Abstract:
An apparatus for doubling the amount of power that can be safely applied to a circulator-based isolator, which is made up of an input hybrid which receives an RF signal and outputs two divided RF signals, a Y-junction circulator that receives one of the divided RF signals at one of the circulator ports and outputs it at another circulator port, an output hybrid which receives the two divided RF signals, combines them, and sends them on, the output hybrid simultaneously receiving a returned signal, dividing it, and outputting two divided returned signals, one divided returned signal going to the circulator and the other divided returned signal going directly to the input hybrid, a phase retarding circuit which takes the half of the divided returned signal that went back to the circulator and retards its phase by 180 degrees and sends it back to the circulator, where it goes to the input hybrid as well, to be recombined with the other half of the returned signal and attenuated.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from provisional application serial no. 60/192,574, filed Mar. 28, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to the isolation of radio frequency (RF) signal amplifiers from returned signals using circulator-based isolators. 
     2. Background and Related Art 
     Circulators are generally ferrite devices composed of permanent magnets. Circulators used as isolators pass RF signals and block returned signals. Some of the power in the RF signal, and nearly all of the power in the blocked returned signals, is dissipated as heat. Dissipating power as heat raises the temperature of a circulator. The performance characteristics of ferrite devices composed of permanent magnets change with temperature. In particular, the frequency response of ferrite, and the coercive effect of permanent magnets, changes as temperatures rise. Uncompensated changes in the frequency response of ferrite and the coercive effect of permanent magnets cause a circulator to suffer higher return losses, or “drift”, in all of its ports. Drift manifests itself as a change in impedance. Changed impedances cause mis-matched impedances, which cause power to be reflected rather than transferred, which leads to further heating, further losses, and ultimately failure of the circulator. Failure of a circulator means upstream components such as amplifiers are no longer being isolated from returned signals, which jeopardizes their lives as well. 
     In general, the higher the temperature a circulator is able to withstand, the higher its rated power level. Circulators used as isolators are normally temperature-compensated to increase the temperature they are able to tolerate. Temperature compensation is costly. Higher operating temperatures, and hence higher rated power levels, may be achieved in return for higher cost and greater complexity. Still, there is a finite limit to the amount of power that can be passed safely through any circulator-based isolator. 
     SUMMARY OF THE INVENTION 
     The present invention provides a solution to the shortcomings of the prior art as discussed above. 
     In particular, the present invention provides an apparatus for doubling the amount of power that can be safely applied to a circulator-based isolator, which is made up of an input hybrid which receives an RF signal and outputs two divided RF signals, a circulator that receives one of the divided RF signals at one of the circulator ports and outputs it at another circulator port, an output hybrid that receives the two divided RF signals, combines them, and sends them on, an output hybrid that simultaneously receives a returned signal, divides it, and outputs two divided returned signals, one divided returned signal going to the circulator and the other divided returned signal going directly to the input hybrid, and a phase retarding circuit that takes the one-half of the divided returned signal that went back to the circulator and retards its phase by 180 degrees and sends it back to the circulator, where it goes to the input hybrid as well, to be recombined with the other half of the returned signal and there attenuated. 
