455,492

PATENT SPECIFICATION

Application Date: March 7, 1935. No. 7206/35.

" " " Nov. 26, 1935 No. 32791/35.

One Complete Specification Left: March 7, 1936.

(Under Section 16 of the Patents and Designs Acts, 1907 to 1932.)

Specification Accepted: Oct. 22, 1936.

-------------------------

PROVISIONAL SPECIFICATION

No. 7206 A.D. 1935.

Improvements in or relating to Electric Signal Transmission Lines

We, ALAN DOWER BLUMLEIN, of 32, Audley Road, Ealing, London, W.5, and JOHN HARDWICK, of 21, Drayton Park Avenue, West Drayton, Middlesex, both British Subjects, do hereby declare the nature of this invention to be as follows:-

The present invention relates to electric signal transmission lines, and more particularly to transmission lines which are required to handle signals covering a wide band of frequencies.

In a transmission line having distributed resistance, inductance and capacity the propagation constant P is given by

EQUAT. HERE

 

where R, L and C are the resistance, inductance and capacity per unit length of the line respectively and w represents the angular frequency of the signal being transmitted and is equal to 2p f where f is the frequency in cycles per second.

When w has a high value so that w L is much greater than R the propagation constant P is given by

P = jw EQUAT. HERE approximately (1)

In this equation EQUAT. HERE represents the wavelength constant and is denoted by b and since the wave velocity W in the line is given by

EQUAT. HERE

 

in this case we have

EQUAT. HERE

The wave velocity W is therefore independent of frequency. Further, the second term in the expression for P in equation (1) denotes the attenuation constant which is also independent of frequency. The line is therefore distortionless, when w is large.

Equation (1), however, can only be applied when w has a high value. If w is small it is found that both the wave velocity and the attenuation constant are functions of w and the line is, therefore, not distortionless, and it is an object of the present invention to provide a transmission line in which, over a wide band of frequencies, distortion is eliminated or substantially reduced.

According to the present invention there is provided a transmission line for transmitting electric current extending over a wide band of frequencies wherein condenses are inserted in series with the line at intervals along the line whereby the dependence on frequency of the attenuation or the wave velocity of the line, or both, is substantially reduced. The condensers may be inserted in series with one or more conductors of the line and may in some cases, be shunted by suitable resistances.

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

Condensers are connected in series with the central conductor of a concentric transmission line comprising a cylindrical sheath coaxial with a central conductor. The condensers have such value and are so spaced apart from one another that the effective series capacity in a unit length has a value which will be denoted by C1.

The effective series impedance term for the line then becomes

EQUAT. HERE

 

If C1 is made equal to EQUAT. HERE then the series impedance becomes

EQUAT. HERE

which equals

EQUAT. HERE

 

The propagation constant P is now given by

EQUAT. HERE

 

The wave velocity and attenuation are then constant for all frequencies even when w is small as well as large provided L, C, C1 and R are independent of frequency.

The characteristic impedance of the line then becomes

EQUAT. HERE

which is an expression for the impedance of a condenser and resistance in series.

In practice it is usually necessary to arrange the series condensers at intervals, which intervals are preferably short compared to the wavelength, at the frequency for which w L=R. for example a required loading of 1 microfarad–kilometre may be made up by introducing condensers every half kilometre. For a concentric line the condensers can be introduced in series with the central conductor, the above loading requiring 2 microfarads every half kilometre. For a balanced line the condensers are preferably put in each conductor, e.g., 4 microfarads in each conductor every half kilometre. For lines without added inductance leading, spacings between the condensers greater than a kilometre may be satisfactory.

If the line has shunt leakance as well as shunt capacity, this can be allowed for by putting resistances in parallel with the series condensers, to equalize the velocity and attenuation.

A further cause of variable attenuation is the change with frequency of effective resistance and inductance of the line. For a line not loaded with inductance, the inductance falls from the "low frequency inductance" to the "high frequency inductance" as frequency is increased. Similarly, the resistance rises with fre-condensers shunted by resistances. These elements tend to neutralise the excess low frequency inductance and also add resistance at low frequencies. For lines working at frequencies up to several megacycles, it is not practicable or advisable to add enough low frequency resistance to make the attenuation constant throughout the range, but a certain amount of low frequency resistance may be added.

In constructing a line therefore, series elements comprising resistances and condensers, (which may have varying values at various loading points), are introduced to reduce the low frequency inductance and increase the low frequency resistance. These elements then give an effectively constant resistance and inductance over the lower part of the frequency range. Series condensers (with shut resistances to allow for leakage) are then added according to the formulae developed above to give a substantially constant velocity and attenuation over the lower part of the frequency range. The increase attenuation for very high frequencies can be equalised at terminal points or at repeater points.

