
     Other modifications are shown in the following figures. Before describing the figures, it should be remembered that all of these modifications can be combined with each other. 
     FIG. 14 is a cut-away view of a single coaxial transverse stub element  70  with a radiating structure. The core element  72  is corrugated, which is also referred to as a slow-wave structure. 
     FIG. 15 is a cut-away view of a single coaxial transverse stub element  74  with a radiating structure. The core element is now comprised of multiple dielectric materials  76  and  78 . FIGS. 14 and 15 describe embodiments in which minimal weight, complex frequency dependence, or precise phase velocity control is required. 
     FIG. 16 is a cut-away view of a single coaxial transverse stub element  80  which shows a step-wise segmented cylindrical segment  82 , and step-wise segmented rims  84 . The step-wise segmentation can go inwards or outwards. This configuration is useful for modifying coupling and/or to broaden frequency bandwidth characteristics. 
     FIG. 17 is a cut-away view of a single coaxial transverse stub element  86  which includes a step-wise segmented core element  88 . 
     FIG. 18 is a cut-away view of a two coaxial transverse stub elements  90 ,  92 , where a first stub element  90  is larger and radiating, and where the second stub element is smaller and reactive  92 . Obviously, the size of the radiating and reactive stub elements can be reversed, and could also include the features of tapering and step-wise segmentation. The pair of radiating  90  and reactive  92  stub elements is illustrative of a matched couplet. This pairing is useful for suppression of coupler-radiated reflections through destructive interference of their individual reflection contributions. Matched couplets are useful in antenna arrays where it is desired to scan the beam at or through a broadside. 
     FIG. 19 is a cut-away view of a single coaxial transverse stub element  94  which includes rims  96  which are tapered. The tapering can occur in either direction, with one or both being tapered. Alternatively, it should now be obvious that step-wise segmentation can be included in the tapering of the cylindrical segment  98  and in the rims  96 . The tapering is able to broaden frequency bandwidth characteristics. 
     FIG. 20 is a cut-away view of a single coaxial transverse stub element  100  which includes the feature of an inclined radiation surface  102  created by rims  104  having different diameters. Adjusting the degree of inclination of the radiation surface  102  results in a change in the radiation pattern of the stub element  100 . Specifically, the peak pattern can be controlled so as to point in a desired axial direction. 
     FIG. 21 is a cut-away view of a single coaxial transverse stub element  106  which combines tapering of the rims  108  with different diameters, thereby creating the inclined radiation surface  110 . The result is the ability to modify the bandwidth and the pattern in the axial direction. 
     FIG. 22 is an end view of a rim  112  of a stub element, where the rim is no longer circular in the plane of the stub element, but is instead oval. Alternative embodiments include making the shape any desired polygon. Changing the shape of the rim  112  enables modification of the radiation pattern in the azimuth. 
     FIGS. 23,  24 ,  25  and  26  are all various illustrations of sectorial rims  114  of a single coaxial transverse stub element  116 . The number of sectors can be modified in order to change phase distribution of the stub element  116 . 
     FIG. 27 is a perspective view of a continuous transverse stub element array structure  120  which forms a helical winding  122  about the core element  124 . In other words, there are only two stub elements  126 ,  128 , but they travel along the entire length of the array structure  120 , winding around the core element  124 . 
     In essence, the preferred and alternative embodiments of the present invention provide a continuous transverse stub element structure which is applicable to both antenna and filter applications, depending on whether the stub elements are radiating or reactive. The array structure provides omnidirectional capability, two dimensional steering capability, gain control, beam shape control, low cost, a phased array antenna design, millimeter wavelength capabilities, small size relative to other designs, and low loss, high Q factor filter designs. 
     It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention. The appended claims are intended to cover such modifications and arrangements.