     DESCRIPTION OF THE DRAWINGS 
     The invention will be described in detail with reference to the following drawings, in which: 
     FIG. 1 is a schematic diagram of a hybrid junction used in connection with the present invention; 
     FIG. 2 is a schematic diagram of a Y-junction circulator used in connection with the present invention; 
     FIG. 3 is a schematic diagram of the Y-junction circulator shown in FIG. 2 configured as an isolator; 
     FIG. 4 is a block diagram of the forward signal path of a first embodiment of a circulator-based isolator power capacity doubling apparatus according to the present invention; 
     FIG. 5 is a block diagram of the reverse signal path of the embodiment of a circulator-based isolator power capacity doubling apparatus shown in FIG. 4; 
     FIG. 6 is a block diagram of the forward signal path of a second embodiment of a circulator-based isolator power capacity doubling apparatus according to the present invention; 
     FIG. 7 is a block diagram of the reverse signal path of the embodiment of a circulator-based isolator power capacity doubling apparatus shown in FIG. 6; 
     FIG. 8 is a block diagram of the forward signal path of a third embodiment of a circulator-based isolator power capacity doubling apparatus according to the present invention; and 
     FIG. 9 is a block diagram of the reverse signal path of the embodiment of a circulator-based isolator power capacity doubling apparatus shown in FIG.  8 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic representation of a conventional device  10  known alternatively as a hybrid, quadrature hybrid, hybrid junction, π- hybrid, “magic tee”, or cross-coupled hybrid of the coaxial type. A hybrid junction is generally a four port waveguide or strip line structure having four terminals or ports so arranged that, when properly terminated in external impedances, an RF signal input at port  11  will be coupled to ports  13  and  14 , but not to port  12 . Furthermore, the signal at port  14  will be in quadrature with the signal at port  13 . Similarly, an RF signal input port  12  will be coupled to ports  13  and  14  (assuming proper load impedances) but not to port  11 . And in that case the signal at port  13  will be in quadrature with the signal at port  14 . In particular, an RF signal S 11  entering hybrid  10  at port  11  will divide and emerge from the two opposite ports  13  and  14  as two output signals S 13   a  and S 14   a , with S 14   a  in quadrature with S 13   a , assuming that ports  13  and  14  are terminated with appropriate equal characteristic impedances, but will be unable to reach the adjacent port  12 . Conversely, if signals S 13   b  and S 14   b , where S 14   b  is in quadrature with S 13   b , enter hybrid  10  at ports  13  and  14 , respectively, the signals will be recombined as a single signal S 12  at port  12 . It is important to note that the signals S 13   b  and S 14   b  will be recombined and output from port  12  rather than port  11  because signal S 14   b  is in quadrature with signal S 13   b . Hybrids with phase relationships other than quadrature can be substituted and configured to similarly split and combine the signal as will be known to people skilled in the art. Examples of known hybrid junctions are disclosed, for example, in U.S. Pat. Nos. 3,818,385 and 4,413,242. The term “hybrid” as used hereinafter shall include a hybrid, hybrid junction, or other equivalent coupler and/or splitter device as known in the art. 
     FIG. 2 is a schematic representation of a particular form of hybrid known as a circulator  20 . This particular form of circulator  20  is known as a Y-junction circulator because it has three ports. In general, however, a circulator may have more than three ports. Circulator  20  is a three port waveguide or transmission line structure having three terminals or ports so arranged that, when properly terminated in external impedances, an RF signal entering circulator  20  at any given port will emerge from the nearest port in the clockwise direction, but will be unable to reach the nearest port in the counter-clockwise direction. In particular, an RF input signal S 21  entering circulator  20  at port  21  will be coupled to port  22 , but not to port  23 . Similarly, an RF input signal S 22  entering circulator  20  at port  22  will be coupled to port  23 , but not to port  21 . And an RF input signal S 23  entering circulator  20  at port  23  will be coupled to port  21 , but not to port  22 . Although a clockwise signal rotation was assumed for the above description, a circulator having a counter-clockwise signal rotation would work in an analogous manner. 
     FIG. 3 is a schematic representation of a circulator  30  being used as an isolator. In this case port  33  is terminated with a matched impedance  34 . An RF input signal S 31  entering circulator  30  at port  31 , that is, in the forward direction, will be coupled to port  32  and emerge. A returned signal R 32  entering circulator  30  at port  32 , on the other hand, will be coupled to port  33  and then be completely attenuated by matched impedance  34 . A device connected to port  31  of circulator  30  is thus isolated from a returned signal R 32  appearing at port  32 . 
     FIG. 4 shows the forward signal path of a first embodiment of the circulator-based isolator power capacity doubling apparatus according to the present invention. In FIG. 4, RF signal RF 41  from an input device  41  which may be, for example, an amplifier, enters input hybrid  42  at first hybrid port  42   a  of input hybrid  42  and is divided, with one-half of signal RF 41  emerging at third hybrid port  42   c  of input hybrid  42  as signal RF 42   c , while the other half of signal RF 41  emerges at fourth hybrid port  42   d  of input hybrid  42  as signal RF 42   d . Signal RF 42   d  is in quadrature with signal RF 42   c . Signal RF 42   c  enters circulator  43  at first circulator port  43   a  of circulator  43  and emerges from second circulator port  43   b  of circulator  43  as signal RF 43   b . In a preferred embodiment circulator  43  is a Y-junction circulator but a circulator with more than three ports could be used as well by appropriately terminating the unused ports. RF 43   b  then enters output hybrid  44  at second hybrid port  44   a  of output hybrid  44 . Signal RF 42   d , meanwhile, bypasses circulator  43  and is input directly to second hybrid port  44   b  of output hybrid  44 . Signal RF 43   b  is then recombined with signal RF 42   d  in output hybrid  44  and emerges at fourth hybrid port  44   d  of output hybrid  44  as signal RF 44   d . Signal RF 44   d  is then transmitted to an output load which may be, for example, an antenna. Circulator  43  thus sees only half of the forward signal power. Hybrid port  44   c  of output hybrid  44  is terminated with matched impedance  48 . Circulator port  43   c  of circulator  43  is terminated with an appropriate impedance, as described below. 
