Abstract:
Apparatus and method for filtering an electromagnetic signal employs the feeding of the signal via a rectangular waveguide to a plural mode cavity filter with coupling available at plural modes. The cavity has a mode-coupling structure such as a mode-coupling screw. The waveguide operates at a first mode of lower cutoff frequency and a second mode of higher cutoff frequency. The feeding is at a frequency for propagation in the first mode. The cavity radiates back into the waveguide, or into another waveguide, below the higher cutoff frequency to provide an evanescent reactive loading which reduces the required penetration of tuning screws for reduced dissipation of power in the tuning screws.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to a filter for an electromagnetic signal and, more particularly, to a construction of a microwave filter with reduced penetration of tuning screws for a corresponding reduction in heat dissipated within the tuning screws.  
           [0002]    Microwave filters are used widely in communication systems, as well as in other applications. A form of microwave filter, of considerable interest herein, is constructed of a right cylindrical outer electrically-conductive wall terminated by opposed electrically-conductive end walls that define a cylindrical cavity, the cavity having tuning screws extending inwardly through the cylindrical wall. Slots in the end walls are used for coupling electromagnetic signals to and from the filter for connection with a further filter stage or to an external waveguide.  
           [0003]    In the use of such filters for the transmission of electromagnetic signals at high power, such as in a transmitter for a satellite-based communication system, a situation arises in which the tuning screws become heated due to the dissipation of electromagnetic power within the tuning screws. In cases of very high electric field strength, arcing may be observed between a screw and the filter wall. Such arcing becomes more pronounced with increased heating of the tuning screws. Also, the energy dissipated in the heating of the screws would be more beneficially employed for the transmission of the electromagnetic signal. Therefore, the dissipation of heat within the tuning screws presents a disadvantage in the operation of the filter.  
         SUMMARY OF THE INVENTION  
         [0004]    The aforementioned disadvantage is overcome and other benefits are provided by a construction of the microwave filter, and a procedure for use of the filter in a manner, in accordance with the invention, in which there is a reduction in the length of penetration of each of the tuning screws in the cylindrical wall of a cavity of the filter. This provides for the dual benefits of inhibition of arcing and the reduction of heat dissipation within a tuning screw of the filter cavity.  
           [0005]    In a preferred embodiment of the invention, reduction in the length of penetration of a tuning screw into the cavity is accomplished by employing a further tuning element, in addition to the tuning screws, namely, a section of waveguide operated in a mode below the transmission cutoff frequency wherein coupling of electromagnetic signals from the filter cavity into the waveguide is by an evanescent mode. In the evanescent mode, no power is propagated in the waveguide, but a reactive loading is produced upon the filter cavity, which loading aids in the tuning of the filter. In the case of a filter having a cylindrical cavity, coupling of electromagnetic power between the cavity and the waveguide is accomplished by an iris in an end wall of the cavity. In a dual mode filter cavity, there are two tuning screws and a mode coupling screw. The waveguide loads the filter reactively to effect the tuning of the filter in conjunction with the tuning operations of the tuning screws. In the resulting filter, the reactive loading enables the tuning operation of both tuning screws of a dual-mode cavity to be accomplished with a reduction in the length of penetration of the tuning screws into the cavity. This reduces heat dissipation in both of the tuning screws. In the case of a filter cavity with two waveguides coupled to opposite end walls of the cavity, both of the waveguides may provide the reactive loading.  
           [0006]    In the preferred embodiment, the filter is a dual mode filter. The cavity is provided with first and second tuning screws positioned perpendicularly in a transverse plane of the cavity for tuning, respectively, first and second modes of the dual modes of electromagnetic waves within the cavity. A third screw is positioned at 45 degrees relative to the first two screws for a coupling of electromagnetic power between the two modes. A cross-shaped iris is employed for coupling electromagnetic power through the end wall between the cavity and the waveguide. One leg of the cross is aligned with the first screw and the second leg of the cross is aligned with the second screw for coupling the respective modes of electromagnetic waves between the cavity and the waveguide.  
