US2036045A - Shielding - Google Patents

Description

H. J. HARRIS March 3l', 1936.

SHIELDING Filed Feb. 8, 1933 /N VE N TOR H. J. HA RR/.S

i v. M

Y ATTORNEY Patented Mar. 31, 1936 SHIELDING Henry J. Harris, West Point, Va., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application February 8, 1933, Serial No. 65531914 10 Claims. (Cl. 173-81) This invention relates to shields for electrical circuits and more particularly to shields for'signaling circuits comprising a plurality of conductors bound together.

An object of the invention is to shield a circuit effectively Awithout thereby introducing a high energy loss in the circuit shielded.

Another and more particular object of the invention is to reduce cross-talk between the circuits of a signaling cable,

It has been suggested heretofore that to protect a. signaling circuit from the effects of externalelectrostatic and electromagnetic elds it be inclosed in a continuous metallic sheath, as of tinfoil. The interfering field induces in the shielding material circulating currents which reduce theintensity of the field Within the sheath. Signals traversing the shieldedcircuit, however, likewise create currents in the sheath and the loss resulting therefrom causes an attenuation of the signals. At high frequencies, particularly, the energy loss in the shielded circuit tends to become excessive'and to preclude this type of shielding. l

In accordance with the present invention there is provided for signaling circuits a shielding system which effectively protects the signaling circuits from external interference and which at the same timeintroduces but little loss in the circuits shielded. The system depends for its operation on the fact that the spacephase relations of the fields generated by the signaling circuit generally vary with respect to those of extraneous flelds. The conducting paths for circulating current essential-to the shielding action can therefore be so arranged with respect to the shielded circuit that the elec.- tromotive force which the signals at one point along the circuit induce in a given circulating path may be neutralized by an oppositely-phased electromotive force induced in that path at another point. The circulating current resulting is accordingly small and the loss of energy from the signaling circuit is likewise small,

In one aspect, it may be said that the invention has the effect that the shielding efficiency is greater with respect to fields arising outside of the shielded system vthan to those arising within it.

As applied to the common form of signaling circuit comprising a pair of conductors twisted together, in which each of the conductors occupies a series of positions constituting a number of complete and similar cycles, the invention contemplates inclosing the circuit in a metallic `vector resultant electrostatic or electromagnetic field extending outward through any one of these loops will be approximately zero, and conse- A,quently there willbe little or no energy loss in the screen or netting resulting from the fields originating in the shielded pair. The presence 1 of thescreen or netting will therefore not have any very significant effect upon transmission losses in the shielded circuit'or circuits. On the other` hand, stray fields from other sources, un-

less such fields are similar to those caused by 15 the shielded circuit itself, will set up eddy currents around the conducting loops and as a result of'the accompanying energy loss and opposing fields created, the strength of the interfering eld reaching the shielded twisted pair will be very small.

It is only where the disturbing fields are similar to the elds created by the shielded circuit that a screen or lnetting in accordance with the present invention will be of little effect. That is, if over oney complete conducting loop of the screen the vectorial resultant vof the field linking that loop is zero, no circulating currents will be induced in the screen and the shielding action will be small. In amulti-circuit cable ernbodying the invention, it is accordingly preferred that conductor pairs having equal lengths of twist be separated in order to avoid excessive interference between them.

Referring to the drawing:

Fig. 1 shows schematically a structure adapted for shielding a twisted-pair circuit;

Figs. 2, 3 and 4 show specic forms which the shield may take; and

Fig. 5 represents a multi-circuit cable shielded in accordance with the present invention.

In Fig. 1 there is represented schematically a pair of twisted signaling conductors l inclosed between two shielding Wires 2. The shielding wires are connected by conducting bands 3 which are 45 spaced along the circuit at intervals substantially equal to the pitch of the twist of the conductors. With respect to the loop formed by two consecutive bands 3 and the portion of the shielding wires 2 included between them, it will be noted that the sense of the field created in one-half of the included section of the signaling circuit is opposite to that in the other half. The electromotive force induced in one half of the conducting loop is accordingly neutralized by that induced in' the Since `the general form of any one conductorf of a twisted circuit is that of a helix in space,

rather than that of a plane curvejit is not necessary that the loops bear any particular phase however, that the distance between the successive conducting bands 3 be uniformly equal to the length of twist and that the longitudinal conducting paths 2 be fairly symmetrical circum- 4ferentially.