     FIG. 5 shows the reverse signal path of the first embodiment of the circulator-based isolator power capacity doubling apparatus according to the present invention that was shown in FIG.  4 . The returned signal P REFL  returns to output hybrid  44  at fourth hybrid port  44   d  of output hybrid  44 . Returned signal P REFL  is divided by output hybrid  44 , with one-half of returned signal P REFL  emerging from first hybrid port  44   a  of output hybrid  44  as returned signal PR 44   a  while the other half of returned signal P REFL  emerges from second hybrid port  44   b  of output hybrid  44  as returned signal PR 44   b . Returned signal PR 44   a  is in quadrature with returned signal PR 44   b . Returned signal PR 44   a  enters circulator  43  at second circulator port  43   b  of circulator  43  and is circulated to third circulator port  43   c  of circulator  43 , where it emerges as returned signal PR 43   c . Returned signal PR 43   c  then travels down quarter-wavelength stub  47 . Quarter-wavelength stub  47  is preferably a transmission line with an electrical length equal to one-quarter of the wavelength of the RF input signal RF 41  (shown in FIG.  4 ), but its electrical length may be any odd multiple of one-quarter of the wavelength of the RF input signal RF 41  as will be known to persons skilled in the art. The phase angle of returned signal PR 43   c  will thus be retarded 90 degrees over the course of quarter-wavelength stub  47 . The phase angle of returned signal PR 43   c  is thus approximately the same as the phase angle of returned signal PR 44   b  after traversing quarter-wavelength stub  47 . Quarter-wavelength stub  47  is terminated by short circuit (ground)  46 . Returned signal PR 43   c  is thus reflected by the impedance mis-match of short circuit (ground)  46  and re-traverses quarter-wavelength stub  47  to third circulator port  43   c , retarding the phase angle of returned signal PR 43   c  a further 90 degrees along the way. Quarter-wavelength stub  47  and short circuit (ground)  46  thus retard the phase angle of returned signal PR 43   c  a total of 180 degrees. The phase angle of returned signal PR 43   c  is now approximately 90 degrees behind the phase angle of returned signal PR 44   b . Thus returned signal PR 44   b  is now in quadrature with returned signal PR 43   c . Returned signal PR 43   c  is circulated to first circulator port  43   a  of circulator  43  where it emerges as returned signal PR 43   a  and enters input hybrid  42  at third hybrid port  42   c  of input hybrid  42 . Returned signal PR 44   b , in contrast, bypasses circulator  43  and is input directly to fourth hybrid port  42   d  of input hybrid  42 . Circulator  43  thus sees only half of returned signal P REFE . Returned signal PR 43   a , which entered input hybrid  42  at third hybrid port  42   c  of input hybrid  42 , recombines with returned signal PR 44   b  entering input hybrid  42  at fourth hybrid port  42   d  of input hybrid  42 , and emerges from input hybrid  42  at second hybrid port  42   b  of input hybrid  42  as returned signal PR 42   b . Returned signal PR 42   b  exits input hybrid  42  at second hybrid port  42   b  of input hybrid  42  and not first hybrid port  42   a  of input hybrid  42  because returned signal PR 44   b  is now in quadrature with returned signal PR 43   a . Second hybrid port  42   b  of input hybrid  42  is terminated with matched impedance  45 , thus completely attenuating returned signal PR 42   b . The input load attached to first hybrid port  42   a  of input hybrid  42  thus sees none of the returned signal P REFL . Since circulator  43  sees only half of the forward signal and half of the returned signal, the input load can be doubled. 