           [0007]    The waveguide has a rectangular cross section composed of two opposed broad walls and two opposed narrow walls. The widths of each broad wall is greater than the width of each narrow wall to provide different transmission cutoff frequencies for a first mode in which the transverse electric field component is parallel to the narrow walls and a second mode in which the transverse electric field component is parallel to the broad walls. For example, in the case of a waveguide having a ratio of 2×1 in the widths of broad wall relative to narrow wall, the first of the foregoing modes has a cutoff frequency essentially one-half that of the second mode. A signal at the first waveguide mode is inputted to the filter cavity at a frequency greater than the cutoff frequency of the first waveguide mode, but less than the cutoff frequency of the second waveguide mode. The mode-coupling screw enables generations of the second mode within the filter cavity, which second mode is coupled by the iris into the waveguide. However, the second-mode frequency is below the second-mode cutoff frequency, and propagates in the waveguide only in an evanescent mode to produce a reactive loading upon the cavity. Thus, electromagnetic power travels within the waveguide in only the first mode, while the second mode aids in the tuning of the cavity of the filter. As a result, the tuning screws are set for a reduced depth of penetration into the cavity. The tuning can be facilitated further, in accordance with a further feature of the invention, by the provision of additional screws diametrically opposite the foregoing three screws. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0008]    The aforementioned aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawing figures wherein:  
         [0009]    [0009]FIG. 1 shows a side view of a cavity filter with attached waveguide assembly, constructed in accordance with the invention, with connection to a communication system indicated diagrammatically;  
         [0010]    [0010]FIG. 2 shows a view of the cavity filter with attached waveguide assembly taken along the line  2 - 2  in FIG. 1;  
         [0011]    [0011]FIG. 3 is an exploded view of the cavity filter with attached waveguide assembly;  
         [0012]    [0012]FIG. 4 is a graph of frequencies for explaining operation of a waveguide of FIG. 1;  
         [0013]    [0013]FIG. 5 shows a cavity filter of an assembly of plural cavities for use in the practice of the invention;  
         [0014]    [0014]FIG. 6 shows diagrammatically a construction of cavity filter with plural tuning and mode-coupling screws in conjunction with corresponding balancing screws;  
         [0015]    [0015]FIG. 7 shows an elongated cavity with an array of tuning and balancing screws;  
         [0016]    [0016]FIG. 8 shows an alternative arrangement of waveguide assembly connected to the cavity filter for practice of the invention; and  
         [0017]    [0017]FIG. 9 is a flow chart of steps in the practice of the invention. 
     
    
       [0018]    Identically labeled elements appearing in different ones of the figures refer to the same element but may not be referenced in the description for all figures.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    FIGS.  1 - 3  show a microwave filter  10  constructed in accordance with the invention, and comprising a cavity  12  electromagnetically coupled to a first waveguide  14  and to a second waveguide  16 . The cavity  12  is defined by an outer cylindrical wall  18  terminated by end walls  20  and  22 , each of the walls  18 ,  20  and  22  being fabricated of an electrically conductive material such as aluminum or copper. The waveguides  14  and  16  are also constructed of an electrically conductive material such as aluminum or copper. Each of the waveguides  14  and  16  includes two broad walls  24  and  26  interconnected by two narrow walls  28  and  30 . Irises  32  and  34  are located in the centers respectively of each of the end walls  20  and  22  of the cavity  12 . The waveguides  14  and  16  are joined respectively to the end walls  20  and  22  of the cavity  12 , and are arranged coaxially with an axis  36  of the cavity  12  for alignment with the respective irises  32 ,  34 . The irises  32  and  34  serve for coupling, inductively, electromagnetic signals between the cavity  12  and the waveguides  14  and  16  respectively.  
         [0020]    Each of the waveguides  14  and  16  has, in cross section, a rectangular configuration wherein the width of a broad wall is greater, typically by a factor of 2, than the width of a narrow wall. Each iris  32 ,  34  has the configuration of a crossed slot with one of the arms being parallel to the narrow walls of the waveguides  14 ,  16 , and the other of the arms being parallel to the broad walls of the waveguides  14 ,  16 . The cavity  12  is a dual-mode cavity with two tuning screws  38  and  40  aligned with respective ones of the arms of each of the irises  32 ,  34  for tuning respective orthogonal modes of electromagnetic waves within the cavity  12 . The tuning screws  38  and  40  are located in the cylindrical wall  18  in a plane transverse to the cylindrical axis  36 , and project inwardly by amounts which are adjustable for tuning the respective modes. Also provided in the cavity  12  is a mode-coupling screw  42  for coupling electromagnetic energy between the two orthogonal modes. A diameter of the cavity  12  is greater than or equal to a diagonal of either one of the waveguides  14  and  16 . The arm of the coupling iris  32 ,  34  which is perpendicular to the wide dimension of either one of the waveguides couples to the waveguide mode below cutoff, the evanescent mode, while the arm on the coupling iris  32 ,  34  which is parallel to the wide dimension of either one of the waveguides couples to a propagating mode within the waveguide.  
