Patent Abstract:
The present invention features a broadband radio frequency (RF) device in the form of a power splitter. A broadband spiral transmission line power divider is used to divide power into two powers with a constant phase difference between the two divided powers. The power divider produces large bandwidths.

Full Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefore. 
    
    
     CROSS REFERENCE TO OTHER PATENT APPLICATIONS 
     None. 
     BACKGROUND OF INVENTION 
     (1) Field of the Invention 
     The present invention relates generally to transmission lines and more particularly, to broadband spiral transmission line power splitters. 
     (2) Description of Prior Art 
     The use of ¼ wavelength 90 degree power splitters is well known in the prior art. For example,  FIG. 1   a  and  FIG. 1   b  show a prior art ¼ wavelength 90 degree power splitters comprising two “hot” lines L 1  and L 2  that are two unbalanced transmission lines running side-by-side for a ¼ wavelength distance on a dielectric surface sheet  108 . The lines share a common ground plane and have a characteristic impedance, Z 0 , to ground, which is usually 50 ohms. Also referring to  FIG. 1   c , the ground plane consists of a top metal ground plane gp 1  and a bottom metal ground plane gp 2 . The two ground planes are held together by metal sides  110 . If the power splitter is made symmetrical about a horizontal plane  2014  through the centers of the lines L 1  and L 2 , so that the thickness  2001  of the space above the lines L 1  and L 2  equals the thickness  2002  of the space below the lines, and the unused space inside the cavity  2003  formed by the two ground planes gp 1 , gp 2  and the two metal sides  110  is filled with the same dielectric material as dielectric  108 , then the structure is strip line. 
     If as shown in  FIG. 1   b , thickness  2001  is appreciably larger than thickness  2002  and dielectric  108  only exists between the lines L 1  and L 2  and ground plane gp 2 , the structure is microstrip. In this case, there is little or no coupling between ground plane gp 2  and lines L 1  and L 2 , and thus ground plane gp 2  serves more as a shield than a ground plane. 
     A first line L 1  has a first end  100  as an input (port relative to ground) and a second end  102  as an output (port relative to the ground). A second line L 2  has a first end  104  and is coupled to power of the first line L 1  wherein the amount of power coupled thereto increases as the coupling between the two lines increases and the separation between the two lines decreases. Even higher degrees of coupling occur if the lines L 1  and L 2  start to overlap each other without touching. The coupled output port relative to ground or first end  104  of the second line L 2  is on the same end as the input end  100  of the first line L 1 . A second end  106  of the second line L 2  is an isolation or dump port relative to ground, and terminates to ground in Z 0 . Ideally, when all of the ports are properly matched to Z 0 , the resultant phase difference between the outputs of the first and second lines L 1  and L 2  is 90 degrees, and all of the input power is divided between the two output ports, with none of it going to the isolation port  106 . 
     If the power splitter is made with microstrip, then access to the ports at the ends of the lines are made by placing connectors on the outside of the splitter located below the ends and below the ground plane gp 2 , such as for example at locations  2006  and  2007  for respective ports  104  and  106  for line L 2 . If the splitter is made with strip line, to maintain symmetry, the locations are moved up the sides  110  of the splitter to lie coincident with axis of the lines L 1  and L 2 , such as for example locations  2008  and  2009  for respective ports  104  and  106  of lines L 2 . Since the two adjacent ends of the two lines L 1  and L 2  are usually very close to each other, the locations are spread out to allow two corresponding connectors to be placed adjacent to each other, and added lengths of transmission line are used to connect the connectors to the ends of the lines. 
     If the coupled power is less than one half the input power, the power splitter is also called a directional coupler, because the coupled power depends on the direction of the wave travelling along the line L 1 . When power is applied to input port  100 , it travels in a forward direction from the port to output port  102  and some of it is coupled to line L 2  at coupled port  104 . No power is coupled to isolation port  106 . If instead power were inserted at output port  102 , it would flow backwards from the output port  102  to input port  100  and the roles of the coupled and isolation ports would become reversed. Power from the backward travelling wave would couple to port  106 , which is now a coupled port, and none of the power would couple to port  104  which is now an isolation port. In general, if any given port of any first line serves as an input port, the opposite port of the first line will be an output port, the adjacent port on the second line will be a coupled port, and the opposite port of the adjacent port on the second line will be an isolation port. 
