466,418

PATENT SPECIFICATION

Application Date: Nov 22, 1935. No. 3247/35

Complete Specification Left: Nov 23, 1936.

Complete Specification Accepted: May 24, 1937.

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PROVISIONAL SPECIFICATION

Improvements in or relating to Electric Signal Transmission Systems

I ALAN DOWER BLUMLEIN, a British Subject of 32, Audley Road, Ealing, London W.5, do hereby declare the nature of this invention to be as follows:-

The present invention relates to electric signal transmission systems.

If a concentric cable is employed for transmitting frequencies in the audio range, considerable interference may be experienced from neighbouring circuits or adjacent power lines etc. This is avoided if a balanced circuit is employed, but this involves the laying of two concentric conductors in two sheaths or in a common sheath.

Such an arrangement however, does not for the space occupied, give as low attenuation for the high frequencies as can be obtained with a single concentric air-spaced circuit. The balanced arrangement eliminates noise at the lower frequencies in a manner analogous to a normal balanced telephone circuit, in that inductive pick-up appears equally on both conductors and so produces no difference in voltage between the conductors. The problem of noise however, does not arise at the upper frequencies because the outer conductor of a concentric circuit forms an effective shield even for inductive circuits at frequencies where it is electrically thick. It is the object of the present invention to produce a circuit having freedom from interference at low frequencies and, at the same time, a space efficiency for high frequencies which is nearly as good as that of a single concentric circuit.

According to the present invention a transmission line for transmitting a wide band of frequencies which may include audio frequencies comprises a concentric cable of relatively low high frequency attenuation and associated with the cable an auxiliary conductor of relatively high high-frequency attenuation, the concentric cable being adapted to transmit satisfactorily the higher frequencies of satisfactorily the higher frequencies of said band and the auxiliary conductor and the central conductor of the concentric cable being arranged to form the conductors of a balanced circuit for the lower frequencies of said band.

According to a feature of the present invention there is provided a transmission line for transmitting a wide band of frequencies, the transmission line comprising a main conductor and an auxiliary conductor, the main conductor being provided with a metal sheath, wherein the attenuation frequency characteristics of the two conductors are comparable at low frequencies but wherein at frequencies at the upper end of the said band the attenuation of the main conductor with its sheath acting as return is much less than that of the auxiliary conductor. Preferably the auxiliary conductor is also provided with a metal sheath, the two metal sheaths being arranged in electrical contact with one another.

According to a further feature of the present invention there is provided a circuit comprising two conductors arranged within a single sheath wherein one of said conductors has a much greater high frequency attenuation than the other of said conductors and wherein it is arranged that, at low frequencies, the attenuations and propagation constants of the two conductors are of the same order of magnitude. The two conductors may be used, particularly at the lower frequencies in the band, as an insulated loop circuit, that is to say a circuit in which the two conductors respectively and both are insulated from earth.

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

A transmission line comprises a conductor enclosed within a sheath to form a concentric cable and substantially air spaced from the sheath and an auxiliary conductor of much greater high frequency attenuation than the concentric cable. The concentric cable may be of the type described in co-pending Applications Nos. 35102/34 (Serial No. 452,713) and 6038/35 (Serial No.452,772). The auxiliary conductor may be of standard telephone construction. The two cables preferably have a common lead sheath which forms the outer conductor of the concentric cable and surrounds the inner sheath thereof. The line thus comprises a high frequency concentric cable having a central conductor surrounded by an inner sheath which is in turn surrounded by an outer sheath, an auxiliary conductor, having paper insulation for example, being arranged between the inner and outer sheaths. The two cables may be combined in this way by making the cross-section of the sheath appear like a pointed egg, the concentric cable being in the rounded part of the egg and the auxiliary conductor being in the pointed end of the egg. Alternatively, the auxiliary conductor may be enclosed in a separate lead sheath which is preferably arranged helically around the sheath of the concentric cable, the sheaths being in electrical contact. At the receiving end the voltages between the air-spaced and auxiliary conductors are fed to an amplifier and since, at low frequencies, these conductors are subject to substantially the same inductive interference, inductive interference is substantially eliminated, the two conductors forming a substantially balanced feeder. At higher frequencies the central conductor of the concentric feeder is not subject to interference on account of the shielding effect of the sheath. So that high frequencies are transmitted efficiently to the amplifier, the sheath may be connected to the auxiliary conductor through a condenser having a low impedance at these frequencies.

