High-frequency balun

Abstract

To provide a high-frequency balun that relaxes a limit of an impedance ratio on a balanced side and an unbalanced side while performing an unbalanced and balanced conversion at a low loss, so as to be compatible with a high frequency.SOLUTION: A high-frequency balun 1 comprises: a first coaxial cable 10 having a first outer conductor parting part GAP1; and a second coaxial cable 20 having a second outer conductor parting part GAP2. In the second coaxial cable 20, a length between a third end 201 and the second outer conductor parting part GAP2, and a length between a fourth end 202 and the second outer conductor parting part GAP2 are the same. In a state where the third end 201 and the second end 102 are arranged in alignment and in parallel to the first coaxial cable 10. The first and second outer conductors are electrically connected via a conductive bonding material 70, the second end 102 is an open end. In the third end 201 and the fourth end 202, a second center conductor is connected to the second outer body. The first end 101 is an unbalance terminal, and both end parts of the second outer conductor parting part GAP2 are balance terminals.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] A balun and an antenna feeding circuit are described in Patent Document 1. As shown in Patent Document 1, when a balanced antenna is used and a signal is fed from an unbalanced circuit, a balun is provided.

[0003] Conventional examples of such baluns include a balun that uses ferrite and a winding, and a balun that performs Q matching using a coaxial cable. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2019 / 155624 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the above-mentioned conventional baluns have strict limitations on the ratio of the impedance on the balanced side to the impedance on the unbalanced side when performing low-loss unbalanced-to-balanced conversion, and are unsuitable for frequencies higher than VHF.

[0006] Therefore, an object of the present invention is to provide a high-frequency balun that can perform low-loss unbalanced-to-balanced conversion while easing restrictions on the ratio of the impedance on the balanced side to the impedance on the unbalanced side and is compatible with high frequencies. [Means for solving the problem]

[0007] The high-frequency balun of the present invention comprises a first coaxial cable having a first center conductor, a first dielectric layer, and a first outer conductor, and having a first end and a second end, and a second coaxial cable having a second center conductor, a second dielectric layer, and a second outer conductor, and having a third end and a fourth end.

[0008] The first coaxial cable has a first outer conductor divided portion not including a first outer conductor at a position of a predetermined length from the second end toward the first end. The second coaxial cable has a second outer conductor divided portion not including a second outer conductor at a position of a predetermined length from the third end toward the fourth end. In the second coaxial cable, the length between the third end and the second outer conductor divided portion is the same as the length between the fourth end and the second outer conductor divided portion.

[0009] The second coaxial cable is arranged parallel to the first coaxial cable with the third end aligned with the second end of the first coaxial cable. The first outer conductor and the second outer conductor are electrically connected by a conductive bonding material. The second end of the first coaxial cable is an open end. At the third and fourth ends of the second coaxial cable, the second central conductor is connected to the second outer conductor and is at approximately the same potential as the ground potential.

[0010] The first end of the first coaxial cable is an unbalanced terminal, and an end of the second outer conductor on the third end side of the second outer conductor divided portion and an end of the second outer conductor on the fourth end side of the second outer conductor divided portion are balanced terminals.

[0011] In this configuration, by appropriately setting the predetermined length, a balun (2:1 balun) with a balanced side impedance to unbalanced side impedance ratio of 2:1 in the first frequency band can be realized with low loss characteristics. Also, a balun (1:1 balun) with a balanced side impedance to unbalanced side impedance ratio of 1:1 in the second frequency band lower than the first frequency band can be realized with low loss and wideband characteristics. For example, if the first coaxial cable and the second coaxial cable are both coaxial cables with a characteristic impedance of 50Ω and a shortening rate of 70%, and the predetermined length is 80 mm, the center frequency of the first frequency band can be set to about 955.4 MHz with a 2:1 balun, and the center frequency of the second frequency band can be set to about 570.4 MHz with a 1:1 balun.

[0012] This allows the high-frequency balun to perform low-loss unbalanced-to-balanced conversion while relaxing the restrictions on the ratio of the impedance on the balanced side to the impedance on the unbalanced side and being compatible with high frequencies.

[0013] In the high frequency balun of the present invention, the dielectric constant of the first dielectric layer is the same as that of the second dielectric layer. In this configuration, the wavelength shortening rate of the first coaxial cable is the same as that of the second coaxial cable, and the above characteristics can be realized with a simpler configuration and with higher accuracy.

[0014] In the high frequency balun of the present invention, the second characteristic impedance of the second coaxial cable is the same as the first characteristic impedance of the first coaxial cable. With this configuration, the above-mentioned 2:1 balun and 1:1 balun can be realized more reliably.

[0015] In the high frequency balun of the present invention, the second characteristic impedance of the second coaxial cable is higher than the first characteristic impedance of the first coaxial cable, making it possible to configure baluns with a wider variety of impedance ratios.

[0016] In the high frequency balun of the present invention, the amount of deviation between the positions of the second end and the third end is 10% or less of the predetermined length. With this configuration, while realizing the above-mentioned characteristics, positional deviation errors during manufacturing can be tolerated, making it easier to manufacture.

[0017] In the high frequency balun of the present invention, the predetermined length is approximately 0.367 times the wavelength of the high frequency signal transmitted through the first coaxial cable and the second coaxial cable. When the balanced side impedance of the present invention is required to be twice the characteristic impedance of the second coaxial cable, the predetermined length varies slightly depending on whether the impedances of the first coaxial cable and the second coaxial cable are the same or different. For example, when the characteristic impedance of the first coaxial cable is 50Ω and the characteristic impedance of the second coaxial cable is 75Ω and the balanced side impedance is required to be 150Ω, the predetermined length is approximately 0.381 times the wavelength of the center frequency of the high frequency signal to be transmitted.

