Patent Publication Number: US-6211750-B1

Title: Coaxial waveguide feed with reduced outer diameter

Description:
FIELD OF THE INVENTION 
     This invention relates to antenna receiving apparatus for receiving or transmitting radio signals, and more particularly to waveguide feeds for receiving or transmitting orthogonally polarized radio signals. 
     BACKGROUND OF THE INVENTION 
     Parabolic as well as other circularly shaped reflectors are typically used in satellite communications for receiving radio signals from or transmitting radio signals to satellites. When receiving signals, the reflectors are used to gather incoming signals and then reflect them to a region, designated as the focal point, where the signals are concentrated and received by the antenna feed. Reciprocally, when transmitting signals, the signals are transmitted first through the antenna feed, after which, the signals disperse in such a manner as to illuminate the reflector, after which, the signals are directed by the reflector towards a satellite or other target. For simplicity the following discussion will be in terms of the antenna used to receive signals. 
     Because of the heavy demand for signal bandwidth, it has become a practice in the industry, in particular satellite to ground transmissions, to double the available bandwidth by simultaneously transmitting orthogonal signals. This can be accomplished by transmitting electric fields, which are linearly polarized as would be the case for a first bandwidth, which is vertically polarized, and a second bandwidth, which is horizontally polarized. Likewise, the two bandwidths can be elliptically or circularly polarized with a first bandwidth with clockwise polarization and a second bandwidth with counter-clockwise polarization. 
     A waveguide feed operating as an antenna feed commonly consists of a waveguide cavity to gather the signals which have been concentrated by the reflector and a probe or probes to couple the signals from the cavity to a circuit board which will usually have electronic devices to amplify and process the signals. 
     In many instances, as in the case for a coaxial waveguide feed, two frequencies are supplied to a single feed through the use of two probes aligned with the electrical fields of orthogonal TE 11  coaxial waveguide modes. However, care should be taken to ensure that the probes are not aligned with the electric fields of the TEM mode. By coupling to the TEM mode the probes couple to each other, which causes them to have poor polarization isolation. The alignment of the probes needs to be such that they excite the primary coaxial waveguide mode, TE 11 , while suppressing the excitation of the TEM or coaxial mode. 
     In order for a single probe in circular shaped coaxial waveguide to be perfectly orthogonal to the TEM mode it would have to be circular in shape with perfect symmetry. There is no such shape that can also be used to excite the TE 11  mode. A common method is to use multiple probes or probes oriented in a complementary fashion. If complementary probes are excited with equal amounts of power but with a 180° phase difference, the TE 11  waveguide mode will be excited with no excitation of the TEM mode. A similar excitation of orthogonal complementary probes will likewise excite a second but orthogonal TE 11  mode with no excitation of the TEM mode. Such a scheme is complicated and, when receiving weak satellite signals, the necessary higher losses results in higher noise figures. 
     Many of the newer satellites transmit multiple bandwidths, which vary considerably in frequency. A good example is the use of the C-Band (typically 3.7 GHz to 4.2 GHz) and the Ku-Band (typically 11.7 GHz to 12.7 GHz) bandwidths. With approximately a 3:1 frequency ratio between the bandwidths, both bandwidths can be simultaneously received by waveguide cavities that are co-located. This is accomplished by placing the Ku-Band waveguide cavity within and with its axis aligned with the C-Band waveguide, which has a cross-sectional dimension of approximately three times the size of the Ku-Band waveguide. 
     While this co-location of waveguide cavities reduces the overall area required by the antenna feed, it is still rather large and cumbersome since each of the waveguide cavities is constructed as an individual or stand-alone cavity and they are then simply placed in axial alignment. 
     Accordingly, it would be desirable to provide a waveguide feed which overcomes this drawback. 
     It is an object of the present invention to provide a new and improved waveguide feed. 
     It is another object of the present invention to provide a new and improved waveguide feed with reduced external dimensions. 
     It is still another object of the present invention to provide a new and improved waveguide feed with reduced external dimensions which provides improved isolation and VSWR. 
     It is a further object of the present invention to provide a new and improved waveguide feed with a coaxial waveguide having a reduced outer dimension. 
