Abstract:
A tuneable E-plane waveguide filter is presented. Tuning is achieved using sliders inserted into the cavities of the insert of the waveguide filter. The sliders are inserted through gaps or notches in the insert, or through notches in the waveguide housing. The positions of the sliders is adjusted to fine-tune the frequency response of the waveguide filter, overcoming limits on narrow relative bandpasses imposed by manufacturing tolerances. When a desired frequency response is achieved, the sliders are fixed in position. Assembly and tuning is less expensive and less complex than tuneable H-plane waveguide filters.

Description:
FIELD OF THE INVENTION 
     This invention relates to waveguide filters, and more particularly to tuneable E-plane waveguide filters. 
     BACKGROUND OF THE INVENTION 
     E-plane waveguide filters consist of a waveguide, formed by two halves of a rectangular parallelepiped housing, and an insert. The insert is a relatively thin sheet of electrically conductive material, typically copper, of uniform thickness and etched or stamped with patterns. The insert is placed between the two halves of the housing so that when the waveguide is assembled the insert lies along the longitudinal axis of the waveguide and is oriented in a plane parallel to the short dimension of the cross-section of the waveguide. The patterns in the insert consist of spacings, or cavities, separated by remaining portions of the insert called fins, all of which run the full interior height of the waveguide. The cavities have resonant frequencies defined by their geometry and the fins have inverting properties defined by their geometry. The frequency response of the filter depends on the lengths of the cavities and fins, on the thickness of the insert, and on the dimensions of the waveguide housing. 
     Manufacturing tolerances on the waveguide housing and on the etching or stamping of the insert place limits on the precision of the filter dimensions, and existing E-plane waveguide filters are unable to provide the precise frequency response needed for narrow bandwidth filters. One solution is to improve the manufacturing process for creating the waveguide housing and the insert in order to improve the precision in the dimensions of the waveguide filter. However this is expensive for the precision needed for narrow bandwidths. Another solution is to fine-tune the filter after manufacture to achieve the desired frequency response from the filter. H-plane filters can be tuned after manufacture, but these are more expensive than E-plane filters due to the more complex assembly required. Furthermore, the tuning of H-plane filters is complex, requiring the adjustment of many tuning screws. There is a need for tuneable E-plane waveguide filters, as these would be less expensive than H-plane filters yet would allow narrower bandwidth filters. 
     SUMMARY OF THE INVENTION 
     The present invention provides a waveguide filter comprising an electrically conductive waveguide housing containing a longitudinally extending rectangular channel having spaced sides, the housing being constructed of at least two housing portions assembled together, and at least one electrically conductive relatively thin planar insert extending along and spaced from the sides of the waveguide channel. The upper and lower edges of the insert are sandwiched between two of the housing portions. The insert has at least one cavity located between the upper and lower edges of the insert and situated in the waveguide channel. A recess is provided in the insert, extending from one of the upper and lower edges of the insert into the cavity, and a tuning slider of electrically conductive material is received in the recess and extends into the cavity a distance determined by the desired frequency response of the waveguide filter. The presence of the slider alters the resonant frequency of the cavity, thereby changing the frequency response of the filter. The thickness of each slider and the approximate distance each is to extend into a cavity in the insert is determined analytically. Once inserted, the position of each slider is finely adjusted until the measured frequency response is as desired, and the sliders are then fixed in position. 
     The recess in a preferred embodiment passes through the entire thickness of the insert to form a gap. It is noted that because the tuning technique is effected by modifying the insert, it is not necessary to modify or alter the housing portions, thus permitting the use of a universal housing for a range of filter designs. 
     In an alternative embodiment, instead of providing a recess in the insert, a notch is provided in a wall of a housing portion and tuning can be achieved by receiving the slider in the notch. This solution is less preferable because it requires modification of the standard housing. 
     This construction of waveguide filter allows very precise frequency response curves to be obtained, overcoming the limits imposed by manufacturing tolerances, without the complexity and cost of an H-plane waveguide filter. Furthermore, a particular waveguide filter can later be tuned to a slightly different frequency response by adjusting the sliders. 
     Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in greater detail with reference to the accompanying diagrams, in which: 
     FIG. 1 is an exploded perspective view of an E-plane waveguide filter of the invention; 
     FIG. 2 is a lateral view of the E-plane waveguide filter of the invention with one half of the waveguide housing removed; 
     FIG. 3 is an end view of the E-plane waveguide filter of the invention; 
     FIG. 4 is an exploded perspective view of an alternate embodiment of the E-plane waveguide filter of the invention; and 
     FIG. 5 is an end view of yet another alternate embodiment of the E-plane waveguide filter of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows the E-plane waveguide filter of the invention. The waveguide housing consists of two halves  10  of a parallelepiped made of an electrically conductive material having a low coefficient of thermal expansion. When assembled, the waveguide will have a longitudinal direction  4  along which an electromagnetic wave is transmitted, a transverse direction  6  lying along the long dimension of a cross-section of the waveguide, and a vertical direction  8  lying along the short dimension of a cross-section of the waveguide. The two halves  10  of the waveguide housing are substantially identical, having a C-shaped cross-section as shown in FIG.  3 . Each half  10  is defined by an outer vertical wall  42  and two spaced transverse walls  44 . The transverse walls  44  end in mating surfaces  48  (in FIG. 1) adapted to mate with the mating surfaces  48  of the other half  10 . However when the waveguide filter is assembled, the mating surfaces  48  of each half  10  will be separated by an insert  12 , which is sandwiched between the mating surfaces  48 . Two doweling holes  19  are provided and extend transversely through the mating surfaces  48  and part way through the transverse walls  44 . The C-shape of the two halves  10  together form a waveguide channel  46 . 
     The insert  12  is formed of a sheet of electrically conductive, easily etched or stamped material, such as copper. The insert  12  is substantially planar and is relatively thin, having two large surfaces, and a transverse dimension, or thickness, significantly less than the longitudinal edges and the vertical edges of the insert  12 , and the longitudinal edges being longer than the vertical edges. A number of cavities  14  are etched or stamped into the insert  12  so as to lie between an upper and a lower edge of the insert  12 . The cavities  14  are separated by remaining portions of the insert, called fins  16 . The number and location of the cavities  14  and fins  16  will depend on the desired frequency response of the filter, and can be determined analytically using well known techniques. The cavities  14  extend the full height of the waveguide channel  46 , such that the upper surface  20  of each cavity  14  lies flush with the upper surface  22  of the waveguide channel  46 , and the lower surface  24  of each cavity  14  lies flush with the lower surface  26  of the waveguide channel  46 . 
     The insert  12  has gaps  28  above or below one or more of the cavities, into each of which a slider  30  can be inserted along the plane of the insert during assembly. It can be seen that each gap  28  extends from the cavity to the upper (or lower) edge of the insert. The sliders  30  are made of a highly conductive, easily etched or stamped material, such as copper. The thickness of each slider  30  is determined from the desired frequency response of the waveguide filter using well known analytic techniques. Two doweling holes  17  pass transversely through the insert  12 . The choice of whether a particular gap  28  will lie in the upper or lower edge of the insert  12  will depend on the positions of the doweling holes  17 . 
     The two doweling holes  17  in the insert  12  are aligned with the doweling holes  19  in the housing halves  10 . When assembled, dowels  18  pass from the doweling holes  19  in one half of the housing  10 , through the doweling holes  17  in the insert  12 , and into the doweling holes  19  of the other half of the housing  10 . The insert  12  is held in vertical and longitudinal position by the dowels  18 , and is held in transverse position by being sandwiched between the mating surfaces  48  of the two halves of the housing  10 . The two halves of the housing  10  are held in position using fasteners (not shown). The fasteners may be, for example, screws passing transversely through the transverse walls  44  and the insert  12 , or may be clamps situated outside the waveguide housing  10 . 