[0018] In the high frequency balun of the present invention, the structure consisting of the first coaxial cable and the second coaxial cable is annular. The fourth end is electrically connected to the second end and the third end in close proximity. In this configuration, the difference between the ground potential of the fourth end and the ground potential of the second end and the third end is small. This makes it possible to realize even better transmission characteristics.

[0019] The high frequency balun of the present invention includes a ground conductor. The second and third terminals are electrically connected to the fourth terminal through the ground conductor. In this configuration, the ground conductor shortens the high frequency connection distance between the second and third terminals and the fourth terminal, and reduces the difference between the ground potential of the fourth terminal and the ground potential of the second and third terminals. This makes it possible to achieve excellent transmission characteristics. [Brief description of the drawings]

[0020] [Figure 1] FIG. 1(A) is a first side view of the high-frequency balun according to the first embodiment, FIG. 1(B) is a second side view of the high-frequency balun according to the first embodiment, and FIG. 1(C) is a top view of the high-frequency balun according to the first embodiment. [Diagram 2] FIG. 2 is an external perspective view of the high-frequency balun according to the first embodiment. [Diagram 3] FIG. 3A is a side view showing a linear structure for forming the high-frequency balun according to the first embodiment, and FIG. 3B is a side cross-sectional view thereof. [Figure 4] 4A and 4B are graphs showing the reflection characteristics of the high-frequency balun according to the first embodiment. [Diagram 5] 5(A), 5(B), and 5(C) are graphs showing the transmission characteristics of the high-frequency balun according to the first embodiment. [Figure 6] FIG. 6 is an external perspective view of the high-frequency balun according to the second embodiment. [Figure 7] FIG. 7 is an external perspective view of the high-frequency balun according to the third embodiment. [Figure 8] FIG. 8 is an external perspective view of the high-frequency balun according to the fourth embodiment. [Figure 9] FIG. 9 is an external perspective view of the high-frequency balun according to the fifth embodiment. [Figure 10] 10A and 10B are graphs showing the reflection characteristics of the high-frequency balun according to the fifth embodiment. [Figure 11] FIG. 11 is an external perspective view of the high-frequency balun according to the sixth embodiment. [Figure 12] 12A and 12B are graphs showing the reflection characteristics of the high-frequency balun according to the sixth embodiment and a comparative high-frequency balun. [Figure 13] FIG. 13 is an external perspective view of the high-frequency balun according to the seventh embodiment. [Figure 14] FIG. 14 is a side cross-sectional view showing a linear structure for forming a high-frequency balun according to the eighth embodiment. [Figure 15] FIG. 15 is an external perspective view of the high-frequency balun according to the ninth embodiment. [Figure 16] 16(A) and 16(B) are graphs showing the reflection characteristics of the high-frequency balun according to the ninth embodiment and a comparative high-frequency balun. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] [First embodiment] A high frequency balun according to a first embodiment of the present invention will be described with reference to the drawings. FIG. 1(A) is a first side view of the high frequency balun according to the first embodiment, FIG. 1(B) is a second side view of the high frequency balun according to the first embodiment, and FIG. 1(C) is a top view of the high frequency balun according to the first embodiment. Note that the names of the first side view, the second side view, and the top view here are not names that specify the direction during use, but are names that are conveniently given to explain the configuration of the high frequency balun. In other words, for example, the surface visible in the top view does not have to be used as the upper side. FIG. 2 is an external perspective view of the high frequency balun according to the first embodiment. FIG. 3(A) is a side view showing a linear structure for forming the high frequency balun according to the first embodiment, and FIG. 3(B) is a side cross-sectional view thereof.

[0022] As shown in Figures 1(A), 1(B), 1(C), 2, 3(A), and 3(B), the high-frequency balun 1 includes a first coaxial cable 10, a second coaxial cable 20, and a coaxial connector 80.

[0023] (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. The first central conductor 11 is a linear conductor made of a metal such as copper. 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 a metal such as copper, and is tubular to a predetermined thickness. The first outer conductor 13 covers the outer peripheral surface of the first dielectric layer 12.

[0024] The dielectric constant (relative dielectric constant) of the first dielectric layer 12 is a first dielectric constant εr1. The characteristic impedance of the first coaxial cable 10 is set to a first characteristic impedance Z10. For example, the first characteristic impedance Z10 is set to 50Ω.

[0025] 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. The first end 101 of the first coaxial cable 10 is connected to the coaxial connector 80. The first central conductor 11 and the first outer conductor 13 are spaced apart, and the second end 102 of the first coaxial cable 10 is a high-frequency open end.

[0026] The first coaxial cable 10 has a first outer conductor division portion GAP1 at a position a predetermined length LA in the direction from the second end 102 to the first end 101. The first outer conductor division portion GAP1 is a portion where the first outer conductor 13 is not formed over the entire circumference, and the first dielectric layer 12 is exposed.

[0027] As a result, the first coaxial cable 10 includes a first portion SEC11 located closer to the first end 101 than the first outer conductor dividing portion GAP1, and a second portion SEC12 located closer to the second end 102 than the first outer conductor dividing portion GAP1. The first portion SEC11 and the second portion SEC12 are connected by the first outer conductor dividing portion GAP1.

