463,111

PATENT SPECIFICATION

Application Date: Sept. 17, 1935. No. 25814/35.

Complete Specification Left: Feb. 14, 1936.

Complete Specification Accepted: March 17, 1937.

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PROVISIONAL SPECIFICATION

Improvements in or relating to Frequency Selective Electric Cables

We, ALAN DOWER BLUMLEIN, of 32, Audley Road, Ealing, London, W.5, EDWARD CECIL CORK, of 50A, Kenilworth Road, Ealing, London, W.5, and JOSEPH LADE PAWSEY, of 3, Dale Avenue, Hounslow West, Middlesex, all British Subjects, do hereby declare the nature of this invention to be as follows:-

The present invention relates to frequency selective electric cables which have a low attenuation for relatively low radio frequency, audio or low frequency, or direct current and a high attenuation for a relatively high radio frequency current.

It is n object of the present invention to provide an improved cable of the above mentioned type.

According to the present invention there is provided a cable comprising an unbroken conductor associated with one or more resonant circuits, said resonant circuits being so constructed and arranged that the attenuation by said cable of a high radio frequency (40 megacycles per second, for example) is relatively high and the attenuation of low radio frequency, audio or low frequency or direct current is relatively low.

Preferably the resonant circuits comprise sections of conducting wire wound helically around the inner conductor of a concentric feeder and within the outer conductor, each section being conductively connected to the inner conductor at one or both ends; in either case the length of the section is so chose with respect to its disposition about the inner conductor that the cable offers a high impedance at a high radio frequency.

In carrying the invention into effect we may proceed as follows:-

The central conductor of a concentric feeder has one end of piece of copper wire conductively attached thereto and the wire is then wound helically around the central conductor, suitable insulation being provided between the wire and the central conductor.

The other end of the wire is conductively attached to the central conductor at a point displaced along the length thereof from the first-mentioned point of attached. The electrical length of the wire between the two points of attached is arranged to exceed the electrical length of central conductor between these points by an odd number of half wavelengths (preferably one half wavelength) of the wave which it is desired to attenuate. The time of travel of the wave to be attenuated through the wire then exceed the time of travel along the central conductor in the manner described above but has approximately one half the length of the wire in the previous case. One end of the wire is connected as before to the inner conductor but the other end of the wire is left free.

In both the above-described examples a metal sheath is arranged to surround and is suitably insulated from the central conductor and the wire. Preferably a plurality of wires is arranged in the manner described. The wires may all be alike, in which case a high attenuation may be obtained in the neighbourhood of one frequency or the wires may be arranged to attenuate different frequencies in which case it is possible to obtain a substantial attenuation of all frequencies within a considerable band of frequencies. In the case in which the wire is connected at both its ends to the central conductor the wire may be a continuous helix from end to end of the cable, the wire being insulated from the central conductor over the greater part of its length but being conductively connected to the central conductor at suitably spaced points.

In cables according to this invention the wires, together with the portions of the central conductor with which they are associated, may be considered as forming tuned circuits which serve to increase the loss of the cable at the frequency or frequencies to which they are tuned.

The attenuation of the cable is dependent on the number of sections of wire employed and on the efficiency per section, the efficiency per section being dependent on the gauge of wire and the spacing between the conductors. The selectivity of the attenuation also depends on these factors.

A cable according to this invention offers the advantage over an attenuating cable loaded with high permeability magnetic material in that the copper wire loading does not require the annealing required by magnetic alloys. Further, the copper wire loading is not affected by direct or low frequency currents in the cable, which currents may cause magnetic saturation if the cable is loaded with magnetic material.

Means may be provided for adjusting the selectivity of the sections and a cable may be constructed which is substantially non-attenuating at all frequencies up to several megacycles per second.

The invention may be applied to a two wire cable of other than concentric type resonant circuits being associated with one or both of the conductors. Further, in the case of a concentric cable helically wound wires may be applied to the outer conductor, the wires being located either inside or outside the outer conductor.

Dated this 17th day of September, 1935.

REDDIE & GROSE,

Agents for the Applicants,

6, Breamís Buildings, London, E.C.4.

COMPLETE SPECIFICATION

Improvements in or relating to Frequency Selective Electric Cables

We, ALAN DOWER BLUMLEIN, of 32, Audley Road, Ealing, London, W.5, EDWARD CECIL CORK, of 50A, Kenilworth Road, Ealing, London, W.5, and JOSEPH LADE PAWSEY, of 3, Dale Avenue, Hounslow West, Middlesex, all British Subjects, do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement:-

The present invention relates to frequency selective electric cables.

In wireless transmitters and receivers, thermionic amplifiers and similar apparatus it is known that much relatively high radio frequency interference is introduced into the apparatus by way of cables connected thereto for the purpose of carrying relatively low radio frequency, audio or low frequency, or direct current. such cables may, for example, constitute power supply leads, or low radio frequency or audio frequency output or input leads.