In order to make this cable as effective as possible for the reduction of interference, the attenuation and the velocity of propagation of the two circuits (the concentric circuit and the balanced circuit) must be as nearly equal to one another as possible at low frequencies. If the concentric cable is arranged to have substantially uniform attenuation and propagation velocity as describe in co-pending Application No. 7206/35 (Serial No. 455,492), its low frequency attenuation is high. If necessary, series resistances may then be added in the auxiliary conductor (these resistances being shunted by suitable condensers) in order to make the attenuations of the two circuits equal to one another. The low frequency effect may then be corrected in both circuits so that over the lower part of the frequency effect the attenuations and propagation velocities of the two circuits are substantially equal to one another. By arranging the auxiliary conductor within the outer sheath of the concentric cable, the sheath resistance which is common to the two circuits tends to maintain the balance (as far as interference is concerned) of the two conductors of the balanced circuit, provided that the attenuations and propagation constants of the two circuits are very nearly equal to one another.

The effect of making the velocity and attenuation of the two circuits similar at lower frequencies, while obviously their capacity per unit length differs greatly, is to make the two circuits in balance, but not in symmetrical balance, i.e., the natural impedance to earth of one circuit will be much greater than that of the other. The effect of inductive interference however, is to produce voltage drops along the outer sheath which are injected into each circuit equally, and appear equally at the receiving terminals of the line.

If desired, the input may be applied to both conductors as in balanced circuit working. At the low frequencies it is propagated in the normal manner of a balanced circuit. At the high frequencies however, it substantially only propagates through the concentric cable. It is advantageous at these higher frequencies to remove the impedance of the auxiliary conductor and this may be done by shunting condensers between it and the sheath at each end of the cable.

The egg-shaped form of the cable above referred to (which may be obtained by extrusion) can be used for packing say four such cables to form a multiple cable. This, however, has the disadvantage that it does not neutralise the low frequency inductive cross-talk between the circuits. If a number of such circuits are contained in a multiple cable, arrangements must be made to rotate the positions of the auxiliary and central conductors to neutralise cross-talk.

As an alternative to making the outer sheath egg-shaped, the auxiliary conductor with its insulation of the concentric cable and squeezed into a circular lead sheath. This is particularly applicable to cables of the kind described in the above referred to co-pending Applications nos. 35102/34 (Serial No. 452,713) and 6038/35 (Serial No. 452,772), in which the inner conductor is insulated, with paper. The additional paper insulated conductor is wound round the outside of the paper sheath of the air-spaced conductor and the whole is forced into a circular lead sheath.

It is advantageous to make the increase of length due to the helical form of the auxiliary conductor approximately equal to the increase of length of the central conductor due to deformation thereof. This will then tend to give each circuit the same velocity of propagation when suitably loaded, assuming air dielectric.

Although it is advantageous to load these conductors after the manner described in co-pending Application No. 7206/35 (Serial No. 455,492), a large degree of cross-talk and interference reduction can be obtained without loading the auxiliary conductor, or even the central conductor. If the cable is to be used for very high frequencies, the repeater sections will have to be so short that for the lower audio frequencies the cable is electrically short and can be operated as a two-wire balanced circuit with a fair approximation to perfect (but not equal) balance.

If a number of such cables is arranged together, the near end cross-talk can be considerably reduced by feeding the input to the cable equally on to each conductor in opposite phase (balanced input), the equality in this case being an equality of current.

As a further alternative, the auxiliary conductor may consist of two or more thin conductors wound round the insulation of the air-spaced conductor. It may be preferable for constructional symmetry, for example, to employ two low quality conductors in place of the one described above for the cable with the deformed central conductor.

Dated this 22nd day of November, 1935.

F. W. CACKETT,

Chartered Patent Agent.

COMPLETE SPECIFICATION

Improvements in or relating to Electric Signal Transmission Systems

I, ALAN DOWER BLUMLEIN, a British Subject, of 32 Audley Road, Ealing, London, W.5, do hereby declare the nature of the 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 electric signal transmission systems and has particular reference to transmission lines employed in short wave signalling systems.

