Abstract:
A quasi-lumped resonator apparatus that makes use of an inductive portion having a plurality of spines extending therefrom along at least a portion of a length thereof, and a capacitive portion electrically and physically coupled to an end of the inductive portion. The capacitive portion has a plurality of spaced apart capacitive fringe plates extending therefrom. A housing is included for enclosing the inductive and capacitive portions. In another aspect a method is disclosed for forming a quasi-lumped resonator.

Description:
FIELD 
       [0001]    The present disclosure relates to resonators, and more particularly to an electromagnetic wave resonator structure and method of forming same that is highly resistant to multipactor breakdown under high power applications. 
       BACKGROUND 
       [0002]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0003]    Currently available coaxial resonators have significant difficulty sustaining operation under high power without the use of high risk and costly break down preclusion techniques. At present, a coaxial resonator often needs to be either pressurized with a gas or be DC biased to avoid breaking down, due to the multipactor phenomenon, when high power is applied to it. The multipactor phenomenon is a secondary electron resonance phenomenon that involves a recurrent RF breakdown of the resonator. More specifically, the recurrent RF breakdown involves the emission of secondary electrons that are stripped off the capacitive portion of the resonator structure, thus rendering the resonator useless, and potentially destroying the resonator. 
         [0004]    Typical coaxial resonators used in high power applications often use smooth surfaced cylindrical sections that form electromagnetic field lines of minimal curvature. Such resonators often need to be either gas pressurized or electrically DC biased to prevent them from breaking down under an application of high power. Pressure vessels or auxiliary DC biasing circuitry is therefore needed. Both of these conventional means add additional mass, equipment, complexity and cost to the resonator structure. The need to use a gas pressurized vessel can also inherently add risk to the resonator design and limit its lifetime. 
       SUMMARY 
       [0005]    In one aspect the present disclosure relates to a quasi-lumped, resonator apparatus. The apparatus may comprise: an inductive portion having a plurality of spines extending therefrom along at least a portion of a length thereof; a capacitive portion electrically and physically coupled to an end of the inductive portion, the capacitive portion having a plurality of spaced apart capacitive fringe plates extending therefrom; and a housing for enclosing the inductive and capacitive portions. 
         [0006]    In another aspect the present disclosure relates to a quasi-lumped, coaxially based resonator apparatus comprising: a tubular inductive portion having a plurality of spines extending radially therefrom along a major portion of a length thereof; a capacitive portion electrically and physically coupled to an end of the inductive portion, the capacitive portion having a plurality of spaced apart and radially extending capacitive fringe plates extending generally perpendicularly from the tubular inductive portion; and a housing for enclosing the inductive and capacitive portions. 
         [0007]    In still another aspect of the present disclosure a method is disclosed for forming a quasi-lumped resonator. The method may comprise: forming an inductive portion as a cylindrical component having a plurality of spines extending therefrom along at least a portion of a length thereof; electrically and mechanically coupling a capacitive portion having a plurality of spaced apart capacitive fringe plates extending radially therefrom, to an end of the inductive portion; and enclosing the inductive and capacitive portions in a housing. 
         [0008]    Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
           [0010]      FIG. 1  is a side view of a quasi-lumped, coaxially based resonator apparatus in accordance with an embodiment of the present disclosure, with a housing thereof shown in dashed lines so that the interior components are visible: 
           [0011]      FIG. 1A  is a partial cross-sectional view of the apparatus of  FIG. 1  showing the tuning element and its coupling to the upper housing portion in greater detail; 
           [0012]      FIG. 2  is a perspective view of an inductive portion of the apparatus; 
           [0013]      FIG. 3  is a top perspective view of the capacitive portion of the apparatus; 
           [0014]      FIG. 4  is a bottom perspective view of the capacitive portion, where it is to be attached to the inductive portion; 
           [0015]      FIG. 4A  is a view of another shape that may be used for the outer portion of the capacitive fringe plates; 
           [0016]      FIG. 5  is a cross sectional view of the apparatus of  FIG. 1  coupled to a quasi transverse electromagnetic (TEM) microstrip line and illustrating the curving electromagnetic fringe field lines radiating from the capacitive fringe plates; 
           [0017]      FIG. 6  is a perspective illustration of a pair of the apparatuses of the present disclosure being used to form a second order filter; and 
           [0018]      FIG. 7  is a flowchart of operations that may be used to form the apparatus. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
         [0020]    Referring to  FIG. 1  there is shown a quasi-lumped, coaxially based resonator apparatus  10  in accordance with one embodiment of the present disclosure. The apparatus  10  includes a housing  12  within which is housed an inductive portion  14  and a capacitive portion  16 . The housing  12  may be a hollow metal structure having a sidewall portion  12   a  and an integrally formed bottom wall portion  19 , with the bottom wall portion  19  having an opening  19   a . An upper end of the sidewall portion  12   a  may be fixedly secured to an upper housing portion  18  either via suitable mechanical fasteners or via adhesives or by any suitable method of attachment. The upper housing portion  18  has a generally half toroidal shape having a central opening  18   a  for a tuning element, which will be discussed in the following paragraphs. The upper housing portion  18  may be secured to the housing sidewall  12   a  once the inductive portion  14  and capacitive portion  16  are fully assembled and positioned within the housing  12 . Alternatively, housing portions  12  and  19  may be formed as separate components and fastened together using suitable fastening elements or adhesives. 
