Abstract:
A filter includes an electromagnetic wave supporting structure having a cavity defined by an encircling sidewall, and microwave feeds coupled to the cavity for inputting and for outputting electromagnetic power to and from the wave supporting structure. At least a portion of an interior surface of the sidewall has a succession of corrugations. Successive ones of the corrugations are spaced apart by a distance less than approximately 0.2 wavelength of the electromagnetic wave, and each of the corrugations has a height greater than the spacing distance but less than approximately 0.5 wavelength of the electromagnetic wave. The geometry of the corrugated sidewall reduces interaction between the electromagnetic wave and the sidewall to inhibit dissipation of power of the electromagnetic wave within the sidewall.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to construction of cavities for microwave filters, particularly dual mode cavity filters both with and without dielectric resonator loading, wherein improved performance in terms of reduced energy loss is obtained by a configuration of cavity wall with reduced interaction with electromagnetic waves within a cavity.  
           [0002]    A filter, typically incorporating plural filter stages, is often employed in a communication system such as a microwave system communicating via satellite. Such filter stages may be constructed of only an air dielectric, or may include also a ceramic dielectric resonator for reduction in an overall size of the filter. In addition, such a filter stage may include tuning screws extending an adjustable amount into the cavity of the filter stage for tuning the filter stage and, furthermore, may include a mode coupling screw for conversion between a single mode and a dual mode of operation of the filter stage. The use of a dual mode filter is advantageous in that the characteristics of a higher order filter function can be obtained within a reduced number of filter stages, thereby providing a saving in overall physical size of the filter.  
           [0003]    This is advantageous in the construction of a filter to have freedom of designing the filter with sharp skirts at the end of a filter pass band for optimal use of available spectrum for multiple communication channels. However, the sharpness of such skirts, as well as the implementation of other pass band characteristics which may require sharp resonance, are limited by propagation losses within the filter, a significant source of the lost power being the dissipation of microwave energy within the cavity walls of the filter. Therefore, improved performance of the filter, both in terms of the ability to attain a desired spectrum characteristic, as well as in its Q (quality factor), should be obtainable in a filter constructed in a manner which reduces the dissipation of microwave energy in the filter sidewalls. Existing filter construction does not provide for this feature.  
         SUMMARY OF THE INVENTION  
         [0004]    The aforementioned problems are overcome and other advantages are provided by a construction of the sidewall of a filter cavity, in accordance with the invention, wherein reduced interaction of electromagnetic waves with the cavity wall is obtained by introducing corrugations within the wall. Preferably, as viewed in a cross-sectional view of the wall, the corrugations have a rounded or sinuous form. The corrugations need be provided only on the inside of the sidewall. The distance between corrugations should be significantly less than the wavelength, preferably less than approximately 0.2 wavelength of the electromagnetic radiation resonant within the cavity. The height (or depth) of a corrugation is less than approximately 0.5 wavelength but is greater than the distance between the corrugations.  
           [0005]    In the theory of operation of the invention, the corrugations, with the cross-sectional dimensions substantially smaller than a wavelength, may be likened to an electrically conductive wall with small holes therein. The holes have cross-sectional dimensions substantially less than a wavelength. In such an electromagnetic structure, there is little penetration of electromagnetic energy through the holes with the result that an electromagnetic wave interacting with the wall interacts with a reduced surface region of the wall. By way of example of such interaction, a component of the magnetic vector parallel to the surface of the wall may induce a surface current in the wall resulting in a power loss proportional to the product of the current and resistance of the wall. The presence of numerous small holes in the wall reduces the amount of wall surface available for interaction with the electromagnetic wave, with a consequent reduction in the amount of power loss. In similar fashion, the presence of the corrugations reduces the amount of surface current and the power loss associated therewith. Performance of the cavity of a stage of the filter is improved by the use of the corrugations, the performance being characterized by reduced power loss and insignificant generation of higher order modes of the electromagnetic waves within the filter stage. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0006]    The aforementioned aspects and other features of the invention are explained in the following description, taken in connection with the accompanying drawing figures wherein:  