The series capacitative loading may be introduced for example either by lumped condensers or by forming the cable conductor or conductors of twisted wires thinly insulated from one another (e.g. by an oxide film) which wires are broken at intervals (the breaks in various wires being out of step) so that the current flows from wire to wire through the thin insulation.

The cables so formed are with advantage fed from terminations formed of a resistances and condenser in series. For example, with a resistance and condensers across each end, a constant current feed into the termination will give a constant voltage across the resistive portion of the termination at the other end.

In order to get favourable matching conditions at the terminations, it may be arranged that the impedance of the line is stepped up for the highest frequencies. Thus the line may be terminated by a resistance and the primary of an auto transformer in series (with a series condenser in addition if desired, to match the line impedance at low frequencies).

The primary of the transformer has a low inductance which only becomes effective at very high frequencies.

The input or output device (which may be a thermionic valve) is thus directly connected for low frequencies, but at the highest frequencies the transformer gives an effective step up. The resistance of the termination can be shunted by a condenser of such value as effectively to remove it at the highest frequencies.

Such lines as here described are particularly suitable for transmitting television signals.

For very short cable lengths all the loading may be applied at the ends.

Dated this 7th day of March, 1935.

REDDIE & GROSE,

Agents for the Applicants,

6, Bream’s Buildings, E.C.4.

PROVISIONAL SPECIFICATION

No. 32791 A.D. 1935.

Improvements in or relating to Electric Signal Transmission Lines

We, ALAN DOWER BLULEIN, of 32, Audley Road, Ealing, London, W.5, and JOHN HARDWICK, of 21, Drayton Park Avenue, West Drayton, Middlesex, both British Subjects, do hereby declare the nature of this invention to be as follows:-

The present invention relates to electric signal transmission lines and more particularly to transmission lines which are required to handle signals covering a wide band of frequencies.

In a transmission line have distributed resistance, inductance and capacity the propagation constant P is given by

EQUAT. HERE

 

where R, L and C are the resistance, inductance and capacity per unit length of the line respectively and w represents the angular frequency of the signal being transmitted and is equal to 2p f where f is the frequency in cycles per second.

When w has a high value so that w L is much greater than R the propagation constant P is given by

P = jw EQUAT. HERE approximately (1)

In this equation EQUAT. HERE represents the wavelength constant and is denoted by b and since the wave velocity W in the line is given by

EQUAT. HERE

 

in this case we have

EQUAT. HERE

 

The wave velocity W is therefore independent of frequency. Further, the second term in the expression for P in equation (1) denotes the attenuation constant which is also independent of frequency. The line is therefore distortionless, when w is large.

Equation (1), however, can only be applied when w has a high value. If w is small it is found that both the wave velocity and the attenuation constant are functions of w and the line is, therefore, not distortionless. Even when w L is large compared with R, there is another effect which may introduce distortion. This is the change of the so-called cable constants L and R at high frequencies due to the skin effect. A change of these constants with frequency introduces changes in the propagation constant with frequency and distortion results.

It is an object of the present invention to provide an improved transmission line in which, over a wide band of frequencies distortion is eliminated or substantially reduced.

According to the present invention there is provided a transmission line for transmitting electric currents extending over a wide band of frequencies, wherein inductances and resistances in series with one another are connected in shunt with the line at intervals along the line whereby attenuation of the line is increased over at least a part of the band of frequencies to be handled. In this way it may be arranged that the dependence on frequency of the attenuation or the wave velocity or both the attenuation and the wave velocity of the line are substantially reduced.

The arrangement of this invention for reducing the distortion of a line may be used in conjunction with other arrangements which are used for this purpose. One such arrangement is described in co-pending Application No. 7206/35.

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

A cable of know type may be considered as having shut capacity C, series resistance R, series distributed inductance L (due to air inductance and the high frequency skin), series distributed inductances L1, L2, L3 . . . . shunted respectively by resistances R1, R2, R3 . . . . and shunt leakance G. The inductances L1, L2, L3 . . . . represent the low frequency inductance of the cable and the resistances R1, R2, R3 . . . . represent the high frequency skin resistance. The number of these inductances and resistances which it is necessary to consider depends on the frequency range to be handled and on the degree approximation which is required. In the following description it will be assumed that only one series inductance L1 shunted by a resistance R1 need be considered.

In a cable of this type, at high frequencies, variations of the propagation constant and attenuation appear due to the changes of inductance and resistance with frequency. These variations may be largely reduced by connecting in shunt with the line a resistance of value A in series with an inductance of value B where the values of A and B are given by

EQUAT. HERE

These expressions for the values of A and B are obtained from a binomial expansion which can only be made on certain assumptions which may not always be justified. The values must therefore be taken as approximate and correct practical values may be found by experiment.