It has been proposed to eliminate this interference by providing low pass filters in the cables. Such filters are difficult to arranged, especially if a wide band of frequencies is to be excluded and their effect is often neutralised if an incorrect terminating impedance be employed.

It is an object of the present invention to provide a simple means whereby the introduction of high frequency interference into electrical apparatus in the above mentioned way may be eliminated or substantially reduced.

The present invention in one aspect provides an electric cable comprising a main conductor associated with an auxiliary conductor in the form of a helix co-axially arranged with respect to the main conductor, the auxiliary conductor being electrically connected at its two ends to the main conductor and being otherwise insulated therefrom and the electrical length of said auxiliary conductor exceeding the electrical length of the part of the main conductor between the points of connection by an odd integral multiple of one half wavelength of a high radio frequency signal or disturbance to be attenuated.

In another aspect the present invention provides an electric cable comprising a main conductor associated with an auxiliary conductor in the form of a helix co-axially arranged with respect to the main conductor, the auxiliary conductor being electrically connected at one of its ends to the main conductor and being otherwise insulated therefrom, and the electrical length of the auxiliary conductor being arranged to exceed the electrical length of that part of the main conductor over which the auxiliary conductor extends by an odd integral multiple of one quarter of the wavelength of a high radio frequency signal or disturbance to be attenuated.

The main conductor referred to above may be either the inner or the outer conductor of a concentric cable, for example, or one of the conductors of a multi-conductor cable.

In cables according to this invention, a plurality of auxiliary conductors may be provided, which may have either of the forms referred to above, and the auxiliary conductors may be of such electrical lengths, relatively to the corresponding parts of the main conductor, that they all attenuate signals of the same frequency, or that each auxiliary conductor or group of auxiliary conductors attenuates signals of different frequencies.

The invention will now be described by way of example with reference to the accompanying drawing, wherein

Figs. 1 and 2 show by way of example two alternative constructions of cables according to the invention.

In the construction which is sown in Fig. 1, the central conductor 1 of a concentric feeder has one end of an auxiliary conductor in the form of a piece of copper wire 7 conductively attached thereto at a point 8. The wire 7 is then wound helically around the central conductor 1, suitable insulation (not shown) being provided between the wire and the central conductor and between adjacent turns of the wire 7.

The other end of the wire 7 is conductively attached to the central conductor 1 at a point 9 displaced along the length thereof from point 8. The electrical length of the wire between the two points 8, 9 of attachment is arranged to exceed the electrical length of central conductor between these points by an odd number of half wavelengths (preferably one half wavelength) of the wave which it is desired to attenuate. The time of travel of the wave to be attenuated through the wire 7 then exceeds the time of travel along the central conductor 1 between the two points 8, 9 of attachment by an odd number of half periods.

In another arrangement shown in Fig. 2 wires 10 are wound around the central conductor 1 in the manner described above but each has approximately one half the length of the wire 7 of Fig. 1. One end 11 of each wire 10 is connected as before to the inner conductor 1 but the other end 12 of the wire 10 is left free. In this case the electrical length of the auxiliary conductor 10 exceeds the electrical length of the main conductor 1 between point 11 and a point 13 adjacent point 12 by an odd integral number of quarter wavelengths of the signal to be attenuated.

In the arrangements of Figs. 1 and 2 a metal sheath 5 is arranged to surround and is suitably insulated by means not shown from the central conductor 1 and the wire 7 or 10. Preferably a plurality of wires is arranged in the manner described with reference to Fig. 1 or Fig. 2. Alternatively some wires may be arranged as in Fig. 1 and other as in Fig. 2. The wires may all be alike, in which case a high attenuation may be obtained in the neighbourhood of one frequency of the wires may be arranged to attenuate difference frequencies in which case it is possible to obtain a substantial attenuation of all frequencies within a considerable band of frequencies. In the case of Fig. 1 in which the wire 7 is connected at both its ends 8, 9 to the central conductor the wire 7 may be a continuous helix from end to end of the cable, the wire 7 being insulated from the central conductor 1 over the greater part of its length but being conductively connected to the central conductor at suitably spaced points such as the points 8, 9 shown in Fig. 1, to form a plurality of auxiliary conductors.

In the above examples the helically wound wires, together with the portions of the central conductor with which they are associated, may be considered as forming rejector circuits which are effectively in series in the cable and therefore serve to increase substantially the attenuation of the cable at the frequency or frequencies to which they are tuned.

The attenuation of the cable is dependent on the number of sections of wire employed and on the efficiency per section, the efficiency per section being dependent on the gauge of wire and the spacing between the conductors 1 and 5. The selectivity of the attenuation also depends on these factors. When the selectivity is great the cable will be substantially non-attenuating up to frequencies of the order of the suppressed frequency.

In making measurements on the attenuation of cables according to the present invention it was found that at a frequency of 45 megacycles per second, from 6 to 20 feet of cable were required to reduce the voltage to EQUAT. HERE of its applied value. In one particular case 20 feed to cable caused the voltage to fall to EQUAT. HERE of its applied value at the above-mentioned frequency. These figures are given by way of example only and cables with widely different attenuations may be constructed.