         [0007]    [0007]FIG. 1 shows a partially stylized view of a filter assembly having resonant cavities at least one of which has a corrugated sidewall constructed in accordance with the invention;  
         [0008]    [0008]FIG. 2 is a set of three graphs showing operation of a group-delay equalizer in FIG. 1 in compensating for the delay in signals propagating through a set of cavity filter stages of a bandpass filter of FIG. 1;  
         [0009]    [0009]FIG. 3 is a cross-sectional view of the group-delay equalizer taken along the line  3 - 3  FIG. 1, an arrangement of tuning screws within a cavity sidewall of FIG. 3 being representative of a corresponding arrangement of tuning screws in sidewalls of other cavities of FIG. 1 and of cavities in FIG. 4;  
         [0010]    [0010]FIG. 4 is a side view of a further embodiment of a bandpass filter comprising two circular cavities arranged in series between two rectangular waveguides serving as input and output ports of the filter, the cavities having corrugated sidewalls in accordance with the invention;  
         [0011]    [0011]FIG. 5 is a sectional view of an input waveguide taken along the line  5 - 5  of FIG. 4, and showing an iris coupling an input signal into a first of the cavities;  
         [0012]    [0012]FIG. 6 is a fragmentary axial view of a cavity of FIG. 4 showing corrugations and a tuning screw, the showing of the corrugations being representative also of corrugations in a cavity of FIG. 1;  
         [0013]    [0013]FIG. 7 is a fragmentary exploded view of a cavity of FIG. 4 showing connection of a circular iris plate to a flange of the cavity;  
         [0014]    [0014]FIG. 8 is a view similar to FIG. 7 but showing an alternative configuration of cavity and iris plate having a rectangular cross-section;  
         [0015]    [0015]FIG. 9 is a fragmentary view of cavity sidewall showing an embodiment wherein the thickness of a corrugation rib is greater than, or approximately equal to, a trough between two successive ribs of the corrugation; and  
         [0016]    [0016]FIG. 10 is a view similar to that of FIG. 9, but showing corrugation wherein the thickness of a rib is less than the width of a trough of the corrugations of the sidewall. 
     
    
       [0017]    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  
       [0018]    The invention applies to a filter comprising: a single cavity, as well as to multiple cavity filters, and applies also to a filter wherein the cavity is supporting only a single mode of electromagnetic wave as well as to a filter wherein the cavity is supporting plural modes of electromagnetic waves. In order to demonstrate the construction of the invention in a filter, or filter assembly, which is useful in a system for communication via satellite, FIGS.  1 - 3  show construction of a filter assembly  10  which, by way of example, comprises a bandpass filter  12  and a group-delay equalizer  14 . As will be described in further detail in the ensuing description, microwave cavities are employed in the construction of the filter  12  and the equalizer  14 , each of these cavities having the configuration of a right circular cylinder in a preferred embodiment of the invention, and wherein each of these cavities has an encircling sidewall which may include a corrugated inner surface for reduced interaction between an electromagnetic wave of the cavity and the encircling cavity wall.  
         [0019]    While benefits of the invention can be obtained by placing the corrugations in only one of the foregoing cavities, a maximum benefit of the invention is obtained by placing the corrugations in all of the cavities. In any one of the cavities, a maximum benefit is obtained by forming the corrugated surface along and entire sidewall, however, a reduced benefit is available even if only a portion of the sidewall is formed with the corrugated surface. Also, as will be described in the ensuing description, the cavities may be provided with dielectric resonators, and also are provided with tuning screws and mode coupling screws to demonstrate utilization of the invention in a practical communication system. It is understood that the invention may be employed also in a simple cavity in which no resonator and no tuning screw are present.  
         [0020]    As shown in FIG. 1, the bandpass filter  12  is constructed as an eight-pole filter; however, the filter may be constructed with some other number of poles such as six poles, if desired. An input microwave port  16  of the filter assembly  10  connects with the filter  12 , and an output microwave port  18  of the filter assembly  10  connects with the equalizer  14  via a circulator  20 . The circulator  20  is connected between the filter  12  and the equalizer  14  to couple electromagnetic signals between an output of the filter  12  and an input of the equalizer  14 . Directions of signal flow are indicated by arrows. The filter  12  and the equalizer  14  are each constructed with at least one cavity having tuning elements, such as tuning screws disposed therein. The four cavities shown in the filter  12  provide for the eight-pole characteristic, and only three cavities would be required to provide for a six-pole characteristic. A single cavity is shown in the equalizer  14 . The ceramic resonators shown in all of the cavities are useful in tuning the passband of the filter assembly  10  for the transmission of microwave signals, and also permit construction of the microwave cavities with a smaller physical size.  