     Because the widths of lines L 1  and L 2  and their separation  2005  are much smaller than their λ/4 length, the size of the power splitter is primarily determined by the length of the lines L 1  and L 2 . At lower frequencies this length can become excessive. One well known method to reduce the length of the lines is meandering. The lines L 1  and L 2  can be meandered about a center line between the two lines L 1  and L 2 .  FIG. 1   d  shows the prior art splitter as  FIG. 1   a  where the lines L 1  and L 2  are separated less and are meandered. Reducing the line widths and their separation further would allow more meander cycles and a much smaller device. Care must be taken to avoid allowing adjacent section of a line, e.g.  1002  and  1003  from coming too close to each other. If the separation  1001  becomes too small, broadside coupling between the sections  1002  and  1003  occurs, and a second mechanism of power transfer between the line ends is introduced along the straight line direction  1004  between the line ends. Only the primary mechanism of power transfer along the transmission line paths of L 1  and L 2  should be allowed. 
     The coupling for the ¼ wavelength 90 degree splitter is frequency dependent wherein maximum coupling occurs every ½ wavelength starting at ¼ wavelength. Nulls in coupling occur every ½ wavelength starting at zero wavelengths between the maximum points. Normally the splitter is used at ¼ wavelength. Because the maximum at ¼ wavelength is between nulls at zero and ½ wavelengths, the ¼ wavelength 90 degree splitter is narrowband as far as constant coupling with frequency is concerned. To obtain broader bandwidth, additional ¼-wavelength sections of two coupled lines are added to the splitter, making it more complex. 
     A disadvantage of the ¼ wavelength 90-degree splitter of the prior art is that its length results in narrowband performance. In the alternative, a bifilar spiral appears lengthless in the radiation domain above a cut-in frequency. Different circumferential lengths radiate at different frequencies, so that radiation always occurs from a circumferential electrical length of one wavelength. When used as a transmission line, a length of a bifilar spiral is difficult to define. Starting at a center feed point of the bifilar spiral and moving outwardly in a circular direction the filar of the spiraled transmission line making up the bifilar spiral eventually couples broadside to itself (via the other filar) at the next turn and then at each succeeding turn with the circumferential length between each turn increasing. If the one-dimensional transmission line is of low Z 0  and highly coupled, the filar of the transmission line becomes highly coupled to itself, and the transmission line starts to appear to be a two-dimensional instead of a one-dimensional transmission line. Power is transferred in the circumferential direction via the two line transmission line and in the radial direction via broadside coupling. This differs from the λ/4 90 degree splitter where power transfer is only via the two line transmission line. Thus the two-dimensional size and different circumferential lengths may allow broadband behavior. 
     U.S. Pat. No. 6,133,891, hereby incorporated by reference, describes spiral transmission lines. This patent describes two spirals that are crossed to form two crossed transmission lines comprising elements for feeding and matching a quadrifilar helix. The two transmission lines are approximately balanced and are of constant or smoothly changing Z 0  with length except for the last ½ of a turn of any given element on the outermost circumference. For a given transmission line length, the given filar has filars on both of its sides. However for the last ½ turn, the given filar has only one opposite filar, which is on the side closest the feed points (the central region) of the spiral. This increases the Z 0  of the transmission line along this ½ turn causing a mismatch. 
     The mismatch shows up as a small increased antenna mismatch when the transmission line is used to feed and match the antenna. If the width of the filar is increased in the area of the ½ turn to increase capacitance to the opposite filar, the Z 0  between the ½ turn of filar and its opposite transmission line filar decreases back to normal. But now the capacitance between the opposite filar and its two surrounding opposite filars, which includes the widened ½ turn of filar, becomes larger than normal resulting in its Z 0  becoming lower than normal. Thus, this attempt at fixing the first mismatch of the ½ turns of filar creates a second mismatch. 
     SUMMARY OF THE INVENTION 
     The present invention features a broadband radio frequency (RF) device in the form of a power splitter. A broadband spiral transmission line power splitter is used to divide power into two powers with a constant phase difference between the two divided powers. The power splitter produces large bandwidths. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will be better understood in view of the following description of the invention taken together with the drawings wherein: 
