US2153174A - Electric cable - Google Patents

Description

April 4, 1939. J. COLLARD 2,153,174

ELECTRIC CABLE Filed Feb. 19, 1937 /N VEN TOR JOHN COLLARD A TTORNEY Patented Apr. 4, 1939 UNITED sT ATES PATENTv OFFICE ELECTRIC CABLE Great Britain Application February 19, 1937, Serial No. 126,612 In Great Britain February 22, 1936 6Claims.

This invention relates to electricl transmission systems and is concerned with the elimination or reduction of interference in cables due to adjacent cables or any electrical circuits. The invention is concerned with the transmission systems employing cables comprising a central conductor which is air-spaced from a surrounding conducting sheath for the greater part of its length. Cables of this kind are employed for the 10 transmission of television signals and other signals of very high frequencies and it will be appreciated that in view of the wide range of frequencies transmitted, extending from very low frequencies-including direct current, up to frequenl5 cies of some megacycles, the signals transmitted are liable to interference caused by adjacent cables or other electric circuits or devices producing interfering signals having a wide frequency range.

One method of reducing interference in cables of the kind to which the present invention relates, requires an auxiliary conductor associated with the central conductor to constitute a balanced circuit. This arrangement, however, is of itself 25 found not to be practicable in respect of frequencies greater than 2,000 cycles per second.

it is also known that the provision o f lead sheathing on cables is effective in reducing interference, but the cost of the lead sheathing renders it uneconomical for frequencies below about 50,000 cycles per second. It is also well known that the addition of iron tape round the lead sheath will increase the shielding. If such tape is to be edective at the low frequencies for which it has hitherto usually been employed, a considerable thickness is required and the construction becomes costly. At high frequencies, if a thick tape is used, its effectiveness becomes relatively o less, due to eddy currents in the cross-section oi the tape. For high frequencies, therefore, a very thin tape is required but this is ineffective at the low frequencies and at very high frequencies such tape is inconveniently thin.

The chief object of the present invention is to provide an improved transmission system and an improved cable for use in such a system for the purpose oi reducing or eliminating interference over a greater frequency range than that to which 5o any one of the above-mentioned methods is economically applicable.

According to the main feature of the invention an electric transmission system is provided comprising a co-axial cable in which, for the purpose 55 of reducing or eliminating nterferenceover a wide frequency range, different means are associated with the cable which are effective in reducing or eliminating interference over different portions of the total frequency range. In the preferred form of the invention the cable is associated with an auxiliary conductor and is so connected as to be effective in reducing or eliminating interference at frequencies up to 2,000 cycles per second and is provided with lead sheathing or a sheath of equivalent material for the purl0 pose of reducing or eliminating interference at frequencies above about 50,000 cycles per second, the lead sheathing having applied to the outer surface thereof tape of soft iron or other suitable material to dealwith frequencies between 2,000 and 50,000 cycles'per second. I

It is found with a transmision system constructed in accordance with the invention, that interference can be reduced or eliminated over a frequency range extending from very low frequencies up to some two megacycles or more.

In order that the said invention may be clearly understood and readily carried into eect the same will now be more fully described with reference to the accompanying drawing inl which the figure illustrates a preferred form of cable for use in the invention.

As shown, the cable comprises a central conductor l disposed substantially centrally in an 3@ insulating sheath of paper 2, the sheath of paper and the central conductor I being surrounded by a conducting sheath 3 in the form of copper tape suitably applied to the paper sheath 2. This tape comprises a plurality of strips placed side by side as shown, to cover the paper sheath 2. Laid helically between the conductor 3 and the paper sheath is an auxiliary conductor 4 surrounded by individual insulation 5 of paper. The conductor 3 is in turn surrounded by a sheath 6 of lead or other suitable material and to the outer surface of the sheath 6 tape of soft iron or other suitable material 'i is applied, the cable assembled being then covered by woven textile material 8.

