470,408

PATENT SPECIFICATION

Application Date: Feb. 13, 1936. No. 4420/36

Complete Specification Left: Dec. 23, 1936.

Complete Specification Accepted: Aug. 13, 1937.

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PROVISIONAL SPECIFICATION

Improvements in or relating to Electric Signal Transmission Lines

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 lines.

In an electric signal transmission line comprising two conductors one of which is earthy, either due to connection to earth, or due to large capacity to earth, trouble arises from interference, either from power systems or from similar circuits. A typical example of such a case is a co-axial cable for transmission up to very high frequencies.

In such a cable the inner conductor has substantially no capacity to earth, whereas the outer conductor is either earthed conductively, or effectively earthed through the capacity of any insulation between the outer conductor and earth. In this type of cable however, the distance between amplifiers is usually very short – say not more than 10 – 15 miles. At low frequencies where interference from power systems is most troublesome, the section of cable between adjacent amplifiers is electrically short, both because of the short physical length of the section, and also due to the high natural velocity of the cable, this velocity being of the order of the velocity of light. For example, a cable section some 10 miles long will be shorter than an 1/8th of a wavelength at frequencies below say 1500 to 2000 herz.

It is the object of the present invention to provide terminal equipment associated with a cable, the arrangement being such that this electrical shortness is used to reduce the interference experienced when operating the cable.

According to the present invention a section of cable which is electrically short within the range of power interference frequencies (say up to 1000 herz.) is connected at the receiving terminal to an impedance which is comparable with the surge impedance of the cable, whereas at the transmitting end the cable is fed, at least at low frequencies (below say 1000 herz.) from an impedance which is high compared with the surge of the cable. A section of cable may be said to be electrically short if its electrical length is less than one eighth of a wave-length.

According to a feature of this invention the cable is coupled at the transmitting end to the anode of a tetrode, pentode, or similar valve having an impedance high compared with the surge impedance of the cable through a coupling capable of passing at least the alternating components of signals to be fed to the cable.

The invention may be carried into effect as follows:

A section of concentric cable has its central conductor connected at the transmitting end through a large condenser to the anode of a high impedance valve such as a tetrode or pentode. The valve is provided with an anode load resistance of large magnitude. The outer conductor is connected to the cathode. It will be seen that the cable is effectively terminated at the transmitting end by a high valve impedance and a high load resistance in parallel with one another. It is arranged that these provide a sending terminal impedance very much larger than the characteristic impedance of the same order of magnitude as the characteristic impedance of the cable.

At frequencies at which the cable is electrically short, power interference operates by causing current to flow in the outer or earth conductor of the cable, thus causing a resistive potential drop in this conductor. This resistive potential drop is the disturbing e.m.f. in the circuit and causes current to flow in a looped circuit comprising the earth conductor, the impedance of the receiving terminal (receiving impedance), the resistance of the non-earthy conductor and the impedance from which it is fed at the sending terminal (sending impedance). Since the sending impedance is very much larger than the receiving impedance, then a much greater proportion of the interference voltage appears across the sending impedance than appears across the receiving impedance, with the result that the interfering voltage across the receiving impedance is greatly reduced.

If the cable is not required for as long a distance as 10 or 15 miles, but for a much shorter distance, this system of feeding and receiving will reduce the noise up to yet higher frequencies, so that for very short distances such as a mile, the noise may be reduced effectively at all frequencies below say 10000 herz. Above this frequency the thickness of the outer conductor of a co-axial cable will effectively help to shield the cable from interference. If however the outer conductor is not sufficiently thick, this shielding action may be increased by an application of iron tape binding and, if necessary, a further outer conductor, to provide in well known manner an efficient shield. If however the noise at the lower frequencies is reduced in the manner of this invention, the thickness of iron binding etc., required, can be materially reduced as compared with what would have been required if the cable had been fed from an impedance equal to its surge impedance.

Dated this 13th day of February, 1936.

REDDIE & GROSE,

Agents for the Applicant,

6, Bream’s Buildings, E.C.4.

COMPLETE SPECIFICATION

Improvements in or relating to Electric Signal Transmission Lines

I, ALAN DOWER BLUMLEIN, a British Subject, of 32, Audley Road, Ealing, London, W.5, 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 electric signal transmission lines.

In an electric signal transmission line comprising two conductors one of which is earthy, either due to connection to earth, or due to large capacity to earth, trouble arises from interference, either from power systems or from similar circuits. A typical example of such a case is a co-axial cable for transmission up to very high frequencies.

In such a cable the inner conductor has substantially no direct capacity to earth, whereas the outer conductor is either earthed conductively, or effectively earthed through the capacity of any insulation between the outer conductor and earth. In this type of cable however, the distance between amplifiers is usually short and may not be more than 10 –15 miles. At low frequencies where interference from power systems is most troublesome, the section of cable between adjacent amplifiers is electrically short, both because of the short physical length of the section, and also due to the high natural velocity of the cable, this velocity being of the order of the velocity of light. For example a cable section some 10 miles long will be shorter than one eighth of a wavelength at frequencies below say 1500 to 2000 cycles per second.

It is the subject of the present invention to provide terminal equipment associated with a cable, the arrangement being such that advantage is taken of this electrical shortness to reduce the interference experience when operating the cable.

According to the present invention, in a system of signal transmission employing a concentric cable, reduction of electrical interference of relatively low frequency is effected by providing between the conductors at the transmitting end of the cable an impedance which is high for said low frequency compared with the surge impedance of the cable.