[0028] An end portion of the first outer conductor divided portion GAP1 on the first portion SEC11 side (an end portion connected to the first portion SEC11) is one end 103 of the first outer conductor divided portion GAP1. An end portion of the first outer conductor divided portion GAP1 on the second portion SEC12 side (an end portion connected to the second portion SEC12) is the other end 104 of the first outer conductor divided portion GAP1.

[0029] The length of the second portion SEC12 is a predetermined length LA. In other words, the other end 104 of the first outer conductor divided portion GAP1 is located at a position that is the predetermined length LA from the second end 102. The length LA is set based on the wavelength λ of the high-frequency signal transmitted by the high-frequency balun 1. Specifically, the length LA is, for example, about 0.367 times the wavelength λ (when used as a 2:1 balun).

[0030] The length of first outer conductor separation portion GAP1 (the distance between one end 103 and the other end 104) is preferably as short as possible, and is at least a distance that does not cause contact between first balanced terminal 31 and second balanced terminal 32, which will be described later.

[0031] The length LD of the first portion SEC11 only needs to be longer than the length LA of the second portion 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 a high-frequency front-end circuit), and the like.

[0032] (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 23. 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 to a predetermined thickness. The second outer conductor 23 covers the outer circumferential surface of the second dielectric layer 22.

[0033] The dielectric constant (relative dielectric constant) 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 rate in the line of the second coaxial cable 20 is the same as the wavelength shortening rate in the line of the first coaxial cable 10. 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, similar to the first characteristic impedance Z10.

[0034] The second coaxial cable 20 has a third end 201 at one end in the extending direction, and a fourth end 202 at the other end. At the third end 201 and the fourth end 202 of the second coaxial cable 20, the second central conductor 21 and the second outer conductor 23 are connected, and the third end 201 and the fourth end 202 of the second coaxial cable 20 are short-circuited ends in terms of high frequency.

[0035] The second coaxial cable 20 has a second outer conductor division portion GAP2 at a position of a predetermined length LB in the direction from the third end 201 to the fourth end 202. The second outer conductor division portion GAP2 is a portion where the second outer conductor 23 is not formed over the entire circumference, and the second dielectric layer 22 is exposed.

[0036] As a result, the second coaxial cable 20 includes a third portion SEC21 located closer to the third end 201 than the second outer conductor division portion GAP2, and a fourth portion SEC22 located closer to the fourth end 202 than the second outer conductor division portion GAP2. The third portion SEC21 and the fourth portion SEC22 are connected by the second outer conductor division portion GAP2.

[0037] An end portion of the second outer conductor division portion GAP2 on the third portion SEC21 side (an end portion connected to the third portion SEC21) is one end 203 of the second outer conductor division portion GAP2. An end portion of the second outer conductor division portion GAP2 on the fourth portion SEC22 side (an end portion connected to the fourth portion SEC22) is the other end 204 of the second outer conductor division portion GAP2.

[0038] The third portion SEC21 has a predetermined length LB. In other words, one end 203 of the second outer conductor division portion GAP2 is located at a position that is the predetermined length LB from the third end 201. The fourth portion SEC22 has a predetermined length LC. In other words, the other end 204 of the second outer conductor division portion GAP2 is located at a position that is the predetermined length LC from the fourth end 202. The lengths LB and LC are the same as the length LA.

[0039] (Fixing structure of the first coaxial cable 10 and the second coaxial cable 20, and connecting structure of the balanced terminal) First, in order to make the fixing structure easier to understand, a description will be given with reference to a linear structure 1S shown in FIG.

[0040] The second coaxial cable 20 is disposed in parallel to the first coaxial cable 10. The second coaxial cable 20 is disposed at a position where the third end 201 is aligned with the second end 102 of the first coaxial cable 10, more specifically, where the third end 201 and the second end 102 are flush with each other.

[0041] Incidentally, the term "flush" as used herein includes an attachment error that does not affect the characteristics of the high frequency balun 1, and the amount of deviation in the attachment position may be approximately 10% or less of the predetermined length LA.

[0042] The second coaxial cable 20 is disposed along the first coaxial cable 10 such that the fourth end 202 is closer to the first end 101 of the first coaxial cable 10 than the third end 201 is.

[0043] As a result, the second outer conductor division portion GAP2 of the second coaxial cable 20 runs parallel to the first outer conductor division portion GAP1 of the first coaxial cable 10. One end 203 of the second outer conductor division portion GAP2 and the other end 104 of the first outer conductor division portion GAP1 are located at approximately the same position. The other end 204 of the second outer conductor division portion GAP2 and one end 103 of the first outer conductor division portion GAP1 are located at approximately the same position.

[0044] In such a configuration, the second outer conductor 23 of the second coaxial cable 20 is fixed and electrically connected to the first outer conductor 13 of the first coaxial cable 10 using a conductive bonding material 70 such as solder.

[0045] More specifically, the second outer conductor 23 and the first outer conductor 13 are fixed and electrically connected at least at the following locations using the conductive bonding material 70.

[0046] The second outer conductor 23 at the position of the third end 201 of the second coaxial cable 20 and its surrounding area is fixed and electrically connected to the first outer conductor 13 at the position of the second end 102 of the first coaxial cable 10 and its surrounding area using a conductive bonding material 70.

[0047] The second outer conductor 23 at the position of the fourth end 202 of the second coaxial cable 20 and in the vicinity thereof is fixed and electrically connected to the adjacent first outer conductor 13 by using a conductive bonding material 70.

[0048] The second outer conductor 23 at one end 203 of the second outer conductor division portion GAP2 in the second coaxial cable 20 and in its surrounding area is fixed and electrically connected to the first outer conductor 13 at the other end 104 of the first outer conductor division portion GAP1 in the first coaxial cable 10 and in its surrounding area using a conductive bonding material 71.