         [0021]    The filter  10  may be employed in a communication system  44 , as depicted in FIG. 1, wherein a signal source  46  establishes an electromagnetic signal in a transverse electric mode TE 10  within the first waveguide  14 . The electromagnetic signal is filtered in the cavity  12 , and is then outputted by the cavity  12  to the second waveguide  16  in the TE 10  mode. The electromagnetic signal then travels from the second waveguide  16  by a communication channel  48  to a receiver  50 .  
         [0022]    With reference also to FIG. 4, the frequency axis shows the low transmission frequency cutoff value for a transverse electric wave TE 10  with electric field parallel to the narrow wall of a waveguide  14 ,  16 , and the high transmission frequency cutoff value for a transverse electric wave TE 01  with electric field parallel to the broad wall of a waveguide  14 ,  16 . The frequency of the signal of the source  46  has a value between the values of the two cutoff frequencies, namely, above the low-frequency cutoff but below the high frequency cutoff. Therefore, electric power at the source frequency can propagate as a TE 10  mode but not as a TE 01  mode. The TE 01  mode can exist only as an evanescent mode.  
         [0023]    In operation of the filter  10 , the signal from the first waveguide  14  is coupled by the iris  32  into the cavity  12  to produce a first mode of vibration of electromagnetic wave within the cavity  12 . By operation of the mode coupling screw  42 , a second and orthogonal mode of vibration of electromagnetic wave is also established within the cavity  12 . The mode-coupling screw  42  serves to couple energy between the two modes so that filtering is accomplished at both of the modes to provide for a filter spectral response having twice the number of poles as for a cavity operating at only a single mode. A filtered wave is outputted by the iris  34  into the second waveguide  16  for communication as a filtered signal to the receiver  50 .  
         [0024]    By virtue of the crossed slot configuration of each of the irises  32  and  34 , electromagnetic energy is radiated into the waveguides  14  and  16  from the electromagnetic wave at the second mode in the cavity  12 . The second mode radiation of the cavity  12  induces the TE 01  evanescent mode in the waveguides  14  and  16 . The evanescent mode does not withdraw power from the cavity  12  but simply presents a reactive load to the tuning operation of the cavity  12 . The result of the reactive loading is that both of the mode tuning screws  38  and  40  are effective to tune their respective modes with reduced amounts of penetration of the tuning screws into the cavity  12 . Therefore, there is less power dissipated in a tuning screw from interaction of the electric field of a respective one of the waveguide modes with the tuning screw. The electric field, E, of the evanescent mode decays rapidly with increasing distance from the cavity and walls  20  and  22 , as shown in the graph  52  appended to the waveguide  16  in FIG. 1. The section of each of the waveguides  14  and  16  in which the significant decay of the electric field occurs is shown at  54  for each of the waveguides. Therefore, the cavity  12  in conjunction with the waveguide sections  54  and the crossed-slot irises  32  and  34  constitute the filter  10  which provides for the reduced dissipation of electromagnetic energy in the tuning screws.  
         [0025]    [0025]FIG. 5 shows a filter cavity assembly  56  composed of a plurality of cavities, two cavities  58  and  60  being shown by way of example. The cavities  58  and  60  are separated by an iris plate  62  having a crossed-slot iris  64 . The iris plate  62  serves as a divider wall between the two cavities  58  and  60 , and the iris  64  provides for coupling of electromagnetic energy at both of the orthogonal modes between the two cavities  58  and  60 . The cavity assembly  56  terminates in the same end walls  20  and  22  as has been described above cavity  12  (FIG. 1) and may be substituted for the cavity  12  in an alternative embodiment for the construction of the filter  10 . Each of the two cavities  58  and  60  is provided with its own set of tuning and mode-coupling screws  38 ,  40  and  42 .  
