Patent Publication Number: US-6211751-B1

Title: Microstrip broadband balun with four ground plates

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a balun used in telecommunications for feeding electric power to a balanced line from an unbalanced circuit, a power feeder consisting of a microstrip line. 
     2. Description of Related Art 
     When using a high frequency band in telecommunications, a microstrip line is often employed as a transmission line for conveying a signal. On the other hand, there is a balanced circuit such as an antenna for transmitting or receiving the signal by radio via space. A balun is used to connect such a balanced circuit with the microstrip line. 
     FIG. 18 shows a conventional balun. In FIG. 18, the reference numeral  1  designates a balanced line; and  2  designates a microstrip center conductor of a microstrip line which constitutes an unbalanced circuit together with a ground plate not shown to transmit a signal. The reference numeral  3  designates a connecting point of the balanced line  1  and microstrip center conductor  2 ; and  4  designates an additional line for short-circuiting the balanced line  1  at a position one quarter wavelength apart from the connecting point  3 . The reference numeral  5  designates a grounding terminal for grounding the balanced line  1  at a position half wavelength apart from the connecting point  3 . Because the balanced line  1  is terminated by the short-circuit at one quarter wavelength apart and by the ground at half wavelength apart from the connecting point  3  on the additional line  4  side, a current (unbalanced current) fed from the microstrip center conductor  2  is transformed to a current (balanced current) flowing through the balanced line  1  in opposite directions. With such a structure, the conventional balun must comprise on the balanced line additional circuits such as the quarter-wave transmission line, half-wave transmission line and additional transmission line, and this presents a problem of increasing the dimension of the structure. Furthermore, because the additional circuits are determined in accordance with the wavelength of the signal to be transmitted, and cannot transform signals of other frequencies from an unbalanced to balanced current, a problem arises of restricting the application of the balun fabricated to a particular frequency. 
     The present invention is implemented to solve the problems. Therefore, an object of the present invention is to provide a compact, rather frequency independent balun. 
     SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided a balun comprising: a balanced line including a first conductor and a second conductor; a first microstrip center conductor connected to the first conductor of the balanced line; and a second microstrip center conductor connected to the second conductor of the balanced line for supplying the second conductor with a current with a phase opposite to a phase of a current flowing through the first conductor. This makes it possible to reduce the size of the balun because it is unnecessary to provide the additional circuit on the balanced line, and to apply the balun to signals with different frequencies. 
     Here, the first microstrip center conductor may be formed on one side of a ground plate, and the second microstrip center conductor may be formed on the other side of the ground plate. This makes it possible to simplify the structure of the balun. 
     The second microstrip center conductor may comprise a delay circuit for delaying an radio wave propagating through the second microstrip center conductor by half wave. This obviates the need for preparing the input signal to the balun with the opposite phase, thereby simplifying an input interface. 
     The balun may further comprise a feeder for supplying power from the first microstrip center conductor to the second microstrip center conductor through a hole provided in the ground plate. This makes it possible to reduce the input signal to the balun to a single signal. 
     The balun may further comprise a ground plate of the first microstrip center conductor, a ground plate of the second microstrip center conductor, and a conductor for connecting the two ground plates which are mounted in parallel. This enables the balun to have a space therein. 
     The first microstrip center conductor and the second microstrip center conductor may be mounted on a side face of a cylindrical groundplate via a dielectric layer. This enables the balun to have a circular space therein. 
     The balun may further comprise a ground plate on a surface of which the first microstrip center conductor and the second microstrip center conductor are each mounted via a dielectric layer. This makes it possible to form the unbalanced circuit section of the balun on the plane. 
     The balun may further comprise a ground plate of the first microstrip center conductor, a ground plate of the second microstrip center conductor and a conductor for connecting the two ground plates which are mounted separately on a plane. This makes it possible to form the unbalanced circuit section of the balun on the plane, and to form a space within the balun. 
     According to a second aspect of the present invention, there is provided a balun comprising a plurality balun components, each of which includes: a balanced line including a first conductor and a second conductor; a first microstrip center conductor connected to the first conductor of the balanced line; and a second microstrip center conductor connected to the second conductor of the balanced line for supplying the second conductor with a current with a phase opposite to a phase of a current flowing through the first conductor, wherein two phases of an radio wave fed to one of the balun components differ from two phases of an radio wave fed to another of the balun components. This makes it possible to rotate the plane of polarization of the radio wave which is radiated from the balanced line. 
     The one of the balun components may comprise for its two microstrip center conductors two ground plates which are facing to each other, and the another of the balun components may comprise for its two microstrip center conductors two ground plates which are facing to each other and are disposed alternately with the ground plates of the one of the balun components, wherein the four ground plates adjacent to each other may be connected by a conductor. This makes it possible to reduce the size of the individual conductors for connecting the ground plates. 
     The four microstrip center conductors of the one of the balun components and of the another of the balun components may be connected to one microstrip center conductor and three microstrip center conductors each including a delay line branching from the one microstrip center conductor. This makes it possible to simplify the signal input interface. 
     According to a third aspect of the present invention, there is provided a balun comprising: a balanced line including a first conductor and a second conductor; a microstrip center conductor connected to the first conductor of the balanced line; and a quarter-wave long conductor plate which is connected at its first edge to a ground plate of the microstrip center conductor and at its second edge to the second conductor of the balanced line. This enables the microstrip center conductor to be reduced to one, thereby improving the productivity. 
     The conductor plate may connected to the ground plate via a quarter-wave long conductor plate facing the ground plate. This enables the balun to have a space in its unbalanced circuit section. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are a perspective view and a cross-sectional view showing an embodiment 1 of a balun in accordance with the present invention; 
     FIG. 2 is a schematic diagram illustrating the operation of the embodiment 1 of the balun in accordance with the present invention; 
     FIGS. 3A-3C are a perspective view, a cross-sectional view and a perspective view showing another structure of the embodiment 1 of the balun in accordance with the present invention with an additional delay line; 
     FIGS. 4A-4C are a perspective view, a cross-sectional view and a schematic operational diagram showing an embodiment 2 of the balun in accordance with the present invention; 
     FIG. 5 is a perspective view showing another structure of the embodiment 2 of the balun in accordance with the present invention; 
     FIG. 6 is a perspective view showing a still another structure of the embodiment 2 of the balun in accordance with the present invention; 
     FIG. 7 is a perspective view showing an embodiment 3 of the balun in accordance with the present invention; 
     FIG. 8 is a perspective view showing another structure of the embodiment 3 of the balun in accordance with the present invention; 
     FIGS. 9A and 9B are a perspective view and a cross-sectional view showing an embodiment 4 of the balun in accordance with the present invention; 
     FIG. 10 is a diagram showing a 1-to-4 splitting microstrip line constituting a signal input circuit to the embodiment 4 of the balun in accordance with the present invention; 
     FIGS. 11A and 11B a perspective view and a plane view of another structure of the embodiment 4 of the balun in accordance with the present invention; 
     FIGS. 12A and 12B are an extended view and a perspective view showing a 1-to-4 splitting microstrip line constituting a signal input circuit to the embodiment 4 of the balun in accordance with the present invention; 
     FIGS. 13A and 13B are an extended view and a cross-sectional view showing a double 1-to-4 splitting microstrip line constituting a signal input circuit to the embodiment 4 of the balun in accordance with the present invention; 
     FIGS. 14A-14C are perspective views showing another structure of the embodiment 4 of the balun in accordance with the present invention; 
     FIGS. 15A-15C are a perspective view, a cross-sectional view and a schematic operational diagram showing an embodiment 5 of the balun in accordance with the present invention; 
     FIG. 16 is a perspective view showing another structure of the embodiment 5 of the balun in accordance with the present invention; 
     FIG. 17 is a perspective view showing a still another structure of the embodiment 5 of the balun in accordance with the present invention; and 
     FIG. 18 is a schematic diagram showing a structure of a conventional balun. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will now be described with reference to the accompanying drawings. 
     EMBODIMENT 1 
     FIGS. 1A and 1B are a perspective view and a cross-sectional view showing a structure of an embodiment 1 of a balun in accordance with the present invention. In FIG. 1A, reference numerals  6  and  7  designate parallel conductors; and the reference numeral  8  designates a parallel-wire balanced line consisting of the conductors  6  and  7 . The reference numeral  9  designates a ground plate of a microstrip line;  10  designates dielectric layers formed on both side of the ground plane  9 ; and reference numerals  11  and  12  each designate a microstrip center conductor formed on the dielectric layers. In FIG. 1B, the reference numeral  13  designates a connecting point of the conductor  6  of the balanced line and the microstrip center conductor  11 ; and  14  designates a connecting point of the conductor  7  of the balanced line and the microstrip center conductor  12 . Reference numerals  15  and  16  designate signal input terminals to the microstrip center conductors  11  and  12 . The ground plane  9  is common to the microstrip center conductors  11  and  12 , so that the structure as shown in FIGS. 1A and 1B couples the unbalanced circuit consisting of the microstrip center conductors  11  and  12  to the balanced line  8  including the conductors  6  and  7 . 
     The operation of the converter will now be described with reference to FIG.  2 . Signals of opposite phases with 0 and −180 degrees are applied to the signal input terminals  15  and  16 . Assume that the signal is fed to the signal input terminal  15 , and a current I flows through the microstrip center conductor  11  in the direction as shown in FIG.  2 . In this case, a current −I flows through a surface  9   a  of the ground plane  9  on the side of the microstrip center conductor  11 . The signal of the opposite phase applied to the signal terminal  16  causes a current −I to flow through the microstrip center conductor  12 , which causes a current I to flow through a surface  9   b  of the ground plane  9  on the side of the microstrip center conductor  12 . The current I flowing through the microstrip center conductor  11  flows into the conductor  6  of the balanced line  8  via the connecting point  13 , which causes a current −I to flow through the other conductor  7  of the balanced line  8  through a displacement current across the conductors  6  and  7 . The current −I flowing through the conductor  7  is connected to the current −I flowing through the microstrip center conductor  12 . Thus, the current I flows through the closed loop in the converter, and the current (balanced current) flows through the conductors  6  and  7  in the opposite direction in the balanced line  8 . As a result, the unbalanced current flowing through the microstrip line can be converted into the balanced current. 
     With such an arrangement, the balun can achieve conversion from the microstrip line to the balanced line without using the half-wave transmission line, quarter-wave transmission line and additional line which are needed for the balanced line in the conventional technique, thereby reducing the size of the balun. In addition, because the structural components are independent of the frequency, the balun is independent of the frequency of the input signal, which makes it possible to increase the frequency range of the input signal. 
     Furthermore, the balanced conversion can also be achieved by providing a delay circuit on the microstrip line as shown in FIGS. 3A-3B. In FIG. 3A, the reference numeral  17  designates a signal input terminal,  18  designates a splitting point, and  19  designates a connecting conductor passing through the dielectric layer and the ground plate. In FIG. 3B, the reference numeral designates a hole made through the ground plate; and in FIG. 3C, the reference numeral  12   a  designates a half-wave delay circuit. A signal applied to the signal input terminal  17  is delivered at the splitting point  18  to the microstrip center conductor  11  shown in FIG.  3 A and to the microstrip center conductor  12  shown in FIG.  3 C. The signal fed from the splitting point  18  to the microstrip center conductor  12  passes through the connecting conductor  19 . The hole  20  through the ground plate is provided so that the connecting conductor  19  passes through the dielectric layer and the ground plate to apply the signal to the microstrip center conductor  12 . Because of the half-wave delay circuit  12   a  connected with the microstrip center conductor  12 , the signal passing through the microstrip center conductor  12  has the phase opposite to the signal passing through the microstrip center conductor  11 . This circuit as shown in FIGS. 3A-3C is connected to the balanced line via the connecting points  13  and  14 . With the foregoing structure, the number of the signal input terminals is reduced to one, which simplifies the signal input. 
     A structure is also possible which provides one of the two microstrip center conductors with a half-wave delay circuit without the connecting conductor  19 . Although this structure requires two signal input terminals, they can be provided with the signal of the same phase. This has an advantage of being able to obviate the necessity for a pre-stage circuit of the balun for generating the opposite phase signals. 
     EMBODIMENT 2 
     FIGS. 4A-4C are views showing another structure of the balun in accordance with the present invention. In FIGS. 4A and 4B, the reference numeral  21  designates a ground plate for the microstrip center conductor  11 ;  22  designates a ground plate for the microstrip center conductor  12 ; and  23  designates a connecting conductor for connecting the ground plates  21  and  22 . 
     In FIG. 4C, assume that the signal input terminals  15  and  16  are supplied with signals of opposite phases, and that a current I flows through the microstrip center conductor  11  in the direction as shown in FIG.  4 C. In this case, a current −I flows through the ground plate  21 . The signal of the opposite phase applied to the signal point  16  causes a current −I to flow through the microstrip center conductor  12 , which in turn causes a current I to flow through the ground plate  22 . The current I flowing through the microstrip center conductor  11  flows into the conductor  6  of the balanced line  8 , which causes a current −I to flow through the other conductor  7  of the balanced line  8  through a displacement current across the conductors  6  and  7 . The current −I flowing through the conductor  7  is connected to the current −I flowing through the microstrip center conductor  12 . Thus, the current I flows through the closed loop via the connecting conductor  23  connecting the ground plates  21  and  22 . As a result, the current (balanced current) flows through the conductors  6  and  7  in the opposite direction in the balanced line  8 , and hence the unbalanced current flowing through the microstrip line can be converted into the balanced current. 
     With such an arrangement, the balun can achieve conversion from the microstrip line to the balanced line without using the half-wave transmission line, quarter-wave transmission line and additional line which are needed for the balanced line in the conventional technique, thereby reducing the size of the balun. In addition, because the structural components are independent of the frequency, the balun is independent of the frequency of the input signal, which makes it possible to increase the frequency range of the input signal. Furthermore, a space can be provided in the balun by separating the two ground plates  21  and  22 . The spacing between the conductors  6  and  7  of the balanced line  8  can be varied by adjusting the spacing between the ground plates  21  and  22  independently of the spacing between the microstrip center conductor  11  and the ground plate  21 , and of the spacing between the microstrip center conductor  12  and the ground plate  22 . 
     Alternatively, connecting conductors  23  for connecting the ground plates can be placed apart from the balanced line  8  as shown in FIG.  5 . In this case also, the current flowing through the balun forms a closed loop as in the above. This causes a balanced current to flow through the balanced line  8 , offering the same advantage as the foregoing. 
     Besides, the ground plate can have a cylindrical structure as shown in FIG. 6, in which the reference numeral  24  designates a cylindrical ground plate, and  25  designates a dielectric layer formed on an outer surface of the ground plate  24 . The balun is constructed by forming the microstrip lines on the dielectric layer  25 , and by connecting them to the balanced line  8 . In this case also, the current flowing through the balun forms a closed loop as in the above. This causes a balanced current to flow through the balanced line  8 , offering the same advantage as the foregoing. 
     EMBODIMENT 3 
     FIG. 7 is a perspective view showing another structure of the balun in accordance with the present invention. In FIG. 7, the reference numeral  26  designates a ground plate, and  27  each designate one of two dielectric layers formed on a surface of the ground plate  26 . The microstrip center conductors  11  and  12  are formed on the dielectric layers  27 , respectively, and are connected to the balanced line  8 . 
     Supplying the signal input terminals  15  and  16  with signals opposite in phase causes currents to flow through the microstrip center conductors  11  and  12 , which induces currents flowing in the same direction through portions of the ground plate  26  facing the transmission lines, thereby causing a current to flow through the ground plate  26  in one direction. The unbalanced currents flowing through the microstrip center conductors  11  and  12  flow into the conductors  6  and  7  of the balanced line  8 , thereby generating in the conductors  6  and  7  balanced currents equal in magnitude and opposite in direction. Because the unbalanced circuit section, which comprises the ground plate  26 , dielectric layers  27  and microstrip center conductors  11  and  12 , is formed on the ground plate  26 , it can be made thinner. 
     Alternatively, as shown in FIG. 8, the ground plate  27  can be separately provided for each of the microstrip center conductors  11  and  12 , and the ground plates can be connected by the connecting conductors  23 . This enables the currents flowing through the ground plates to flow in one direction through the connecting conductors  23 . With such an arrangement, the unbalanced circuit portion can be made thinner, and besides, a hollow space can be established in the unbalanced circuit portion between the two conductors of the balanced line  8 . 
     EMBODIMENT 4 
     FIGS. 9A and 9B are views showing another arrangement of the balun in accordance with the present invention: FIG. 9A is a perspective view; and FIG. 9B is a cross-sectional view of a rectangular tube, in which reference numerals  28  and  29  designate a pair of conductors, and  30  designates a balanced line consisting of the conductors  28  and  29 . Thus, the arrangement in the figures includes two pairs of the balanced lines: one is the balanced line  8  consisting of the conductors  6  and  7 ; and the other is the balanced line  30  consisting of the conductors  28  and  29 . Reference numerals  31  and  32  designate microstrip center conductors, which are connected to the conductors  28  and  29 , respectively. Reference numerals  33  and  34  designate ground plates corresponding to microstrip center conductors  31  and  32 , respectively, which are formed on the dielectric layer  10 . A connecting conductor  23  for interconnecting the ground plates  21 ,  22 ,  33  and  34  connect the adjacent ground plates. Reference numerals  35  and  36  designate signal input terminals of the microstrip center conductors  31  and  32 . The signal input terminals of the microstrip center conductors  11  and  12  are designated by reference numerals  15  and  16 . In FIG. 9A, the balanced lines  8  and  30  are disposed such that surfaces including their conductors are orthogonal to each other, and the rectangular tube has a square cross section as shown in FIG.  9 B. 
     Next, the operation of the balun as shown in FIGS. 9A and 9B will be described. As described before in connection with the structure of the embodiment 2 as shown in FIGS. 4A-4C, the signals opposite in phase cause balanced currents to appear in the conductors  6  and  7  of the balanced line  8 , when they are applied to the signal input terminals  15  and  16  of the balun comprising the balanced line  8 , microstrip center conductors  11  and  12 , ground plates  21  and  22  and connecting conductor  23 . On the other hand, signals opposite in phase cause balanced currents to appear in the conductors  28  and  29  of the balanced line  30 , when they are applied to the signal input terminals  35  and  36  of the balun comprising the balanced line  30 , microstrip center conductors  31  and  32 , ground plates  33  and  34  and connecting conductor  23 . When the signals applied to the signal input terminals  15 ,  35 ,  16  and  36  have a phase shifted by 90 degrees each, the phases of the displacement currents induced in the balanced lines  8  and  30  are shifted by 90 degrees. Thus, when sinusoidal waves are applied to the signal input terminals, it will be seen that the composite vector of the displacement currents in the balanced lines  8  and  30  rotates. With such an arrangement, the balun can not only convert the unbalance current to balanced current, but also generate a circularly polarized wave because of the rotation of the displacement current in the balanced lines. 
     Although the ground plates  21  and  22 , and  33  and  34  are placed such that they face each other in the balun as shown in FIGS. 9A and 9B, each pair of the microstrip center conductors, dielectric layers and ground plates can be arranged as shown in FIG. 7 or  8 , and a pair of the baluns with such an arrangement can be combined. In this case, the thickness of the unbalanced circuit portion can be made thinner. 
     Although the balanced lines  8  and  30  are constructed such that the planes including their conductors are orthogonal to each other, they can be disposed such that they form an obtuse angle. With such an arrangement, three pairs of balanced lines can constitute the balun, for example. 
     FIG. 10 is a schematic diagram showing a 1-to-4 splitting delay microstrip line for supplying the four microstrip center conductors of FIGS. 9A and 9B with input signals. A microstrip line from the signal input terminal  37  is split into four lines: one line with a phase of 0 degrees; and the remaining three lines with a phase 90 degrees each shifted through delay lines. The four outputs are connected to the signal input terminals  15 ,  35 ,  16  and  36  of FIG.  9 A. 
     FIGS. 11A and 11B are views showing a structure comprising two sets of balanced lines arranged as shown in FIGS. 9A and 9B for generating circularly polarized waves, in which two sets A and B of the baluns are depicted. The two sets can constitute a double-frequency circularly polarized wave balun when they are supplied with signals of different frequencies. The connecting conductor  23  for connecting the ground plates of the set A and the connecting conductor  23  for connecting the ground plates of the set B are not connected electrically, although they seem to intersect in this figure. 
     FIGS. 12A and 12B are views showing a structure of a feeder to the double-frequency circularly polarized wave balun, which is composed of a 1-to-4 splitting microstrip line. As shown in FIG. 12A, the microstrip line starting from a signal input terminal  38  is split into four lines to form a microstrip line for delaying phases according to distances to signal output terminals  39 . An octagonal pipe as shown in FIG. 12B is constructed by bending the board. The four phase signal output terminals are placed at about the center of the edges of every other four sides of the octagonal pipe. By connecting the 1-to-4 splitting circuit to the signal input terminals of the double-frequency circularly polarized wave balun, the number of the signal input terminals for one frequency balun can be reduced to one. Incidentally, in FIGS. 12A and 12B, the microstrip line is formed on a dielectric board, on the inside wall of which a ground plate is formed. 
     FIGS. 13A and 13B are views showing a feeder to the double-frequency circularly polarized wave balun, which is composed of a double 1-to-4 splitting microstrip line. In FIG. 13B, the reference numeral  40  designates a ground plate, on both sides of which dielectric layers are attached to form a board. A 1-to-4 splitting microstrip line is formed on both surfaces of the board. In FIG. 13A, the microstrip line starting from a signal input terminal  41  is split into four lines to form a microstrip line for delaying its phase according to the distance to four signal output terminals  43 . Likewise, the microstrip line starting from a signal input terminal  42  is split into four lines to form a microstrip line for delaying its phase according to the distance to four signal output terminals  44 . A feeder to the double-frequency circularly polarized wave balun is constructed by bending the board into an octagonal pipe such that the eight signal output terminals  43  and  44  are placed at the center of edges of its sides. The feeder can reduce the number of signal input terminals to one per frequency, that is, the number of signal input terminals to the double-frequency circularly polarized wave balun to two. 
     FIGS. 14A-14C are views showing a double-frequency circularly polarized wave balun that is composed by combining a couple of baluns with an unbalanced circuit section being placed on a surface for generating circularly polarized waves. In FIG. 14A, the reference numeral  45  designates a ground plate;  46  designates a microstrip line formed on the ground plate via a dielectric layer;  47  designates a signal input terminal to the microstrip line  46 ; and  48  designates a couple of balanced lines consisting of four conductors for generating a circularly polarized wave. The microstrip line  46  seen from the signal input terminal  47  is split into four lines, so that their phases are shifted every 90 degrees at the inputs to the balanced lines  48 . The reference numeral  49  designates four holes provided in the board consisting of the ground plate and dielectric layer. A balun C consisting of the foregoing elements is placed on a balun D with a structure like that of the balun C except for the holes  49  as shown in FIG. 14B, in which balanced lines  50  of the balun D are passed through the holes  49  provided in the balun C. In addition, the microstrip line on the balun D is insulated from the ground plate of the balun C by providing a space or by inserting an insulator between them. 
     When two signals with different frequencies are supplied to the baluns C and D, circularly polarized waves with different frequencies are generated in the balanced lines  48  and  50 . 
     EMBODIMENT 5 
     FIGS. 15A-15C are views showing another structure of the balun in accordance with the present invention, in which reference numerals  6  and  7  designate conductors; and the reference numeral  8  designates a balanced line consisting of the conductors  6  and  7 . The reference numeral  11  designates a microstrip center conductor;  10  designates a dielectric layer; and  21  designates a ground plate corresponding to the microstrip center conductor  11 . The reference numeral  51  designates a quarter wave long parallel plane, which is connected to the conductor  7  at its one end. The reference numeral  52  designates a short-circuit plate for short-circuiting the other edge of the parallel plane  51  to the ground plate  21 ; and  53  designates an open edge of the parallel plane  51 . 
     Next, the operation of the balun will be described with reference to FIG.  15 C. When a current I is supplied to the signal input terminal  15  of the microstrip center conductor  11 , a current −I flows through the ground plate  21 . The current I flowing into the conductor  6  causes a current −I to flow through the conductor  7  due to a displacement current. Although no current flows through the parallel plane  51  because it is a quarter-wave parallel plane circuit with its edge shorted, the current −I flowing through the conductor  7  flows through the ground plate via a displacement current at the open edge  53  of the parallel plane  51 . Thus, the current supplied to the signal input terminal  15  flows with forming a closed loop, so that the balanced current flows in the opposite direction in the balanced line  8 . This has an advantage of being able to obviate the necessity for generating the opposite phase signals at the signal input of the balun. 
     Alternatively, a configuration as shown in FIG. 16 is possible in which the ground plate  21  and the parallel plane  51  are connected by a conductor  54 . In this case, a parallel plane  55  is provided such that it is parallel to the ground plate, and is short-circuited with the parallel plane  51 . An edge of the parallel plane  55  is connected to the ground plate  21  by the conductor  54 . A signal supplied to the signal input terminal  15  causes a displacement circuit at the open edge of the parallel plane  51  as in the foregoing configuration as shown in FIG. 15, thereby causing a current to flow through the balanced line  8 . This configuration has an advantage of being able to provide spaces between the conductors constituting the balanced line. 
     FIG. 17 shows a balun arranged by combining the balun as shown in FIG. 16 with the balun as shown in FIG.  4 . The signal input terminals of the balun as shown in FIG. 17 are three points denoted by  56 ,  15  and  57 , to which three signals are supplied with their phase shifted by 0, −90 and −180 degrees, for example. This enables a vector of a displacement current generated in the balanced line to rotate, thereby generating a circularly polarized wave. 
     As described above, the balun in accordance with the present invention is suitable to construct a small, frequency independent balun for converting into a balanced current an unbalanced current flowing through a microstrip line which is employed as a transmission circuit for transmitting a signal when a high frequency band is used in electric communications.