DE2754169A1 - SHEATHING FOR ELECTRIC CABLES, IN PARTICULAR FOR HIGH-FREQUENCY AND HIGH-FREQUENCY CABLES - Google Patents

Description

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FILECA
Route National No. 1
Sainte Genevieve (Oise)
France

Sheathing for electrical
Cables, in particular for high-frequency and ultra-high frequency cables

The invention relates to a sheathing for electrical cables, in particular for high-frequency or ultra-high frequency cables.

There are already single »and multiple sheaths for electrical cables known, which serve as armouring and / or shielding. The known shields, however, fulfill their protective function, which consists on the one hand of emitting energy from the cable to the outside
prevent and on the other hand an impairment of
Avoiding current conduction inside the cable due to external interference is not completely satisfactory in all cases.

The invention is based on the object of specifying a cable sheathing which, in comparison to the known

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Coverings, especially shields, protection of a cable is significantly improved even in an environment in which there is very strong interference, and which is easy and inexpensive to manufacture.

This object is achieved in a casing of the type described at the beginning according to the invention in that that they have at least one energy-absorbing energy-absorbing device that is in operation between two conductive structures Has layer made of a material whose conductivity is lower than that of the material conductive structures and in which electrical energy in the range of frequencies at which the sheathing must provide effective protection, can be converted into heat to such an extent that the Gap between the conductive structures over the useful length of the cable an attenuation of at least 6 dB (decibel) results.

It is advantageous if at least one energy-absorbing layer contains materials which are underneath the operating conditions of the cable lead to dielectric losses over the useful length of the cable contribute to the intended attenuation in the area between the conductive structures.

It is also advantageous if at least one energy-absorbing layer contains materials the losses under the operating conditions of the cable in the form of eddy current losses or hysteresis losses have, over the useful length of the cable to the intended attenuation in the area between the conductive Structures contribute.

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It is also advantageous if at least one energy-absorbing layer contains materials that lead to dielectric losses under the operating conditions of the cable that extend over the useful length of the Cable contribute to the intended attenuation in the area between the conductive structures.

Finally, it is advantageous if one or more energy-absorbing layers are provided, which are designed in terms of their strength, conductivity and relative permeability such that they contribute at least half to the determination of the tilting frequency of the casing.

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The invention will now be further elucidated using some mathematical relationships extracted from the simplified theory of jacketed electrical cables, in conjunction with drawings.

Show it:

Fig. 1 longitudinal sections through known electrical cables with single or Multiple shielding, each for one half of the cable,

3 shows longitudinal sections through preferred exemplary embodiments of cables with Sheaths according to the invention, each for one half of the cable.

The transmission impedance Z "of a shielded cable is defined as the quotient of the interference voltage ν occurring between two points of the shielding at a distance of 1 m and the current intensity i in the shielding; the following applies:

² T "i ^(1}

Kirchhoff has shown that the transmission impedance is a Property of the shield and symmetrical to it

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is; the transmission impedance is therefore independent of Type and number of conductors - live or non-live - enclosed by the shield are.

The transmission impedance Z_ comprises an imaginary one Part and a real part for which the following equation applies:

Z,

T ro

/ ch

(2)

χ - cos χ

whereby

and where

ro e

f ch

(3)

Resistance coating of the reinforcement Strength (thickness) of the reinforcement

*) Permeability of empty space; relative permeability of the material Conductivity of the material Frequency of the cosine - hyperbolic signal

are.

So the transfer impedance Z ^ i _st a function of χ rapidly decreasing function.

If you want the transmission impedance Z _T to be small for a given frequency, then you can use materials for the shielding for which the product / ^ ₁ - <3 * has an increased value

*) magnetic field constant

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Owns value. The use of ferromagnetic. materials thus opens up the possibility of high values of the relative permeability at relatively low frequencies and the conductivity.