The result of connecting resistance A and inductance B in series with one another and in shut across the line is to produce a cable with constant wave velocity and attenuation at the higher frequencies but, at low frequencies, the series inductance remains L + L1. The shunt capacity is however decreased by the effective negative capacity introduced by A and B. The shunt leakance G is increased by EQUAT. HERE.

A cable is therefore corrected for variations of its inductance and resistance at high frequencies by resolving its constants into as many R1, L1, R2, L2 sections as are necessary for the required degree of approximation and frequency range and then inserting an A B element for each of these. The cable need not be fully corrected in this manner over the whole of the required frequency range. It is always possible to introduce a certain amount of correction in the repeaters which are necessary at intervals along the line. The low frequency constants of the modified cable may then be deduced.

Assuming that A and B are at the same temperature as the cable, it is preferably arranged that A has a temperature coefficient equal in magnitude but opposite in sign to the temperature coefficient of the series resistance of the cable, and that B has a temperature coefficient of magnitude double that of the series resistance of the cable and of opposite sign.

At low frequencies (frequencies at which w L is not large compared with R) it is also necessary to apply correction, because it is in general impossible to construct a wide range cable with sufficient series inductance to be fully effective at low frequencies. At low frequencies the cable behaves as if it has series resistance R, series inductance L, shunt capacity C and shunt leakance G. these values are not necessarily equal to the values mentioned above in connection with high frequency correction. The low frequency correction is then effected by connecting resistance A0 and inductance B0 in series with one another and in shunt with the line. The values of A0 and B0 are given by

EQUAT. HERE

 

The propagation constant of a cable treated in this way is then equal to

EQUAT. HERE

for all frequencies. The first term shows the wave velocity to be independent of frequency and the other terms show that the attenuation is also independent of frequency. This correction is called the low frequency correction whilst the correction for variations of inductance and resistance with frequency is (since it only becomes noticeable at high frequencies) called the high frequency correction.

Either the high or the low frequency correction may be applied separately to modify the characteristics of the cable over a part of the whole of the frequency bands over which these corrections are effective. The two corrections may also be applied simultaneously.

The high frequency correction according to this invention may be applied in conjunction with low frequency correction as described in co-pending Application No. 7206/35.

The low frequency correction according to this invention may be applied in conjunction with high frequency correction as described in co-pending Application No. 7206/35.

High frequency correction can also be affected in part according to this invention and in part according to co-pending Application No. 7206/35. If this correction is effected half by the one arrangement and half by the other method it is found that a closer approximation to continuous loading is obtained and it is therefore possible to space the loading points further apart from one another than when all the correction is applied by one only of these arrangements. By suitably distributing the correction between the two arrangements it may be possible to arrange that EQUAT. HERE in which case the cable is uniform for all frequencies and no low frequency correction is necessary.

Similarly the low frequency correction may be applied in part by the arrangement of this invention and in part as described in co-pending Application No. 7206/34, whether or not high frequency correction according to this invention or according to co-pending Application No. 7206/35 has been applied.

The theory of cables is worked out for continuous loading but in practice lumped loading is generally applied at loading points suitably spaced along the cable. For the method of lading of this invention it is found that if a frequency w is involved such as A = w B, then the distance between consecutive loading points must be quite small compared with the wavelength corresponding to frequency w . The actual distance employed depends on the accuracy of correction of the cable which is required. The maximum distance which will satisfy any given requirements is best found by trial and error. The cable constants are measured and the cable is transformed to its equivalent p network. This network is then terminated at each end by a load equal to one half the load of the cable. The new propagation constant is then determined. It is found that a bump or hollow in the transmission curve of the cable is most likely to occur at a frequency corresponding to a wavelength one quarter or one half of which is equal to the loading distance. If the curve is substantially flat in the neighbourhood of both these frequencies, then it is likely to be flat over the frequency range for which the cable has been corrected.

It is to be understood that the calculated values of A and B are express in ohms and henries per unit length. Since these loading elements are connected in shut with the cable the units should be the reciprocals of these quantities. It follows that, if the loading distance be halved, the values of the loading elements (A and B etc.) must be doubled.

The invention may be applied either to a balanced or unbalanced cable.

Dated this 26th day of November, 1935.

REDDIE & GROSE,

Agents for the Applicants,

6, Bream’s Buildings, London, E.C.4.

COMPLETE SPECIFICATION

Improvements in or relating to Electric Signal Transmission Lines

We, ALAN DOWER BLUMLEIN, of 32, Audley Road, Ealing, London, W.5, and JOHN HARDWIC, of 21, Drayton Park Avenue, West Drayton, Middlesex, both 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 electric signal transmission lines, and is more particularly concerned with transmission lines which are required to handle signals covering a wide band of frequencies.