     SUMMARY OF THE INVENTION 
     The above problems and others are at least partially solved and the above objects and others are realized in coaxial waveguide apparatus for conducting radio signals within a range of frequencies including a waveguide having an open end and a rear wall opposite the open end positioned along an axis of the waveguide and defining a waveguide cavity with a fundamental waveguide mode within the waveguide. The waveguide is constructed with a cutoff frequency for the fundamental waveguide mode which is at least 95% of the lowest frequency for which the apparatus operates. A center conductor is positioned within the waveguide cavity and along the axis of the waveguide so that the center conductor and the waveguide define a coaxial waveguide cavity, which will have a different fundamental mode than the above waveguide cavity. 
     Generally, at least one waveguide probe is mounted within said coaxial waveguide cavity so as to extend through either the waveguide or the rear wall into the coaxial waveguide cavity for exciting a first primary coaxial waveguide mode electromagnetic signal in the waveguide cavity. The waveguide probe is oriented to couple to a primary coaxial waveguide mode of the coaxial waveguide cavity and is further oriented to be substantially orthogonal to the TEM mode of the coaxial waveguide cavity. A second waveguide probe may be mounted within the coaxial waveguide cavity so as to extend through one of the waveguide and the rear wall for exciting a second primary coaxial waveguide mode electromagnetic signal in the waveguide cavity. The second probe is mounted orthogonal to the first probe and so that the second primary coaxial waveguide mode electromagnetic signal is orthogonal to the first primary coaxial waveguide electromagnetic signal and to the TEM mode of the coaxial waveguide cavity. 
     In a preferred embodiment of the present invention, the waveguide is constructed with a cutoff frequency for the fundamental waveguide mode in a range of 100% to 200% of the lowest frequency for the apparatus. In a specific example, the apparatus is constructed to conduct C-Band frequencies (typically 3.7 GHz to 4.2 GHz) and the waveguide is constructed to have an inner diameter in a range of approximately 2 inches to 1 inch. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring to the drawings: 
     FIG. 1 is a cross-sectional view of a prior art dual band coaxial waveguide feed; 
     FIG. 2 is a cross-sectional view of the prior art coaxial waveguide feed of FIG. 1, portions thereof removed, illustrating a TE 11  or primary coaxial waveguide mode; 
     FIG. 3 is a cross-sectional view of the prior art coaxial waveguide feed of FIG. 1, portions thereof removed, illustrating the TEM or coaxial mode; 
     FIG. 4 is a cross-sectional view of a prior art coaxial waveguide feed with multiple orthogonal complementary probes; 
     FIG. 5 is a cross-sectional view of a coaxial waveguide feed illustrating the placement of probes in alignment with the primary waveguide mode and orthogonal to the TEM mode; 
     FIG. 6 is a cross-sectional view of a coaxial waveguide feed in accordance with the present invention; 
     FIG. 7 is an isometric view of the coaxial waveguide feed of FIG. 6, portions thereof broken away; 
     FIG. 8 is a cross-sectional view of a coaxial waveguide feed illustrating the placement of probes through the rear wall; 
     FIG. 9 is a longitudinal sectional view of the coaxial waveguide feed of FIG. 8; 
     FIG. 10 is a cross-sectional view of a coaxial waveguide feed illustrating the placement of probes through the side wall; 
     FIG. 11 is a longitudinal sectional view of the coaxial waveguide feed of FIG. 10; 
     FIGS. 12 and 13 are longitudinal sectional views of coaxial waveguide feed with different shaped probes; and 
     FIG. 14 is a cross-sectional view of a coaxial waveguide feed with probes entering the feed at adjacent entries. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Many of the newer satellites transmit multiple bandwidths, which vary considerably in frequency. A good example is the use of the C-Band (typically 3.7 GHz to 4.2 GHz) and the Ku-Band (typically 11.7 GHz to 12.7 GHz) bandwidths. With approximately a 3:1 frequency ratio between the bandwidths, both bandwidths can be simultaneously received by waveguide cavities that are co-located. Referring to FIG. 1, it can be seen that this co-location is accomplished by placing a Ku-Band waveguide  10  defining a cavity  11  within and with its axis aligned with a C-Band waveguide  12  defining a cavity  13 . C-Band waveguide  12  has an inner diameter approximately three times the inner diameter of Ku-Band waveguide  10  as shown in FIG.  1 . Waveguide  10  positioned within waveguide  12  forms a coaxial waveguide which is designated generally  16 . 