         [0021]    With reference to each of the three graphs of FIG. 2, group delay is shown on the vertical axis, and frequency is shown on the horizontal axis. The bandpass filter  12  introduces a group delay to signals passing through the filter  12 , and the equalizer  14  tends to compensate for the group delay to overcome distortion in signal transmission associated with group delay. The graph on the left shows that the group delay of the bandpass filter  12  varies with frequency, and has maximum values of delay at both ends of the passband with a lesser amount of delay in the mid-band region. The middle graph shows that the equalizer introduces a group delay which varies with frequency such that a maximum amount of delay is introduced in the mid-band region, with lesser delay being introduced at both ends of the passband. The signal propagating through the filter assembly  10  (FIG. 1) experiences the contributions of the group delay of both the bandpass filter  12  and the equalizer  14 , this resulting in the sum of the delays as is portrayed in the graph at the right of FIG. 2. The graph at the right shows that the equalizer  14  is operative to flatten the central portion of the filter passband, which portion is employed in the transmission of the signal through the filter assembly  10 .  
         [0022]    A similar form of construction may be utilized for the cavities of the bandpass filter  12  and for the cavity of the equalizer  14 , the form of construction being demonstrated readily for the single cavity of the equalizer  14 . With reference to FIGS. 1 and 3, the group-delay equalizer  14  is constructed of a housing  22  of an electrically conductive material such as copper or aluminum, and having an exterior wall  24  enclosing a right circular cylindrical cavity  26 . A disk shaped ceramic resonator  28  is disposed along a central cylindrical axis  30  of the cavity  26 . End walls  32  of the equalizer  14  connect with the wall  24  and close off the cavity  26 , one of the end walls  32  serving to support and to locate the resonator  28  within the cavity  26 . For ease of reference, the wall  24  may be described (with reference to the orientation of the equalizer shown in FIG. 3) as having an outer surface composed of a top surface  34 , a bottom surface  36 , a right surface  38  and a left surface  40  which are joined together by inclined surfaces  42  and  44  respectively at the left and the right edges of the top surface  36 , and by inclined surfaces  46  and  48  respectively at the left and the right edges of the bottom surface  36 . The inclined surfaces are inclined at  45  degrees relative to the top and the bottom surfaces.  
         [0023]    Three tuning elements in the form of electrically conducting screws  52 ,  54 , and  56  are shown disposed in the exterior wall  24 , and being oriented with their respective axes intersecting the cylindrical axis  30 . The screw  52  is located in the top surface  34 , the screw  54  is located in the inclined surface  44 , and the screw  56  is located in the right surface  38 . By rotation of the screws  52 ,  54 , and  56 , respective ones of the screws can be advanced into the cavity  26  a desired amount for tuning the group-delay equalizer  14  for transmission of each of two orthogonal electromagnetic waves propagating within the equalizer  14 . The tuning screw  52  interacts with the electric field of a vertically polarized one of the waves, and the tuning screw  56  interacts with the electric field of a horizontally polarized one of the waves. The tuning screw  54  is oriented at  45  degrees relative to the screws  52  and  56  for interaction with both of the waves to serve as a mode coupling screw for coupling electromagnetic energy between the two waves.  
         [0024]    As shown in FIG. 1, in the equalizer  14 , the resonator  28  is held at a predetermined distance from one of the end walls  32  by a supporting structure, which may be referred to as a pedestal  58 , and is constructed of a cylindrical low-loss ceramic element secured by a suitable means, such as an adhesive, to an end surface of the resonator  28  and to the end wall  32 . The resonator  28  is constructed of a ceramic disk in the shape of a right-circular cylinder wherein the ratio of the diameter of the disk to the thickness of the disk is greater than  2 . The exterior wall  24  and the end wall  32  of the equalizer  14  may be fabricated of aluminum. The dielectric constant of the resonator  28  has a value typically in the range of  30 - 36 . Most of the energy of the field is located within the resonator  28 , and a relatively small amount of the energy is located within an evanescent mode within the cavity and outside of the resonator  28 . The presence of the resonator  28  within the cavity  26  allows for the construction of a much smaller cavity for resonating at the desired frequency as compared to the physical size of such a cavity in the absence of the resonator. By way of example, the cavity of an equalizer which is dielectrically loaded with the ceramic resonator, as is the case with the equalizer  14  of the invention, is approximately ⅓ to ¼ the size of an unloaded cavity.  