         FIG. 1   a  is a top view schematic of a 90-degree power splitter known in the prior art; 
         FIG. 1   b  is a width side cross sectional view schematic of a 90 degree power splitter known in the prior art; 
         FIG. 1   c  is a schematic of a length side cross sectional view through the center of one of the filars of a 90 degree power splitter known in the prior art; 
         FIG. 1   d  is a side cross sectional view schematic of a 90 degree power splitter known in the prior art after it has undergone meandering; 
         FIG. 2   a  is a top view of a bifilar spiral mounted above a ground plane according to one embodiment of the present invention; 
         FIG. 2   b  is a side cross sectional view of a bifilar spiral mounted above a ground plane according to one embodiment of the present invention; 
         FIG. 3  is the bifilar spiral power splitter of  FIG. 2   a  composed of a bifilar spiral mounted above a ground plane slightly modified and in more detail; 
         FIG. 4  is the backside of the ground plane of the bifilar spiral power splitter shown in  FIG. 3 ; 
         FIG. 5   a  is a cross-sectional view of the bifilar spiral power splitter shown in  FIGS. 3 and 4 ; and 
         FIG. 5   b  is an alternative embodiment of the bifilar spiral power splitter shown in  FIGS. 3 and 4  with some of the connectors flipped. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A bifilar spiral power splitter  40 , as illustrated in  FIG. 2   a  and  FIG. 2   b , is made by modifying the traditional ¼ wavelength 90 degree power splitter shown in  FIG. 1   a . A bifilar spiral  43  composed of interleaved filars  12   a  and  12   b , is placed between a common ground plane formed by a top ground plane  422  and a bottom ground plane  42  both of which are supported by metal sides  423 . Ends  41  of the filars  12   a  and  12   b  are fed or feed against the common ground in a 50 ohm system, as opposed to the filars  12   a  and  12   b  being fed against each other at the center of the spiral  43 . 
     If the bifilar spiral power splitter  40  is made of microstrip, the distance  3001  between the top ground plane  422  and the filars  12   a  and  12   b  is significantly larger than the distance  3002  between the filars  12   a  and  12   b  and the bottom ground plane  42 , and the space  3003  above the spiral  43  is air. This causes little coupling between spiral  42  and top ground plane  422  and thus the top ground plane  422  is only a shield. Between the spiral  43  and bottom ground plane  42  is an insulating dielectric substrate  44  through which the spiral  43  mostly couples to the bottom ground plane  42 . Connectors are placed on the outside bottom of the bottom ground  42  to connect to the filar ends  41  and port locations at locations  3006 ,  3007 ,  3008  and  3009 . 
     If the bifilar spiral power splitter  40  is made of stripline, the top and bottom separations  3001  and  3002  are equal, and insulating dielectric substrate  44  is on both sides of the spiral  43  and the adjacent ground planes  42  and  422 . Coupling to both ground planes  42  and  422  is equal. The connectors are moved to be in line with the spiral plane. The two connectors on the outer edges of the spiral  43  are moved up the metal sides  423  to connect to the outer spiral ends at locations  3010  and  3013 . For the inner ends of the spiral  43 , a metal cylinder  424  of radius less than the radius of an inner end of a filar  12   a  or  12   b  is centered on the vertical axis of the spiral  43  to connect to the top and bottom ground planes  42  and  422 . Material from the two ground planes  42  and  422  and part of the insulating dielectric substrate  44  are removed to allow for the presence of the metal cylinder  424 . The two connectors on the inner edges of the spiral  43  are moved up the sides of the metal cylinder  424  to connect to the inner spiral ends at locations  3011  and  3012 . To allow for the size of the connectors, the inner radius of the spiral  43  is increased. As an alternative, an added length of coaxial cable can be used to extend the connection point to the bottom of the bottom ground plane  42 , where connectors would be added. 
     The bifilar spiral power splitter  40  may have broadband properties, since a first filar  12   b  is coupled on both sides by a second filar  12   a , and the transmission lines composed of coupled filars  12   a  and  12   b  relative to ground take on a two-dimensional form, where a radial cross-section crosses continuously alternating sections between the first filar  12   b  and the second filar  12   a . As the spiral radius increases, the bifilar spiral  43  has circumferences of transmission line that increase in length. The broadside coupling between the varying circumferential lengths of transmission line may allow broad banding or large bandwidths (e.g., 2:1). 
     Referring now to  FIGS. 3-5   a  there is illustrated a slightly modified and detailed embodiment of the present invention wherein one of the bifilar spirals of the previous described two open crossed bifilar spirals of U.S. Pat. No. 6,133,891 is modified by placing it above ground plane  57  to form bifilar spiral power splitter  51  having broadband properties, as described in the previous paragraph. 
