JPH09147634A - Coaxial cable - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively and sufficiently restrict heat transmission without deteri orating the electric characteristics by forming grooves which are opened to the outside and continued in the circumferential direction in the outer conductor. SOLUTION: An outer conductor 10 of a coaxial cable is formed with a groove 13. This groove 13 is formed by large barrel parts 10a of the outer conductor 10 and narrow barrel parts 10b, which are respectively pinched between the large barrel parts 10a. In such a coaxial cable, heat transmission is mainly performed through the outer conductor 10. A center conductor 11 and a dielectric 12 are also concerned to the heat transmission, but since the degree thereof is about 1/10 in the case of the conductor 11, and about 1/1000-1/5000 in the case of the dielectric 12, it can be disregarded. Cross sectional area of the narrow barrel part 10b is smaller than the cross sectional area of the large barrel part 10a. Transmission of heat can be restricted by the groove 13 on the basis of the Wiedemann-Franz's law between the heat conductivity and the electric conductivity, and the transmission of signal is performed through the inner peripheral surface of the outer conductor 10 by the skin effect independently of the influence of the groove 13.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coaxial cable, and more particularly to a coaxial cable suitable for transmitting a signal to a device placed under a constant temperature environment. A coaxial cable is a cable that bundles a coaxial core consisting of a cylindrical outer conductor and a central conductor at the center of the outer conductor. It has excellent characteristics such as easy impedance uniformity.

[0002]

2. Description of the Related Art FIG. 5 is a diagram showing an example of use of a coaxial cable. In this use example, one end of a coaxial cable 4 having a structure in which a coaxial core composed of a cylindrical outer conductor 1 and a center conductor 2 at the center thereof, and a dielectric 3 insulating between the two conductors 1 and 2 is bundled. , Connected to a device (not shown) provided inside the thermostat 5.

[0003] As is well known, the thermostatic bath 5 is for keeping the internal environmental temperature constant. Naturally, a sufficient heat insulation is applied to the bath wall surrounding the four sides. The structure is such that the internal temperature tends to fluctuate due to the effect of heat transmitted through it. Therefore, in order to suppress the transmission of heat from the coaxial cable 4,
Measures were taken such as (1) reducing the diameter of the coaxial cable 4 and (2) increasing the total length of the coaxial cable 4.
According to (1), the cross-sectional area of the outer conductor 1 can be reduced, and according to (2), the heat transfer path length can be lengthened.
In either case, the effect of suppressing heat transfer can be obtained.

[0004] The constant temperature environment is not limited to the inside of the thermostat. For example, the environment in a vacuum insulated container that has been cooled at a very low temperature (operating environment of a superconductor) also corresponds to the environment.

[0005]

However, although the measures (1) and (2) are effective in suppressing the heat transfer, the smaller the diameter and the longer the total length are. However, since the amount of signal attenuation increases, there is a problem to be solved inevitably accompanied by deterioration of electrical characteristics. This is because the transfer of heat and the transfer of signals are performed by the same mechanism in terms of a wave phenomenon, and thus suppressing heat transfer is, in other words, suppressing signal transfer.

Therefore, an object of the present invention is to obtain a sufficient heat transfer suppressing effect without deteriorating the electrical characteristics.

[0007]

The above object is to provide a coaxial cable provided with a cylindrical outer conductor, a central conductor located at the axial center of the outer conductor, and a dielectric material interposed between the two conductors. This can be achieved by forming a groove that is open to the outside and is continuous in the circumferential direction in the outer conductor, or by providing a plurality of the grooves in the longitudinal direction of the cable.

The reason is that the cross-sectional area of the outer conductor, which is the main heat transfer path, is reduced in the groove, so that the heat resistance is increased and the heat transfer is suppressed. This is because the signal propagates on the inner peripheral surface of the outer conductor due to the skin effect, so that the electrical characteristics do not deteriorate.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is an external view showing an embodiment of the coaxial cable according to the present invention, and FIG. 2 is a sectional view thereof.
In FIGS. 1 and 2, 10 is a cylindrical outer conductor, 11 is a center conductor, and this center conductor 11 is arranged concentrically with the outer conductor 10 via a dielectric 12.

The coaxial cable of the present embodiment has an outer conductor 10
The feature is that the groove 13 is formed in the groove. This groove 13
The outer conductor 10 is formed by the thick body portion 10a and the thin body portion 10b sandwiched between the thick body portion 10a. The thin body portion 10b has a circular shape around the waist, like the thick body portion 10a, and has a diameter smaller than that of the thick body portion 10a.
The outer peripheral surface of is exposed to the outside together with the outer peripheral surface of the thick body portion 10a. Here, the outside is the atmosphere (atmosphere or vacuum or any gas) around the coaxial cable. In the illustrated example, the groove 13 has a concave shape,
Not limited to this. For example, it may have an inverted trapezoidal shape as shown in FIG. 3A, or may have a semicircular or U-shaped shape as shown in FIG. 3B. Alternatively, it may be V-shaped as shown in FIG.