     In FIG. 6 is shown the forward signal path of a second embodiment of the circulator-based isolator power capacity doubling apparatus according to the present invention. The second embodiment is generally the concept of the first embodiment that was shown in FIGS. 4 and 5, extended to a plurality of input devices. RF signals RF 51 - 1 , RF 51 - 2 , RF 51 - 3 , . . . RF 51 -n from input devices  51 - 1 ,  51 - 2 ,  51 - 3 , . . .  51 -n, respectively, enter hybrids  52 - 1 ,  52 - 2 ,  52 - 3 , . . .  52 -n at first hybrid ports  52 - 1   a ,  52 - 2   a ,  52 - 3   a  . . .  52 -na, respectively, where they are divided into signals RF 52 - 1   c , RF 52 - 2   c , RF 52 - 3   c  . . . RF 52 -nc and signals RF 52 - 1   d , RF 52 - 2   d  , RF 52 - 3   d  . . . RF 52 -nd. Signals RF 52 -l d , RF 52 - 2   d , RF 52 - 3   d  . . . RF 52 -nd are in quadrature with signals RF 52 - 1   c , RF 52 - 2   c , RF 52 - 3   c  . . . RF 52 -nc, respectively. Signals RF 52 - 1   c  , RF 52 - 2   c , RF 52 - 3   c  . . . RF 52 -nc are multiplexed in multiplexer  59 - 1 , forming signal RF 59 - 1 , while signals RF 52 - 1   d , RF 52 - 2   d , RF 52 - 3   d  . . . RF 52 -nd are multiplexed in multiplexer  59 - 2 , forming signal RF 59 - 2 . Multiplexers  59 - 1  and  59 - 2  can be time, frequency, or code division multiplexers, or any equivalent type of signal combination means, such that the multiplexed signals can also be de-multiplexed. Signal RF 59 - 2  is in quadrature with signal RF 59 - 1 . Signal RF 59 - 1  enters first circulator  53  at port  53   a  of circulator  53  and emerges from second circulator port  53   b  of circulator  53  as signal RF 53   b . In this embodiment circulator  53  is a Y-junction circulator but a circulator with more than three ports could be used as well by appropriately terminating the unused ports. Signal RF 53   b  then enters output hybrid  54  at first hybrid port  54   a  of output hybrid  54 . Signal RF 59 - 2 , meanwhile, bypasses circulator  53  and is input directly to second hybrid port  54   b  of output hybrid  54 . Signal RF 53   b  then recombines with signal RF 59 - 2  in output hybrid  54  and emerges from fourth hybrid port  54   d  of output hybrid  54  as signal RF 54   d . Signal RF 54   d  is then transmitted to an output load which may, for example, be an antenna. The circulator  53  thus sees only half of the forward power. Third hybrid port  54   c  of output hybrid  54  is terminated with matched impedance  58 . Circulator port  53   c  of circulator  53  is terminated with an appropriate impedance, as described below. 
     In FIG. 7 is shown the reverse signal path of the second embodiment of the circulator-based isolator power capacity doubling apparatus according to the present invention that was shown in FIG.  6 . Returned signal P REFL  returns to output hybrid  54  at fourth hybrid port  54   d  of output hybrid  54 . Returned signal P REFL  is divided by output hybrid  54 , with one-half of returned signal P REFL  emerging from first hybrid port  54   a  of output hybrid  54  as returned signal PR 54   a  while the other half of returned signal P REFL  emerges from second hybrid port  54   b  of output hybrid  54  as returned signal PR 54   b . Returned signal PR 54   a  is in quadrature with returned signal PR 54   b . Returned signal PR 54   a  enters circulator  53  at second circulator port  53   b  of circulator  53  and is circulated to third circulator port  53   c  of circulator  53 , where it emerges as PR 53   c . Returned signal PR 53   c  then travels down quarter-wavelength stub  57 . The electrical length of quarter-wavelength stub  57  is preferably equal to one-quarter of the wavelength of the RF input signals RF 51 - 1 , RF 51 - 2 , RF 51 - 3  . . . RF 51 -n (shown in FIG.  6 ), but it may be any odd multiple of one-quarter of the