     A pair of orthogonally oriented probes  14  are positioned in cavity  11  for transmitting or receiving Ku-Band signals and a pair of orthogonally oriented probes  15  are positioned in cavity  13  for transmitting or receiving C-Band signals. A waveguide orthomode-transducer (OMT) is often used in place of probes  14  shown in waveguide cavity  11 . However, waveguide probes are often used in lieu of a waveguide OMT in cavity  13 . 
     Referring specifically to FIG. 2, the alignment of electric fields (illustrated as lines  17  with arrowheads to indicate the polarity of the field) for a TE 11  or primary waveguide mode for coaxial waveguide  16 . Coaxial waveguide  16  will support a second TE 11  mode (not shown), which is orthogonal to the first TE 11  but has its fields rotated 90 degrees relative to the fields shown in FIG.  2 . 
     Turning now to FIG. 3, the alignment of electric fields (illustrated as lines  18  with arrowheads to indicate the polarity of the field) for the TEM mode of coaxial waveguide  16  is illustrated. The TEM mode is commonly referred to as the coaxial mode and, contrary to the nature of the TE 11  mode, only one TEM mode can exist in coaxial waveguide  16 . 
     Returning to FIG. 1 with additional reference to FIGS. 2 and 3, it can be seen that probes  15  are aligned with the electrical fields  17  of a first and second orthogonal TE 11  modes. However, probes  15  are also aligned with electric fields  18  of the TEM mode. In this position it is obvious that probes  15  will couple to or excite both of the TE 11  and the TEM modes. By coupling to the TEM mode probes  15  will couple to each other, which will cause them to have poor polarization isolation. The coupling to the TEM mode will also cause beam squint and loss of efficiency when the waveguide apparatus is used as an antenna feed. The alignment of probes  15  needs to be such that they excite the primary coaxial waveguide mode, while suppressing the excitation of the TEM or coaxial mode. 
     In order for a single probe to be perfectly orthogonal to the TEM mode shown in FIG. 3, it would have to be circular in shape with perfect symmetry. There is no such shape that can also be used to excite the TE 11  mode. A common method is to use multiple orthogonal complementary probes, which are shown in FIG.  4 . If complementary probes A and C are excited with equal amounts of power but with a 180° phase difference, the TE 11  waveguide mode will be excited with no excitation of the TEM mode. A similar excitation of complementary probes B and D will likewise excite a second but orthogonal TE 11  mode with no excitation of the TEM mode. Such a scheme is complicated and, when receiving weak satellite signals, the necessary higher losses will result in higher noise figures. All of the probes of the A through D type in coaxial waveguide, lie along a plane that bisects the waveguide and as such are aligned with the fields of the TEM mode. 
     Turning now to FIG. 5, a cross-sectional view of an coaxial waveguide  20  is illustrated in which an inner conductor  21  is coaxially positioned within an outer conductor  22  to define a waveguide cavity  23 . Further, the alignment of the electric fields for the TEM mode of coaxial waveguide  20  is illustrated as lines  22  with arrowheads to indicate the polarity of the fields. The alignment of a probe  25 , in FIG. 5 shows how a single probe can be positioned to be aligned with a TE 11  mode and to be orthogonal or non-aligned with the TEM mode. When an E-Field vector crosses a conductor or probe at an angle which is non-orthogonal to the probe it will excite electrical currents which flow along the probe. E-Field vectors can be resolved into x and y components, where one of the components is orthogonal to the probe while the other component is tangential to the probe. The tangential component will excite currents, which will flow along the probe. 
     In Probe  25 , E-Field vectors a, c, and d excite currents i a , i c , and i d , while a vector b, being orthogonal to probe  25 , excites no currents. Current i a  excited by vector a is opposed to currents i c , and i d , which are excited by E-Field vectors c and d. If current i a  excited by E-Field vector a is equal and opposite in phase to currents i c , and i d , excited by E-Field vectors c and d, then Probe  25  is orthogonal to the TEM mode. Although, FIG. 5 demonstrates how a probe can be positioned to be orthogonal to an electro-magnetic field, the actual phenomenon is more complicated. The field intensity near center conductor  21  is more intense than the fields near outer conductor  22 , and equal fields along probe  25  do not necessarily equally excite currents. For example, it may be easier to excite a current near the end of probe  25  then at the mid section or vice-versa depending on the impedances present. However, it is necessary that field vectors of the TEM mode are present which excite currents that oppose one another. A second probe  26  is illustrated which has some orthogonality to the E-Fields, but currents induced into probe  26  by tangential components (i.e. E-Field vectors e and f) all flow in the same direction. Probe  25  can be considered to be substantially orthogonal to the TEM mode while probe  26  is not substantially orthogonal. 