[0049] The second outer conductor 23 at the other end 204 of the second outer conductor division portion GAP2 in the second coaxial cable 20 and in its surrounding area is fixed and electrically connected to the first outer conductor 13 at the one end 103 of the first outer conductor division portion GAP1 in the first coaxial cable 10 and in its surrounding area using a conductive bonding material 72.

[0050] Such a structure 1S is curved into an annular body as shown in FIGS. 1(A), 1(B), 1(C) and 2, thereby realizing the shape of the high frequency balun 1.

[0051] At this time, the second end 102 of the first coaxial cable 10, the third end 201 of the second coaxial cable 20, and the fourth end 202 of the second coaxial cable 20 are disposed in positions adjacent to each other. The first outer conductor 13 and the second outer conductor 23, including the positions where the second end 102, the third end 201, and the fourth end 202 are adjacent to each other, are fixed and electrically connected using the conductive bonding material 70.

[0052] (Characteristics of high frequency balun 1) In this configuration, the first outer conductor 13 and the second outer conductor 23 are connected to the ground potential.

[0053] The high-frequency signal, which is an unbalanced signal input through the coaxial connector 80, is transmitted through the first coaxial cable 10. With the above-mentioned configuration, the third portion SEC21 and the second portion SEC122 of the second coaxial cable 20 are electromagnetically (high-frequency) coupled to the first coaxial cable 10, respectively.

[0054] As a result, the high-frequency signal is coupled to an electromagnetic field generated by the high-frequency signal transmitted through the first coaxial cable 10, and a high-frequency signal with a predetermined phase difference is transmitted to the third portion SEC21 and the second portion SEC122 of the second coaxial cable 20. The high-frequency signal transmitted to the third portion SEC21 and the second portion SEC122 of the second coaxial cable 20 has a predetermined phase difference, and can be taken out from the first balanced terminal 31 connected to one end 203 of the second outer conductor division portion GAP2 and the second balanced terminal 32 connected to the other end 204 of the second outer conductor division portion GAP2. In other words, the high-frequency signal of the balanced signal transmitted through the second coaxial cable 20 is output from the second outer conductor 23 at both ends of the second outer conductor division portion GAP2.

[0055] In this way, the first end 101 of the first coaxial cable 10 to which the coaxial connector 80 is connected becomes an unbalanced terminal of the high-frequency balun 1. In addition, the second outer conductor 23 at both ends of the second outer conductor division portion GAP2 of the second coaxial cable 20 becomes balanced terminals of the high-frequency balun 1. This allows the high-frequency balun 1 to achieve an unbalanced-to-balanced conversion function.

[0056] Furthermore, with this configuration, the high-frequency balun 1 can make the impedance of the balanced terminal twice the impedance of the unbalanced terminal (which is the same as the first characteristic impedance Z10 of the first coaxial cable 10). This allows the high-frequency balun 1 to set the ratio of the impedance of the balanced terminal to the impedance of the unbalanced terminal to 2:1. In other words, the high-frequency balun 1 realizes a 2:1 balun.

[0057] Figures 4(A) and 4(B) are graphs showing the reflection characteristics of the high frequency balun according to the first embodiment. In Figure 4(A), the vertical axis is shown in dB, and in Figure 4(B), the vertical axis is shown in VSWR. The frequency ft in Figures 4(A) and 4(B) is the center frequency of the high frequency signal transmitted by the high frequency balun 1. The wavelength of this frequency ft is the wavelength that determines the above-mentioned length LA.

[0058] The solid line in Fig. 4(A) represents the characteristics when a terminating resistor of 100 Ω is connected to the terminal pair of the first balanced terminal 31 and the second balanced terminal 32, and the dashed line in Fig. 4(B) represents the characteristics when a terminating resistor of 50 Ω is connected to the terminal pair of the first balanced terminal 31 and the second balanced terminal 32. Fig. 4(A) and Fig. 4(B) show the case where the first characteristic impedance Z10 of the first coaxial cable 10 and the second characteristic impedance Z20 of the second coaxial cable 20 are 50 Ω.

[0059] As shown in Figure 4(A), the return loss is greatly reduced at frequency ft. Also, at frequency ft (for example, 955.4MHz in this case), the VSWR is 1.0 when a 100Ω termination resistor is connected, and 2.0 when a 50Ω termination resistor is connected.

[0060] In this way, the high-frequency balun 1 can achieve unbalanced-to-balanced conversion with low loss at the frequency ft with an impedance ratio of 2:1 between the balanced side and the unbalanced side.

[0061] In this case, if a VSWR of 2.0 or less is allowed when connected with pure resistance, the high frequency balun 1 can be used in a wide band from 300 MHz to 1030.4 MHz. In addition to the center frequency of 955.4 MHz, the VSWR of the high frequency balun 1 drops between 300 MHz and 570 MHz as a 2:1 balun. Therefore, if this level of VSWR is allowed, the frequency ft can be increased even further, and the high frequency balun 1 can be made smaller.

[0062] The high-frequency balun 1 further has the following characteristics. Figures 5(A), 5(B), and 5(C) are graphs showing the transmission characteristics of the high-frequency balun according to the first embodiment. Figure 5(A) shows the reflection characteristics, with the vertical axis shown in dB, Figure 5(B) shows the reflection characteristics, with the vertical axis shown in VSWR, and Figure 5(C) shows the pass characteristics, with the vertical axis shown in dB. The solid line in Figure 5(A) shows the reflection characteristics when a 50Ω termination resistor is connected to the terminal pair of the first balanced terminal 31 and the second balanced terminal 32, and the dashed line in Figure 5(B) shows the reflection characteristics when a 100Ω termination resistor is connected to the terminal pair of the first balanced terminal 31 and the second balanced terminal 32. Figure 5(C) shows the pass characteristics when the impedance on the balanced side is 50Ω.