At very high frequencies, the real part of the transmission impedance Z_ tends to zero. However, the transmission impedance Z _T does not become zero overall; Rather, an inductive value (imaginary part) remains, which is a complicated function of the type of shielding (bracing, tape, cable tie, pipe, etc.), its composition (number of winding layers and conductors) and its geometry (thickness, angle, etc.) .) is.

In the context of the present application, any system of metallic sheaths without a single shield is used an interruption in the electrical conductivity between successive layers in the frequency range under consideration designated.

As a multiple shield, there are two or more single shields considered as defined above, which are separated by a layer by which a considerable Interruption of the electrical conductivity at the relevant frequencies between at least two successive ones Shielding layers is brought about.

For the sake of simplicity, one can therefore assume that a multiple shield is isolated from one another Single shields is formed, where ■ the insulation can be (almost) complete insulation or also

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a simple discontinuity in the sense of a decrease in conductivity.

It can be shown that with η shields / that is, with multiple shielding with η single shields with the transmission impedances Z _T1 , ZT-, ... Z and a length L, the following value of the transmission impedance Z _{T of} the overall arrangement is obtained:

L ⁿ , Z _T1 , Z _T2 , ...> Z _Tn

TZ ₁ 'Z ₀ -, ··. . . · Z „τ

12 2 J oil, η

where Z. = Transmission impedance of the single armouring i with i = 1 ... η

Z. = characteristic impedance of a line

1 | 1t I

with the single armoring i as the core and with the single armoring i + 1 as the return conductor.

The validity of the (simplified) equation (4) is restricted to the cases in which the values of Z ... are much higher than their assigned values of Z _Ti L and ^Z Ti ₊ 1 ^L

At first glance it appears obvious that multiple shields offer considerable advantages and allow a very significant reduction in the transmission _{impedance Z T} , since the usual values of ZL (a few tenths of an ohm to a few thousandths of an ohm) are generally much lower than the usual values of Z .,. * (a few ohms).

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In fact, there is only a real advantage if the distances between the individual shields (Shielding gaps) are matched to their characteristic impedance. In this case - and only in this case - namely has a line with a shielding space and the two adjoining conductive ones Layers or shields have a constant impedance.

Otherwise, the impedance Z fluctuates depending on the length of such a line and the excitation frequency which cause considerable fluctuations in the shielding effect.

In particular, the following considerations apply in this case:

If the line is short-circuited at one of its ends, the following applies to the impedance occurring at the other end:

where y = Oi ⁺ iß **

and Z = characteristic impedance of the shield gap;

if, on the other hand, the line is open at one end (idle), the following variable impedance results:

Z = Z coth f ^L
at their other end.

*) th = hyperbolic tangent

**) coth = cotangent hyperbolic - 11

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Depending on the frequency, the impedance goes through a series of very high maxima and close to zero Minima.

Pulses that are very rich in harmonics can cause serious interference in such a cable the transmission impedance Z ", for the wavefront (or the Pulse leading edge) is not defined.

In practice, the adaptation of the shielding spaces (or the associated cables) is a costly, difficult and extraordinary process; There are namely special connectors or couplings required that are suitable for separating the individual single shields from one another, as well as special resistance forms for the adaptation. After all, making the connections themselves is very difficult because those at the innermost Conductor connected transmitting or receiving device may under no circumstances have a common electrical connection point with the outermost shield.

The result is that the advantages that result from individual shields isolated from one another or from multiple shielding could arise, apart from exceptional cases resulting from the adjustment deficiencies can be completely canceled.

The basic concept of the invention, which will be explained in more detail below, makes it possible to benefit from the advantages a multiple shield to pull without the above

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listed complicated, time-consuming and insecure adaptation would have to be carried out.

Let us consider a line of very great length L.
If such a line has sufficient attenuation, as is well known, the state of its end - whether matched or not - has no influence on its input impedance. In fact, it is sufficient that the product OiL der
Conduction is sufficient to cause the reflected wave to be a minimal part of the incident wave
represents. If Oi is sufficiently large, then L can be small.