     The best and simplest way to position a probe is to align it while making electrical measurements. Often times after a set of dual waveguide probes are aligned for the optimum orthogonality or polarization isolation, they will not necessarily be mechanically positioned at an angle of 90° to each other. This is due to the fact that the asymmetrical presence of the probes upsets the symmetry of the waveguide structure and consequently the symmetry of the fields. 
     Thus, as described above, a coaxial waveguide feed can be utilized as a feed for a larger or primary antenna reflector, among other uses, and has both an outer conductor and an inner conductor, which together comprise a coaxial waveguide. A single waveguide probe is mounted within the coaxial waveguide and is oriented such that it couples to a primary coaxial waveguide mode while being orthogonal to the TEM or coaxial mode. A second waveguide probe can be mounted for coupling to a second primary coaxial waveguide mode, orthogonal to the first primary waveguide mode. The center conductor can also be utilized as a second waveguide antenna feed, which would operate at a higher frequency than would the larger coaxial waveguide. 
     In all of the prior art coaxial waveguide feeds or co-located waveguide cavities, such as for receiving C-Band and Ku-Band, the diameter of the outer conductor is approximately three times the diameter of the inner conductor when receiving frequency bands which are approximately 3:1. However, as will be described in detail below, the presence of the inner conductor allows the cross-sectional dimensions of the outer conductor to be smaller than the dimensions which would be allowable without the inner conductor. This reduction in the cross-sectional dimensions of the outer conductor allows the new and novel antenna feed to be utilized in tight fitting situations such as multi-beaming applications, where multiple antenna feeds are fitted to a single primary reflector. 
     Turning now to FIG. 6, a cross-sectional view of a coaxial waveguide feed  30  is illustrated. In this discussion it will be assumed for convenience that coaxial waveguide feed  30  is used for an antenna feed although it will be understood by those skilled in the art that many other uses are possible. Coaxial waveguide feed  30  includes an inner conductor  32  (or waveguide) and a coaxially mounted outer conductor  35  (or waveguide) defining a coaxial waveguide cavity  36  therebetween with a fundamental waveguide mode. Referring in addition to FIG. 7 which is an isometric view with portions broken away, it can be seen that outer conductor  35  is a waveguide with an open end  40  and an opposite end  41  terminating in a rear wall  42 . Open end  40  and opposite end  41  are positioned along an axis  43  of coaxial waveguide feed  30 . As will be explained in more detail presently, inner conductor  32  can be a solid conductor or it can be an waveguide defining a second waveguide cavity therein. A pair of orthogonal probes  37  and  38  are positioned in waveguide cavity  36  for exciting first and second primary coaxial waveguide mode electromagnetic signals in waveguide cavity  36 . 
     In designing coaxial waveguide feeds, the engineer must select a waveguide size, which will accommodate certain desired waveguide modes. Most often the size of the waveguide is selected such that it is large enough to allow the fundamental waveguide mode to propagate, but small enough to suppress all of the other waveguide modes, which are referred to as higher order modes. For example, a waveguide feed, which is designed to receive the 3.7 GHz-4.2 GHz satellite transmission band (the C-Band), must be large enough to allow 3.7 GHz to propagate in the fundamental mode and small enough to suppress higher order modes at 4.2 GHz. 
     For circular waveguide as, for example, waveguide  35  (without center conductor  32 ), the minimum diameter needed to propagate the fundamental mode, TE 11 , is determined by the equation, 
     
       
           D=λc/ 1.706   (I)  
       
     
     where: 
     λc is the cutoff wavelength for 3.7 GHz, and 
     D is the diameter of the circular waveguide. 
     Using equation I, the diameter of a circular waveguide needed to propagate the fundamental waveguide mode at frequencies of 3.7 GHz and higher is 1.87 inches. 
     The first higher order mode is the TM 01  mode, which is allowed to propagate when the diameter of a circular waveguide is determined by the equation, 
     
       
           D=λc/ 1.3065   (II)  
       
     
     where: 
     λc is the cutoff wavelength for 4.2 GHz, and 
     D is the diameter of the circular waveguide. 
     Using equation II, the diameter of a circular waveguide needed to propagate the TM 01  mode at frequencies of 4.2 GHz and higher is 2.15 inches. 