[0063] As shown in Figures 5(A), 5(B), and 5(C), when the impedance on the balanced side is set to 50 Ω, low-loss transmission characteristics can be achieved in the frequency range lower than the above-mentioned frequency ft. For example, in the case of Figures 5(A), 5(B), and 5(C), if a VSWR of 2.0 or less is allowed when connected with pure resistance, the high-frequency balun 1 can be used as a 1:1 balun in a wide frequency band from 338.4 MHz to 955.4 MHz. Also, if a VSWR of 1.5 or less is allowed when connected with pure resistance, the high-frequency balun 1 can be used as a 1:1 balun in a wide frequency band from 396.4 MHz to 862.4 MHz.

[0064] In addition, the frequency ft of the high-frequency balun 1 and the physical size (magnitude) of the high-frequency balun 1 are inversely proportional to each other. Therefore, by appropriately changing the size of the high-frequency balun 1, similar characteristics can be obtained in a frequency band including the desired frequency ft. For example, to configure a 1:1 balun with a wide frequency band with a center frequency (frequency ft) of 570.4 MHz, the lengths LA, LB, and LC should be set to approximately 0.2173 times the in-line wavelength of the first coaxial cable 10 and the second coaxial cable 20. In addition, to configure a 1:1 balun with a wide frequency band with a center frequency of 629.4 MHz and a VSWR of 1.5 or less, the lengths LA, LB, and LC should be set to approximately 0.2398 times the in-line wavelength of the first coaxial cable 10 and the second coaxial cable 20.

[0065] It is preferable that the ring shape formed by the high-frequency balun 1 is close to a perfect circle, but it may be a circle that is somewhat flattened. For example, if the aspect ratio of the high-frequency balun 1 is a circle (ellipse) of 1:2 to 2:1 (the vertical diameter is 1 / 2 to 2 times the horizontal diameter), it will have characteristics similar to a perfect circle (1:1).

[0066] [Second embodiment] A high-frequency balun according to a second embodiment of the present invention will be described with reference to Fig. 6. Fig. 6 is an external perspective view of the high-frequency balun according to the second embodiment.

[0067] 6, the high-frequency balun 1A according to the second embodiment differs in overall shape from the high-frequency balun 1 according to the first embodiment in that it includes a ground conductor 90. Other configurations of the high-frequency balun 1A are similar to those of the high-frequency balun 1, and descriptions of similar parts will be omitted.

[0068] The high frequency balun 1A includes a ground conductor 90. The ground conductor 90 is, for example, a flat metal plate.

[0069] The structure made up of the first coaxial cable 10 and the second coaxial cable 20 does not form a loop, but has a shape that is bent midway in the extending direction.

[0070] The second end 102 of the first outer conductor 13 of the first coaxial cable 10 and its vicinity, the third end 201 of the second outer conductor 23 of the second coaxial cable 20 and its vicinity, and the fourth end 202 of the second outer conductor 23 of the second coaxial cable 20 and its vicinity are fixed to and electrically connected to the ground conductor 90 by the conductive bonding material 70. The portion of the first outer conductor 13 of the first coaxial cable 10 adjacent to the fourth end 202 is also fixed to and electrically connected to the ground conductor 90 by the conductive bonding material 70.

[0071] The first outer conductor 13 at one end 103 and the other end 104 of the first outer conductor separation portion GAP1, and the second outer conductor 23 at one end 203 and the other end 204 of the second outer conductor separation portion GAP2 are spaced a predetermined distance from the ground conductor 90 (a distance that is such that there is almost no electromagnetic coupling with the ground conductor 90).

[0072] With this configuration, the second end 102 of the first outer conductor 13, the third end 201 of the second outer conductor 23, and the fourth end 202 of the second outer conductor 23 can be considered to be at the same potential through the ground conductor 90 at the frequency of the high-frequency signal transmitted by the high-frequency balun 1A.

[0073] As a result, the high-frequency balun 1A exhibits the same effects as the high-frequency balun 1.

[0074] [Third embodiment] A high-frequency balun according to a third embodiment of the present invention will be described with reference to Fig. 7. Fig. 7 is an external perspective view of the high-frequency balun according to the third embodiment.

[0075] 7, the high-frequency balun 1B according to the third embodiment differs from the high-frequency balun 1 according to the first embodiment in the fixing manner (connection manner) of the second end 102 and the third end 201. Other configurations of the high-frequency balun 1B are similar to those of the high-frequency balun 1, and descriptions of similar parts will be omitted.

[0076] In the high frequency balun 1B, the second end 102 and the third end 201 are disposed so as to face the first end 101, and are fixed and electrically connected to the conductive bonding material 70. Even in this fixing mode (connection mode), the high frequency balun 1B achieves the same effects as the high frequency balun 1.

[0077] [Fourth embodiment] A high-frequency balun according to a fourth embodiment of the present invention will be described with reference to the drawings. Fig. 8 is an external perspective view of the high-frequency balun according to the fourth embodiment.

[0078] 8, the high-frequency balun 1C according to the fourth embodiment differs from the high-frequency balun 1 according to the first embodiment in the fixing manner (connection manner) of the second end 102 and the third end 201. Other configurations of the high-frequency balun 1C are similar to those of the high-frequency balun 1, and descriptions of similar parts will be omitted.

