High-frequency balun
The coaxial cable-based balun design addresses bandwidth and power limitations by providing a stable, low-loss, and broadband conversion circuit with a simple configuration, enhancing performance across various frequencies.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- FURUNO ELECTRIC CO LTD
- Filing Date
- 2024-11-14
- Publication Date
- 2026-05-26
AI Technical Summary
Existing balanced-unbalanced conversion circuits (high-frequency baluns) face challenges such as narrow bandwidth and power limitations due to magnetic saturation, especially when using ferrite, and the length of windings becomes significant at higher frequencies, affecting performance.
A high-frequency balun design comprising three coaxial cables with specific configurations, including open and short-circuit ends, parallel arrangements, and grounded outer conductors, allowing for a 1:1 impedance ratio and stable characteristics, forming a simple and broadband conversion circuit.
The design achieves stable, low-loss transmission with a wide bandwidth and simplified structure, utilizing coaxial cables without transformers, and maintains optimal performance across a broad frequency range.
Smart Images
Figure 2026086061000001_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to a high-frequency balun that performs unbalanced-to-balanced conversion of high-frequency signals.
Background Art
[0002] Non-Patent Document 1 describes a balanced-unbalanced conversion circuit using a coaxial line. The balanced-unbalanced conversion circuit of Non-Patent Document 1 is composed of two coaxial lines.
Prior Art Documents
Non-Patent Documents
[0003]
Non-Patent Document 1
Summary of the Invention
Problems to be Solved by the Invention
[0004] However, when a balanced-unbalanced conversion circuit (high-frequency balun) with a ratio of the impedance on the balanced side to the impedance on the unbalanced side of 1:1 is configured using the configuration shown in Non-Patent Document 1, the low-loss frequency band becomes narrow.
[0005] In addition, there is also a balanced-unbalanced conversion circuit (high-frequency balun) using ferrite, but the shape of the ferrite is restricted, and due to this shape restriction, a power upper limit occurs due to magnetic saturation. Also, as the frequency increases, the length of the winding becomes non-negligible compared to the wavelength, making it difficult to obtain good characteristics.
[0006] Therefore, an object of the present invention is to realize a balanced-unbalanced conversion circuit (high-frequency balun) with a wide bandwidth and a simple configuration.
Means for Solving the Problems
[0007] A high-frequency balun according to one embodiment of this invention comprises: a first coaxial cable having a first central conductor, a first dielectric layer, and a first outer conductor, and having a first end and a second end; a second coaxial cable having a second central conductor, a second dielectric layer, and a second outer conductor, and having a third end and a fourth end; and a third coaxial cable having a third central conductor, a third dielectric layer, and a third outer conductor, and having a fifth end and a sixth end.
[0008] The first coaxial cable has an outer conductor section without a first outer conductor at a predetermined length from the second end towards the first end. The first coaxial cable comprises a first portion on the first end side of the outer conductor section and a second portion on the second end side of the outer conductor section. The second end of the first coaxial cable is an open end. The second and third coaxial cables have the same length as the second portion. The second coaxial cable is arranged parallel to the second portion with its third end adjacent to the second end and its fourth end adjacent to the end on the outer conductor section side. The third end of the second coaxial cable is a short-circuit end. The second central conductor protrudes from its fourth end and is connected to the first outer conductor of the first portion. The third coaxial cable is arranged parallel to the first portion with its sixth end adjacent to the end on the outer conductor section side. The fifth end of the third coaxial cable is a short-circuit end. The third central conductor protrudes from the sixth end and connects to the first outer conductor of the second section. The first end of the first coaxial cable forms an unbalanced terminal. The end of the first section on the side of the outer conductor division and the end of the second section on the side of the outer conductor division form a balanced terminal.
[0009] This configuration allows the ratio of the impedance on the unbalanced side to the impedance on the balanced side to be 1:1.
[0010] In a high-frequency balun according to one embodiment of this invention, the first outer conductor, the second outer conductor, and the third outer conductor are grounded.
[0011] In this configuration, the characteristics of the high-frequency balun become stable.
[0012] In one embodiment of this invention, a high-frequency balun has a structure comprising a first coaxial cable, a second coaxial cable, and a third coaxial cable, which is configured in an annular shape. The second, third, and fifth ends are located in close proximity to each other. The first outer conductor, the second outer conductor, and the third outer conductor are electrically connected at the second, third, and fifth ends.
[0013] In this configuration, the potential difference between the second, third, and fifth terminals is suppressed, stabilizing the characteristics of the high-frequency balun.