     The power splitter  51  illustrated in  FIGS. 3 ,  4 , and  5   a  comprises a fiberglass board  50  having a thickness of 1/16 of an inch. The bifilar spiral  43  has two filars  52 ,  53  shaped as an Archimedean spiral. The filars  52 ,  53  are copper plated and attached to a surface of the fiberglass board  50 . The bifilar spiral  43  has a width of 4.3 inches. The filars  52 ,  53  have a width “H” of 3/32 of an inch, and the separation between the filars  52 ,  53  is approximately 3/32 of an inch except for the last ⅛ of a turn, designated as “C”. This is so because the widths of the filars  52 ,  53  taper down to a fine strip at their ends most distant from a center axis  75  of the bifilar spiral  43 . The filars  52 ,  53  have beginning points  62 ,  63 , near the center of the spiral, commonly used as the feed points for a normal bifilar spiral by itself. The beginning points  62 ,  63  are operatively connected to extended center conductors  54 ,  55  of coaxial connectors  542 ,  552 . The coaxial connectors  542 ,  552  are preferably SMA coaxial connectors. The center conductors  54 ,  55  protrude through a backside  56  of the fiberglass board  50  to the filar ends. The outer conductors  541 ,  551  of coaxial connectors  542 ,  552  are soldered to a copper tape or copper plated ground plane  57 , which covers the entire backside  56  of the fiberglass board  50 . 
     The ends of the filars  52 ,  53  that are furthest from the beginning points  62 ,  63  at the outermost circumference of the bifilar spiral  43  are operatively connected to center conductors  60 ,  61 , respectively of 2 coaxial connectors  602  and  612 . The coaxial connectors are preferably SMA connectors. The center conductors  60 ,  61  protrude through the backside  56  of the fiberglass board  50  to the filar ends. The outer conductors  601 ,  611  are soldered to the ground plane  57 , which covers the backside  56  of the fiberglass board  50 . However, the end lengths of the filars  52 ,  53  starting at points  64 ,  65  are tapered at the last ⅛ of the turn and add excessive inductance to the impedance of the filars  52 ,  53  when the power splitter  51  is matched to 50 ohms with the center conductors  60 ,  61  of connectors  602  and  612  connected to filar ends J 1  and J 2  located at H 1  and H 2 . Thus, to correct this, the filars  52 ,  53  are terminated before the taper starting at  64 ,  65  by adding gaps G 1 , G 2  in the filars  52 ,  53 , and connectors  602  and  612  are moved before the gaps where their center conductors  60  and  61  are connected to filars  52  and  53 . The gaps G 1 , G 2  are made across the filars  52 ,  53 . The gaps G 1 , G 2  disconnect the last ⅛ turn of the filars  52 ,  53 . Disconnecting the filars  52 ,  53  before the taper ensure that the filars  52 ,  53  have a constant width. The tapers starting at points  64 ,  65  of the filars  52 ,  53  are not removed to maintain a gradual transition from conductor to nonconductor along the circumferential and radial directions. 
     A second ground plane  66  is placed over and insulated with a dielectric layer  67  from the bifilar spiral  43 , thereby placing the filars  52 ,  53  between two ground planes  57 ,  66  similar to a stripline configuration. If dielectric layer  67  is removed and ground plane  66  is placed high enough above the spiral  43  to prevent coupling to the spiral  43 , then the configuration is microstrip. By placing the filars  52 ,  53  between the ground planes  57 ,  66 , the bifilar spiral  43  is precluded from radiating. However, the impedance of either of the filars  52  or  53  with respect to the first ground plan  57  is greater than 50 ohms because of the smaller than required width of the filars  52  and  53 . To lower the impedance to 50 ohms, the distance of the second ground plane  66  from the bifilar spiral  43  is adjusted until a low VSWR is realized at any of the inputs of any connectors attached to the bifilar spiral power splitter  51  (any given port) with all other connectors (ports) terminating in 50 ohms. 
     The ground planes  57  and  66  are normally operatively connected together by placing metal sides  68 ,  69  around the entire perimeter of the fiberglass board  50 . The combination of the ground planes  57 ,  66  and the metal sides  68 ,  69  encloses and shields the power splitter  51 . 