If a concentric cable is employed for transmitting frequencies in the audio range, considerable interference may be experienced from neighbouring circuits or adjacent power lines, etc. This is avoided if a balanced circuit is employed, but this involves the laying of two concentric conductors in two sheaths or in a common sheath.

Such an arrangement, however, does not for the space occupied, give as low attenuation for the high frequencies as can be obtained with a single concentric air-spaced circuit. The balanced arrangement eliminates noise at the lower frequencies in a manner analogous to a normal balanced telephone circuit, in the inductive pick-up appears equally on both conductors and so produces no difference in voltage between the conductors. The problem of noise, however, does not arise at the upper frequencies because the outer conductor of a concentric circuit forms an effective shield even for inductive circuits at frequencies where it is electrically thick.

It has previously been proposed to construct a concentric conductor having an outer sheath of copper and two conductors semi-circular in cross section supported centrally of the outer sheath by means of a strip of dielectric material placed between the semi-circular conductors and twisted to form a helix. The semi-circular conductors were to be connected in parallel to form a transmission circuit with the outer sheath and an independent low frequency transmission circuit was to be provided by the semi-circular conductors.

It is the object of the present invention to produce a circuit having freedom from interference at low frequencies, and, at the same time, a space efficiency for high frequencies which is nearly as good as that of a single concentric circuit.

According to the present invention a transmission line capable of transmitting signals covering a wide range of frequencies is constituted by a concentric cable comprising an outer conducting sheath, a main central conductor and an auxiliary conductor, the outer conducting sheath and the main central conductor providing an unbalanced circuit for the high frequencies in said range, and the main central conductor and said auxiliary conductor providing a balanced circuit for the transmission of the low frequencies in said range, the transition from the low frequency circuit to the high frequency circuit being progressive with increasing frequency of the signals transmitted.

The main and auxiliary conductors may both be contained within a common conducting sheath which serves as a return path for high frequency currents or the main conductor is contained within a conducting sheath while the auxiliary conductor is arranged outside the sheath. In a particular transmission line according to the invention, condensers shunted by resistances are inserted in series with the main conductor for the purpose of maintaining a predetermined relationship between the resistance/capacity ratio of the main conductor to earth and the resistance/capacity ratio of the auxiliary conductor to earth. Further condensers may be connected in series with the main or auxiliary conductors or both for the purpose of maintaining a predetermined relationship between the frequency/attenuation characteristic of the main conductor at low frequencies and that of the auxiliary conductor. The transmission line is provided at its termination with a resistance connected to the main and auxiliary conductors in series through condensers. In a particular constructional form of transmission line according to the invention, the main conductor is supported substantially centrally of a sheath of insulating material by deformation of the conductor and the auxiliary conductor is contained within a separate insulating sheath and wound helically about the sheath containing the main conductor. The sheath common to both the conductors and forming the return path may be of a material of low conductivity such as lead, the conductive function of which is assisted by an auxiliary sheath of highly conductive material such as copper foil. The two conductors of the transmission line may be used particularly at the lower frequencies in that range as an insulated loop circuit, that is to say, a circuit in which the two conductors respectively and both are insulated from earth.

In order that the nature of the invention may be more clearly understood and readily carried into effect, a constructional form of transmission line embodying the invention and the equivalent electrical circuit will now be described by way of example with reference to the accompanying drawings in which:-

Fig 1 shows a fragment of transmission line showing portions broken away to reveal the interior constructional details of the elements.

Fig 2 shows a transmission line including transmitting and receiving elements.

Referring to Fig 1 of the drawings, a portion 1 of a transmission line is shown comprising a central conductor 2 enclosed within a wound paper sheath 3 to form a concentric cable substantially air-spaced from the sheath. The central conductor itself may be of the type described in the specification of Patent Nos. 452,772 or 452,713. An auxiliary conductor 4 which is of standard telephone construction is provided with a separate paper insulating covering 5 and lies within a helical groove in the paper sheath 3. The increase of length due to the helical form of the auxiliary conductor is made approximately equal to the increase of length of the central conductor due to deformation thereof. This matching of lengths tends to give each circuit the same velocity of propagation when suitably loaded, assuming air to be the dielectric. The outer conductor of the cable is composite, including a thin wound layer of copper foil 6 and a relatively thick lead sheath 7 in close contact with the copper foil. The outer covering 8 consists of a layer of woven textile material.