In the coaxial cable having the improved structure as described above, heat is mainly transferred through the outer conductor 10. The central conductor 11 and the dielectric 12 also participate in heat transfer, but the degree of involvement is about 1 in the case of the central conductor 11.
/ 10 and the dielectric 12 is approximately 1/1000 to 1 /
Since it is 5000, it can be sufficiently ignored. Assuming that heat is being transferred from one end side to the other end side of the cable, some of the thick body portions 10 are included in the heat transfer path.
a and some thin barrel portions 10b intervene. Comparing the cross-sectional areas S of these two types of body portions 10a and 10b, the cross-sectional area of the thin body portion 10b is smaller than that of the thick body portion 10a.

In general, in a metal, a heat flow is carried by free electrons that contribute to electric conduction. Therefore, between the thermal conductivity λ and the electrical conductivity σ, the Wiedemann-Franz law (metal temperature at a constant temperature A empirical rule known as “the ratio λ / σ of the conductivity λ and the electric conductivity σ is constant regardless of the type of metal” is established. Therefore, even for “thermal resistance”, which is the reciprocal of thermal conductivity λ, approximately Mathisen's rule (the electrical resistivity ρ of a metal is uniformly determined by the type of the metal and the concentration of impurities contained in the metal) ) Is satisfied, the equation of the electric resistance R shown below can be applied, and the thermal resistance is in inverse proportion to the cross-sectional area S of the heat transfer path.

R = ρ (Ι / S) [Ω] Here, ρ is the electrical resistivity, Ι is the length of the conducting wire, and S is the cross-sectional area of the conducting wire. Therefore, since the minimum cross-sectional area of the heat transfer path in this embodiment is given by the cross-sectional area of the thin barrel portion 10b, the heat resistance can be increased by that much and the heat transfer can be suppressed. On the other hand, since the signal is transmitted along the inner peripheral surface of the outer conductor 10 (that is, the contact surface with the dielectric 12) by the so-called skin effect, the groove 13 is not affected. The skin effect extends from the inner peripheral surface of the outer conductor 10 to a predetermined depth depending on the frequency of the signal, so at least when the thickness D from the bottom surface of the groove 13 to the dielectric 12 is set. It must be more than the depth of reach.

In the above embodiment, the number of the grooves 13 is plural. This is because an exponential proportional relationship is established between the effect of suppressing heat transfer and the ratio of the groove 13 to the total cable length. FIG. 4 is a characteristic diagram showing a result of an experiment conducted by the inventor of the present application. The vertical axis represents the amount of heat q (so-called heat flux) that moves per unit area / unit time, and the horizontal axis represents the ratio P of the groove 13 to the total cable length. In addition, in the experiment, the outer conductor 10 is 0.26 mm.
Thick Cu (copper), 0.58 mm thick PTF on the dielectric 12
E (poly, tetra, fluoro, ethylene), central conductor 11
Was performed using a silver-plated copper-clad steel wire with a radius of 0.255 mm and D was 40 μm.

When P = 0%, that is, when the heat flux q when the groove 13 is not formed (prior art) is 100% and P is gradually increased, the heat flux q tends to decrease exponentially. Was done. According to this characteristic diagram, the minimum heat flux q
Is obtained when P is 100%. If attention is paid only to suppression of heat transfer, P should be as close to 100% as possible. However, as P increases, the strength of the outer conductor 10 is lost, deformation of the dielectric 12 such as thermal expansion cannot be suppressed, and electrical characteristics deteriorate. Therefore, the practical range of P is Although not particularly limited, it will be around 50%.

The value of P can be adjusted by the number of the grooves 13 or the width L with one groove 13. Especially,
When the number of the grooves 13 is adjusted, the characteristic line moves downward (see arrow A) as the number increases, so that a smaller heat quantity flux q can be obtained even with the same value P. Further, in the above embodiment, the example of application to the single outer conductor 10 is shown, but the present invention is not limited to this. In the case of a double structure or a multiple structure of more than that, a groove may be formed in the outermost outer conductor.

[0017]

According to the present invention, the heat resistance of the outer conductor, which is the main heat transfer path for heat, can be increased, and heat transfer can be suppressed without causing electrical deterioration, which is an advantage over the prior art. The effect is obtained.

[Brief description of the drawings]

FIG. 1 is an external view of a coaxial cable according to an embodiment.

FIG. 2 is a cross-sectional view of the coaxial cable of one embodiment.

FIG. 3 is another shape view of the groove.

FIG. 4 is a characteristic diagram of a coaxial cable according to an embodiment.

FIG. 5 is a diagram showing a conventional coaxial cable and an example of its use.

[Explanation of symbols]

 10: outer conductor 11: center conductor 12: dielectric 13: groove

Claims (2)

[Claims] 1. A coaxial cable comprising a cylindrical outer conductor, a central conductor located at the axial center of the outer conductor, and a dielectric material interposed between the two conductors, the outer conductor being open to the outside. A coaxial cable characterized in that a continuous groove is formed in the circumferential direction. 2. The coaxial cable according to claim 1, wherein a plurality of the grooves are provided in a longitudinal direction of the cable.