     For the configuration of power splitter  51  to be made truly stripline, then it is made symmetrical about horizontal plane  4001  that passes through the plane of the spiral  43 . The distance  4002  between the spiral  43  and top ground plane  66  is made the same as the distance  4003  between the spiral  43  and bottom ground plane  57 . The connectors are moved up and rotated so that their center conductors lie horizontally in plane  4001 . Connector  602  is moved up to location  4006  and its outer conductor  601  is instead soldered to metal side  68 . Connector  61  is moved up to location  4009  and its outer conductor  611  is instead soldered to metal side  69 . For the inner connectors, a vertical metal surface is provided by drilling a hole about the vertical axis  75  in the structure to allow insertion of a metal cylinder  4005 . The top of cylinder  4005  is soldered to the edge of the resultant hole in ground plane  66 . The bottom of cylinder  4005  is soldered to the edge of the resultant hole in ground plane  57 . Connector  54  is moved up to location  4007  in the metal cylinder  4005  and its outer conductor  541  is instead soldered to the inside surface of metal cylinder  4005 . In the same fashion connector  55  is placed at location  4008 . If there is no room inside the cylinder  4005  for connectors  54  and  55 , then they are placed outside of the splitter  51  and short lengths of coaxial cable are used to connect them to their respective locations  4007  and  4008 . 
     In an alternative embodiment, shown in  FIG. 5   b , the filars  52 ,  53  are required to be overlapped because of higher desired coupling between the filars  52 ,  53 . Overlapping is done by flipping over one of the filars  53  to lie on the dielectric layer  67  which lies on the second ground  66 . Optionally the inner center conductor  55  and outer connector  551  of connector  552 , and the outer center conductor  61  and outer conductor  611  of connector  612  are flipped over so that the outer conductors of the connectors lie on and connect to the second ground  66 . The locations of the optionally flipped over connectors is shown in  FIG. 5   b ; the locations of the not flipped over connectors is shown in  FIG. 5   a . The filar  53  becomes filar  70 , and optionally center conductors  55  and outer conductor  551  of connector  552  becomes alternate center conductor  71  and outer conductor  711  of connector  712 , and center conductor  61  and outer conductor  611  of connector  612  becomes alternate center conductor  72  and outer conductor  721  of connector  722 . In essence, the filars  53 , and optionally center conductor  55  and outer conductor  551  of connector  552 , and center conductor  61  and outer conductor  611  of connector  612  are replaced by filar  70 , and optionally the alternate center conductor  71  and outer conductor  711  of connector  712 , and the alternate center conductor  72  and outer conductor  721  of connector  722 . Further, the filars  52 ,  70  are widened by width “HH”. The dielectric layer  80  is inserted between the filars  52  and  70  to keep them from touching each other. 
     With four connectors placed on the bifilar spiral power splitter  51  and the structure adjusted for a 50 ohm system, S parameters were measured between all ports that are not symmetrical, between 0 and 400 MHz, for the power splitter shown in  FIG. 5   a , without dielectric  67  and sides  68  and  69 . 
     The power splitter  51  has ports  1  ( 64 ) and  4  ( 65 ) located near outside ends of the bifilar spiral  43  at the truncated end of the filars before the gaps and are shown further apart than ports  2  ( 62 ) and  3  ( 63 ), which are the beginning points  62 ,  63  of the bifilar spiral  43 . Since the ports must physically connect to the outside world, the coaxial connectors connected to these port points are more appropriately called the ports. Thus, e.g., connector  602  can be considered the port at  64  instead of  64 . 
     Measurements are described in the table below. Unused ports for a particular measurement are shown terminated to ground. The port order for the “S” parameters is reversed and not in the normal convention (e.g., S 21  is written as S 12 ). This is to emphasize the physical paths from one port to another, whereas normal convention emphasizes power at an output port relative to power at an input port. A measurement is taken with one ground ( 57 ) and then a measurement is taken with two grounds ( 57  and  66 ). For lower frequencies, radiation from the one ground measurement is insignificant. However, at higher frequencies, radiation is significant and the presence of the second ground is needed to prevent unwanted radiation. 
     The removal of the metal sides also allowed some adjusting of coupling between filars by adjusting the height of the top ground planes  66  relative to the spiral  43 . The more a filar is surrounded by ground, the less it couples to adjacent filars. The top ground plane  66  is more of a floating ground than a true ground, because with the sides missing it is not connected to the bottom ground plane  57  and the cables&#39; outer conductors. However, the filars  52  and  53  can still couple to ground via coupling to the top ground plane  66  since the top ground plane  66  itself is highly coupled to the bottom ground plane  57  when it is close to it. When the top ground plane  66  is far enough from the spiral  43  or missing, the structure is microstrip. When it is close enough so the spiral  43  can couple to it, the structure is a modified form of strip line. Physically, the second ground  66  was implemented by holding a metal plate at a short distance above the bifilar spiral  43 ; or by inserting a thin layer of foam plastic between the spiral and the metal plate. The following measurements were taken: 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                 MEASURE- 
                   
