Abstract:
An impedance matching feed is disclosed for use in a ridge waveguide which allows a coaxial transmission line, generally having an impedance of fifty ohm, to be matched to a ridge waveguide of arbitrary impedance. The matching feed consist of a transformer which is located inside the ridge of the waveguide, a probe and a quarter wave choke.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to a ridge waveguide. More specifically, the present invention relates to a ridge waveguide resistive type feed with a matching transformer within the ridge of the waveguide which matches a standard coaxial transmission line to a ridge waveguide.  
         [0003]     2. Description of the Prior Art  
         [0004]     Typically, in a simple transition feed for a waveguide the probe does not touch the upper surface and may require additional elements for impedance matching. One such probe design that extends partially into the waveguide is illustrated in U.S. Pat. No. 5,867,073, to Sander Weinreb and Dean Bowyer which issued Feb. 2, 1999. Disclosed in U.S. Pat. No. 5,867,073 is a transition between a waveguide and a transmission line in which a probe portion of the transmission line extends into the waveguide to electrically field couple signals between the waveguide and transmission line. The transmission line is a coplaner fuse and includes a substrate having conductors disposed therein to prevent energy from propagating into the substrate from the waveguide. Since the probe is formed as an integral element of the transmission line, direct coupling of the waveguide&#39;s signals to the transmission line occurs.  
         [0005]     The probe heights of the type illustrated in U.S. Pat. No. 5,867,073 and in other simple probe transition feeds are generally dimensionally sensitive and often impractical in ridge waveguides when the space from the top of the ridge to the top or upper face of the waveguide is relatively small.  
         [0006]     Further, conventional probes are often shaped to successfully match the transmission line&#39;s impedance. Other prior well known art resistively matched transitions would require an external impedance matching network when the waveguide impedance differs from the coaxial transmission line impedance.  
         [0007]     Accordingly there is a need for a relatively compact, simple in design yet highly effective feed which does not require substantial probe shaping and/or an external matching network to impedance match the waveguide to a coaxial transmission line.  
       SUMMARY OF THE INVENTION  
       [0008]     The impedance matching feed comprising the present invention overcomes some of the difficulties of the past including those mentioned above in that it is a relatively simple in design, yet highly effective for matching the input transmission line impedance, which is generally fifty ohms, to the waveguide impedance. The impedance of the ridge waveguide is an arbitrary impedance, that is it will generally be different than the impedance of the coaxial transmission line.  
         [0009]     The impedance matching feed consist of a matching transformer located within the ridge of the waveguide. The feed matches a standard coaxial transmission line, which is generally fifty ohms, and does not require an external matching network. A probe extends, from the transformer, vertically upward within the waveguide&#39;s interior to the upper wall of the waveguide and is electrically connected to the waveguide. One end of the waveguide is terminated in a quarter wave choke, which is a short approximating λ g /4. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a cross sectional view of an impedance matching feed partially located in a ridge waveguide comprising one embodiment of the present invention;  
         [0011]      FIGS. 2   a  and  2   b  are electrical equivalent circuit diagrams for the impedance matching feed of  FIG. 1 ;  
         [0012]      FIG. 3  is a cross sectional view of an impedance matching feed comprising a second embodiment of the invention which has a tapered transformer; and  
         [0013]      FIG. 4  is a cross sectional view of an impedance matching feed comprising a third embodiment of the invention which has a stepped transformer. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]     Referring first to  FIG. 1 , there is shown a probe  10  which couples a coaxial transmission line  14 , which is generally a connector, to a hollow metallic waveguide  16 . As depicted in  FIG. 1 , coaxial transmission line  14  is mounted on the bottom surface of waveguide  16 . The waveguide  16  may also be a dielectric filled metallic waveguide.  
         [0015]     The waveguide  16  is formed of a hollow interior  18  with open ends to receive and deliver radio frequency signals. Waveguide  16 , which has a rectangular shape, includes an upper or top wall  20 , a lower or bottom wall  22  and a pair of side walls  24  and  26 . A ridge  28 , which is located at or near the center of the waveguide  16 , runs the length of waveguide  16 , and extends vertically upward from bottom or lower wall  22  of the waveguide  16 . One end of the waveguide  16  is terminated with a quarterwave choke, which is a short approximately λ g /4.  
         [0016]     A transformer  30  located within ridge  28  electrically connects the probe  10  to the coaxial transmission line  14 . Coaxial transmission line  14  typically has an impedance of fifty ohms. Coaxial transmission line  14  includes an inner conductor  32  which may be any electrically conductive material, a dielectric  34  which may be any well known dielectric material, and an outer conductor  35 .  
         [0017]     As shown in  FIG. 1 , the transformer  30  consist of a circular inner conductor  36  and a dielectric  38  which surrounds the conductor  36  and is shielded by the metallic ridge  28 . Probe  10  is a conductor which extends vertically upward from ridge  28  to the upper wall  20  of waveguide  16 . The upper end of probe  10  is electrically connected to the bottom surface  40  of upper wall  20 . The conductor  36  of transformer  30  and probe  10  may be fabricated from any well known electrical conductor. Probe  10  couples radio frequency electrical signals between the waveguide  16  and the transmission line  14 .  
         [0018]     Transformer  30  is shown in  FIG. 1  as being positioned above reference plane  42 - 42 . The coaxial transmission line  14  is connected to waveguide  16  below reference plane  42  as shown in  FIG. 1 . The diameter of transformer  30  is configured to provide an impedance match with the coaxial transmission line  14  at reference plane  42 - 42 .  
         [0019]     Referring now to  FIGS. 1, 2   a  and  2   b , an electrical equivalent circuit for the feed to the waveguide is depicted in  FIGS. 2   a  and  2   b . In  FIGS. 2   a  and  2   b , L 1  is the length for the shorted end of waveguide  16  and L 2  is the length for transformer  30 . Z 44-44  is the impedance looking into transformer  30  when transformer  30  is terminated with the characteristic impedance for the coaxial transmission line  14 . Z g  is the waveguide impedance. Z coax  is the impedance of coaxial transmission line  14  which is normally fifty ohms but Z coax  may have another value. Z t (L 2 ) is the impedance of the transformer  30  which can be variable as a function of transformer length, or Z t (L 2 ) can be a constant impedance.  
         [0020]     To obtain an impedance match with coaxial transmission line  14  at reference plane  42 - 42 , the reactances must be tuned out. The diameter of probe  10  may be shaped to tune reactances to a desired level, when needed. Shunt susceptance is made zero by terminating the waveguide with a quarterwave choke. A match occurs when Z 44-44  is the same as the waveguide impedance Z g . Since Z 44-44  is the impedance looking into transformer  30 , the impedance profile Z t (L 2 ) can be selected to make Z 44-44  match the waveguide impedance Z g .  
         [0021]     Thus, the coaxial feed impedance, which is normally fifty ohms, does not have to be the same as the waveguide impedance to obtain a match between the waveguide  16  and the coaxial transmission line  14 .  
         [0022]     For the relatively simple case of a single step quarter wave transformer, the impedance Z t (L 2 ) is kept constant and the length L 2  is selected to be λ/4 at the operating frequency. The impedance looking toward the short is: 
 