         [0021]    The inductive portion  14  may include a tubular main body portion  20  having a conical base  22  for mounting and as an area for electrical connections. The conical base  22  is positioned over the opening  19   a  in the bottom wall  19 . The conical base  22  may also have a flange or like structure that permits it, and thus the housing  12 , to be fixedly secured to another support surface. 
         [0022]    The tubular main body portion  20  may include a plurality of spines  24  that extend from the main body portion  20  radially outwardly from an axial center  26  of the main body portion  20 . The capacitive portion  16  is electrically and mechanically coupled, such as by a friction fit or suitable fasteners or adhesives, to an upper portion  28  of the main body portion  20 . A cylindrical tuning element  30  having a central opening  34  is positioned over a boss portion  32  formed in the upper housing portion  18  and secured against an inside surface of the upper housing portion  18  adjacent to, but not in contact with, the capacitive portion  16 . The tuning element  30  enables the resonant frequency of the apparatus  10  to be fine tuned. An opening  36  may be formed in the housing  12  to enable electromagnetic coupling of the resonant frequency electromagnetic wave energy produced by the apparatus  10  to an external component, for example a microstrip or coaxial transmission line. 
         [0023]    Referring to  FIG. 2  the inductive portion  14  is shown in greater detail. The main body portion  20  may include a hollow tubular wall portion  38  that forms a bore  40 . However, the tubular main body portion  20  could just as readily be formed as a solid component. Incorporating the bore  40 , however, will provide a degree of weight savings. The spines  24  may extend along a major portion of the length of the wall portion  38 , and preferably may extend along the full length of the tubular wall portion  38 . The spines  24  may vary in shape and dimension to suit specific applications. The spacing of the spines  24 , as well as the overall number employed, may also be varied to enable the inductive portion  14  to be designed for a specific application. However, in one embodiment preferably about 20-30 spines  24  are formed to extend radially from the tubular wall portion  38 . The inductive portion  14  may be formed as a single piece component from metal or as a multipiece component. The main body portion  20  may be coated with silver plating to reduce loss. The overall length of the main body portion  20  will be selected to meet the needs of a specific application, but in one example it may be about four to six inches (102 mm-152 mm) in length when the apparatus  10  is being used in a UHF application to form a passband filter centered at a frequency of 250 MHz. The overall diameter of the main body portion  20  may also vary as needed to tune the apparatus  10  for a specific application. But when used to form a passband filter with a center frequency about 250 MHz, the diameter will typically be between about four to eight inches (101 mm-203 mm), depending on absolute loss requirements. 
         [0024]    Referring to  FIGS. 3 and 4 , the capacitive portion  16  is shown in greater detail. The capacitive portion  16  may be formed as a single piece component from a single piece of metal, or alternatively as a multipiece component. The capacitive portion  16  may form a generally toroidal shape that fits partially within the generally toroidal shape of the upper portion  18  of the housing  16  when the apparatus  10  is fully assembled. The capacitive portion  16  may include a central support portion  42  having a flange  44  and a coaxially aligned coupling sleeve  46 . The flange  44  has an axial center  48  at which a threaded hole  50  may be formed. Extending radially outwardly from the axial center  48  of the flange  44  is a plurality of spaced apart capacitive fringe plates  52 . The fringe plates  52  may each have a generally circular outer portion  54  and a notched inner portion  56 . The notched inner portion  56  enables a recess  58  to be formed at which the capacitive portion  16  may be attached to the inductive portion  14  with at least one threaded fastener  43 . It is important to note that fastener  43  needs to be non-metallic or that some form of insulating sleeve needs to be used so that no metallic (i.e., conductive) connection is formed by the use of the fastener  43  between the inductive portion  14  and the capacitive portion  16 . Such metallic contact would short out the apparatus  10 . 
         [0025]    Referring briefly to  FIG. 1A , the tuning element  30  may be fastened to the upper housing portion  18  at its axial center via a threaded stud  32 ′ that extends through a threaded boss portion  32  of the upper housing portion  18 , and held on the boss by a threaded nut  32 ″. The threaded boss  32  receives the threaded stud  32 ′ and the tuning element  30  is secured via a separate non-metallic nut or suitable fastener  32 ″ over one end of the threaded shaft  32 ′. Thus, the tuning element  30  is held adjacent to the capacitive portion  16  but not in contact with the capacitive portion  16 . Thus, the tuning element  30  does not touch the capacitive portion  16  once the apparatus  10  is fully assembled, nor while the tuning element  30  is being adjusted. The tuning element  30  may be silver plated, or otherwise metallic or made from a suitable dielectric material. An external frame-like device (not shown) that is coupled to the threaded stud  32 ′ may be used to urge the tuning element  30  either downwardly or upwardly in accordance with arrow  33  to change the longitudinal position of the tuning element  30  relative to the capacitive portion  16 . The entire capacitive portion  16  may be coated with a silver plating or copper plating to reduce losses. The capacitive portion  16  may also be formed from a plurality of independent component parts or as a single piece component from a single piece of metal. 