                 2 ND   
                 MEASUREMENT (all unused  
               
               
                 MENTS 
                 PORTS 
                 GROUND 
                 ports are terminated in 50 ohms) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 1 
                 1 
                 no 
                 S 11 , outer port 
               
               
                 2 
                 1 
                 yes 
                 S 11 , outer port 
               
               
                 3 
                 2 
                 no 
                 S 22 , inner port 
               
               
                 4 
                 2 
                 yes 
                 S 22 , inner port 
               
               
                 5 
                 1 2 
                 yes 
                 S 12  magnitude, S 12  phase set to 0° 
               
               
                 6 
                 1 3 
                 yes 
                 S 13  magnitude, S 13 /S 12  phase 
               
               
                 7 
                 1 4 
                 yes 
                 S 14  magnitude, S 14 /S 12  phase 
               
               
                 8 
                 1 2 
                 no 
                 S 12  magnitude, S 12  phase set to 0° 
               
               
                 9 
                 1 3 
                 no 
                 S 13  magnitude, S 13 /S 12  phase 
               
               
                 10 
                 1 4 
                 no 
                 S 14  magnitude, S 14 /S 12  phase 
               
               
                 11 
                 2 1 
                 yes 
                 S 21  magnitude, S 21  phase set to 0° 
               
               
                 12 
                 2 4 
                 yes 
                 S 24  magnitude, S 24 /S 12  phase 
               
               
                 13 
                 2 4 
                 yes 
                 S 24  magnitude, S 24 /S 12  phase 
               
               
                   
                   
                   
                 measured with second ground  
               
               
                   
                   
                   
                 tighter to spiral, about 1/64″ 
               
               
                 14 
                 2 3 
                 yes 
                 S 23  magnitude, S 23 /S 12  phase 
               
               
                 15 
                 2 1 
                 no 
                 S 21  magnitude, S 21  phase set to 0° 
               
               
                 16 
                 2 4 
                 no 
                 S 24  magnitude S 24 /S 21  phase 
               
               
                 17 
                 2 3 
                 no 
                 S 23  magnitude, S 23 /S 21  phase 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                 MEASURE- 
                 MEASURE-  
                 MEASURE-  
               
               
                   
                   
                 MENT 
                 MENT 
                 MENT 
               
               
                 PORT 
                 PART 
                 1-4, PORT 
                 5-10, PORT  
                 11-17, PORT  
               
               
                 NUMBER 
                 NUMBER 
                 TYPE 
                 TYPE 
                 TYPE 
               
               
                   
               
             
             
               
                 1 
                 602 
                 input 
                 input 
                 output 
               
               
                 2 
                 542 
                 input 
                 output 
                 input 
               
               
                 3 
                 552 
                   
                 isolation 
                 coupled 
               
               
                 4 
                 612 
                   
                 coupled 
                 isolation 
               
               
                   
               
             
          
         
       
     