 Z   s   =jZ   g tan BL   1   (1) 
 
 which is an open circuit and the input impedance for the equivalent circuit of  FIG. 2   a  becomes: 
 
 Z   in   =−jX   c   +jX   1   +Z   44-44   ( 2 ) 
 
 When probe  10  is shaped such that the reactances cancel, an impedance match is obtained when Z 44-44  equals Z g . For the single step quarter wave transformer, Z t (L 2 ) is found from the following equation: 
 
 Z   t ( L   2 )={square root}{square root over ( Z   g ( Z   coax ))}  (3) 
 
 which is constant as a function of length L 2 . 
 
         [0023]     The matching feed of  FIG. 1  works well even when the waveguide impedance is substantially different than the coaxial input impedance due to the transformer contained within the ridge of waveguide  16 . The matching feed of  FIG. 1  also works well when the space between the top of the waveguide&#39;s ridge and the top of the waveguide is relatively short, i.e. substantially less than λ/4.  
         [0024]     Referring to  FIGS. 3 and 4 ,  FIG. 3  depicts a tapered transformer  50  which has a tapered conductor  52  and a dielectric  54  with an outer diameter which is uniform.  FIG. 4  depicts a transformer  60  which has a stepped conductor  62  and a dielectric  64  which has a uniform outer diameter. The transformer  60  of  FIG. 4  has a plurality of steps  66 ,  68  and  70  with each step  66 ,  68  and  70  having a different diameter. The lengths of each step  66 ,  68  and  70  of transformer  60  are usually equal as shown in  FIG. 4 .  
         [0025]     The impedance of the transformers  50  and  60  is Z t (L 2 ) which may vary along the length of the transformers  50  and  60 . It should be understood that the outer diameters of transformers  50  and  60  can also be made variable stepped or nonuniform with their respective conductors  52  and  62  being constant or variable stepped or nonuniform.  
         [0026]     For the stepped version, the number of steps is arbitrary and can be different than the three steps as shown in  FIG. 4 . Probe and transformer diameters may also be non-circular.  
         [0027]     While  FIGS. 3 and 4 , show the outer dielectric diameters of the transformer being constant and the inner conductor diameters varying, the inner conductor and the outer dielectric or both may be varied in any manner to obtain the impedance profile needed for the transformer. The impedance matching feed may be used with single and double ridge waveguides, or other waveguide geometries, such as waveguides which are asymmetric. The probe diameter may also be shaped and can have a dielectric material around it. The probe diameter may be different than the diameter of the transformer&#39;s inner conductor and it may be shaped such that its radius varies as a funtion of length.  
         [0028]     From the foregoing, it is readily apparent that the present invention comprises a new, unique and exceedingly useful and effective impedance matching feed partially located in a waveguide ridge which constitutes a considerable improvement over the known prior art. Many modifications and variations of the invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims that the invention may be practiced otherwise than as specifically described.