[0079] In the high-frequency balun 1C, the second end 102 and the third end 201 are disposed to face the surface of the first outer conductor 13, and are fixed and electrically connected to the conductive bonding material 70. Even in this fixing mode (connection mode), the high-frequency balun 1C achieves the same effects as the high-frequency balun 1.

[0080] [Fifth embodiment] A high-frequency balun according to a fifth embodiment of the present invention will be described with reference to Fig. 9. Fig. 9 is an external perspective view of the high-frequency balun according to the fifth embodiment.

[0081] 9, the high-frequency balun 1D according to the fifth embodiment differs from the high-frequency balun 1 according to the first embodiment in that it includes a second coaxial cable 20D. Other configurations of the high-frequency balun 1D are similar to those of the high-frequency balun 1, and descriptions of similar parts will be omitted.

[0082] The second coaxial cable 20D has a second characteristic impedance Z20D. The second characteristic impedance Z20D is different from the first characteristic impedance Z10 of the first coaxial cable 10. Specifically, the second characteristic impedance Z20D is different from the first characteristic impedance Z10, and as an example, the first characteristic impedance Z10 is 50Ω, whereas the second characteristic impedance Z20D is 75Ω. Note that the dielectric constant (relative dielectric constant) of the second dielectric layer 22 of the second coaxial cable 20D is preferably the same as the dielectric constant (relative dielectric constant) of the first dielectric layer 12 of the first coaxial cable 10.

[0083] In this configuration, the impedance on the balanced side of the high-frequency balun 1D is 150Ω (=75Ω×2). The above-mentioned predetermined length is approximately 0.392 times the wavelength of the center frequency of the high-frequency signal to be transmitted.

[0084] Fig. 10(A) and Fig. 10(B) are graphs showing the reflection characteristics of the high-frequency balun according to the fifth embodiment. In Fig. 10(A), the vertical axis is shown in dB, and in Fig. 10(B), the vertical axis is shown in VSWR. The solid line in Fig. 10(A) shows the characteristics when a terminating resistor of 75Ω is connected to the terminal pair of the first balanced terminal 31 and the second balanced terminal 32, and the dashed line in Fig. 10(B) shows the characteristics when a terminating resistor of 150Ω is connected to the terminal pair of the first balanced terminal 31 and the second balanced terminal 32. Fig. 10(A) and Fig. 10(B) show the case where the first characteristic impedance Z10 of the first coaxial cable 10, i.e., the impedance on the unbalanced side, is 50Ω, and the second characteristic impedance Z20D of the second coaxial cable 20D is 75Ω.

[0085] As shown in Figures 10(A) and 10(B), the high-frequency balun 1D has a center frequency of 537.6 MHz (approximately 0.381 times the wavelength within the line), and if a VSWR of 1.5 or less is allowed when connected with pure resistance, a 3:1 balun with a passband of 507.0 MHz to 557.5 MHz can be realized by using a coaxial cable with a shortening rate of 70% and lengths LA, LB, and LC of 150 mm.

[0086] Even when the second characteristic impedance Z20D is set to 75Ω like the high frequency balun 1D, a low VSWR can be achieved at frequencies lower than the center frequency and over a wide frequency range, just as when the second characteristic impedance Z20 is 50Ω. As a result, the high frequency balun 1D can achieve a 75:50 balun in the frequency band from 161.6MHz to 517.1MHz by allowing a VSWR of 1.5 or less when connected with pure resistance, with 200.0MHz (0.143 times the wavelength in the line) being the frequency with the best characteristics.

[0087] [Sixth embodiment] A high-frequency balun according to a sixth embodiment of the present invention will be described with reference to the drawings. Fig. 11 is an external perspective view of the high-frequency balun according to the sixth embodiment.

[0088] 11, the high-frequency balun 1E according to the sixth embodiment differs from the high-frequency balun 1 according to the first embodiment in that it includes a first coaxial cable 10E. Other configurations of the high-frequency balun 1E are similar to those of the high-frequency balun 1, and descriptions of similar parts will be omitted.

[0089] The first coaxial cable 10E includes a first portion SEC11E and a second portion SEC12E.

[0090] A length LD (see FIG. 3) between one end 103 and the second end 102 of the first outer conductor divided portion GAP1 in the first portion SEC11E is shorter than a length LC of the fourth portion SEC22 of the second coaxial cable 20. For example, the length LD is about 90% of the length LC.

[0091] A length LA (see FIG. 3) between the other end 104 and the second end 102 of the first outer conductor divided portion GAP1 in the second portion SEC12E is shorter than a length LB of the fourth portion SEC22 of the second coaxial cable 20. For example, the length LA is about 90% of the length LB. The lengths LA and LD are approximately the same.

[0092] With this configuration, the high-frequency balun 1E has the following characteristics. Figures 12(A) and 12(B) are graphs showing the reflection characteristics of the high-frequency balun according to the sixth embodiment and a comparative high-frequency balun. In Figure 12(A), the vertical axis is shown in dB, and in Figure 12(B), the vertical axis is shown in VSWR. The comparative high-frequency balun has a configuration in which the lengths LA, LB, LC, and LD are the same. In Figures 12(A) and 12(B), the solid lines show the characteristics of the high-frequency balun 1E (shortened), and the dashed lines show the characteristics of the comparative balun (not shortened).

[0093] 12A and 12B, the reflection characteristics of both the high-frequency balun 1E and the comparative high-frequency baluns have two peaks with low return loss at different frequencies. In the comparative high-frequency baluns corresponding to the above-described embodiments, the peak on the higher frequency side has a lower return loss than the peak on the lower frequency side.