     The first four sets of measurements were performed to ascertain how well the power splitter  51  was matched. The tapered outer ends of the filars  52  and  53  added excessive inductance. Thus, the ends of the filars  52  and  53  were terminated prior to the taper starting at  64 ,  65 . With only the first ground plane  57  on the power divider  51 , the impedance on a Smith Chart was found to be centered at approximately 65 ohms. Adding the second ground increased shunting between the filars  52 ,  53 , and ground plane  57  lowering the impedance to the desired 50 ohms, although the impedance locus was not as tight as for the case of 65 ohms. VSWR measurements indicated that the low frequency area of 0 to 500 MHz was reasonably matched. It is desirable to have the power splitter  51  operating in a well-matched area where it does not radiate. For a general idea of where the spiral  43  can be expected to radiate, for the bifilar fed by itself in free space, with a diameter of 4.3 inches, radiation starts at 874 MHz. This is defined as a radiation cut-in frequency. The addition of one ground plane to the bifilar spiral  43  raises cut-in a large amount. The addition of two grounds will ensure almost no radiation, making an even higher cut-in. A higher radiation cut-in allows raising the power divider&#39;s operational frequency. Thus with radiation starting well above 500 MHz, it is desirable to improve the match beyond 500 MHz if possible, if operation at higher frequencies is desired. 
     Measurements 5-7, referenced in the table of 17 measurements above, were taken when an outer (radius) port  1  (connector  602 ) is an input port feeding through to an inner radius port  2  as an output port or  3  as an isolation port (connector  542  or  552 ) or to the other outer port  4  as a coupled port (connector  612 ). Measurement 5 is of S 12  with a reference phase being set at 0-degrees between port  1  and port  2 , and indicates the loss through a filar path. The loss at 0 Hz is 0 db, but it increases to 2 db at 400 MHz. Measurement 6 is the difference when the output is switched to the isolation port  3 . From about 40 to 400 MHz, there was seen an approximately flat response. The phase of S 13  relative to S 12  is 110 degrees, and the S 13  power transfer is −28 db+/−2 db. This is a broadband power divider because 400 MHz/40 MHz is a 10:1 bandwidth. However, its power level is significantly lower than the −19 db of the couple port discussed below. Further measurements found that small adjustments of the distance between the second ground plane and the spiral could bring the phase difference to 90 degrees. 
     A cut-in frequency was seen in S 13  at 40 MHz. (The cut-in frequency is defined as the frequency below which the amplitude response drops significantly with decreasing frequency.) This is due to the fact that at 0 Hz, the response is being measured across two lines that are simply a capacitor. Above 400 MHz the flat response was lost. This may be due to the size of the bifilar spiral  43 . 
     S 14  measurement 7 of the coupled port showed a ½ wavelength cyclic amplitude response of about −19 db starting at a filar ¼ wavelength length at 35 MHz. The filars  52 ,  53  are approximately 57.6 inches in length, which is ¼ wavelength at 51.3 MHz. Dielectric loading due to the fiberglass brought this frequency down to 35 MHz. Nulls occur every ½ wavelength of filar length. The phase response was not the constant phase seen in a prior art 90 degree splitter. From 0 to 400 MHz, phase cycled every ½ wavelength about 145 degrees, with overall phase decreasing with each cycle. 
     Overall when comparing the spiral splitter  51  with the prior art 90 degree splitter, there are similarities and differences. Isolation power is low but its phase is constant which allows it to be used as an inefficient broadband power splitter. The prior art splitter isolation phase is not constant. Coupled power is significantly higher and cycles every ½ wavelength, but its phase is not the constant 90 degrees of the prior art splitter, which allows the coupled port of the prior art splitter to be used as a narrow band power splitter. This disallows the coupled port from being used for power splitting. Thus the spiral splitter  51  has a better constant phase and amplitude response on what would be the isolation port on a normal prior art 90 degree splitter. 
     The second ground plane was removed to see the effects of increased broadside coupling. Thus measurements 8-10 repeat measurements 5-7 but without the second ground plane. Differences are: isolation response S 13  had a flatter broadband phase response and it swapped to negative at −90+/−10 degrees. S 13  amplitude increased significantly from −28 db (+/−2 db) to −12 db (+/−2 db) from 50 to 290 MHz with a null at 370 MHz. Flatness bandwidth decreased. The null and reduced flatness may be due to some radiation at higher frequencies. S 13  had a much lower cut-in frequency. Coupling port S 14  amplitude changed from −19 db to −125 db. The significant increase in S 13  and S 14  shows removal of the second ground allows significant increase in broadside coupling. More importantly, the power level of the broadband isolation port is slightly more than the power of the unusable coupled port, making the splitter more usable. 