If it is possible to convert all of the energy available in the transmission lines formed by the shielding spaces into heat, at least in the spectrum in which shielding of the cable is to be achieved, then a pseudo adjustment of the shielding spaces has been achieved and the ends of the short-circuit the relevant line as required
or leave it open without running the risk of the disadvantages explained above occurring.

So the advantages of multiple shielding are achieved without the disadvantages occurring and conventional coaxial cable couplings can be used, with all
Layers of the cable jacket treated as a unit.

Several types of materials are now known which are capable of absorbing the energy they receive.

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when subjected to fields originating in an electrical, magnetic or electromagnetic field Have appearance.

As preferred examples of materials which swallow energy by converting it into heat, ie as preferred materials for an energy-absorbing layer according to the invention, insulation materials with a high tg £, in which dielectric losses, ferromagnetic materials, in which eddy current losses and materials in which slight conductance losses occur are mentioned, it being pointed out at the same time, however, that the invention is not limited to energy-absorbing layers made of these materials.

It is assumed that a shield gap has a large attenuation CX. At the end of a path 1 .. »
the product <X 1 is so small that the residual current is practically zero or negligible, so that one goes through
Applying the mean value theorem can assign the following value at the same time:

an average value of the current i ^- <£. ±

m
a traversed mean distance l _m ^ L,

so that for a given frequency one can write equation (1) as follows:

^{V = Z} T ¹ H ₁ ^{= 21} T ¹ (5)

From this it follows because i <i

^Z T (6)

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On the other hand, l _{m takes} the place of L in equation (4), so that the following equation ultimately results for the equivalent transmission impedance of a cable with multiple shielding:

i_-. ^ζ1 τ ^/ ΐ · ^ζ1 τ? ** · ** ^ζ 'τη ⁿ "1 ^1J · ^lJ - ^Tn

Z · Z ^ -, ♦. .. "Z _Λ 12 23 n-1, n

This results in: Z ¹ L ■ £ Z _r

It can be seen that the dependence of the transmission impedance _z on the (ni) th power of the length L of the cable, the

represented the serious disadvantage of cables with multiple shielding, no longer exists.

As a result, these cables can be given a greater length without having to fear that the values Z .L exceed the impedance values Z ... .... of the armouring distances, since 1 <£. L there all Z ' _T. < ^ζ τ

If you fill the reinforcement spaces with an energy-absorbing material, etc. With insulation materials with a high tg <f with ferromagnetic materials, etc., the advantages of multiple shielding can be fully utilized without the difficulties previously discussed above arising.

If one now looks at Fig. 1 of the drawing more into Going single, one recognizes .that the known cable

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according to FIG. 1 starting from its longitudinal center axis 0-0

has the following structure:
a soul 1 /

an insulation layer 2, in the case of a coaxial cable e.g. an air layer / a single shield 3 with η layers, made of electrically conductive material, the shield inevitably not only protection against parasitic electric fields, but also one forms mechanical protection and thus also serves as cable armouring.

Fig. 2 shows in a corresponding manner a known cable which differs from that according to FIG. 1 in that between two shields 3, 3 ', each with η and n ¹ layers of conductive material, an intermediate layer 4 is provided, which the two shields 3, 3 'insulated from one another, but not designed as an energy-swallowing layer according to the invention

3 to 6 show halved longitudinal sections through preferred embodiments of cables according to the invention, which at least one when used as intended of the cable, i.e. when the cable is in operation, it is an energy-absorbing layer between two conductive structures exhibit.

In detail, Fig. 3 shows a particularly simple cable in which the core 1 of a sheath in the form of a energy-absorbing layer 5 is surrounded, which in turn during operation, i.e. when the cable is used, directly

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bar is surrounded by the ground 6, which in this case as electrically conductive structure forms the reference potential or the return line.