[0094] On the other hand, in the high-frequency balun 1E of the present embodiment, the peak on the low frequency side has a lower return loss than the peak on the high frequency side. Therefore, in the high-frequency balun 1E, the frequency band of the high-frequency signal transmitted by the high-frequency balun 1E is set so as to include the peak on the low frequency side.

[0095] This allows the first coaxial cable 10E of the high-frequency balun 1E to be shortened, thereby realizing miniaturization.

[0096] [Seventh embodiment] A high-frequency balun according to a seventh embodiment of the present invention will be described with reference to the drawings. Fig. 13 is an external perspective view of the high-frequency balun according to the seventh embodiment.

[0097] 13, the high-frequency balun 1F according to the seventh embodiment differs from the high-frequency balun 1 according to the first embodiment in the arrangement of the first coaxial cable 10 and the second coaxial cable 20. Other configurations of the high-frequency balun 1F are similar to those of the high-frequency balun 1, and descriptions of similar parts will be omitted.

[0098] In the high-frequency balun 1F, the first coaxial cable 10 and the second coaxial cable 20 are not arranged in parallel. The third end 201 and the fourth end 202 of the second coaxial cable 20 are spaced apart from the first coaxial cable 10 and are not physically or electrically connected to the first outer conductor 13 of the first coaxial cable 10.

[0099] Even with this configuration, the high-frequency balun 1F can achieve the same effects as the high-frequency balun 1.

[0100] [Eighth embodiment] A high frequency balun according to an eighth embodiment of the present invention will be described with reference to the drawings. Fig. 14 is a side cross-sectional view showing a linear structure for forming the high frequency balun according to the eighth embodiment.

[0101] 14, the high-frequency balun structure 1GS according to the eighth embodiment differs from the high-frequency balun 1 structure 1S according to the first embodiment in that it includes an insulating shielding film. Other configurations of the high-frequency balun structure 1GS are similar to those of the high-frequency balun 1 structure 1S, and descriptions of similar parts will be omitted.

[0102] The structure 1S includes a first coaxial cable 10G and a second coaxial cable 20G.

[0103] The first coaxial cable 10G differs from the first coaxial cable 10 according to the first embodiment in that it includes an insulating shielding film 19. In the first coaxial cable 10G, the outer circumferential surface of the first outer conductor 13 is covered with the insulating shielding film 19 except for a portion. The insulating shielding film 19 is formed on the first outer conductor 13 of the first coaxial cable 10G at a portion that does not require connection with the second coaxial cable 20G.

[0104] The second coaxial cable 20G differs from the second coaxial cable 20 according to the first embodiment in that it includes an insulating shielding film 29. In the second coaxial cable 20G, the outer circumferential surface of the second outer conductor 23 is covered with the insulating shielding film 29 except for a portion. The insulating shielding film 29 is formed in a portion of the second outer conductor 23 of the second coaxial cable 20G that does not require connection with the first coaxial cable 10G.

[0105] The portion requiring connection between the first outer conductor 13 of the first coaxial cable 10G and the second outer conductor 23 of the second coaxial cable 20G is bound by a conductive wire WR, for example, as shown in FIG.

[0106] Even with this configuration, the high-frequency balun made up of the structure 1GS achieves the same effects as the high-frequency balun 1.

[0107] [Ninth embodiment] A high-frequency balun according to a ninth embodiment of the present invention will be described with reference to the drawings. Fig. 15 is an external perspective view of the high-frequency balun according to the ninth embodiment.

[0108] 15, the high-frequency balun 1H according to the ninth embodiment is shorter than the length LC of the fourth portion SEC22 of the second coaxial cable 20H, and the length LB of SEC21 is also shorter than that of the fourth portion SEC22 of the second coaxial cable 20H in comparison with that of the fifth embodiment shown in FIG. For example, the length LC is about 90% of the length LD, and the length LB is about 90% of the length LA. The other configuration of the high-frequency balun 1H is similar to that of the high-frequency balun 1D, and a description of similar parts will be omitted.

[0109] With this configuration, the high-frequency balun 1H has the following characteristics. Figures 16(A) and 16(B) are graphs showing the reflection characteristics of the high-frequency balun according to the ninth embodiment and a comparative high-frequency balun with a balanced side impedance of 75Ω. In Figure 16(A), the vertical axis is shown in dB, and in Figure 16(B), the vertical axis is shown in VSWR. The comparative high-frequency balun has a configuration in which the lengths LA, LB, LC, and LD are the same. In Figures 16(A) and 16(B), the dashed lines show the characteristics of the comparative balun (without shortening), and the solid lines show the characteristics of the high-frequency balun 1H (with shortening of LC and LB).

[0110] As shown in Figures 16(A) and 16(B), when the comparative high-frequency balun is not shortened, the reflection characteristics have two peaks with low return loss at different frequencies, with the peak on the lower frequency side having a lower return loss than the peak on the higher frequency side.

[0111] In this case, the reflection loss of the peak on the low frequency side is not -∞ dB, and the VSWR is not 1. However, according to the embodiment of the ninth embodiment, as shown by the solid lines in Fig. 16(A) and Fig. 16(B), the reflection loss of the peak on the low frequency side is improved to be less, and although the frequency band becomes narrower, low loss can be achieved.

[0112] In the above-described embodiments, the conductive bonding material 70 such as solder is used. However, as long as a certain level of fixation strength and low-resistance electrical connection can be achieved, the present invention is not limited to the conductive bonding material 70 such as solder. For example, spot welding, crimping, etc. can be used, and further, a cable fixing method using a conductive wire or the like may be used.