     FIG. 2 is a transverse view of the waveguide filter when assembled, with one half of the waveguide housing removed to expose details of the filter. The sliders  30  extend into the cavities  14  and alter the resonant frequency of each cavity  14 , thereby altering the frequency response of the waveguide filter. The approximate depth  32  that each slider  30  extends into a cavity  14  is determined from the desired frequency response of the waveguide filter using well known analytic techniques. The depth  32  is finely adjusted, either manually or using mechanical means, until the measured frequency response of the waveguide filter matches the desired frequency response. The slider  30  is then fastened in position, using for example glue. If the sliders  30  are thin enough to enter the gap without significant friction against the mating surfaces  48  when the waveguide housing  10  has been fastened together, then the sliders can be inserted, adjusted and fastened in position after the waveguide housing  10  has been fastened. If the sliders are thick enough to encounter significant friction against the mating surfaces  48  when the waveguide housing  10  has been fastened together, then the waveguide housing  10  is not fastened completely until the sliders have been inserted, adjusted and fastened in position. 
     FIG. 4 shows an alternate embodiment of the E-plane waveguide filter of the invention. In this embodiment there are no gaps in the insert  12 . Rather notches  40  are located in one of the halves  10  of the waveguide housing. The notches  40  are recessed into the mating surface  48  of either half  10  of the waveguide housing, and extend from an outer surface  52  of the waveguide housing to an inner surface  54  of the waveguide housing. The choice of whether a particular gap  40  will lie in the upper or lower transverse wall  44  will depend on the positions of the doweling holes  19 . When a slider  30  is inserted into a notch  40 , the slider  30  lies in a plane parallel to and adjacent to the insert  12 . In this embodiment, the method of tuning the filter is the same as in the embodiment shown in FIG.  1 . The approximate depth that each slider  30  extends into the waveguide channel  46  is determined from the desired frequency response of the waveguide filter using well known analytic techniques. The depth is finely adjusted until the measured frequency response of the waveguide filter matches the desired frequency response. The slider  30  is then fastened in position, using for example glue. A disadvantage of this embodiment over the first embodiment, however, is that universal housings cannot be used since the location of the notches  40  will depend on the desired frequency response. 
     In the preferred embodiment of the invention, the walls  42  of the waveguide channel  46  to which the insert  12  is parallel are shorter than the walls  44  of the waveguide channel  46  to which the insert  12  is perpendicular. The converse could be the case, but the filter would then only function for unconventional propagation modes of the electromagnetic signal. 
     The embodiment shown results in an optimal Q-factor for the filter. Variations resulting in a reduced Q-factor are nevertheless also included in the scope of the invention. The insert  12  could also lie offset from the longitudinal axis of the waveguide housing  10 . The cavities  14  need not reach the full height of the waveguide channel  46 . The longitudinal dimension, or width, of a slider need not be substantially equal to that of the gaps  28  or notches  40 . 
     Other alternative embodiments are also included within the scope of the invention. The cavities  14  need not be rectangular as shown in the preferred embodiments. The gaps  28  or the notches  40  need not be vertical, but could extend from the upper or lower edge of the insert  12  to the cavity  14  and intersect the cavity  14  at an angle other than 90 degrees. Instead of complete gaps  28  in the insert  12 , recesses or notches which do not pass through the entire plane of the insert  12  could be used to receive the respective sliders. However these embodiments may complicate the design of the filter. 
     A gap may be thought of as a specific form of recess which passes through the insert, and thus the term “recess” is used hereinafter to denote either a recess (notch) which does not pass completely through the insert thickness or a recess (gap) which does pass completely through the insert thickness. 
     As a further alternative, more than one insert could be used. In such an embodiment, an end view of which is shown in FIG. 5, the inserts  12  are placed parallel to each other and separated by housing spacers  50  aligned with the transverse walls  44 . The housing spacers  50  are held in place by the dowels  18 . Sliders  30  can be inserted either through gaps  28  in the inserts  12 , as in FIG. 1, through notches  40  in the transverse walls  44 , as in FIG. 4, or through notches in the housing spacers  50 . 
     As yet a further alternative, the dowels  18  can be replaced by pins projecting contiguously from one of the housing halves  10  rather than being separate pieces inserted into doweling holes in both halves of the housing. 
     The outer form of the waveguide housing need not be exactly as depicted. In particular, flanges or flange holes could be provided at the longitudinal ends of the housing to allow the waveguide filter to be fastened to other components in a communication path. 
     What has been described is merely illustrative of the application of the principles of the invention. Other arrangements and methods can be implemented by those skilled in the art without departing from the spirit and scope of the present invention. For example, other methods of fastening the housing and the insert can be implemented, as long as they do not interfere in the placement of the sliders.