[0014] In a high-frequency balun according to one embodiment of this invention, the first characteristic impedance of the first coaxial cable, the second characteristic impedance of the second coaxial cable, and the third characteristic impedance of the third coaxial cable are the same.
[0015] This configuration allows for the implementation of a high-frequency balun with a simpler design. [Brief explanation of the drawing]
[0016] [Figure 1] Figure 1 is a perspective view of a high-frequency balun according to an embodiment. [Figure 2] Figure 2(A) is a side view showing a linear structure for forming a high-frequency balun according to an embodiment, and Figure 2(B) is a side cross-sectional view thereof. [Figure 3] Figure 3 is a graph showing the reflection characteristics (dB) of the high-frequency balun according to the embodiment. [Figure 4] Figure 4 is a graph showing the reflection characteristics (VSWR) of the high-frequency balun according to the embodiment. [Modes for carrying out the invention]
[0017] A high-frequency balun according to an embodiment of the present invention will be described with reference to the figures. Figure 1 is a perspective view of the high-frequency balun according to the embodiment. Figure 2(A) is a side view showing a linear structure for forming the high-frequency balun according to the embodiment, and Figure 2(B) is a side cross-sectional view thereof.
[0018] As shown in FIGS. 1, 2(A), and 2(B), the high-frequency balun 1 includes a first coaxial cable 10, a second coaxial cable 20, and a third coaxial cable 30.
[0019] The first coaxial cable 10 includes a first central conductor 11, a first dielectric layer 12, and a first outer conductor 13.
[0020] The first central conductor 11 is made of a metal such as copper and is a linear conductor. The first dielectric layer 12 has a predetermined dielectric constant. The first dielectric layer 12 covers the periphery of the first central conductor 11 with a predetermined thickness. The first outer conductor 13 is a metal such as copper and is cylindrical with a predetermined thickness. The first outer conductor 13 covers the outer peripheral surface of the first dielectric layer 12.
[0021] The dielectric constant (relative permittivity) of the first dielectric layer 12 is the first dielectric constant εr1. The characteristic impedance of the first coaxial cable 10 is set to the first characteristic impedance Z10. For example, the first characteristic impedance Z10 is set to 50 Ω.
[0022] The first coaxial cable 10 has a first end 101 at one end in the extending direction and a second end 102 at the other end.
[0023] At the second end 102, the first central conductor 11 and the first outer conductor 13 are not connected. The second end 102 is a high-frequency open end of the first coaxial cable 10.
[0024] The first coaxial cable 10 has an outer conductor disconnect portion GAP at a position of a predetermined length in the direction from the second end 102 to the first end 101. The outer conductor disconnect portion GAP is a portion where the first outer conductor 13 is not formed over the entire circumference, and the first dielectric layer 12 is exposed.
[0025] As a result, the first coaxial cable 10 comprises a first portion SEC11 located on the first end 101 side of the outer conductor splitting portion GAP, and a second portion SEC12 located on the second end 102 side of the outer conductor splitting portion GAP. The first portion SEC11 and the second portion SEC12 are connected by the outer conductor splitting portion GAP. The first portion SEC11 has a connection end 103 on the outer conductor splitting portion GAP side. The second portion SEC12 has a connection end 104 on the outer conductor splitting portion GAP side.
[0026] The length of the second section SEC12 is set based on the wavelength λ of the high-frequency signal transmitted by the high-frequency balun 1. For example, the length of the second section SEC12 is approximately 1 / 4 the wavelength λ. However, the length of the second section SEC12 is not limited to approximately 1 / 4 the wavelength λ, and can be adjusted and set based on the characteristics of the high-frequency balun 1.
[0027] The length of the outer conductor gap (the distance between connection end 103 and connection end 104) is preferably as short as possible, and is at least a distance at which the first balanced terminal 41 and the second balanced terminal 42, described later, do not come into contact.
[0028] The length of the first part SEC11 only needs to be longer than the second part SEC12, and can be set appropriately based on the arrangement of the high-frequency balun 1, the connection to other circuit elements (such as a balanced antenna or high-frequency front-end circuit), etc.
[0029] The second coaxial cable 20 comprises a second central conductor 21, a second dielectric layer 22, and a second outer conductor 23.
[0030] 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 with a predetermined thickness. The second outer conductor 23 is made of a metal such as copper and is cylindrical with a predetermined thickness. The second outer conductor 23 covers the outer surface of the second dielectric layer 22.
[0031] The dielectric constant (relative permittivity) of the second dielectric layer 22 is the second dielectric constant εr2. The second dielectric constant εr2 is the same as the first dielectric constant εr1. As a result, the wavelength shortening factor in the second coaxial cable 20 is the same as the wavelength shortening factor in the first coaxial cable 10.