wavelength of the RF input signals RF 51 - 1 , RF 51 - 2 , RF 51 - 3  . . . RF 51 -n as will be known to persons skilled in the art. The phase angle of returned signal PR 53   c  is thus retarded 90 degrees over the course of quarter-wavelength stub  57 . The phase angle of returned signal PR 53   c  is thus approximately the same as the phase angle of returned signal PR 54   b  after traversing quarter-wavelength stub  57 . Quarter-wavelength stub  57  is terminated by short circuit (ground)  56 . Returned signal PR 53   c  is thus reflected by the impedance mis-match of short circuit (ground)  56  and re-traverses quarter-wavelength stub  57  back to third circulator port  53   c  of circulator  53 , retarding the phase angle of returned signal PR 53   c  a further 90 degrees along the way. Quarter-wavelength stub  57  and short circuit (ground)  56  thus retard the phase angle of returned signal PR 53   c  a total of 180 degrees. Returned signal PR 54   b  is now in quadrature with returned signal PR 53   c . Returned signal PR 53   c  is circulated to first circulator port  53   a  of circulator  53  where it emerges as returned signal PR 53   a  and is input to multiplexer  59 - 1 . Multiplexer  59 - 1  de-multiplexes returned signal PR 53   a  into returned signals PR 59 - 11 , PR 59 - 12 , PR 59 - 13 , . . . PR 59 - 1   n . Returned signals PR 59 - 11 , PR 59 - 12 , PR 59 - 13 , . . . PR 59 - 1   n  enter input hybrids  52 - 1 ,  52 - 2 ,  52 - 3 , . . .  52 -n at third hybrid ports  52 - 1   c ,  52 - 2   c ,  52 - 3   c , . . .  52 -nc, respectively. Returned signal PR 54   b , in contrast, bypasses circulator  53  and enters multiplexer  59 - 2 , where it is de-multiplexed into returned signals PR 59 - 21 , PR 59 - 22 , PR 59 - 23 , . . . PR 59 - 2   n . Returned signals PR 59 - 21 , PR 59 - 22 , PR 59 - 23 , . . . PR 59 - 2   n  then enter fourth hybrid ports  52 l d ,  52 - 2   d ,  52 - 3   d , . . .  52 -nd, respectively. Circulator  53  thus sees only half of returned signal PREFL. Returned signals PR 59 - 11 , PR 59 - 12 , PR 59 - 13 , . . . PR 59 - 1   n , which entered input hybrids  52 - 1 ,  52 - 2 ,  52 - 3 , . . .  52 -n at third hybrid ports  52 - 1   c ,  52 - 2   c ,  52 - 3   c , . . .  52 -nc, respectively, recombine with returned signals PR 59 - 21 , PR 59 - 22 , PR 59 - 23 , . . . PR 59 - 2   n  entering input hybrids  52 - 1 ,  52 - 2 ,  52 - 3 , . . .  52 -n at fourth hybrid ports  52 - 1   d ,  52 - 2   d ,  523   d , . . .  52 -nd, respectively, and emerge from input hybrids  52 - 1 ,  52 - 2 ,  52 - 3 , . . .  52 -n at second hybrid ports  52 - 1   b ,  52 - 2   b ,  52 - 3   b , . . .  52 -nb as returned signals PR 52 - 1   b , PR 52 - 2   b , PR 52 - 3 b, . . . PR 52 -nb, respectively. Returned signals PR 521   b , PR 52 - 2   b , PR 52 - 3   b , . . . PR 52 -nb exit input hybrids  52 - 1 ,  52 - 2 ,  52 - 3 , . . .  52 -n at second hybrid ports  52 - 1   b ,  52 - 2   b ,  52 - 3   b , . . .  52 -nb and not first hybrid ports  52 -l a ,  52 - 2   a ,  52 - 3   a , . . .  52 -na because returned signals PR 59 - 21 , PR 59 - 22 , PR 59 - 23 , . . . . PR 59 - 2   n  are in quadrature with returned signals PR 59 - 11 , PR 59 - 12 , PR 59 - 13 , . . . PR 59 - 1   n . Second hybrid ports  52 - 1   b ,  52 - 2   b ,  52 - 3   b , . . . .  52 -nb are each terminated with matched impedances  55 - 1 ,  55 - 2 ,  55 - 3 , . . .  55 -n, respectively, thus completely attenuating returned signals PR 52 - 1   b , PR 52 - 2   b , PR 52 - 3   b , . . . PR 52 -nb. The input loads  51 - 1 ,  51 - 2 ,  51 - 3 , . . . .  51 -n attached to first hybrid ports  52 -l a ,  52 - 2   a ,  52 - 3   a , . . .  52 -na, respectively, thus see none of the returned signal P REFL . Since circulator  53  sees only half of the forward signal and half of the returned signal, the input load can be doubled. 