         [0026]    [0026]FIG. 6 shows a cavity  12 A which is a further embodiment of the cavity  12  wherein additional tuning screws  38 A and  40 A (balancing screws) are positioned diametrically opposite the tuning screws  38  and  40  to facilitate turning and also, if desired, an additional mode-coupling screw  42 A (balancing screw) is positioned opposite the mode-coupling screw  42  to facilitate coupling of power between waves at the two orthogonal modes. In the case of the use of a longer cavity, supporting a higher order mode in a direction along a cylindrical axis of the cavity, namely for TE 11x  modes in the cavity, a series of the balancing screws  38 A can be placed along the length of the cavity opposite the tuning screws  38 , in a longitudinal axial plane, as is depicted diagrammatically for a cavity  12 B in FIG. 7. Similarly, a series of the balancing screws  42 A can be placed along the length of the cavity opposite the mode-coupling screws  42 , in a longitudinal axial plane, as is depicted in FIG. 7. The balancing screws facilitate tuning and enhance a balancing between the two modes.  
         [0027]    [0027]FIG. 8 shows a further connection of the cavity  12  between an input waveguide  66  and an output waveguide  68  by means of a circulator  70 . The circulator  70  has three ports, a first of which connects with the input waveguide  66 , a second of which connects via the waveguide  14  to the cavity  12 , and the third of which connects with the output waveguide  68 . In the arrangement of FIG. 8, the cavity end wall  20  is provided with the iris  32  (not shown in FIG. 8, but shown in FIG. 3) while the end wall  22  is provided without its iris. Thus, an input signal in the waveguide  66 , as may be provided by the source  46  (FIG. 1), is coupled by the circulator  70  into the waveguide  14  and the filter  12  and, after filtering by the filter  12  which reflects the filtered signal back into the circulator  70 , is coupled further by the circulator  70  into the output waveguide  68  for transmission, by way of example, to the receiver  50  (FIG. 1).  
         [0028]    [0028]FIG. 9 is a flowchart showing steps in the procedure for carrying forth the operation of the filter  10  in the communication system  44  of FIG. 1. In FIG. 9, the procedure begins at block  72  wherein the signal source  46  excites the transverse electric mode in the first waveguide  14 . Then, at block  74 , after reception of the microwave energy from the transverse electric wave by the cavity  12 , there is a coupling of energy from a first of the two orthogonal wave modes to the second of the two orthogonal wave modes by use of the mode-coupling screw  42 . There follows, at block  76 , a generation of the evanescent modes in the waveguides  14  and  16  by a coupling of radiant energy from the cavity  12  via the irises  32  and  34  into the waveguides  14  and  16 . At block  78 , the tuning screws  38  and  40  are adjusted in the presence of the reactive loading produced by the evanescent modes. Thereupon, at block  80 , the filtered signal is outputted to the receiver  50  via the transverse electric wave in the waveguide  16 .  
         [0029]    By way of alternative embodiments, it is noted that the invention can be practiced by connection of the cavity (FIG. 1) or assembly of cavities (FIG. 5) to various arrangements of the waveguides. For example, it is possible to radiate the evanescent mode into only the second waveguide  16  by use of the crossed-slot configuration of coupling aperture in the second end wall  22  while employing only a linear single mode slot (not shown) in the first end wall  20 , in which case the first waveguide  14  could have a square cross section. The second waveguide  16  would still function as disclosed in FIG. 1 for extracting the filtered signal from the cavity  12  and for providing the reactive loading of the evanescent mode. Alternatively, such an arrangement of waveguides could be employed, by way of example, with the cavity  12  in conjunction with the circulator of FIG. 8 in which the second waveguide (not shown in FIG. 8) could be connected to the second end wall, as shown in FIG. 1, to serve the sole function of providing the reactive loading of the evanescent mode to the cavity  12 , while the first waveguide  14  is used for applying the input signal to the cavity and for extracting the output filtered signal from the cavity. In a more general sense, the invention requires only that there be at least one waveguide connected to at least one cavity of the cavity assembly to produce the reactive loading of the evanescent mode.  
         [0030]    It is to be understood that the above-described embodiments of the invention are illustrative only, and that modifications thereof may occur to those skilled in the art. Accordingly, this invention is not to be regarded as limited to the embodiments disclosed herein, but is to be limited only as defined by the appended claims.