         [0025]    The foregoing construction of the equalizer  14  may be applied also in the construction of the bandpass filter  12 . Thus, the bandpass filter  12 , in the preferred embodiment of the invention, comprises four cavities  60  separated by end walls  62 , with two additional end walls  62  located at opposite ends of the assembly of the filter  12 , the assembly of the four cavities  60  being enclosed by an encircling cylindrical wall  64  which contacts the end walls  62 . To reduce the overall size of the bandpass filter  12 , it is advantageous to load the cavities  60  with dielectric resonators  66 , the resonators  66  being positioned by pedestals  68  relative to respective ones of the end walls  62 . The pedestals  68  are essentially transparent to the microwave radiation at the frequency of operation of the filter assembly  10 , such as at a frequency of 12 Ghz (gigahertz) employed in the preferred embodiment of the invention. Accordingly, it is convenient to couple the cavities  60  by means of irises  70  located in respective ones of the end walls  62  beneath individual ones of the pedestals  68 . The irises may be circular or cross shaped, by way of example, in accordance with the usual practice in construction of microwave filters.  
         [0026]    The number of the poles of the filter  12  can be doubled by generation of two orthogonal modes of wave propagation within the filter  12 . This can be accomplished by introducing the screws  52 ,  54  and  56 , described above with reference to the construction of the equalizer  14 , into the cavities  60  located at the ends of the filter  12 , some of these screws being indicated in FIG. 1. For example, in the top cavity  60  of the filter  12 , a screw  52  can interact with a microwave signal input by a probe  72  of the port  16  to generate a first mode of microwave propagation while interacting with screws  54  and  56  to introduce a second orthogonal mode of microwave propagation. Screws  52  and  56  in the other ones of the cavities  60  participate in the propagation of the two orthogonal modes. In the bottom cavity  60 , the screw  54 , mounted at the  45  degree angle position, enables coupling of microwave energy from both of the modes via the screw  52  to a probe  74  at a port  76  of the filter  12  for coupling, by a single mode, microwave signals between the filter  12  and the circulator  20 . A similar port  76 A with a probe  74 A are located on the wall  24  of the equalizer  14  for coupling electromagnetic power into and out of the cavity  26  of the equalizer  14 .  
         [0027]    With respect to the operation of the circulator  20  (FIG. 1), the circulator  20  is a three-port circulator, wherein a first port connects with the filter port  76  to receive a signal, indicated at arrow  78 , and a second port serves to output a signal, indicated at arrow  80 , to the equalizer  14 . The circulator  20  is constructed in well-known form such as a stripline structure with ferromagnetic material and an applied magnetic field to provide for a circulating propagation path indicated by arrow  82 . Signals reflecting back from the equalizer  14  via its port  76 A, indicated at arrow  84 , output the circulator  20  at port  18 . The equalizer cavity  26  resonates at the same frequency as does the bandpass filter  12 , and thus the reflection causes a time delay only for those frequencies which are contained in the passband of the filter  12 . Both the signals at arrows  80  and  84  are single mode signals, while the two orthogonal modes appear within the equalizer  14 .  
         [0028]    In accordance with the invention, the provision of a corrugated surface on the inner surface of the encircling wall  64  of a cavity  60  in the filter  12  is demonstrated in FIG. 1 for the case of the third cavity  60  wherein a thickened portion  86 , of the wall  64  is provided with a set of the grooves or troughs  88  separated by ribs  90 . The troughs  88  and the ribs  90  are parallel to the end walls  62  and perpendicular to a central cylindrical axis  92  of the filter  12 . The succession of troughs  88  and ribs  90  provide for a corrugation  94  of an inner sidewall surface of the cavity  60 .  