[0032] The characteristic impedance of the second coaxial cable 20 is set to the second characteristic impedance Z20. The second characteristic impedance Z20 is the same as the first characteristic impedance Z10. For example, the second characteristic impedance Z20 of the second coaxial cable 20 is set to 50Ω, just like the first characteristic impedance Z10.
[0033] The second coaxial cable 20 has a third end 201 at one end in the direction of extension and a fourth end 202 at the other end.
[0034] The length of the second coaxial cable 20 is the same as the length of the second section SEC12 of the first coaxial cable 10. The second coaxial cable 20 is arranged parallel to the second section SEC12. The third end 201 is adjacent to the second end 102 of the second section SEC12, and the fourth end 202 is adjacent to the connection end 104 of the second section SEC12. The second outer conductor 23 of the second coaxial cable 20 and the first outer conductor 13 of the second section SEC12 are connected.
[0035] At the third end 201, the second central conductor 21 and the second outer conductor 23 are connected. The third end 201 is the high-frequency short-circuit end of the second coaxial cable 20.
[0036] At the fourth end 202, the second central conductor 21 and the second outer conductor 23 are not connected. The second central conductor 21 protrudes from the fourth end 202. The second central conductor 21 is connected to the first outer conductor 13 at the connection end 103 of the first portion SEC11 of the first coaxial cable 10.
[0037] Furthermore, the second central conductor 21 does not protrude from the fourth end 202, and it is also possible to connect the second central conductor 21 at the fourth end 202 to the first outer conductor 13 using a separate linear conductor.
[0038] The third coaxial cable 30 comprises a third central conductor 31, a third dielectric layer 32, and a third outer conductor 33.
[0039] The third central conductor 31 is made of a metal such as copper and is a linear conductor. The third dielectric layer 32 has a predetermined dielectric constant. The third dielectric layer 32 covers the periphery of the third central conductor 31 with a predetermined thickness. The third outer conductor 33 is made of a metal such as copper and is cylindrical with a predetermined thickness. The third outer conductor 33 covers the outer surface of the third dielectric layer 32.
[0040] The dielectric constant (relative dielectric constant) of the third dielectric layer 32 is the third dielectric constant εr3. The third dielectric constant εr3 is the same as the first dielectric constant εr1 and the second dielectric constant εr2. As a result, the wavelength shortening factor in the third coaxial cable 30 is the same as the wavelength shortening factor in the first coaxial cable 10 and the second coaxial cable 20.
[0041] The characteristic impedance of the third coaxial cable 30 is set to the third characteristic impedance Z30. The third characteristic impedance Z30 is the same as the first characteristic impedance Z10 and the second characteristic impedance Z20. For example, the third characteristic impedance Z30 of the third coaxial cable 30 is set to 50Ω, just like the first characteristic impedance Z10 and the second characteristic impedance Z20.
[0042] The third coaxial cable 30 has a fifth end 301 at one end in the direction of extension and a sixth end 302 at the other end.
[0043] The length of the third coaxial cable 30 is the same as the length of the second section SEC12 of the first coaxial cable 10 and the second coaxial cable 20. The third coaxial cable 30 is arranged parallel to the first section SEC11. The sixth end 302 is adjacent to the connection end 103 of the first section SEC11. The third outer conductor 33 of the third coaxial cable 30 is connected to the first outer conductor 13 of the first section SEC11.
[0044] At the fifth end 301, the third central conductor 31 and the third outer conductor 33 are connected. The fifth end 301 is the high-frequency short-circuit end of the third coaxial cable 30.
[0045] At the sixth end 302, the third central conductor 31 and the third outer conductor 33 are not connected.
[0046] The third central conductor 31 protrudes from the sixth end 302. The third central conductor 31 is connected to the first outer conductor 13 at the connection end 104 of the second portion SEC12 of the first coaxial cable 10. Alternatively, the third central conductor 31 may not protrude from the sixth end 302, and a separate linear conductor may be used to connect the third central conductor 31 at the sixth end 302 to the first outer conductor 13.
[0047] In this configuration, the first end 101 of the first coaxial cable 10 is connected to the coaxial connector 80. The first end 101 constitutes the unbalanced terminal of the high-frequency balun 1.
[0048] Both sides of the outer conductor separation gap, namely the ends where the connection end 103 of the first outer conductor 13 and the sixth end 302 of the third outer conductor 33 are connected, and the ends where the connection end 104 of the first outer conductor 13 and the fourth end 202 of the second outer conductor 23 are connected, constitute the balanced terminals of the high-frequency balun 1.