     With the above information we now know that, if we want a circular waveguide to propagate only the fundamental mode TE 11  from 3.7 GHz to 4.2 GHz, the diameter must be between 1.87 inches and 2.15 inches. Since it is difficult to match impedances when the diameter is close to the cutoff of the fundamental waveguide mode, the diameter selected needs to be closer to 2.15 inches rather than 1.87 inches. 
     Good engineering practice dictates that the diameter of the circular waveguide should be no larger than 98% of a diameter, which would allow a higher order mode to propagate at the highest frequency used in the band. It should also be at least 10% larger than the diameter at which the waveguide will no longer allow the lower band edge to propagate as a fundamental waveguide mode. Using these criteria, a diameter of 2.1 inches to 2.125 inches is a good selection. C-Band waveguide feeds currently in operation for the 3.7 GHz-4.2 GHz band have diameters close to 2.1 inches and are represented in FIG. 6 by waveguide  35 ′ illustrated in broken lines. The diameter selected is determined by the upper band edge, F 2 , and it&#39;s relationship to a first higher order waveguide mode and by the lower band edge, F 1 , and it&#39;s relationship to the fundamental waveguide mode. A diameter of 2.1 inches converts to 0.747 λ at 4.2 GHz and 0.658λ at 3.7 GHz. 
     The cutoff frequency, F c , for the fundamental mode for a 2.1 inch diameter circular waveguide (e.g. waveguide  35 ′, which has a diameter approximately 3 times the diameter of center conductor  32 ) is 3.294 GHz. The ratio between F c  and the lower band edge, F 1 , is 3.294/3.7, which is 0.89, or the cutoff frequency is 89% of the lowest operating frequency. Here it should be noted that an F 1  equal to F c  (a cutoff frequency approximately 100% of the lowest operating frequency) will not propagate in the circular waveguide. 
     By adding center conductor  32  to outer conductor  35 , the circular waveguide becomes a coaxial waveguide. Coaxial waveguide feed  30  is also able to conduct the TEM mode which has no cutoff frequency and which is not considered a waveguide mode. With the diameter of outer conductor  35  being designated D a  and the diameter of inner conductor  32  being designated D b , for the condition where D a  equals three times D b , the cutoff wavelength for the fundamental coaxial waveguide mode TE 11  is determined by the following equation, 
     
       
         λ c= 1.873(Π/4)( D   a   +D   b ); D a =3D b .  
       
     
     The cutoff wavelength for the next higher order mode, TE 21 , is determined by the equation, 
     
       
         λ c= 1.023(Π/4)( D   a   +D   b ); D a =3D b .  
       
     
     For a coaxial waveguide cavity with an outer diameter of 2.1 inches and with a 0.7 inch inner diameter, the cutoff frequency (F c ) for the fundamental mode, TE 11 , is 2.865 GHz, while the cutoff frequency (F c ) for the first higher order, TE 21 , mode is 5.246 GHz. For a circular waveguide (without the inner conductor) with a diameter of 2.1 inches, the fundamental mode, TE 11 , cuts off at 3.294 GHz while the first higher order mode, TM 01 , cuts off at 4.302 GHz. 
     It can be seen that coaxial waveguide feed  30  of FIG. 6 allows for a much broader-frequency operating range than that which is allowed by a circular waveguide feed. Thus, the outer diameter of coaxial waveguide cavity  36  is reduced so as to have a cutoff frequency (F c ) for the fundamental circular waveguide mode, TE 11 , which is at least 95% of the lowest operating frequency for coaxial waveguide feed  30 . More specifically, outer conductor  35  is preferably constructed so that coaxial waveguide cavity  36  has a cutoff frequency (F c ) for the fundamental circular waveguide mode, TE 11 , which is in a range of 100% to 200% of the lowest operating frequency for coaxial waveguide feed  30 . In a specific example, assuming coaxial waveguide apparatus  30  is designed for receiving or transmitting radio signals of C-Band frequencies and with an inner conductor of diameter 0.7 inches, outer conductor  35  is constructed to have an inner diameter (outer diameter of coaxial waveguide cavity  36 ) of less than 2 inches. More specifically, when coaxial waveguide apparatus  30  is designed for receiving or transmitting radio signals of C-Band frequencies, outer conductor  35  is preferably constructed to have an inner diameter in a range of 2 inches to 1 inch. 