It will be seen that the main and auxiliary conductors have a common outer conductor 7 the copper foil sheath providing a lower resistance return path than that afforded by the lead sheath at high frequencies.

Fig 2 of the drawings shows in the form of a circuit diagram a method of using the cable shown in Fig 1. The point A represents the transmitting or signal input end, the dotted lines indicate distance, the point B a typical loading point in the line and the point C the receiving end of the line. The thick line 2 represents the main central conductor and the thin line 4 represents the auxiliary conductor. At low frequencies the conductors 2 and 4 are subject to substantially the same inductive interference which is thereby substantially eliminated, the two conductors forming a substantially balanced circuit. At higher frequencies the central conductor 2 is not subject to interference on account of the shielding effect of the sheath 7.

In order to make the line as effective as possible for the reduction of interference the attenuation and the velocity of propagation of the main conductor and of the auxiliary conductor, must be as nearly equal to one another as possible at low frequencies. The main conductor 2 is loaded at intervals by condensers 9 shunted by resistances 10. Several such resistance-condenser combinations may be inserted having different time constants and serve to equalise the resistance and inductance change of this main conductor in accordance with the method described in the specification of Patent No 455,492. Sufficient resistance is added by the insertion of the resistances 10 to make the resistance/capacity ratio of the main conductor equal to the resistance/capacity ratio of the auxiliary conductor at low frequencies. The main conductor is further loaded at intervals, not necessarily as frequently as those employed for the condenser/resistance loading, by condensers such as 11 in accordance with the method described in the previously mentioned specification to correct for the low frequency effect. Condenser 12 and condenser 11 being the same as the ratio between the capacity of the auxiliary conductor to the sheath and the capacity of the main conductor to the sheath. With cables so loaded with plain series condensers, the characteristic impedance is similar to that of a resistance in series with the condenser. The line is therefore terminated by condensers 13 and 14 and resistance 15 in series. A transformer 16 transfers the voltage across the resistance 15 to the grid of a valve 17 at the receiving end. If a very wide frequency range is used the transformer 16 may only have a ratio of perhaps 1:1. If the described method of coupling the line to receiving apparatus does not permit the higher frequencies in the range transmitted to be passed to the valve 17, an additional valve may be used having its grid connected through a small condenser to one terminal of the primary winding of transformer 16 and its output mixed with the output of valve 17. The cathode of such an additional valve should be connected to the sheath 7 and a resistance and bias battery connected between its grid and cathode.

At the transmitting end of the line a termination including resistance 18 and condenser 19 is connected between the conductor 4 and earth. The main conductor 2 is fed through a large condenser 20 from the anode of a high impedance valve 21 and a high resistance 22 is connected between the anode of the valve 21 and a source of high tension current. The value of the resistance 22 is high compared with the impedance of the conductor 2 so that the valve 21 delivers practically constant currents into the circuit. In order that a balance may be obtained at the transmitting end between the main and auxiliary conductors, the main conductor 2 is terminated by resistance 23 and condenser 24.

By arranging the auxiliary conductor 4 within the outer sheath 7 of the line, the sheath resistance which is common to the two circuits tends to maintain the balance, as far as interference is concerned, of the two conductors of the balanced circuit, provided that the attenuation and propagation constants of the two circuits are very nearly equal to one another. The effect of making the velocity and attenuation of the two circuits similar at lower frequencies, while obviously their capacity per unit length differs greatly is to make the two circuits in balance, but not in symmetrical balance, that is to say, the natural impedance to earth of one circuit will be much greater than that of the other. The effect of inductive interference however, is to produce voltage drops along the outer sheath 7 which are injected into each circuit equally and appear equally at the receiving end. Thus, interference voltages will produce equal voltages on both conductors, the propagationís of which at low frequencies have been made equal. Therefore, these interference voltages will not produce a voltage across the resistance 15 if the two circuits are in perfect but not necessarily symmetrical balance. If the transmission line circuit is electrically long, the terminating resistance 18 and condenser 19 and resistance 23 and condenser 24 may be omitted.