[0113] <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 includes a first outer conductor divided portion that does not include the first outer conductor at a position of a predetermined length from the second end toward the first end, the second coaxial cable includes a second outer conductor divided portion that does not include the second outer conductor at a position of the predetermined length from the third end toward the fourth end, a length between the third end and the second outer conductor divided portion of the second coaxial cable is equal to a length between the fourth end and the second outer conductor divided portion of the second coaxial cable, the second coaxial cable is arranged parallel to the first coaxial cable with the third end aligned with the second end of the first coaxial cable; the first outer conductor and the second outer conductor are electrically connected by a conductive bonding material; the second end of the first coaxial cable is an open end, At the third end and the fourth end of the second coaxial cable, the second central conductor is connected to the second outer conductor and has substantially the same potential as a ground potential; the first end of the first coaxial cable is an unbalanced terminal; a high-frequency balun, wherein an end portion of the second outer conductor on the third end side and an end portion of the second outer conductor on the fourth end side are balanced terminals.

[0114] <2> <1> A high frequency balun comprising: A high frequency balun, wherein the dielectric constant of the first dielectric layer and the dielectric constant of the second dielectric layer are the same.

[0115] <3> <1> or <2> A high frequency balun comprising: A high frequency balun, wherein the second characteristic impedance of the second coaxial cable and the first characteristic impedance of the first coaxial cable are the same.

[0116] <4> <1> or <2> A high frequency balun comprising: A high frequency balun, wherein a second characteristic impedance of the second coaxial cable is higher than a first characteristic impedance of the first coaxial cable.

[0117] <5> <1> ~ <4> Any of the high-frequency baluns described above, A high-frequency balun, wherein a deviation between the position of the second end and the position of the third end is 10% or less of the predetermined length.

[0118] <6> <1> ~ <5> Any of the high-frequency baluns described above, A high frequency balun, wherein the predetermined length is approximately 0.367 times the wavelength of a high frequency signal transmitted through the first coaxial cable and the second coaxial cable.

[0119] <7> <1> ~ <6> Any of the high-frequency baluns described above, a structure including the first coaxial cable and the second coaxial cable is annular; The fourth end is proximate and electrically connected to the second end and the third end.

[0120] <8> <1> ~ <6> Any of the high-frequency baluns described above, A ground conductor is provided, The second end, the third end and the fourth end are electrically connected to each other through the ground conductor. [Explanation of symbols]

[0121] 1, 1A, 1B, 1C, 1D, 1E, 1F, 1H: High frequency balun 1S, 1GS: Structure 10, 10E, 10G: 1st coaxial cable 11: First central conductor 12: First dielectric layer 13: First outer conductor 19, 29: Insulating shielding film 20, 20D, 20G, 20H: Second coaxial cable 21: Second central conductor 22: Second dielectric layer 23: Second outer conductor 31: 1st balanced terminal 32: 2nd balanced terminal 70, 71, 72: Conductive bonding material 80: Coaxial connector 90: Ground conductor 101: 1st end 102: 2nd end 103: One end 104: Other end 201: Third end 202: 4th end 203: One end 204: Other end GAP1: First outer conductor gap GAP2: Second outer conductor gap SEC11, SEC11E: Part 1 SEC12, SEC12E: 2nd part SEC21, SEC21H: 3rd part SEC22, SEC22H: 4th part Z10: First characteristic impedance Z20, Z20D, Z20H: Second characteristic impedance

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 includes a first outer conductor divided portion that does not include the first outer conductor at a position of a predetermined length from the second end toward the first end, the second coaxial cable includes a second outer conductor divided portion that does not include the second outer conductor at a position of the predetermined length from the third end toward the fourth end, a length between the third end and the second outer conductor divided portion of the second coaxial cable is equal to a length between the fourth end and the second outer conductor divided portion of the second coaxial cable, the second coaxial cable is arranged parallel to the first coaxial cable with the third end aligned with the second end of the first coaxial cable; the first outer conductor and the second outer conductor are electrically connected to each other by a conductive bonding material; the second end of the first coaxial cable is an open end, At the third end and the fourth end of the second coaxial cable, the second central conductor is connected to the second outer conductor and has substantially the same potential as a ground potential; the first end of the first coaxial cable is an unbalanced terminal; an end portion of the second outer conductor on the third end side and an end portion of the second outer conductor on the fourth end side are balanced terminals. High frequency balun.

2. 2. The high frequency balun according to claim 1, The dielectric constant of the first dielectric layer and the dielectric constant of the second dielectric layer are the same. High frequency balun.

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

4. 2. The high frequency balun according to claim 1, A second characteristic impedance of the second coaxial cable is higher than a first characteristic impedance of the first coaxial cable. High frequency balun.

5. 2. The high frequency balun according to claim 1, A deviation between the position of the second end and the position of the third end is 10% or less of the predetermined length. High frequency balun.

6. 2. The high frequency balun according to claim 1, the predetermined length is approximately 0.367 times the wavelength of a high-frequency signal transmitted through the first coaxial cable and the second coaxial cable; High frequency balun.

7. 2. The high frequency balun according to claim 1, a structure including the first coaxial cable and the second coaxial cable is annular; The fourth end is adjacent to and electrically connected to the second end and the third end. High frequency balun.

8. 2. The high frequency balun according to claim 1, A ground conductor is provided, the second end, the third end and the fourth end are electrically connected to each other through the ground conductor; High frequency balun.

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