The receiving end of the cable may in certain circumstances be terminated by an impedance which is of the same order as that of the surge impedance of the cable. In a particular system according to the invention the cable is a section which is electrically short within the range of interference frequencies to be reduced, such frequencies having a range up to about 1000 p.p.s. and such a cable section may for example be less than one eighth of a wavelength within the range of interference frequencies.

At the transmitting end the cable may be coupled to the anode of a thermionic valve having an impedance which is high compared with the surge impedance of the cable, the coupling being such that at least the alternating components of the signals to be transmitted are fed to the cable. Conveniently a valve of the tetrode or of the pentode type may be employed, the anode of the valve being coupled to the central conductor of the concentric cable through relatively large condenser.

In order that the invention may be more clearly understood and readily carried into effect, reference will now be made to the accompanying drawings in which:

Fig. 1 is an explanatory diagram and Fig. 2 shows an example of a method of connecting a cable in accordance with the invention.

Referring to Fig. 1 of the drawings, a length of concentric cable is shown supplied from a signal generator G which is connected between the central conductor C and the outer conducting sheath S. The impedance of the generator G is arranged to be high compared with the surge impedance of the cable and at the receiving end of the cable the terminating resistance represented by resistance R is given a value which is of the same order as that of the surge impedance of the cable.

In Fig. 2 of the drawings a high impedance tetrode valve V is shown as a signal generator, the anode of the valve being connected through a condenser P of the order of 10 mfds. to the conductor C. The valve V is provided with a load resistance LR of the order of 10000 ohms and the outer sheath S of the cable is connected to the cathode of the valve.

It will be seen that the cable is effectively terminated at the transmitting end by a high valve impedance and a high load resistance in parallel with one another. It is arranged that these provide a sending terminal impedance very much larger that the surge impedance of the cable. At the receiving end the cable is terminated by an impedance of the same order of magnitude as the surge impedance of the cable, the requisite resistance being provided by a resistance R, which may for example be 80 ohms.

At frequencies at which the cable is electrically short, power interference operates by causing current to flow in the outer or earth conductor of the cable, thus causing a resistive potential drop in this conductor. This resistive potential drop is the disturbing e.m.f. in the circuit and causes current to flow in a looped circuit comprising the earthy conductor, the impedance of the receiving terminal (receiving impedance,) the resistance of the non-earthy conductor and the impedance from which it is fed at the sending terminal (sending impedance). Since the sending impedance is very much larger than the receiving impedance, then much greater proportion of the interference voltage appears across the sending impedance than appears across the receiving impedance is greatly reduced.

In the example given the valve anode impedance may be 50000 ohms which is in parallel with a 10000 ohms feed resistance giving a sending impedance of 8300 ohms. The terminating impedance may be 80 ohms so that approximately only one hundredth of the interfering voltage appears at the receiving end. The method of operation described above depends on the shunting effect of the cable capacity being small, and this will be the case if the cable is electrically short, as for example when al is much less that EQUAT. HERE where a is the magnitude of the propagation constant and l is the length of the cable.

The cable is preferably terminated by its surge impedance, but this is not necessary in order to obtain the reduction of noise to signal ratio provided by the method described. If the terminating resistance is larger than the surge impedance then the noise will be greater, but so also will the signal. In practice however, in order to prevent undesirable reflections, the termination at the receiving end is preferably made equal to high frequency surge impedance.

Other methods of coupling at the transmitting end are available such as transformer coupling provided the transformer presents a high impedance to the cable within the range of interfering frequencies to be reduced.

If the cable is not required for as long a distance as 10 or 15 miles, but for a much shorter distance, this system of feeding and receiving will reduce the noise up to yet high frequencies, so that for very short distances such as a mile, the noise may be reduced effectively at all frequencies below say 10000 cycles per second. Above this frequency the thickness of the outer conductor of a co-axial cable will effectively help to shield the cable from interference. If however the outer conductor is not sufficiently thick, this shielding action may be increased by an application of iron tape binding and, if necessary, a further outer conductor, to provide in well known manner and efficient shield. If however the noise at the lower frequencies is reduced in the manner of this invention, the thickness of iron binding or other auxiliary shield required, can be materially reduced as compared with what would have been required if the cable had been fed from an impedance equal to its surge impedance.

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 system of signal transmission employing a concentric cable in which reduction of electrical interference of relatively low frequency is effected by providing between the conductors at the transmitting end of the cable an impedance which is high for said low frequency compared with the surge impedance of the cable.
  2. A system according to claim 1 where in the receiving end of the cable is terminated by an impedance which is of the same order as the surge impedance of the cable.
  3. A system according to Claim 1 or 2 in which the cable is a section which is electrically short within the range of interference frequencies to be reduced, such frequencies having a range up to about 1000 p.p.s.
  4. A system according to Claim 1, 2 or 3 in which the cable is a section having an electrical length less than one eighth of a wave length within the range of interference frequencies to be reduced.
  5. A system according to any one of the preceding claims in which the impedance at the transmitting end of the cable is provided by a thermionic valve and a load resistance.
  6. A system of signal transmission employing a concentric cable in which reduction of electrical interference of relatively low frequency is effected by coupling the cable at the transmitting end to the anode of a thermionic valve having an impedance which is high compared with the surge impedance of the cable, the coupling being such that at least the alternating components of the signals to be transmitted are fed to the cable.
  7. A system according to Claim 5 or 6 in which the thermionic valve is of the tetrode or pentode type.
  8. A system of signal transmission employing a concentric cable having impedances at the transmitting and terminating ends substantially as described with reference to the accompanying drawings.

Dated this 23rd day of December, 1936.

F. W. CACKETT,

Chartered Patent Agent.

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Leamington Spa: Printed for His Majesty’s Stationery Office, by the Courier Press. – 1937.