     FIG. 8 shows the forward signal path of a third embodiment of the circulator-based isolator power capacity doubling apparatus according to the present invention. In FIG. 8, RF signal RF 61  from an input device  61  which may be, for example, an amplifier, enters input hybrid  62  at first hybrid port  62   a  of input hybrid  62  and is divided, with one-half of signal RF 61  emerging at third hybrid port  62   c  of input hybrid  62  as signal RF 62   c  while the other half of signal RF 61  emerges at fourth hybrid port  62   d  of input hybrid  62  as signal RF 62   d . Signal RF 62   d  is in quadrature with signal RF 62   c . Signal RF 62   c  enters circulator  63  at first circulator port  63   a  of circulator  63  and emerges from second circulator port  63   b  of circulator  63  as signal RF 63   b . In this embodiment circulator  63  is a four-port circulator but a circulator with more than four ports could be used as well by appropriately terminating the unused ports. RF 63   b  then enters output hybrid  64  at second hybrid port  64   a  of output hybrid  64 . Signal RF 62   d , meanwhile, bypasses circulator  63  and is input directly to second hybrid port  64   b . Signal RF 63   c  is then recombined with signal RF 62   d and emerges at fourth hybrid port  64   d  of output hybrid  64  as signal RF 64   d . Signal RF 64   d  is then transmitted to an output load which may be, for example, an antenna. The circulator  63  thus sees only half of the forward signal power. Third hybrid port  64   c  of output hybrid  64  is terminated with matched impedance  68 . Third circulator port  63   c  and fourth circulator port  63   d  of circulator  63  and are each terminated with appropriate impedances, as described below. 
     FIG. 9 shows the reverse signal path of the third embodiment of the circulator-based isolator power capacity doubling apparatus that was shown in FIG.  8 . In FIG. 9, the returned signal P REFL  returns to output hybrid  64  at fourth hybrid port  64   d  of output hybrid  64 . Returned signal P RELF  is divided by output hybrid  64 , with one-half of returned signal P REFL  emerging from first hybrid port  64   a  of output hybrid  64  as returned signal PR 64   a  while the other half of returned signal P REFL  emerges from second hybrid port  64   b  of output hybrid  64  as returned signal PR 64   b . Returned signal PR 64   a  is in quadrature with returned signal PR 64   b . Returned signal PR 64   a  enters circulator  63  at second circulator port  63   b  of circulator  63  and is circulated to third circulator port  63   c  of circulator  63 , where it emerges as returned signal PR 63   c . Returned signal PR 63   c  then travels down half-wavelength transmission line  67  to fourth circulator port  63   d  of circulator  63 . Half-wavelength transmission line  67  is preferably a transmission line with an electrical length equal to one-half of the wavelength of the RF input signal RF 61  (shown in FIG.  8 ). The phase angle of returned signal PR 63   c  is thus retarded 180 degrees over the course of half-wavelength transmission line  67 . The phase angle of returned signal PR 63   c  is thus approximately 90 degrees behind the phase angle of returned signal PR 64   b  when it re-enters circulator  63  at third circulator port  63   d  of circulator  63 . Thus returned signal PR 64   b  is now in quadrature with PR 63   c . Returned signal PR 63   c  is circulated to first circulator port  63   a  of circulator  63  where it emerges as returned signal PR 63   a  and enters input hybrid  62  at third hybrid port  62   c  of input hybrid  62 . Returned signal PR 64   b , in contrast, bypasses circulator  63  and is input directly to fourth hybrid port  62   d  of input hybrid  62 . Circulator  63  thus sees only half of returned signal P REFL  Returned signal PR 63   a , which entered input hybrid  62  at third hybrid port  62   c  of input hybrid  62 , recombines with returned signal PR 64   b  entering input hybrid  62  at fourth hybrid port  62   d  of input hybrid  62 , and emerges from input hybrid  62  at second hybrid port  62   b  of input hybrid  62  as returned signal PR 62   b . Returned signal PR 62   b  exits input hybrid  62  at second hybrid port  62   b  of input hybrid  62  and not first hybrid port  62   a  of input hybrid  62  because returned signal PR 63   a  is in quadrature with returned signal PR 64   b . Second hybrid port  62   b  of input hybrid  62  is terminated with matched impedance  65 , thus completely attenuating returned signal PR 62   b . The input load attached to first hybrid port  62   a  of input hybrid  62  thus sees none of the returned signal P REFL  Since circulator  63  sees only half of the forward signal and half of the returned signal, the input load can be doubled. The third embodiment can be extended to multiple input devices in the manner of the second embodiment. 
     The invention having been thus described, it will be apparent to those skilled in the art that the same may be varied in many ways without departing from the spirit and scope of the inventions. All such modifications are intended to be encompassed by the following claims.