In a smooth transmission line, that is, a transmission line (e.g., an unloaded line) which may be regarded as having, over a predetermined working frequency range, distributed resistance, inductance and capacity, the propagation constant P is given by

EQUAT. HERE

 

where R, L and C are the resistance inductance and capacity per unit length of the line respectively and w represents the angular frequency of the signal being transmitted and is equal to 2p f where f is the frequency in cycles per second.

When w has a high value so that w L is much greater than R the propagation constant P is given by

EQUAT. HERE

approximately (1)

In this equation EQUAT. HERE represents the wavelength constant and is denoted by b and since the wave velocity W in the line is given by

EQUAT. HERE

 

in this case we have

EQUAT. HERE

 

The wave velocity W is therefore independent of frequency. Further, the second term in the expression for P in equation (1) denotes the attenuation constant which is also independent of frequency. The line is therefore distortionless, when w is large.

Equation (1) however, can only be applied when w has a high value. If w is small it is found that both the wave velocity and the attenuation constant are functions of w and the line is, therefore, not distortionless. The variation of wave velocity and attenuation constant with w at low frequencies will be termed the low frequency effect. It has already been proposed, in voice-frequency telephone systems, to load a transmission line by means of series inductances, these inductances serving to increase the effective inductance of the line and thus to decrease the attenuation of the line at the higher frequencies, the attenuation being thereby rendered more constant. It is also known to tune a line loaded in this way to a predetermined voice frequency by means of condensers inserted at intervals in series in the line; in such an arrangement, the line has a relatively low attenuation at the frequency to which it is tuned, and it may be arranged that the variation in attenuation of the line over a range of frequencies in the neighbourhood of the resonant frequency tends to compensate for distortion due to an associated telephone instrument.

In another arrangement which has largely been proposed, a resistance shunted by a condenser is arranged in series in a line loaded with series inductances, the condenser serving gradually to short circuit the resistance with increasing frequency; in this arrangement, a regulation of the attenuation of the line results from the loss of energy in the resistance.

In certain transmission systems, such as systems for the transmission of television signals over lines, it is not only important that the attenuation of the line shall be substantially uniform over the wide frequency range involved, but it is also a desideratum, if relative phase shift is to be avoided, that the wave velocity should be substantially uniform over the frequency range.

It is an object of the present invention to provide a new or improved transmission system in which distortion due to the low-frequency effect is eliminated or substantially reduced.

The present invention provides an electric signal transmission system comprising a transmission line which, over a predetermined working range of frequency, simulates a smooth line, characterised in that one or more series circuits each comprising a condenser effectively shunted by a resistance are connected in series in the line at spaced intervals along it, the total effective series capacity (C1) due to said series circuits being equal substantially to that given by the relationship

EQUAT. HERE

 

and the total leakance (G1) due to said series circuits being equal substantially to that given by the relationship

EQUAT. HERE

 

where C, L, R and G respectively the capacity, inductance, resistance and leakance of said transmission line.

The invention also provides an electric signal transmission system comprising a transmission line which, over a predetermined working range of frequency, simulates a smooth line, characterised in that one or more condensers are connected in series in the line at spaced intervals along it, the total effective series capacity (C1) due to said condensers being equal substantially to that given by the relationship

EQUAT. HERE

 

where L and R are respectively the inductance and resistance of said transmission line.

The invention further provides an electric signal transmission system comprising a transmission line which, over a predetermined working range of frequency, simulates a smooth line, characterised in that one or more impedance elements each comprising a resistance and an inductance in series with one another are connected in shut across said line at spaced intervals along it, said impedance elements being so constituted that the attenuation and wave velocity of said line are increased at the lower end of said range of frequency to an extent such that the attenuation and wave velocity are rendered more uniform over a substantial part of said range.

Even when w L is large compared with R, there is another effect which may introduce distortion. This is the change of the so-called cable constants L and R at high frequencies due to the skin effect. A change of these constants with frequency introduces changes in the propagation constant with frequency and distortion results. This will be called the high frequency effect.

A further object of the invention is accordingly to provide a transmission system in which distortion due to the high-frequency effect is eliminated or substantially reduced.

According, therefore, to another feature of the invention, there is provided an electric signal transmission system comprising a transmission line which, over a pre-determined working range of frequency, simulates a smooth line, characterised in that one or more condensers each effectively shunted by a resistance are connected in series in the line at spaced intervals along it, the magnitudes of said condensers and resistances being such that the rise of resistance and fall of inductance of said line with increasing frequency are at least partially neutralised.

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

Fig. 1 shows a transmission system according to the invention,

Figs. 2 to 5 show details of various arrangements according to the invention for equalising a cable, and

Fig. 6 shows an alternative arrangement according to the invention of the cable termination shown in Fig. 1.

Reference