Fig. 4 also shows a relatively simple embodiment of a cable according to the invention, which starting from has the following elements from its longitudinal center axis 0-0 to the outside:

the soul 1,

an insulating layer (possibly an insulating space),

a conductive shield 3 (single layer), an energy-absorbing layer 5, the ground 6 (ground or reference potential)

5 and 6 show more complicated embodiments in which multilayer electrically conductive shields 3, 3 ¹ , 3 ¹ 'etc. are provided outside the insulation layer 2, which are separated from one another by energy-absorbing layers 5, 5 ^{1 etc..}

It goes without saying that the person skilled in the art based on the exemplary embodiments 3 to 6 there are numerous possibilities for changes and / or additions to bids, without having to leave the basic idea of the invention.

It should also be noted that the following applies to the number of layers n, n ¹ , n ¹ 'etc. of the shields: n, n ¹ ", n ¹ ' = 1 ^ 2 ...

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The shields 3/3 ^{1/3 11,} etc. may be of different types, as well as the energy absorbing layers 5, 5 ^1, etc.

In the context of the present invention, the individual shields can be implemented in any conventional manner For example, by braiding, by wrapping with tape, by welding or crimping iron bands through Use of drawn tubes.

Finally, it should be pointed out once again that the invention does not apply to those explained and illustrated above Embodiments is limited, but that based on these other embodiments and Aus, types of management can be provided without having to depart from the basic concept of the invention.

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Claims (10)

Dr.-ing. DipL-Puye. OSKAR KÖNIG patent attorney Deutsche Bank AG Stuttgart Telephone: (07 U) 2lSfiCj 29 G4 61 account no. 89/00 Telegram: Koenigpat 7000 STUTTGART-I, Klüpfelstraße 6 Postscheck Stgi. 84919 P.O. Box 51 4534 Claims Sheathing for electrical cables, especially for high-frequency or high-frequency cables, characterized in that it has at least one in operation between two conductive structures (1, 6; 3, 6; 3, 3 '), has energy-absorbing layer (5) made of a material whose conductivity is lower is as that of the material of the conductive structures and in which electrical energy is in the range of Frequencies at which the sheathing must provide effective protection, in such a Circumference can be converted into heat that is in the space between the conductive structures results in attenuation of at least 6 dB (decibel) over the useful length of the cable. 2. Sheathing according to claim 1, characterized in that at least one energy-absorbing layer (5) Contains materials that are classified under the operating 9098U / 0589 ORIGINAL INSPECTED - 2 - conditions of the cable lead to dielectric losses over the useful length of the cable to the contribute provided attenuation in the area between the conductive structures. 3. Sheathing according to claim 1, characterized in that that at least one energy-absorbing layer (5) contains materials that are under the Operating conditions of the cable Losses in the form of eddy current losses or hysteresis losses have that gwischen over the useful length of the cable to the intended attenuation in the area contribute to the conductive structures. 4. Sheathing according to claim 1, characterized in that that at least one energy-absorbing layer (5) contains materials that are subject to the operating conditions of the cable lead to dielectric losses over the useful length of the cable to the intended Contribute attenuation in the area between the conductive structures. 5. sheath according to one or more of claims 1-4, characterized in that one or more energy absorbing layers are provided, which in terms of its strength, conductivity and relative permeability are designed such that they at least contribute to the half for determining the sweep frequency of the shroud . 9098U / 05Ö9 - 3 - 6. Sheath according to claim 3 or 4, characterized characterized in that at least one energy-absorbing 'layer is made of ferromagnetic materials contains. 7. Sheathing according to one or more of the claims 1-5, characterized in that at least one egg-swallowing layer (5) is insulating materials contains. 8. Sheath according to one or more of claims 1-6, characterized in that one of the one energy-absorbing layer (5) surrounding conductive structures through the ground (6) and / or the neighboring masses is formed. 9. Sheathing according to one or more of the claims 1-7, characterized in that one of the conductive structures between which there is a energy-absorbing layer (5) is formed by the core (1) of the cable. 10. Sheath according to one of the preceding claims, characterized in that at least one conductive structure is a conductive layer or a stack of conductive layers. 9098U / 0589