If desired the input may be applied to both conductors 2 and 4 as in balanced circuit working. Thus, instead of a single valve 21, two valves may be used connected in push-pull, one feeding the main conductor 2 and the other feeding the auxiliary conductor 4. At the low frequencies currents will be propagated in the normal manner of a balanced circuit. At the high frequencies however, currents substantially only propagate through the central conductor. It is advantageous at these higher frequencies to remove the impedance of the auxiliary conductor and this may be done by shunting condensers between it and the sheath at each end of the cable.

Modifications may of course be made within the scope of the invention, thus for example, the auxiliary conductor 4 may be arranged within a separate lead sheath instead of the paper sheath 5. Again, the auxiliary conductor may lie immediately outside the lead sheath 7 instead of within it, the central and auxiliary conductors may have separate lead sheaths and be contained within an outer lead sheath common to both, and so forth.

While in the particular construction of cable described the auxiliary conductor is arranged to lie within a helical groove at the surface of a central paper sheath, the lead sheath may be made substantially egg shaped in cross section, the sheath 5 containing the auxiliary conductor lying at the narrow end of the egg. In such a case the auxiliary conductor is straight and the excess length of the main conductor due to its deformation, compared with the auxiliary conductor, may be compensated for electrically.

Such an egg shaped form of cable which may be obtained by extrusion, can be used for packing four such cables to form a multiple cable. This, however, has the disadvantage that it does not neutralise the low frequency inductive cross talk between the resistance/capacity ratio of such circuits are contained in a multiple cable, arrangements must be made to rotate the positions of the auxiliary and central conductors to neutralise cross talk.

It is possible to obtain some degree of reduction of cross talk and interference without the resistances and condenser loading described with reference to Fig 2 or as described in the specification of Patent No 455,492. If the cable is to be used for very high frequencies the repeater sections will have to be so short that for the lower audible frequencies the cable is electrically short and can be operated as a two-wire balanced circuit with a fair approximation to perfect, but not equal, balance.

In a further modified construction, the auxiliary conductor may consist of two or more different conductors wound around the insulation of the central conductor. It may be preferable for constructional symmetry for example to employ two low quality conductors in place of the one described above for the cable with the deformed central conductor.

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

  1. A transmission line capable of transmitting signals covering a wide range of frequencies and constituted by a concentric cable comprising an outer conducting sheath, a main central conductor and an auxiliary conductor, the outer conducting sheath and the main central conductor providing an unbalanced circuit for the high frequencies in said range, and the main central conductor and said auxiliary conductor providing a balanced circuit for the transmission of the low frequencies in said range, the transition from the low frequency circuit being progressive with increasing frequency of the signals transmitted.
  2. A transmission line according to Claim 1 wherein the main central conductor and the auxiliary conductor are contained within the outer conducting sheath which serves a s a return path for high frequency currents.
  3. A transmission line according to Claim 1 or 2 in which condensers shunted by resistances are inserted in series with the main conductor for the purpose of maintaining a predetermined relationship between the resistance/capacity ratio of the main conductor to earth and the resistance/capacity ratio of the auxiliary conductor to earth.

4. A transmission line according to Claim 3 in which further condensers are connected

in series with the main conductor or the auxiliary conductor or both, for the purpose

maintaining a predetermined relationship between the frequency/attenuation

characteristic of the main conductor at low frequencies and that of the auxiliary

conductor.

  1. A transmission line according to Claim 4 provided at is termination with a resistance connected to the main and auxiliary conductors in series through condensers.
  2. A transmission line according to any one of the preceding claims in which the main conductor is supported substantially centrally of a sheath of insulating material by deformation of the conductor and the auxiliary conductor is disposed about the outer surface of said insulating sheath.
  3. A transmission line according to any one of the preceding claims in which the main conductor is supported substantially centrally of a sheath of insulating material by deformation of the conductor and the auxiliary conductor is contained within a separate insulating sheath and wound helically about the sheath containing the main conductor.
  4. A transmission line according to any one of the preceding claims in which the main and auxiliary conductors are separated from each other by insulating material and contained within a protective sheath of material of low conductivity such as lead, the conductive function of which is assisted by an auxiliary sheath of highly conductive material such as copper foil.
  5. A transmission line constructed substantially as described with reference to Fig 1 of the accompanying drawings.
  6. A transmission line constructed and designed to operate substantially as described with reference to Fig 2 of the accompanying drawings.

Dated this 23rd day of November, 1936

F. W. CACKETT

Chartered Patent Agent,

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