         [0029]    FIGS.  4 - 8  show a further embodiment of bandpass filter, indicated at  96 , wherein the filter  96  comprises two cavities  98  and  100  arranged in series and coupled electromagnetically via an iris  102  centrally located in an iris plate  104  (shown also in FIG. 7). As a further example in the construction of the invention, the cavities  98  and  100  are provided without the ceramic resonators of FIG. 1. In FIGS.  4 - 8 , the inner surface of each of the cavities  98  and  100  is bounded by a sidewall  106 . Each of the cavities  98  and  100  includes the aforementioned set of tuning screws  52 ,  54  and  56  disposed in the sidewall  106  of the cavity, the operation of the tuning screws  52 ,  54  and  56  in the filter  96  being the same as that disclosed above with reference to the filter assembly  10  of FIG. 1. The sidewall  106  in each of the cavities  98  and  100  has the configuration of a right circular cylinder, wherein the outer surface of the sidewall  106  is smooth and the inner surface of the sidewall  106  is corrugated with corrugation  94  (shown in FIGS. 6 and 7). The cylindrical cavities  98  and  100  are disposed about a common cylindrical axis  108 . In each of the cavities  98  and  100 , opposite ends of the sidewall  106  terminates in flanges  110  and  112  which enable connection of the sidewall  106  to other components of the filter  96 .  
         [0030]    An input rectangular cross-sectional waveguide  114  couples electromagnetic signals into the first cavity  98  via an iris  116  centrally located in an iris plate  118 , and an output rectangular cross-sectional waveguide  120  couples electromagnetic signals out of the second cavity  100  via an iris  122  centrally located in an iris plate  124 . Each of the input and the output waveguides  114  and  120  have flanges  126  to enable connection of the waveguides to other components of the filter  96 . Bolts  128  (shown in FIG. 5) pass through the flange  126  of the input waveguide  114  and through the iris plate  118  into the flange  110  of the first cavity  98  to secure the waveguide  114  and the iris plate  118  to the input end of the first cavity  98 . In similar fashion, bolts (not shown) are employed to secure the flange  112  at the output end of the second cavity  100  via the iris plate  124  to the flange  126  of the output waveguide  120 . Also, in similar fashion, bolts (not shown) are employed to secure the flange  112  of the first cavity  98  to the flange  110  of the second cavity  100  via the iris plate  104 . The irises  116  and  122  are each configured as horizontal slots, and are parallel to each other and to the tuning screws  56  in each of the cavities  98  and  100 . The iris  102  is configured as a crossed slot, wherein one portion of the slot is horizontal and parallel to the irises  116  and  122 , and the other portion of the crossed slot is vertical and parallel to the tuning screw  52  in each of the cavities  98  and  100 .  
         [0031]    By way of alternative embodiments of the invention, is noted that the circular cylindrical cross-sectional form of the cavity  98  depicted in FIG. 7 may be replaced with an elliptical cross-sectional form (not shown), or a rectangular cross-sectional form, as depicted for a cavity  98 A in FIG. 8. The cavity  98 A comprises a top broad wall  130  and a bottom broad wall  132 , which are joined together by a right sidewall  134  and a left sidewall  136 . FIG. 8 demonstrates a further option for construction of the corrugation wherein a corrugation  94 A is provided only on the two sidewalls  134  and  136  of the cavity  98 A while the broad walls  130  and  132  remains smooth. The flanges  110 A and  112 A of the cavity  98 A similarly have rectangular configuration for connection with iris plates, such as the iris plate  104 A which connects to the flange  112 A.  
         [0032]    With reference to FIG. 6, which shows corrugation  94  suitable for use in a cavity of the filter assembly  10  (FIG. 1) as well as in the cavity  98  (FIG. 7) and in the cavity  98 A (FIG. 8), the corrugation  94  may have different forms. By way of example, FIG. 6 depicts the corrugation  94  of FIG. 2, wherein tips  138  of the ribs  90  facing the axis  108  are rounded, as by a circular arc, and the sides of the troughs  88  are straight. The outer ends of the troughs  88 , distant from the axis  108 , may be flat, a shown in the sectional view of FIG. 6, or may be provided with a curvature (not shown). The corrugation may be formed either by a process of casting or machining. One of the ribs  90 , further identified at  140 , has a greater width than the width of the other ribs  90  in order to accommodate the width of a tuning screw such as the screw  52 .  