[0049] The first balanced terminal 41 is made of a linear conductor. The first balanced terminal 41 is connected to one end of the outer conductor gap (the end where the connection end 103 of the first outer conductor 13 and the sixth end 302 of the third outer conductor 33 are connected) using a conductive bonding material 71 (see Figure 1). The second balanced terminal 42 is made of a linear conductor. The second balanced terminal 42 is connected to the other end of the outer conductor gap (the end where the connection end 104 of the first outer conductor 13 and the fourth end 202 of the second outer conductor 23 are connected) using a conductive bonding material 72 (see Figure 1).
[0050] By curving the structure described above to form an annular shape, as shown in Figure 1, the shape of the high-frequency balun 1 is realized.
[0051] In this configuration, the second end 102 of the first coaxial cable 10, the third end 201 of the second coaxial cable 20, and the fifth end 301 of the third coaxial cable 30 are positioned in close proximity to each other. The first outer conductor 13, the second outer conductor 23, and the third outer conductor 33, including the positions where the second end 102, the third end 201, and the fifth end 301 are in close proximity, are each fixed to each other using a conductive bonding material 70 and electrically connected. In this section, the first outer conductor 13, the second outer conductor 23, and the third outer conductor 33 are grounded.
[0052] With this configuration, the high-frequency balun 1 incorporates a Roberts balun as its basic configuration. Furthermore, by having a configuration in which the center conductor and the outer conductor are connected in a cross pattern at both ends of the balanced two-conductor cable, the impedance of the balanced two-conductor cable, which is composed of the second coaxial cable and the third coaxial cable, can be halved.
[0053] Therefore, ideally, when the balanced termination resistance is 50Ω, the best characteristics are obtained when the first characteristic impedance Z10 of the first coaxial cable is 50Ω, and the second characteristic impedance Z20 of the second coaxial cable and the third characteristic impedance Z30 of the third coaxial cable are both 100Ω.
[0054] (Characteristics of High-Frequency Balun 1) Figure 3 is a graph showing the reflection characteristics (dB) of the high-frequency balun according to the embodiment. Figure 4 is a graph showing the reflection characteristics (VSWR) of the high-frequency balun according to the embodiment. Figures 3 and 4 show the characteristics when the unbalanced side impedance is 50Ω. In Figures 3 and 4, the solid line shows the characteristics when the balanced side termination resistance is 50Ω, and the dashed line shows the characteristics when the balanced side termination resistance is 100Ω.
[0055] As shown in Figures 3 and 4, the high-frequency balun 1 can achieve lower loss transmission characteristics when the balanced-side termination resistance is 50Ω compared to when it is 100Ω. Furthermore, the high-frequency balun 1 can achieve a sufficient loss suppression effect. In addition, the high-frequency balun 1 can widen the low-loss frequency bandwidth when the balanced-side termination resistance is 50Ω compared to when it is 100Ω.
[0056] Therefore, the high-frequency balun 1 can constitute a 1:1 balanced-unbalanced conversion circuit with broadband and low-loss characteristics.
[0057] Furthermore, the high-frequency balun 1 is constructed using multiple coaxial cables without the need for transformers or other components. This allows the high-frequency balun 1 to form a 1:1 balanced-unbalanced conversion circuit with a simple configuration.
[0058] Furthermore, the high-frequency balun 1 uses multiple coaxial cables with the same characteristic impedance. This allows the high-frequency balun 1 to construct a 1:1 balanced-unbalanced conversion circuit with an even simpler configuration.
[0059] In the embodiments described above, a configuration using a conductive bonding material such as solder was shown. However, the method is not limited to conductive bonding materials such as solder, as long as it can achieve fixing with a predetermined strength and low electrical resistance. For example, spot welding, crimping, etc., can be used, and furthermore, a fixing method using conductive wire or the like may also be employed.
[0060] <1> A first coaxial cable comprising a first central conductor, a first dielectric layer, and a first outer conductor, and having a first end and a second end, A second coaxial cable comprising a second central conductor, a second dielectric layer, and a second outer conductor, and having a third end and a fourth end, A third coaxial cable comprising a third central conductor, a third dielectric layer, and a third outer conductor, and having a fifth end and a sixth end, Equipped with, The first coaxial cable is At a position of a predetermined length from the second end to the first end, the outer conductor is divided into sections, and the first outer conductor is not provided. It comprises a first portion located on the first end side of the outer conductor division portion, and a second portion located on the second end side of the outer conductor division portion, The second end of the first coaxial cable is an open end. The second coaxial cable and the third coaxial cable have the same length as the second portion. The second coaxial cable is arranged parallel to the second portion, with its third end adjacent to the second end and its fourth end adjacent to the end of the second portion on the side of the outer conductor division. The third end of the second coaxial cable is a short-circuit end. The second central conductor protrudes from the fourth end and is connected to the first outer conductor of the first portion. The third coaxial cable is arranged parallel to the first portion, with its sixth end adjacent to the end of the first portion on the side of the outer conductor division. The fifth end of the third coaxial cable is a short-circuit end. The third central conductor protrudes from the sixth end and is connected to the first outer conductor of the second portion. The first end of the first coaxial cable constitutes an unbalanced terminal. The end of the first portion on the side of the outer conductor division and the end of the second portion on the side of the outer conductor division constitute a balanced terminal. The ratio of the impedance on the unbalanced side to the impedance on the balanced side is 1:1. High-frequency balun.