"I'he auxiliary conductor 4 is effective in reducg5 ing or eliminating interference at frequencies up to about 2,000 cycles per second. In the simplest form of application of the invention the conductors i to 4 are connected'to the secondary winding of a transformer having a centre tap which is earthed, as by a connection to the lead sheathing 6, the signals which it is desired to transmit along the cable being applied to the primary of the transformer. It will, however, be understood that various forms of circuits maybe 'atl employed in conjunction with the cable shown in the drawing.

In one specific form of cable the inside diameter of the sheath 3 may be five-sixteenths of an inch and the diameter of the centre conductor I may be about 0.08 of an inch` The centre conductor may be f ormed as described in the specifications of British Patents Nos. 452,713 or 452,772, the paper sheath 2 being replaced if required by the insulating sheath of rubber described in these specifications. The auxiliary conductor 4 is conveniently of about 0.035 of an inch in diameter and this size is found to be economical in use for providing a balance circuit free from interference up to about 2,000 cycles per second. The lead sheathing 6 may be of about one-sixteenth of an inch thick which affords shielding suilicient to reduce or eliminate interference at frequencies above 50,000 cycles per second. The soft iron tape 'l may have a thickness of about 0.01 of an inch and such a thickness is found to be economical and effective in reducing interference over a range of about 2,000 to 50,000 cycles per second.

It is desirable that the conductor 4 be disposed as close to the sheath 3 as is feasible, but in some cases it may be found convenient to insert the conductor I in the insulating sheath 2, the conductor in this case being preferably helically applied to the internal surface of the sheath 2.

It will be appreciated from the above that the invention is directed to the combination of various means for reducing or eliminating interference,

.each of ythe means being effective over only a portion of the total frequency range to be dealt with so that whilst effective in reducing or eliminating interference over a very Wide frequency range, the construction of the cable will be practicable and economical.

I claim:

1. An electric transmission system embodying a co-axial cable, comprising an inner insulating sheath and a centre conductor, an auxiliary conductor helically wound over said sheath in such a manner as to form a balanced circuit for reducing low frequency interference, a conducting sheath oi' lead surrounding the auxiliary conductor, inner sheath and centre conductor for reducing or eliminating interference at high frequencies, and tape of soft iron applied externally to the sheath of l lead for reducing interference at frequencies intermediate the said low and high frequencies.

2. An electric transmission system as in claim l, wherein the auxiliary conductor. lead sheathing, and the soft iron or like tape are so constructed and arranged as to reduce or eliminate interference at frequency ranges of the order of respectively, up to 2,000 cycles per second, above 50,000 cycles per second and between 2,000 and 50,000 cycles per second.

3. A co-axial cable in which for the purpose of reducing or eliminating interference over a Wide frequency range, an auxiliary conductor is wound around the central conductor to form a balanced circuit for reducing low frequency interference and additional metallic means surrounding the auxiliary conductor for reducing interference over the higher frequency portion of the total frequency range.

4. A co-axial cable comprising an inner sheath and a centre conductor, an auxiliary conductor in the form of a Wire wound around said sheath and connected to the centre conductor through an external circuit to form a balanced circuit for reducing low frequency interference, a conducting sheath of lead surrounding the auxiliary conductor and inner sheath for reducing interference at high frequencies, and tape of soft iron applied externally to the sheath of lead for reducing interference at frequencies intermediate the said low and high frequencies.

5. A transmission line for transmitting a Wide range of frequencies comprising an insulating sheath and a central conductor Within said sheath, a conductor helically Wound around said insulating sheath, an electrically conducting sheath surrounding said last conductor and said insulating sheath, a lead sheath spaced from and surrounding said electrically conducting sheath, and a tape of magnetic permeability surrounding said lead sheath.

6. A transmission line for transmitting a wide range of frequencies comprising a paper sheath and a central conductor within said sheath, a Wire helically wound around said paper sheath, copper tape surrounding said wire and paper sheath, a lead sheath spaced from and surrounding said copper tape, wire and paper sheath, and a tape of soft iron surrounding said lead sheath.

JOHN COLLARD.

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