Abstract:
Two or more reflective isolator elements are joined at their outputs to produce an RF combining structure. An RF circulator element is used for each input RF signal as a reflective isolator element to provide a different phase delay according to the direction of propagation of the RF wave. The output of the reflective isolator elements thus exhibits a high output impedance, when looking into the RF output port, preventing back propagation of signals from one input port to another.

Description:
BACKGROUND 
   Efficient combining of RF signals of the same or similar frequencies with varying relative phases and amplitudes cannot be achieved with such devices as Wilkinson or hybrid combiners, as they rely upon specific relative power levels and phases of the input signals to operate effectively and provide isolation between their inputs. 
   In the case of standard quadrature hybrid combiners (such as branch line couplers), the phase of the input signals must differ by 90 degrees with equal amplitudes for optimum combining efficiency. These criteria are required to ensure optimal voltage cancellation at specific nodes within the combining network, thus providing isolation and efficient operation. 
   An ultra high efficiency RF power amplifier could be realized by employing different biasing, device sizing and RF drive levels to the individual high power gain stages. However, the aforementioned combining schemes cannot provide efficient combining and isolation between the stages of such a power amplifier. Therefore, efficiency would be degraded, and portions of the RF energy from a single stage will be delivered to the other stages, effectively causing load impedance shifts, complicating the amplifier&#39;s behavior. 
   In the case of Wilkinson combiners, specific relative amplitudes and phases (usually zero) are required to minimize loss within the isolation resistor. Using such prior combining schemes, failure of a high power gain stage will result in a dramatic drop in efficiency. In the case where two stages are combined by a quadrature hybrid or Wilkinson combiner, at least half of the power of the stage remaining in operation will be lost, due to loss within the combining network. 
   SUMMARY 
   Two or more reflective isolator elements are joined at their outputs to produce an RF combining structure. An RF circulator element is used within each reflective isolator element to provide a different phase delay according to the direction of propagation of the RF wave. The output of the reflective isolator elements exhibits a high impedance to signals propagating backwards into it, thus preventing propagation of signals from one input to another. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a three port RF combiner according to an example embodiment. 
       FIG. 2  is a block diagram of a reflective isolator element used for the RF combiner of  FIG. 1  according to an example embodiment. 
       FIG. 3  is a block diagram illustrating multiple input signals being combined according to an example embodiment. 
   

   DETAILED DESCRIPTION 
   In the following description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural, logical and electrical changes may be made without departing from the scope of the present invention. The following description is, therefore, not to be taken in a limited sense, and the scope of the present invention is defined by the appended claims. 
     FIG. 1  is a block diagram of a three port RF combiner  100  according to an example embodiment. Two input ports,  110  and  115  are shown coupled through two reflective isolator elements  120  and  125  respectively. The input ports are coupled to RF signals, which are combined through the reflective isolator elements to a node  130 . Node  130  is an RF output port, which may be coupled to load  140 , such as an antenna for broadcast of the combined RF signals. The input ports each have an input impedance defined as Z load  and an output port impedance defined as Z, looking into the output port, which in one embodiment is very large, and may essentially be referred to as infinite. The antenna, or output load  140  is also characterized as having an impedance of Z load , matching the input impedances of the reflective isolator elements  120  and  125  in one embodiment. The reflective isolator elements  120  and  125  in one embodiment provide isolation so that sources are not presented with different impedances due to leakage currents. Further, in one embodiment, inputs need not be correlated to be combined efficiently. The isolators also enable use of redundancy for continued operation should components fail, with significantly lower loss than achievable using prior art combining schemes. 
     FIG. 2  is a block diagram of a reflective isolator element  200  used for elements  120  and  125  of the RF combiner  100  of  FIG. 1  according to an example embodiment. Reflective isolator element  200  consists of an RF circulator element  210  in parallel with a transmission line (or waveguide)  215  with a phase delay equal to that of a forward path  220  of the RF circulator element  210 . Many different types of circulator elements may be used, such as those available from SCD Components, Inc. or many other manufacturers. An RF wave entering the reflective isolator element at an RF input  225  is split equally between the RF circulator element  210  and the parallel transmission line  215  because both have a length of ½λ and impedance matched elements. The RF wave in the forward going direction will recombine at an RF output  230  without loss, as phases of the aforementioned paths are equal in one embodiment. Therefore, the impedance of the reflective isolator element, looking into its input port will assume that of the RF load presented to its RF output. 
   The parallel transmission line  215  may be thought of as arm one of the reflective isolator element, with arm two having two paths that contain the RF circulator element  210 . The RF circulator element  210  is coupled to input node  225  by a line  227  having an impedance of Z R  and length of ⅛λ The lines may be transmission lines or waveguides, or other types of lines that are capable of carrying RF signals. Line  227  is coupled to port  1  at  235  of circulator  240 . The circulator  240  has three ports in one embodiment, and restricts RF signals to travel in one direction from port  1  at  235  to port  2  at  245 , and from port  2  to port  3  at  250 , and from port  3  to port  1 . An output portion  255  of the RF circulator element  210  has an impedance of Z R  and length of ⅛λ, and is then coupled to a line  260 , having an impedance of Z R  and length of ¼λ As can be seen, the phase shift in both arms is the same in the forward going direction. The rectangles in the figures are merely symbols used to represent length and impedance of the lines. 
   An RF wave entering the reflective isolator element from the output node  230  will split equally between the RF circulator element  210  and the parallel transmission line  215 . However, the signals will be out of phase at node  225 , and hence no RF current will be delivered to the RF input. Therefore, the impedance of the reflective isolator element  200 , looking into its output port will be infinite. 
   This zeroing of backward traveling waves is accomplished via the path back through the RF circulator element  210  reaching port  2  at  245 , and being directed toward port  3  at  250 . Port  3  is coupled to an RF short  270  via a path  275  having an impedance of Z R  and a length of ¼λ. The backward traveling wave is reflected at the RF short circuit  270  and travels back to the RF circulator element to port  1  at  235 , and from there to input node  225 . The total shift in phase of this reverse path is λ, while that of the reverse path in the parallel arm is ½λ. In one embodiment, the path length difference is an odd multiple of ½λ to obtain cancellation. 
     FIG. 3  is an alternative embodiment illustrating multiple RF input nodes  310 ,  320 ,  330 , and  340  being combined. Each input node in one embodiment is coupled to a reflective isolator element  315 ,  325 ,  335  and  345  respectively, which in turn are coupled to an output node  350 . Since the output impedance of each of the reflective isolator elements is essentially infinite, there is no feedback from other outputs. The output node  350  may be coupled to a load, such as an antenna  355  for transmitting the combined RF input signals. 
   In one embodiment, the bandwidth of the circulator may be chosen to match the application—they are inherently narrow band (frequency) devices. Hence, the combining network will be frequency specific. The insertion phase of the circulator should be considered when designing the combiner (it may dictate the distance from port  3  to the RF short or open circuit, and the phase delays of the other transmission line elements). The insertion loss of the circulator may impact combining efficiency. Optimum combining efficiency may occur when the input signals are of the same frequency. 
   The Abstract is provided to comply with 37 C.F.R. §1.72(b) to allow the reader to quickly ascertain the nature and gist of the technical disclosure. The Abstract is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims.