High-frequency balun

Abstract

To achieve low-loss unbalanced / balanced conversion in a wide band.SOLUTION: In a high-frequency balun 1, first coaxial cables 10 each include a first end 101 connected to an unbalanced line and a second end 102 as an open end, and a first portion SEC11, a first outer-conductor-free portion GAP1, and a second portion SEC12 are connected to each other. Each of the second coaxial cables 20 has a third end 201 and a fourth end 202, which are short-circuited ends, and is connected to the third portion SEC21, the second outer conductor-free portion GAP2, and the fourth portion SEC22. The end portion 103 on the first outer conductor non-formed portion side and the end portion 203 on the second outer conductor non-formed portion side are electrically connected to each other by the conductive bonding material 71 and constitute a first terminal of the balanced terminals. An end portion 104 on the first outer conductor non-formed portion side in the second portion and an end portion 204 on the second outer conductor non-formed portion side in the fourth portion are electrically connected by a conductive bonding material 72 to constitute a second terminal of the balanced terminals. An electrical length of the first portion and an electrical length of the second portion are one fourth of a wavelength λ of a high-frequency signal to be subjected to unbalanced / balanced conversion.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a high-frequency balun that performs unbalanced-to-balanced conversion of a high-frequency signal. [Background technology]

[0002] Non-Patent Document 1 describes a balun that performs unbalanced-to-balanced conversion using a microstrip line. Conventional baluns of this type include baluns that use ferrite and windings, and baluns that use coaxial cables. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Analysis and Design of a High-Performance Planar Marchand Balun,2002 IEEE MTT-S Disgest,CY Ng, M.Chongcheawchamnan, and ID Robertson Summary of the Invention [Problem to be solved by the invention]

[0004] However, it is difficult to create baluns using ferrite and windings at frequencies above 1 GHz due to the winding length being significant compared to the wavelength and the limitations of the ferrite snake.In addition, baluns using coaxial cables have a narrow frequency band in which excellent unbalanced-to-balanced conversion characteristics can be obtained.

[0005] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a high-frequency balun that can realize low-loss unbalanced-to-balanced conversion over a wide band and with a wide impedance conversion ratio range using a limited number of types of coaxial cables. [Means for solving the problem]

[0006] A high-frequency balun according to one embodiment of the present invention includes a first coaxial cable and a second coaxial cable. The first coaxial cable includes a first center conductor, a first dielectric layer, and a first outer conductor, and has first and second ends. The second coaxial cable includes a second center conductor, a second dielectric layer, and a second outer conductor, and has third and fourth ends.

[0007] The first coaxial cable has a shape in which the first portion, the first outer conductor-free portion, and the second portion are connected from the first end to the second end. The first end is an unbalanced terminal that connects to the unbalanced line, and the second end is an open end where the first central conductor and the first outer conductor are not connected.

[0008] The second coaxial cable has a shape in which the third portion, the second outer conductor-free portion, and the fourth portion are connected from the third end to the fourth end, and the third and fourth portions have the same length. The third and fourth ends are short-circuit ends where the second central conductor and the second outer conductor are electrically connected.

[0009] The first end and the second end are electrically connected by a conductive bonding material, and the third end and the fourth end are electrically connected by a conductive bonding material.

[0010] The first terminal of the balanced terminal is formed by electrically connecting the end of the first portion without outer conductor to the end of the third portion without second outer conductor by a conductive bonding material, and the second terminal of the balanced terminal is formed by electrically connecting the end of the second portion without first outer conductor to the end of the fourth portion without second outer conductor by a conductive bonding material.

[0011] The electrical lengths of the first and second sections are 1 / 4 of the wavelength λ of the high-frequency signal to be converted from unbalanced to balanced. The lengths of the third and fourth sections are set based on the characteristic impedance ratio of the balanced and unbalanced lines connected by the high-frequency balun.

[0012] The length of the second portion from the end of the first central conductor on the first outer conductor non-forming portion side is set based on the matching characteristics of the imaginary part of impedance between the balanced line and the unbalanced line.

[0013] In this configuration, in a configuration using coaxial cables, a high frequency balun in which the characteristic impedance of the balanced line is greater than the characteristic impedance of the unbalanced line can be configured over a wide band.

[0014] In the high-frequency balun according to one embodiment of the present invention, the first characteristic impedance of the first coaxial cable and the second characteristic impedance of the second coaxial cable are the same, and it is possible to use coaxial cables with the same specifications.

[0015] In a high-frequency balun according to one embodiment of the present invention, the first characteristic impedance and the second characteristic impedance are 50 Ω. The lengths of the third and fourth portions are 1 / 8 of the wavelength λ. The length of the second portion from the end of the first central conductor on the first outer-conductor-free portion side is approximately 1 / 8 of the wavelength λ.

[0016] In this configuration, a high-frequency balun with a characteristic impedance ratio of 2:1 between the balanced line and the unbalanced line can be constructed using a standard coaxial cable that is relatively easy to obtain.

[0017] In this configuration, there are two frequency bands where it operates as a 2:1 high-frequency balun, and it operates as a 1:1 balun at frequencies between them.

[0018] In the high-frequency balun according to one embodiment of the present invention, the first end, the second end, the third end, and the fourth end are electrically connected by a conductive bonding material and connected to a reference potential.

[0019] In this configuration, stable unbalance-to-balance conversion characteristics can be achieved by connecting the first terminal, the second terminal, the third terminal, and the fourth terminal.

[0020] In the high-frequency balun according to one embodiment of the present invention, the characteristic impedance of the first coaxial cable is different from the characteristic impedance of the second coaxial cable.

[0021] In this configuration, a high-frequency balun with a characteristic impedance of the balanced line and the characteristic impedance of the unbalanced line being n (n>1):1 can be configured over a wide band.

[0022] In a high-frequency balun according to an embodiment of the present invention, the second coaxial cable preferably has the same characteristic impedance as the balanced line. For example, the second coaxial cable is formed by connecting two coaxial cables in parallel, each having the same characteristic impedance as the balanced line.

[0023] In this configuration, two coaxial cables are used to create a pseudo-balanced line, and the characteristic impedance of the balanced line is twice that of the coaxial cables that make up the line. However, by connecting two coaxial cables in parallel, it is possible to create a balanced line with the same characteristic impedance as the coaxial cables that make up the balanced line. For example, to obtain a balun with an impedance of 50 Ω at the unbalanced terminal and 75 Ω at the balanced terminal, it is easy to use a coaxial cable with a characteristic impedance of 50 Ω as the first coaxial cable and two coaxial cables with a characteristic impedance of 75 Ω connected in parallel as the second coaxial cable. Furthermore, this configuration allows for a wide range of n values ​​while suppressing large losses during unbalanced-to-balanced conversion.

[0024] In a high-frequency balun according to one embodiment of the present invention, the connection between the first and second terminals and the connection between the third and fourth terminals are separated, and the connection portions are each individually connected to a reference potential.

[0025] In this configuration, the balun functions in the same way even if the ends of the different coaxial cables are separated. Even in this case, more stable characteristics can be obtained by connecting the first and second ends of the first coaxial cable and the third and fourth ends of the second coaxial cable to a reference potential individually. [Brief explanation of the drawings]

[0026] [Figure 1] FIG. 1 is a perspective view of the appearance of a high-frequency balun according to the first embodiment. [Figure 2] FIG. 2(A) is a graph showing the frequency characteristics of S11 when a pure resistance twice the characteristic impedance of the coaxial cable is connected to the balanced terminal of the high-frequency balun according to the first embodiment of the present invention, and FIG. 2(B) is a graph showing the frequency characteristics of VSWR when a pure resistance twice the characteristic impedance of the coaxial cable is connected to the balanced terminal of the high-frequency balun according to the first embodiment of the present invention. [Figure 3] FIG. 3(A) is a graph showing the frequency characteristics of S11 when the high-frequency balun according to the first embodiment of the present invention is made using a coaxial cable with a characteristic impedance of 50 Ω and a pure resistor of 50 Ω (solid line) and a pure resistor of 100 Ω (dashed line) are connected to the balanced terminals. FIG. 3(B) is a graph showing the frequency characteristics of VSWR when the high-frequency balun according to the first embodiment of the present invention is made using a coaxial cable with a characteristic impedance of 50 Ω and a pure resistor of 50 Ω (solid line) and a pure resistor of 100 Ω (dashed line) are connected to the balanced terminals. [Figure 4] FIG. 4 is a perspective view of the appearance of the high-frequency balun according to the second embodiment. [Figure 5] FIG. 5 is a graph showing a comparison of the VSWR frequency characteristics obtained by simulating the high-frequency balun according to the first embodiment in which all coaxial cables are configured with coaxial cables having a characteristic impedance of 50 Ω when a pure resistance of 75 Ω is connected to the balanced terminals, and the high-frequency balun according to the second embodiment in which the first coaxial cable is configured with a coaxial cable having a characteristic impedance of 50 Ω and the second coaxial cable is configured with two parallel coaxial cables having a characteristic impedance of 75 Ω. [Figure 6] FIG. 6 is a perspective view of the appearance of the high-frequency balun according to the third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0027] [First embodiment] A high-frequency balun according to a first embodiment of the present invention will be described with reference to the drawings. In the first embodiment, a high-frequency balun that realizes a ratio of (the characteristic impedance of the balanced line):(the characteristic impedance of the unbalanced line) of 1:1 and 2:1 will be described.

[0028] 1 is a perspective view of the appearance of a high-frequency balun according to Embodiment 1. As shown in FIG. 1, the high-frequency balun 1 includes a first coaxial cable 10, a second coaxial cable 20, and a coaxial connector 80.

[0029] (first coaxial cable 10) The first coaxial cable 10 includes a first central conductor 11, a first dielectric layer 12, and a first outer conductor 13.

[0030] The first central conductor 11 is a linear conductor made of a metal such as copper. The first central conductor 11 has a cut portion midway along its extension. The first dielectric layer 12 has a predetermined dielectric constant. The first dielectric layer 12 covers the periphery of the first central conductor 11 to a predetermined thickness. The first outer conductor 13 is made of a metal such as copper and is tubular with a predetermined thickness. The first outer conductor 13 covers the outer peripheral surface of the first dielectric layer 12.

[0031] The first coaxial cable 10 has a first end 101 at one end in the extension direction and a second end 102 at the other end. The first end 101 of the first coaxial cable 10 is connected to the coaxial connector 80 via a wiring coaxial cable SEC10. The wiring coaxial cable SEC10 and the first portion SEC11 are continuously formed by a single coaxial cable including a first center conductor 11, a first dielectric layer 12, and a first outer conductor 13.

[0032] The connection portion to this coaxial connector 80 constitutes an unbalanced terminal of the high-frequency balun 1 .

[0033] The second end 102 of the first coaxial cable 10 is an open end in terms of high frequency, with the first central conductor 11 and the first outer conductor 13 spaced apart from each other.

[0034] The first coaxial cable 10 has a shape in which a first portion SEC11, a first outer-conductor-free portion GAP1, and a second portion SEC12 are connected from a first end 101 to a second end 102. The first portion SEC11 and the second portion SEC12 include a first central conductor 11, a first dielectric layer 12, and a first outer conductor 13.

[0035] The first outer conductor-free portion GAP1 includes a first central conductor 11 and a first dielectric layer 12, but does not include a first outer conductor 13. The end of the first portion SEC11 on the first outer conductor-free portion GAP1 side is an end 103. The end of the second portion SEC12 on the first outer conductor-free portion GAP1 side is an end 104.

[0036] The first end 101 of the first portion SEC11 and the second end 102 of the second portion SEC12 are electrically connected by a conductive bonding material 73.

[0037] The cut portion of the first central conductor 11 is formed at a position of length θ192 from the end portion 104 toward the second end 102. In other words, the characteristic-adjusting portion 19 in the first central conductor 11 is the portion from the end portion 104 to the cut end portion E19, and this length is θ192.

[0038] (Second coaxial cable 20) The second coaxial cable 20 includes a second central conductor 21, a second dielectric layer 22, and a second outer conductor .

[0039] The second central conductor 21 is made of a metal such as copper and is a linear conductor. The second dielectric layer 22 has a predetermined dielectric constant. The second dielectric layer 22 covers the periphery of the second central conductor 21 to a predetermined thickness. The second outer conductor 23 is made of a metal such as copper and is tubular with a predetermined thickness. The second outer conductor 23 covers the outer peripheral surface of the second dielectric layer 22.

[0040] The second coaxial cable 20 has a third end 201 at one end in the extension direction and a fourth end 202 at the other end. The third end 201 of the second coaxial cable 20 is a high-frequency short-circuited end where the second central conductor 21 and the second outer conductor 23 are connected. The fourth end 202 of the second coaxial cable 20 is a high-frequency short-circuited end where the second central conductor 21 and the second outer conductor 23 are connected.

[0041] The second coaxial cable 20 has a shape in which the third portion SEC21, the second outer-conductor-free portion GAP2, and the fourth portion SEC22 are connected from the third end 201 to the fourth end 202. The third portion SEC21 and the fourth portion SEC22 each include a second center conductor 21, a second dielectric layer 22, and a second outer conductor 23. The second outer-conductor-free portion GAP2 includes the second center conductor 21 and the second dielectric layer 22, but does not include the second outer conductor 23.

[0042] The third portion SEC21 and the fourth portion SEC22 have approximately the same length. Here, "approximately the same" includes both exact match and manufacturing tolerances, and also includes a range in which the lengths can be finely adjusted to adjust the characteristics as a balun, as will be described later.

[0043] The end of the third portion SEC21 on the second outer conductor-free portion GAP2 side is an end 203. The end of the fourth portion SEC22 on the second outer conductor-free portion GAP2 side is an end 204.

[0044] The third end 201 and the fourth end 202 of the second coaxial cable 20 are electrically connected by a conductive bonding material 73 .

[0045] (Fixing structure of the first coaxial cable 10 and the second coaxial cable 20, and connecting structure of the balanced terminal) The third end 201 and the fourth end 202 of the second coaxial cable 20 are electrically connected to the connection point between the first end 101 and the second end 102 of the first coaxial cable 10 using a conductive bonding material 73. This electrical connection point is connected to a reference potential (ground potential).

[0046] The end 103 of the first portion SEC11 of the first coaxial cable 10 and the end 203 of the third portion SEC21 of the second coaxial cable 20 are arranged flush with each other. Note that flush here includes an installation error that does not affect the characteristics of the high-frequency balun 1, and may be, for example, approximately 10% or less of the length of the third portion SEC21.

[0047] The end 103 and the end 203 are electrically connected using a conductive bonding material 71. A linear terminal conductor 31 is electrically connected to this electrical connection.

[0048] The end 104 of the second portion SEC12 of the first coaxial cable 10 and the end 204 of the fourth portion SEC22 of the second coaxial cable 20 are arranged flush with each other. Note that flush here includes an installation error that does not affect the characteristics of the high-frequency balun 1, and may be, for example, approximately 10% or less of the length of the fourth portion SEC22.

[0049] The end 104 and the end 204 are electrically connected using a conductive bonding material 72. The linear terminal conductor 32 is then electrically connected to this electrical connection.

[0050] The terminal conductor 31 (the connection between the end 103 and the end 203) and the terminal conductor 32 (the connection between the end 104 and the end 204) form a pair to form the balanced input / output terminals of the high-frequency balun 1.

[0051] In this configuration, the high-frequency balun 1 further includes the following configuration.

[0052] The first coaxial cable 10 and the second coaxial cable 20 have the same transmission characteristics. For example, the first characteristic impedance Z10 of the first coaxial cable 10 and the second characteristic impedance Z20 of the second coaxial cable 20 are the same.

[0053] The electrical length of the first portion SEC11 and the electrical length of the second portion SEC12 in the first coaxial cable 10 are the same. The electrical length of the third portion SEC21 and the electrical length of the fourth portion SEC22 in the second coaxial cable 20 are the same.

[0054] The lengths of the first and second sections SEC11 and SEC12, which are on the unbalanced line side, are set to 1 / 4 (λ / 4) of the wavelength of the high-frequency signal to be transmitted (unbalanced-to-balanced converted) by high-frequency balun 1, in order to obtain wideband characteristics, based on the IEEE document "A Wide-Band Balum" (J.W. McLaughlin, D.A. Dunn, and R.W. Grow). Note that the wavelength here corresponds to the representative frequency of the frequency band that high-frequency balun 1 converts from unbalanced to balanced with low loss.

[0055] Furthermore, the electrical length of the third portion SEC21, the electrical length of the fourth portion SEC22, and the length θ192 of the characteristic adjustment portion 19 in the first central conductor 11 are set as follows based on equation (4) in the IEEE document "A New Wide-Band Balum" (WILLMARK K. ROBERTSt). Hereinafter, this equation (4) will be referred to as equation (4) in document A.

[0056] The real part of equation (4) in document A is an equation for calculating the impedance ratio, and the imaginary part of equation (4) in document A is an equation for achieving matching.

[0057] Here, to realize a high-frequency balun with (characteristic impedance of balanced line):(characteristic impedance of unbalanced line)=100Ω:50Ω, i.e., a 2:1 high-frequency balun, the first characteristic impedance Z10 of the first coaxial cable 10 and the second characteristic impedance Z20 of the second coaxial cable 20 are set to 50Ω.

[0058] In the above-described configuration, the unbalanced line of the high-frequency balun 1 is formed by the first coaxial cable 10. Therefore, the characteristic impedance on the unbalanced side of the high-frequency balun 1 is the same as the first characteristic impedance Z10 of the first coaxial cable 10. Therefore, since the first characteristic impedance Z10 of the first coaxial cable 10 is 50 Ω, the characteristic impedance on the unbalanced side can be set to 50 Ω.

[0059] In the above-described configuration, the balanced line of the high-frequency balun 1 is formed by the third portion SEC21 and the fourth portion SEC22 of the second coaxial cable 20 connected in parallel. Therefore, the characteristic impedance on the balanced side is the sum of the characteristic impedance of the third portion SEC21 and the characteristic impedance of the fourth portion SEC22. Specifically, in this case, the characteristic impedance on the balanced side is 100Ω, which is the sum of the characteristic impedance of the third portion SEC21 (50Ω) and the characteristic impedance of the fourth portion SEC22 (50Ω).

[0060] This allows the ratio of the characteristic impedance of the balanced line to the characteristic impedance of the unbalanced line to be the desired 100Ω:50Ω=2:1.

[0061] Furthermore, if we substitute the fact that the characteristic impedance of the balanced line is 100 Ω and the characteristic impedance of the unbalanced line is 50 Ω into the equation for the real part of equation (4) in Reference A, we obtain the following relationship.

[0062] 50=100 / (100 2 / (100 2 (tanθab) 2 )+1) -(A) Here, θab corresponds to the electrical length of the third portion SEC21 and the electrical length of the fourth portion SEC22.

[0063] Solving equation (A), we get θab=45[deg]=π / 4[rad] This becomes:

[0064] Therefore, the electrical length of the third portion SEC21 and the electrical length of the fourth portion SEC22 may be set to 1 / 8 (λ / 8) of the wavelength of the high-frequency signal.

[0065] Furthermore, if θab=45[deg] is substituted into the equation with the imaginary part=0 in equation (4) of reference A, the following relationship is obtained.

[0066] 0=((100 2 )·100·tan45deg) / ((1002 )+(100·tan45deg) 2 )-50·cotθb -(B) Here, θb corresponds to the length θ192 of the characteristic adjustment portion 19.

[0067] Solving equation (B), we get θb(θ192)=45[deg]=π / 4[rad] This becomes:

[0068] Therefore, the length θ192 of the characteristic-adjusting portion 19 may be set to 1 / 8 (λ / 8) of the wavelength of the high-frequency signal.

[0069] With this configuration, the high-frequency balun 1 can obtain the characteristics shown in FIGS. 2(A), 2(B), 3(A), and 3(B).

[0070] Fig. 2(A) is a graph showing the frequency characteristics when a pure resistance twice the characteristic impedance of the coaxial cable is connected to the balanced terminal of S11 of the high-frequency balun according to the first embodiment of the present invention, and Fig. 2(B) is a graph showing the frequency characteristics of the VSWR when a pure resistance twice the characteristic impedance of the coaxial cable is connected to the balanced terminal of the high-frequency balun according to the first embodiment of the present invention. Fig. 2(A) and Fig. 2(B) show the characteristics as a 2:1 balun.

[0071] FIG. 3(A) is a graph showing the frequency characteristics of S11 when the high-frequency balun according to the first embodiment of the present invention is made using a coaxial cable with a characteristic impedance of 50 Ω and a pure resistor of 50 Ω (solid line) and a pure resistor of 100 Ω (dashed line) are connected to the balanced terminals. FIG. 3(B) is a graph showing the frequency characteristics of VSWR when the high-frequency balun according to the first embodiment of the present invention is made using a coaxial cable with a characteristic impedance of 50 Ω and a pure resistor of 50 Ω (solid line) and a pure resistor of 100 Ω (dashed line) are connected to the balanced terminals.

[0072] As shown in Figure 2(A), the high-frequency balun 1 is a 2:1 balun that can ensure a wide frequency band where S11 (return loss) is lower than -10 [dB]. Also, as shown in Figure 2(B), the high-frequency balun 1 is a 2:1 balun that can ensure a wide frequency band where VSWR is lower than 2.0.

[0073] In this way, the high-frequency balun 1 can achieve low-loss unbalanced-to-balanced conversion over a wide band as a 2:1 balun.

[0074] Furthermore, as shown in Fig. 3A, the high-frequency balun 1, as a 1:1 balun, can ensure a frequency band in which S11 (return loss) is lower than -10 [dB] within a predetermined frequency width. Also, as shown in Fig. 3B, the high-frequency balun 1, as a 1:1 balun, can ensure a frequency band in which VSWR is lower than 2.0 within a predetermined frequency width.

[0075] In this way, the high-frequency balun 1 can achieve low-loss unbalanced-to-balanced conversion as a 1:1 balun.

[0076] As described above, the high-frequency balun 1 can achieve wideband, low-loss unbalanced-to-balanced conversion as a 2:1 balun in a configuration using coaxial cables. Furthermore, the high-frequency balun 1 can also be used as a 1:1 balun while achieving the above-mentioned characteristics of a 2:1 balun.

[0077] In the above-described embodiment, a conductive bonding material such as solder is used. However, the bonding material is not limited to solder as long as it can achieve a predetermined strength of fixation and a low-resistance electrical connection. For example, spot welding, crimping, etc. can be used, and a cable fixing method using a conductive wire or the like can also be used.

[0078] Furthermore, the above-described high-frequency balun 1 has a balun configuration in which the characteristic impedance of the balanced line: the characteristic impedance of the unbalanced line is 2:1. However, if n is a real number greater than 1 (n>1), and the high-frequency balun 1 is configured to have a balun in which the characteristic impedance of the balanced line: the characteristic impedance of the unbalanced line is n:1 by appropriately adjusting the electrical length of the third portion SEC21, the electrical length of the fourth portion SEC22, and the length θ192 of the characteristic adjustment unit 192 in accordance with n based on the above-described configuration and theory.

[0079] That is, the high-frequency balun 1 can realize low-loss unbalanced-to-balanced conversion over a wide band using coaxial cables, under the condition that the characteristic impedance of the balanced line is higher than the characteristic impedance of the unbalanced line.

[0080] However, the high-frequency balun 1 can be realized as the widest bandwidth balun by configuring the balun in the above-mentioned shape so that the first characteristic impedance Z10 of the first coaxial cable 10 and the second characteristic impedance Z20 of the second coaxial cable 20 are both 50Ω, and the characteristic impedance on the unbalanced side is 50Ω and the characteristic impedance on the balanced side is 100Ω.

[0081] [Second embodiment] A high-frequency balun according to a second embodiment of the present invention will be described with reference to the drawings. Fig. 4 is a perspective view showing the appearance of the high-frequency balun according to the second embodiment.

[0082] 4, the high-frequency balun 1A according to the third embodiment differs from the high-frequency balun 1 according to the first embodiment in that it includes a second coaxial cable 20A. The basic configuration of the high-frequency balun 1A is conceptually similar to that of the high-frequency balun 1, and a description of similar parts will be omitted.

[0083] The high-frequency balun 1A includes a second coaxial cable 20A. The second coaxial cable 20A has a configuration in which two coaxial cables 291 and 292 are arranged in parallel. Each of the two coaxial cables 291 and 292 has the same configuration as the second coaxial cable 20 according to the first embodiment.

[0084] The characteristic impedances Z291 and Z292 of the two coaxial cables 291 and 292 are set based on the target characteristic impedance of the balanced line side.

[0085] For example, when the characteristic impedance on the balanced line side is set to 75Ω, the characteristic impedances Z291 and Z292 of the two coaxial cables 291 and 292 are each set to 75Ω.

[0086] By setting the characteristic impedance of the coaxial cables 291 and 292 to 75 Ω, the characteristic impedance of the balanced line formed by connecting the coaxial cables 291 and 292 in parallel can be set to 75 Ω, so that the characteristic impedance of the balanced line and the characteristic impedance of the coaxial cables that make up the balanced line become the same.

[0087] On the other hand, the characteristic impedance of the unbalanced line is set to 50 Ω as in the first embodiment, and the characteristic impedance of the first coaxial cable 10 is also 50 Ω. Therefore, even on the unbalanced line side, the characteristic impedance of the unbalanced line side and the characteristic impedance of the coaxial cable that constitutes the unbalanced line are the same.

[0088] With this configuration, the high-frequency balun 1A realizes unbalanced-to-balanced conversion between an unbalanced line with a characteristic impedance of 50Ω and a balanced line with a characteristic impedance of 75Ω.

[0089] FIG. 5 is a graph showing a comparison of the VSWR frequency characteristics obtained by simulating the high-frequency balun according to the first embodiment in which all coaxial cables are configured with coaxial cables having a characteristic impedance of 50 Ω when a pure resistance of 75 Ω is connected to the balanced terminals, and the high-frequency balun according to the second embodiment in which the first coaxial cable is configured with a coaxial cable having a characteristic impedance of 50 Ω and the second coaxial cable is configured with two parallel coaxial cables having a characteristic impedance of 75 Ω.

[0090] As shown in Figure 5, by using the configuration of the high-frequency balun 1A, the frequency band where the VSWR is lower than 2.0 can be wider. Furthermore, by adopting the configuration of the high-frequency balun 1A, the frequency band where the VSWR is lower than 1.5 can be wider.

[0091] In this way, the high-frequency balun 1A can achieve wider bandwidth unbalanced-to-balanced conversion by matching the characteristic impedance of the first coaxial cable 10 on the unbalanced side to the characteristic impedance of an external unbalanced line, and matching the characteristic impedance of the second coaxial cable 20B on the balanced side to the characteristic impedance of an external balanced line.

[0092] In this embodiment, the characteristic impedance of the unbalanced line is 50 Ω and the characteristic impedance of the balanced line is 75 Ω. However, as in the first embodiment, the characteristic impedance ratio is not limited to this, and by appropriately adopting the configuration of the high-frequency balun 1A, it is possible to more flexibly accommodate impedance ratios of n (n>1):1, thereby realizing wideband unbalanced-to-balanced conversion.

[0093] [Third embodiment] A high-frequency balun according to a third embodiment of the present invention will be described with reference to the drawings. Fig. 6 is a perspective view showing the appearance of the high-frequency balun according to the third embodiment.

[0094] 6, the high-frequency balun 1B according to the third embodiment differs from the high-frequency balun 1 according to the first embodiment in that the third end 201 and the fourth end 202 of the second coaxial cable 20 are not connected to the points where the first end 101 and the second end 102 of the first coaxial cable 10 are connected. Other configurations of the high-frequency balun 1B are the same as those of the high-frequency balun 1.

[0095] With this configuration, the high-frequency balun 1B can obtain characteristics similar to those of the high-frequency balun 1. However, as shown in the high-frequency balun 1, by connecting the third end 201 and the fourth end 202 of the second coaxial cable 20 to the point where the first end 101 and the second end 102 of the first coaxial cable 10 are connected and connecting them to a reference potential, more stable characteristics can be obtained.

[0096] <1> a first coaxial cable including a first center conductor, a first dielectric layer, and a first outer conductor, the first coaxial cable having a first end and a second end; a second coaxial cable including a second center conductor, a second dielectric layer, and a second outer conductor, the second coaxial cable having a third end and a fourth end; Equipped with the first coaxial cable has a shape in which a first portion, a first outer conductor-free portion, and a second portion are connected from the first end to the second end, the first end is an unbalanced terminal connected to an unbalanced line, and the second end is an open end at which the first central conductor and the first outer conductor are not connected, the second coaxial cable has a shape in which a third portion, a second outer conductor-free portion, and a fourth portion are connected from the third end to the fourth end, and the third portion and the fourth portion have the same length; the third end and the fourth end are short-circuit ends at which the second central conductor and the second outer conductor are electrically connected, the first end and the second end are electrically connected by a conductive bonding material; the third end and the fourth end are electrically connected by a conductive bonding material; an end of the first portion on the first outer conductor-free portion side and an end of the third portion on the second outer conductor-free portion side are electrically connected by a conductive bonding material to form a first terminal of a balanced terminal; an end of the second portion on the side of the first outer conductor-free portion and an end of the fourth portion on the side of the second outer conductor-free portion are electrically connected by a conductive bonding material to form a second terminal of the balanced terminal; an electrical length of the first portion and an electrical length of the second portion are ¼ of a wavelength λ of a high-frequency signal to be converted from unbalanced to balanced; the lengths of the third and fourth portions are set based on a characteristic impedance ratio between a balanced line and an unbalanced line connected to a high-frequency balun; A high-frequency balun, wherein the length of the second portion from the end of the first central conductor on the first outer conductor non-forming portion side is set based on the matching characteristics of the imaginary part of impedance between the balanced line and the unbalanced line.

[0097] <2> <1> The high frequency balun according to claim 1, A high frequency balun, wherein a first characteristic impedance of the first coaxial cable and a second characteristic impedance of the second coaxial cable are the same.

[0098] <3> <2> The high frequency balun according to claim 1, the first characteristic impedance and the second characteristic impedance are 50Ω; the length of the third portion and the length of the fourth portion are 1 / 8 of the wavelength λ; a length of the second portion from the end of the first central conductor on the first outer conductor-free portion side is approximately 1 / 8 of the wavelength λ;

[0099] <4> <1> ~ <3> The high-frequency balun according to any one of The high-frequency balun, wherein the first end, the second end, the third end, and the fourth end are electrically connected by a conductive bonding material and connected to a reference potential.

[0100] <5> <1> The high frequency balun according to claim 1, A high frequency balun, wherein the characteristic impedance of the first coaxial cable and the characteristic impedance of the second coaxial cable are different.

[0101] <6> <5> The high frequency balun according to claim 1, The second coaxial cable is a high-frequency balun formed by connecting two coaxial cables in parallel, each having the same characteristic impedance as the characteristic impedance of a balanced line.

[0102] <7> <1> ~ <6> The high-frequency balun according to any one of A high-frequency balun, wherein a connection portion between the first end and the second end and a connection portion between the third end and the fourth end are separated from each other, and the connection portions are each individually connected to a reference potential. [Explanation of symbols]

[0103] 1, 1A, 1B: High frequency balun 10: First coaxial cable 11: First central conductor 12: First dielectric layer 13: First outer conductor 19:Characteristics adjustment part 20, 20B: Second coaxial cable 21: Second center conductor 22: Second dielectric layer 23: Second outer conductor 31, 32: Terminal conductor 71, 72, 73: Conductive bonding material 80: Coaxial connector 101: 1st end 102: 2nd end 103:End 104:End 201: Third end 202: 4th end 203:End 204: End 291, 292: Coaxial cable GAP1: First outer conductor-free portion GAP2: Second outer conductor-free portion SEC10: Coaxial cable for wiring SEC11: Part 1 SEC12: 2nd part SEC21: 3rd part SEC22: Part 4

Claims

1. a first coaxial cable including a first center conductor, a first dielectric layer, and a first outer conductor, the first coaxial cable having a first end and a second end; a second coaxial cable including a second center conductor, a second dielectric layer, and a second outer conductor, the second coaxial cable having a third end and a fourth end; Equipped with the first coaxial cable has a shape in which a first portion, a first outer conductor-free portion, and a second portion are connected from the first end to the second end, the first end is an unbalanced terminal connected to an unbalanced line, and the second end is an open end at which the first central conductor and the first outer conductor are not connected to each other, the second coaxial cable has a shape in which a third portion, a second outer conductor-free portion, and a fourth portion are connected from the third end to the fourth end, and the third portion and the fourth portion have the same length; the third end and the fourth end are short-circuit ends at which the second central conductor and the second outer conductor are electrically connected, the first end and the second end are electrically connected by a conductive bonding material; the third end and the fourth end are electrically connected by a conductive bonding material; an end of the first portion on the first outer conductor-free portion side and an end of the third portion on the second outer conductor-free portion side are electrically connected by a conductive bonding material to form a first terminal of a balanced terminal; an end of the second portion on the side of the first outer conductor-free portion and an end of the fourth portion on the side of the second outer conductor-free portion are electrically connected by a conductive bonding material to form a second terminal of the balanced terminal; an electrical length of the first portion and an electrical length of the second portion are ¼ of a wavelength λ of a high-frequency signal to be converted from unbalanced to balanced; the lengths of the third and fourth portions are set based on a characteristic impedance ratio between a balanced line and an unbalanced line connected to a high-frequency balun; a length of the second portion from an end of the first central conductor on the first outer conductor-free portion side is set based on matching characteristics of an imaginary part of impedance between the balanced line and the unbalanced line. High frequency balun.

2. 2. The high frequency balun according to claim 1, a first characteristic impedance of the first coaxial cable and a second characteristic impedance of the second coaxial cable are equal to each other; High frequency balun.

3. 3. The high frequency balun according to claim 2, the first characteristic impedance and the second characteristic impedance are 50Ω; a length of the third portion and a length of the fourth portion are 1 / 8 of the wavelength λ; a length of the second portion from an end of the first central conductor on the first outer conductor-free portion side is approximately 1 / 8 of the wavelength λ; High frequency balun.

4. 4. The high-frequency balun according to claim 1, the first end, the second end, the third end, and the fourth end are electrically connected by a conductive bonding material and are connected to a reference potential; High frequency balun.

5. 2. The high frequency balun according to claim 1, The characteristic impedance of the first coaxial cable is different from the characteristic impedance of the second coaxial cable. High frequency balun.

6. 6. The high frequency balun according to claim 5, The second coaxial cable is configured by connecting two coaxial cables in parallel, each having the same characteristic impedance as the characteristic impedance of a balanced line. High frequency balun.

7. 4. The high-frequency balun according to claim 1, a connection portion between the first end and the second end and a connection portion between the third end and the fourth end are separated from each other, and the connection portions are individually connected to a reference potential. High frequency balun.

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