         [0026]    The radius of curvature of the circular outer portion  54  of each fringe plate  52 , and thus the collective area of the fringe plates  52 , may vary as needed to fine tune the apparatus  10  for specific applications. However, if the apparatus  10  is used as a filter in the UHF band, it is expected that the radius of each fringe plate  52  may typically be between about 25%-35% of the overall radius of the capacitive portion  16 . The thickness of each fringe plate  52  may also vary widely to meet the needs of specific applications, but in one example may be between 0.01 inch-0.06 inch (0.254 mm-1.524 mm). But again, these factors may vary considerably depending on the specific application and resonant frequency which the apparatus  10  is designed to operate at. 
         [0027]    While the capacitive fringe plates  52  are shown as having the generally circular outer portion  54 , this shape could also be tailored to meet the needs of a specific application. For example,  FIG. 4A  shows a generally tear drop shaped outer portion  54   a  for the capacitive fringe plate  52   a . Such a shape for the fringe plates would provide capacitive portion  16  with the shape of an asymmetric toroid. The tear drop shape may be added to further increase the power handling of the resonator apparatus  10  in extreme cases. It may also be useful as a design trade off. Its use may also enable the designer to replace part of the length of the inductive portion  14  with a smooth surface. In that case, inductive portion  14  would have spines over less than 100% of its length with a simple cylindrical surface on a portion of its length. 
         [0028]    Important advantages of the apparatus  10  are the construction, and particularly the shape, of the capacitive portion  16 , as well as the spines  24  on the inductive portion  14 . These features enable high curvature fringing electromagnetic fields having a high gradient to be formed that are much less susceptible to multipactor breakdown under high intensity electromagnetic fields. The use of the capacitive fringe plates  52  and the spines  24  enables the total surface area of the capacitive portion  16 , and the total surface area of the inductive portion  14 , to both be maintained but limits the amount of total surface area from which electron stripping can occur with the apparatus  10 . More specifically, the use of the fringe plates  52  provides an increased surface area via fringe fields to obtain the desired sufficient total capacitance, but since the increased surface area is not provided by a simple flat surface area, the proclivity for increased electron stripping is significantly reduced or eliminated. The high curvature fringing electromagnetic fields are shown in simplified form in  FIG. 5  and denoted by reference numeral  60 .  FIG. 5  also shows an exemplary microstrip transmission line  62  magnetically coupled to the apparatus  10  with the conductive element  64  of the microstrip line extending within the housing  12 . 
         [0029]    The apparatus  10 , due to its high curvature and high gradient electromagnetic fringing fields, does not require a pressurized vessel or auxiliary DC biasing circuitry to resist the occurrence of the multipactor phenomenon. Such components have often been required by previously developed, conventional coaxial transverse electromagnetic (TEM) resonators. This also enables the apparatus  10  to be constructed with less cost, less weight and less complexity than previously developed coaxial resonators. Eliminating the need to use a pressurized vessel also eliminates any risk of explosion in operating the resonator  10  that would otherwise be present when using a pressurized vessel to house the inductive and capacitive subsections. The lack of a pressurized vessel also eliminates the risk that the vessel will leak over time, which can cause a critical pressure to be reached within the vessel during operation that in turn produces corona breakdown within the device. When this happens, the device can be destroyed by the internal plasma breakdown within it. This risk is completely eliminated with the apparatus  10  because of its non-pressurized housing  12 . 
         [0030]      FIG. 6  illustrate an embodiment  100  in which two of the apparatuses  10 ′ and  10 ″ are used to form a second order filter. The housing  12 ′ of the lumped resonator  10 ′ is interfaced to the housing  12 ″ of the resonator  10 ″ by a passageway  102  that enables electromagnetic wave energy from resonator  10 ′ to radiate from one housing to the other. It will be appreciated that even higher order filters may just as easily be constructed. 
         [0031]    Referring to  FIG. 7 , a method  200  is shown for constructing the apparatus  10 . At operation  202  the inductive portion  14  is formed, for example from a single piece of metal. At operation  204  the capacitive portion  16  is formed, for example from a single piece of metal. At operation  206  the housing  12  is formed. At operation  208  the tuning element  30  is formed. At operation  210  the inductive portion  14  is secured to the capacitive portion  16 . At operation  212  the tuning element  30  is fastened to the upper housing portion  18 . At operation  214  the conical base  22  is secured over the opening  19   a  in the bottom wall  19  of the housing  12 . At operation  216  the conical base  22  may be bolted or otherwise secured to a support surface. At operation  218  the upper housing portion  18  is secured to the sidewall  12   a  of the housing  12 . At operation  220  the threaded shaft  32 ′ is used to adjust the longitudinal position of the tuning element  30 , and thus tune the apparatus  10 . 
         [0032]    While various embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the present disclosure. The examples illustrate the various embodiments and are not intended to limit the present disclosure. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.