The embodiment shown in Fig; 2 is similar to the schematic representation of Fig. 1 except .thatl the length of the conducting loops is a multiple of the twisting pitch. This structure may be preferred where the twisting pitch is short or a high shielding eiect is not required. The twisted pair I is inclosed by av pair of longitudinal metallic wiresA II, or as many more as may b e required, which arebonded to the conducting rings 5 that are placed at intervals equal to twice the twisting pitch. Intermediate rings Ii of insulating material may be provided to hold the shielding wires to the signaling conductors where a long twisting pitch is employed. The number and sizeof the shield wires may be varied, of course, according to the degree of shielding required. For the sake of clarity in the drawing,

'no attempt has been made to show the structures to scale. 1

In Fig. 3 is shown another type of shield in accordance with the invention. In this case the shield comprises a hollow metallic tube I in which long narrow slits have been cut to provide the circulating paths for induced currents. The length of the longitudinal portions 8 between the circumferential portions 9 is equal to the length of twist of the inclosed twisted circuit I or to a multiple of that length. The width of the portions 8 may be chosenaccording to the mechanical characteristics required. As shown, the tube may be formed from a tape in which slits or openings have been cut. A serving of paper tape IIJ over the twisted pair may or may not be provided.

In another embodiment, shown in Fig. 4, the shield comprises a lattice work of narrow metallic tapes I2. 'Ihe diamon'deshap'ed metallic loops thus provided may again be equal in length to the twisting pitch of the inclosed circuit I or Ato a multiple --of that pitch. may be wound a tape of paper II for insulating purposesV and a similar wrapping I0 may be applied to the conducting pair.

Fig. 5 represents a cross-section of a multiconductor cable suitable for Y carrier telephone transmission, in which the individual pairs are provided'with shields oi the type hereinbefore described. The shielded pairs I5 may be arranged in a plurality of layers separated by shielding material I 6. A wrapping of heavy paper II and a leadsheath I8 complete the structure. 'I'he twisting pitches ofY adjacent pairs are preferably unequal so that the condition will not exist where the disturbing iield of one pair is similar to that l 'I'he longest twisting pitch to said assemblage resultant electromotive force induced in each of Over the latticework,

generated by signals in an adjacent shielded pair, since the eiiicacy of the shield is then lowest.

three times as great as the shortest.

From the foregoing description of the essential nature of the present invention, it is clear that the invention may be embodied in many forms `other than the specic ones shown herein for purposes of illustration. The invention includes such other forms as come within the scope and spirit of the appended claims.

What is claimed is:

1. An electrical transmission structure comprising an assemblage of conductors .arranged in a periodically repeated conguration, an'electromagnetic screen therefor, said screen consisting relation to the twisted circuit. It is desirable,-

of 'closed conducting loops so disposed with respect of conductors that the vector said loops by currents in stantially zero.

2. A signal transmission circuit comprising a said conductors is subpair of conductors twisted together and a low-loss shield for said circuit, said shield consisting essentially of conducting loops so disposed with respect to said conductors that the signal-induced electromotive force'at one point of each loop tending to circulate current around said loop is counteracted by the electromotive force induced at another point of the same loop.

,3. A transmission circuit comprising a pair of Aconductors twisted together and a loweloss shield essentially for said circuit, saidshield consisting of conducting loopsoi such nature that the electromotive force induced at one point of veach conducting loop bywaves in said circuit is neutralized by the electromotive force induced' at another .point of the same loop.

4. A transmission circuit comprising a pair of conductors twisted together and a low-loss shield enclosing said pair,` said shield consisting of conducting loops for the circulation of currents induced by extraneous elds, the resultant electromotive force induced in one portion of each of said loops by thewaves in said circuit being substantially neutralized by the resultant electromotive force inducedfby said waves in the remaining portion of said loop.

5. An'electrical transmission line comprising a pluralityof conductors twisted together and a shield for said line, said shield comprising a plurality of longitudinal conducting members joined electrically only at intervals equal to the pitch of the twist of said conductors or to a multiple of said pitch;

6. A signaling line comprising a plurality of insulated conductors twisted together and a. shield for said line, said shield comprising a plurality of longitudinal wires and members electrically connecting said wires only at intervals approximating an integral multiple of the twisting pitch of saidv conductors, and means for binding said shield wires to said conductors.

7. A shielded transmission line comprising a plurality of conductors twisted together and a conducting sheath surrounding said conductors. said sheath having portions removed to vprovide a succession of conducting loops each equal in length to the twisting pitch of said conductors or to a multiple of said pitch.

8. A shielded transmission plurality'of insulated wires twisted together and a lattice-work of conductingl strips surrounding said plurality of wires, said latticework providing, for the circulation of induced currents only loops used may be twice or twist of the pair shielded or to a multiple of that length.

10. A multi-circuit signaling cable in accordance with the next preceding claim, in which the twisting pitches of adjacent signaling pairs are r) unequal.

HENRY J. HARRIS.

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