[0061] <2> <1> A high-frequency balun as described above, The first outer conductor, the second outer conductor, and the third outer conductor are grounded. High-frequency balun.
[0062] <3> <1> or <2> A high-frequency balun as described above, The structure comprising the first coaxial cable, the second coaxial cable, and the third coaxial cable is ring-shaped, The second end, the third end, and the fifth end are located in close proximity to each other. The first outer conductor, the second outer conductor, and the third outer conductor are electrically connected at the second, third, and fifth ends, High-frequency balun.
[0063] <4> <1> ~ <3> A high-frequency balun as described in any of the following: The first characteristic impedance of the first coaxial cable, the second characteristic impedance of the second coaxial cable, and the third characteristic impedance of the third coaxial cable are the same. High-frequency balun. [Explanation of Symbols]
[0064] 1: High-frequency balun 10: First coaxial cable 11: First central conductor 12: First dielectric layer 13: First outer conductor 20: Second coaxial cable 21: Second central conductor 22: Second dielectric layer 23: Second outer conductor 30: Third coaxial cable 31: Third central conductor 32: Third dielectric layer 33: Third outer conductor 41: 1st balanced terminal 42: 2nd balanced terminal 70, 71, 72: Conductive bonding material 80: Coaxial connector 101: 1st end 102: 2nd end 103, 104: Connection terminals 201: Third end 202: 4th end 301: 5th end 302: 6th end GAP: Outer conductor division point SEC11: Part 1 SEC12: 2nd part
Claims
1. A first coaxial cable comprising a first central conductor, a first dielectric layer, and a first outer conductor, and having a first end and a second end, A second coaxial cable comprising a second central conductor, a second dielectric layer, and a second outer conductor, and having a third end and a fourth end, A third coaxial cable comprising a third central conductor, a third dielectric layer, and a third outer conductor, and having a fifth end and a sixth end, Equipped with, The first coaxial cable is At a position of a predetermined length from the second end to the first end, the outer conductor is divided into sections, and the first outer conductor is not provided. It comprises a first portion located on the first end side of the outer conductor division portion, and a second portion located on the second end side of the outer conductor division portion, The second end of the first coaxial cable is an open end. The second coaxial cable and the third coaxial cable have the same length as the second portion. The second coaxial cable is arranged parallel to the second portion, with its third end adjacent to the second end and its fourth end adjacent to the end of the second portion on the side of the outer conductor separation portion. The third end of the second coaxial cable is a short-circuit end. The second central conductor protrudes from the fourth end and is connected to the first outer conductor of the first portion. The third coaxial cable is arranged parallel to the first portion, with its sixth end adjacent to the end of the first portion on the side of the outer conductor division. The fifth end of the third coaxial cable is a short-circuit end. The third central conductor protrudes from the sixth end and is connected to the first outer conductor of the second portion. The first end of the first coaxial cable constitutes an unbalanced terminal. The end of the first portion on the side of the outer conductor division and the end of the second portion on the side of the outer conductor division constitute a balanced terminal. The ratio of the impedance on the unbalanced side to the impedance on the balanced side is 1:
1. High-frequency balun.
2. A high-frequency balun according to claim 1, The first outer conductor, the second outer conductor, and the third outer conductor are grounded. High-frequency balun.
3. A high-frequency balun according to claim 1, The structure comprising the first coaxial cable, the second coaxial cable, and the third coaxial cable is ring-shaped, The second end, the third end, and the fifth end are located in close proximity to each other. The first outer conductor, the second outer conductor, and the third outer conductor are electrically connected at the second, third, and fifth ends, High-frequency balun.
4. A high-frequency balun according to claim 1, The first characteristic impedance of the first coaxial cable, the second characteristic impedance of the second coaxial cable, and the third characteristic impedance of the third coaxial cable are the same. High-frequency balun.