     Because unlike the normal 90 degree splitter, the ports on both ends of the filars of the spiral splitter  51  are not symmetrical, measurements 5-10 were repeated with the change of the bifilar spiral  43  being fed from an inner port instead of an outer port. This can be looked upon as corresponding to measuring the S parameters of a 90 degree power splitter in the reverse direction. Measurements 11-17 are the measurements when the bifilar spiral  43  was fed from an inner port to the outer ports or to the other inner port. Thus, the S 21  and S 12  (output ports) measurements correspond, the S 24  and S 13  (isolation ports) measurements correspond, and the S 23  and S 14  (coupled ports) measurements correspond. Measurements of the corresponding responses between inner fed and outer fed cases yield the following results. For the case with the second plane removed, measurements 15 to 17, and 8 to 10, coupled and isolation responses were similar except the coupled phase of S 23  started to cycle every half wavelength about zero degrees. For the case of the presence of the second ground plane, measurements 11 to 14 and 5 to 7, coupled and isolation responses were similar except for the inner fed case of isolation S 24 , S 24  had a wider bandwidth than the outer fed case of S 13 . The bandwidth was from 40 to 500 MHz, at 12.5:1. A change in performance was found for S 24  when the second ground was placed tighter on the bifilar spiral  43 . The cut-in frequency was reduced in half, and the phase response went from approximately 100 degrees to a flatter 90 degree response. The prior flat response, of −27 db (+/−2 db) was found to become −20 db (+/−2 db) for a bandwidth from 20 to 400 MHz, or a 20:1 bandwidth. For the inner fed case of coupled power S 14  the magnitude decreased to about −27 db. Some of the ½ wavelength magnitude nulls were partially filled in and the phase mainly cycled every 4 wave length about zero degrees, and for some part of the zero to 400 MHz band was reasonably flat. 
     Overall, the case that is most usable for a power splitter occurs at the second port where the power to be divided has broadband constant magnitude and phase response and has more power than the other unused port. For the spiral power splitter  51 , the used broadband port is the isolation port, and the unused port becomes the coupled port. The following table determines the case where the used port has the most relative power: 
                                                             ISOLATION   COUPLED               INPUT       PORT   PORT   USED/           PORT   2 ND     POWER   POWER   UNUSED       MEASUREMENTS   LOCATION   GROUND   (+/−2 db)   (+/−2 db)   POWER                   5 to 7   outer   yes   S 13  = −28 db   S 14  = −19 db   −9 db       8 to 10   outer   no   S 13  = −12 db   S 14  = −12 db    0 db       11, 12, 14   inner   yes   S 24  = −27 db   S 23  = −27 db    0 db       13   inner   yes,   S 24  = −20 db   S 23  = −27 db    7 db               tighter                   15 to 17   inner   no   S 24  = −11 db   S 23  = −15 db    4 db                    
It is apparent that feeding the splitter  51  from the inner port gives more power to the used versus unused ports. Measurement 13, where inter-filar coupling is tighter from the tighter second ground plane, is the best case where this ratio is 7 db. Thus the best case of the general configuration investigated has a splitter fed on an inner port with the second floating ground plane highly coupled to the filars. Input port power is divided between the output port and the isolation port, while the coupled port is unused and terminated with 50 ohms
 
     In conclusion, the power splitter  51  provides a design producing approximately flat broadband responses and large bandwidths. The power splitter  51  appears to have little directivity when compared to known 90 degree splitters. In fact, when compared to a known 90 degree power splitter, what would be considered the isolated port performs better at coupling a flat magnitude and phase response than what would be considered the coupled port. Additionally in conclusion, the presence of the second ground plane was found to be a critical factor in determining the phase of the “isolated” port. When this second ground plane is brought close enough to the spiral, the phase shifts from −90 to 90 degrees. 
     In alternative embodiments, the filars  52 ,  53  of the bifilar spiral  43  of splitter  51  are placed closer to each other or even overlapping each other to increase coupling, to the extreme case of even power division for a 3 db splitter. 
     The spirals previously described are Archimedean spirals, however, equiangular spirals or spirals built with other smooth math functions can also be used. Further, the spirals are flat, however, the spirals can also be conical or cylindrical shaped resulting in a three-dimensional power divider. The splitter  51  is also easier to design and build than a prior art broadband 90 degree splitter since there are no multiple sections. 
     In light of the above, it is therefore understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Technology Classification (CPC): 7