     Thus, the present invention allows for the outer conductor of a coaxial waveguide to be smaller than that which would be allowed if the feed were designed for a circular waveguide without the inner conductor. It also allows for the use of waveguide probes which couple to the fundamental waveguide mode while having orthogonality to the TEM mode and which enter the waveguide cavity from either the rear wall or the side wall. The novel coaxial waveguide feed (e.g. feed  30 ) of the present invention can be used with a single probe (i.e. either probe  37  or  38 ) in coaxial waveguide cavity  36 . A single probe will still have the advantage of exciting the TE 11  coaxial waveguide mode while minimizing the excitation of the TEM mode. 
     The probe or probes enter the coaxial waveguide apparatus from either the backwall or from the sidewall. FIGS. 8 and 9 are cross-sectional and longitudinal sectional views, respectively, of a coaxial waveguide feed  50  having an inner conductor  51  and a coaxially positioned outer conductor  52  defining a coaxial waveguide cavity  53  therebetween. A pair of orthogonal probes  54  and  55  enter cavity  53  through a backwall  56 . Having the probes enter from the backwall, as shown in FIGS. 6,  8 , or  9  offers the advantage of allowing the probes to easily exit onto a single circuit board. FIGS. 10 and 11 are cross-sectional and longitudinal sectional views, respectively, of a coaxial waveguide feed  60  having an inner conductor  61  and a coaxially positioned outer conductor  62  defining a coaxial waveguide cavity  63  therebetween. A pair of orthogonal probes  64  and  65  enter cavity  63  through a sidewall of outer conductor  62 . The side wall exit, as shown in FIGS. 10 and 11, offers the advantage of having exits with spatial distance and of allowing separate waveguide flanges if that is what is desired for a particular design. 
     Many different shapes of probes can be used to achieve high isolation from the TEM mode and good coupling to the primary waveguide mode. Referring specifically to FIGS. 12 and 13, two different shapes of probes that work well are illustrated. When using dual probes, the two probes can have different shapes and still perform well. The probes can also enter the waveguide in close proximity to each other as shown in FIG. 14, rather than as shown in FIGS. 8 and 10. 
     Referring again to FIG. 6 for example, inner conductor  32  of coaxial waveguide feed  30  can be a solid or a hollow conductor. In a further variation, inner conductor  32  can be a waveguide defining a circular waveguide cavity therein, which can then be used for a second band of frequencies. By converting inner conductor  32  into a second but smaller waveguide, coaxial waveguide feed  30  is commonly used to make a co-located feed for two separate frequency bands. Synchronous orbit satellites are commonly designed to transmit a C-Band at 3.7 GHz-4.2 GHz and a Ku-Band at 11.7 GHz-12.2 GHz. These frequencies have approximately the same 3:1 ratio used in the above discussion of coaxial waveguide feed  30 . In this fashion, coaxial waveguide feed  30  can be used as a co-located feed to transmit or receive signals for both bands. 
     The common practice in the industry is to take a circular waveguide, which has a diameter that will accommodate a C-Band only feed, and add a smaller concentric waveguide for the Ku-Band signals. The wide spread between the fundamental mode and the next higher order mode allow the designer plenty of room to determine the diameter of the outer conductor for the C-Band, while the diameter of the smaller Ku-Band waveguide has the more stringent constraints of the circular waveguide. 
     It is becoming a common practice to install multiple feeds on a single reflector with each feed positioned to receive signals from different satellites. In some situations the positions of the feeds are so close to each other that they physically interfere with each other. The application of the present invention allows the feeds to be spaced closer than what is allowed with current designs. Further, the application of the present invention to the waveguide probes, as shown for example in FIG. 6, allows the probes to achieve a better matching condition and higher isolation between two orthogonally polarized probes. 
     Although the above discussions have centered on coaxial waveguides with circular shapes for both the outer conductor and the center conductor, it should be understood that the same principles apply to waveguides that may not be circular in shape. For example the outer conductor could be circular, square, rectangular, or some other shape while the inner conductor could be circular, etc. or vice-versus. Or, both the outer and inner conductors could be circular, square, or rectangular. The inner conductor can also be triangular or any other shape imaginable. Also, the center conductor does not need to contain a waveguide to apply the teachings of this invention. 
     While I have shown and described specific embodiments of the present invention, further modifications and improvements will occur to those skilled in the art. I desire it to be understood, therefore, that this invention is not limited to the particular forms shown and I intend in the appended claims to cover all modifications that do not depart from the spirit and scope of this invention.