         [0033]    With reference to both FIGS. 1 and 4, the cavities are formed in a similar fashion. In FIG. 1, the end walls  62  in conjunction with the cylindrical wall  64  serve the function of defining the cavities  60  of the bandpass filter  12 . The centrally located ones of the end walls  62  include the aforementioned irises  70  and, in this sense, may be regarded as iris plates such as the iris plates  104 ,  118  and  124  of FIG. 4. In FIG. 4, the iris plates  118  and  104  serve as end walls of the cavity  98 , and the iris plates  104  and  124  serve as end walls of the cavity  100 . While corrugations appear on the sidewalls of the cavities, no corrugations appear on the surfaces of the end walls, particularly on the surfaces of the iris plates which serve as the end walls. The iris plates must be thin, in terms of a wavelength of the electromagnetic radiation, to properly propagate electromagnetic signals through the irises between adjacent ones of the cavities.  
         [0034]    It is noted that the corrugation  94 , whether located in a cavity of the filter assembly  10  or in a cavity of the bandpass filter  96 , must be rigid so as to have no effect on the tuning of the respective cavities such as might be produced by vibratory motion of a rib  90 , in the event that equipment containing the cavities is subjected to vibration. Such vibratory motion may have the effect of introducing a low level noise spectrum located about a main spectral line to which a cavity is tuned. The thickened portion  86  of the cylindrical wall  64  of the bandpass filter  12  serves as a rigid base for supporting the ribs  90  and inhibiting any movement among the ribs  90  relative to each other and relative to an end wall  62 . Also, the ribs  90  are constructed in solid form, as shown in FIGS. 6, 9 and  10  to provide a rigid interconnection with the thickened portion. In the embodiment of the filter  96  of FIGS.  4 - 8 , the sidewall  106  is constructed with sufficient rigidity to inhibit vibratory movement among the ribs relative to each other and to the iris plates. For example, the rigidity may be provided by sufficient wall thickness, indicated at T in FIG. 6.  
         [0035]    [0035]FIG. 9 shows a sidewall  106 A which is similar in construction to the sidewall  106  of FIG. 6, but differs therefrom in that, in the embodiment of FIG. 9, ribs  90 A have curved sides rather than the straight sides depicted in FIG. 6 for the ribs  90 . Similarly, the sidewall of a trough  88 A of FIG. 9 is curved. FIG. 9 depicts the situation wherein the width of a rib  90 A (indicated at A) is equal to or somewhat greater than the width of the trough  88 A (indicated at B). A sidewall  106 B depicted in FIG. 10 is similar in construction to the sidewall  106 A of FIG. 9 but differs therefrom in that the width of a rib  90 B (FIG. 10) of the wall  106 B is less than the width of a trough  88 B of the wall  106 B. The configuration of sidewall depicted in either FIG. 9 or FIG. 10 may be employed in waveguides configured with circular or rectangular configurations such as depicted in FIGS. 7 and 8.  
         [0036]    The depth of the trough  88  (indicated at C in FIG. 6) is greater than the spacing between ribs  90  (indicated at D). The distance D between the ribs  90 , should be significantly less than the wavelength of the electromagnetic radiation resonant within a cavity such as the cavity  98 , preferably less than approximately 0.2 wavelength of the electromagnetic radiation resonant in the cavity. The height (or depth) C of the trough  88  is less than approximately 0.5 wavelength but is greater than the distance D between the ribs.  
         [0037]    With respect to the theory of operation of the invention, FIG. 9 shows a graphical representation of the electromagnetic field which is shown to have an electric component (E) and a magnetic component (H), the latter being parallel to the wall  106 A. There is interaction of the magnetic component (H) with the region at the tip  138  of a rib  9 OA to produce surface current J. The surface current is produced only at the region of the tip  138  of which the surface is substantially parallel to the direction of the H vector. There is essentially no interaction of the surface of the steep slopes of the trough  88 A with the magnetic component (H). As a result of the production of the surface current, there is resistive loss associated with the flow of the electric current in the electrically conductive maternal of the ribs  90 A with a corresponding loss of power from the electromagnetic wave. Since only a relatively small portion of the corrugated sidewall  106 A interacts with the magnetic component of the electromagnetic wave, as compared to a much larger interaction region in the case of a flat wall or a corrugated wall having flat tops to the ribs, the corrugated sidewall of the invention provides for a more efficient transfer and reduced loss of electromagnetic power. The relatively small spacing D between the ribs of the corrugation enables the electromagnetic characteristics of the corrugated sidewall to approach that of a flat-surface wall with respect to the construction of a resonant chamber, thereby to preserve the mode of resonance within the cavity.  
         [0038]    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.