Cables constructed in accordance with the present invention may be used, for example, for connecting a short wave wireless receiver (working at frequencies of 10 megacycles or more) to a source of direct current in which high frequency disturbances are liable to be superimposed on the direct current. If the short wave receiver is shielded, then the sheath 5 of the cable may be connected to the shielding of the receiver.

It will be observed that in such a case the only interference that can enter must be the interference propagated in the circuit comprising the internal conductor 1 and the sheath 5, and wing to the high attenuation of this circuit, this interference will be small.

The invention has been described with reference to a single conductor inside a conducting sheath which acts as a return conductor. It is equally applicable to a plurality of conductors with or without a sheath. For example, it may be desired to carry a direct current from a source subject to unwanted high frequency induction producing a high frequency voltage between its terminals to an apparatus where such high frequency voltages across the D.C. source would be harmful. A pair of wires forming a looped circuit of large high frequency loss may be employed for this purpose. The wires may be enclosed in a single sheath or separate sheaths and the sheath of sheaths may be of either conducting or insulating material. Similarly, for supplying say 50 cycle mains supply to a short wave wireless receiver, a twin cable with a conducting sheath may be employed, the sheath being connected to the shielding of the receiver and being left floating adjacent the connection with the supply mains. In this case a cable with two insulated conductors is preferably so constructed as to pass freely the 50 cycle currents, but to attenuate any high frequency currents which may enter the cable in either the circuit consisting of the two conductors in parallel with sheath as a return, or in the circuit consisting of the two conductors as a balanced circuit.

A cable according to the invention may be used for supplying power to an apparatus such as an audio frequency amplifier which is shielded but suffers from high frequency disturbances (produced for example by a near-by transmitter) entering it though the power supply wires. Similarly such a cable may be used to reduce the extraneous noises heard in a broadcast receiver, due to disturbances arriving along the mains leads.

The arrangements described may be applied to a multi-conductor cable, auxiliary conductors being associated with one or more of the main conductors. Further, in the case of a concentric cable, helically wound wires forming auxiliary conductors may be applied to the outer conductor, the auxiliary conductors being located with inside or outside the outer conductor. If the auxiliary conductors are outside the outer conductor, they serve to attenuate interfering signals propagating along the outside of the outer conductor; in this case, a sheath, which may be either of insulating or conducting material, is arranged to surround the auxiliary conductors to protect them against displacement or fracture. If the sheath is of conducting material, it is of course insulated from the auxiliary conductors.

The present invention is also applicable to cables which are required to carry relatively low radio frequency currents and to exclude high frequency disturbances. A cable can be constructed so that its attenuation for relatively low radio frequency currents is low compared with its attenuation for disturbing currents of higher frequency and may be used as a feeder connecting an aerial receiving signals at relatively low radio frequencies (up to say 3 megacycles) to a receiving apparatus. The high attenuation of the cable at high radio frequencies (40 megacycles for example) prevents disturbances at these frequencies from reaching the receiving apparatus.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is:-

  1. An electric cable comprising a main conductor associated with an auxiliary conductor in the form of a helix co-axially arranged with respect to the main conductor, the auxiliary conductor being electrically connected at its two ends to the main conductor and being otherwise insulated therefrom and the electrical length of said auxiliary conductor exceeding the electrical length of the part of the main conductor between the points of connection by an odd integral multiple of one half the wavelength of a high radio frequency signal or disturbance to be attenuated.
  2. An electric cable comprising a main conductor associated with an auxiliary conductor in the form of a helix co-axially arranged with respect to the main conductor, the auxiliary conductor being electrically connected at one of its ends to the main conductor and being otherwise insulated therefrom, and the electrical length of the auxiliary conductor being arranged to exceed the electrical length of that part of the main conductor over which the auxiliary conductor extends by an odd integral multiple of one quarter of the wavelength of a high radio frequency signal or disturbance to be attenuated.
  3. An electric cable according to claim 1 or 2, wherein there are provided a plurality of auxiliary conductors, said auxiliary conductors being of such electrical lengths, relatively to those of the corresponding parts of the main conductor, that they attenuate signals or disturbances of the same frequency.
  4. An electric cable according to claim 1 or 2, wherein there are provided a plurality of auxiliary conductors, said auxiliary conductors being of such electrical lengths, relatively to those of the corresponding parts of the main conductor, that they attenuate signals or disturbances of different frequencies.
  5. An electric cable according to any of the preceding claims associated with wireless receiver or transmitter or similar apparatus and adapted to supply thereto or to derive therefrom direct or relatively low radio frequency or audio frequency current, the arrangement being such that high radio frequency signals or disturbances are substantially prevented from entering said apparatus by way of said cable.
  6. Electric cables substantially as described with reference to the accompanying drawings.

Dated this 14th day of February, 1936.

REDDIE & GROSE,

Agents for the Applicants,

6, Breamís Buildings, London, E.C.4.

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Leamington Spa: Printed for His Majestyís Stationery Office, by the Courier Press. Ė 1937.