461,324

PATENT SPECIFICATION

Application Date: Aug. 12, 1935. No. 22724/35.

Complete Specification Left: July 17, 1936.

Complete Specification Accepted: Feb. 12, 1937.

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PROVISIONAL SPECIFICATION

Improvements in or relating to High Frequency Electric Transmission Lines

I, ALAN DOWER BLULEIN, 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 high frequency electric transmission lines, and more particularly to lines which are required to feed a number of loads distributed there along.

In such systems it is important that the connection of one load across the line shall not disturb adjacent loads. At power frequencies (e.g. 50 cycles per second) this result is obtained by making the impedance of the generator low compared with the impedance of the load and by by employing a line of low inductance and resistance. At telephone and radio frequencies, however, efficient operation requires the use of loads of the same order of impedance as the generator and successful working of electrically long feeders requires that they shall be properly matched.

It is an object of the present invention to provide a transmission line which substantially satisfies these requirements.

According to the present invention there is provided a transmission line comprising a feeder adapted to be fed at one end from a generator having an impedance of the same order of magnitude as the characteristic or surge impedance of the feeder, the feeder being provided with a tapping point at which a tapped load is connected to the feeder and the arrangement being characterised in that there is provided at the tapping point a bridge circuit comprising two tightly-coupled inductive arms and a loading impedance, the arrangement being such that a balance is established between the incoming part of the feeder and the loading impedance, so that the impedance presented and the power supplied to the succeeding feeder is substantially unaffected by the impedance of the tapped load. The feeder may have the same or different characteristic impedances on the two sides of the tapping point and the tapped load may be a signal responsive device or a further feeder or any other desired device.

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

Fig. 1 is an explanatory diagram illustrative of the principle of the present invention and Figs. 2 and 3 show circuits of two embodiments of the present invention.

Referring to Fig. 1, four impedance elements A, B, P, Q, and an auto-transformer 1 are connected as shown. The auto-transformer comprises S turns of wire and it is assumed that the inductance is so high and that the coupling between the turns is so tight that it operates as a perfect auto-transformer.

The number of turns in the part of the auto-transformer 1 between points 2 and 3 is nS where n has a suitable value less than unity. The number of turns between points 3 and 4 is therefore (1 – n)S. If A, B, P and Q represent the magnitudes of the impedances denoted by these references, and if (1 – n)A=nB then there will be no transmission from P to Q. Similarly if n(1 – n)P=Q then there will be no transmission from A to B.

If the former of the above relations is satisfied then the impedance facing P is equal to A/n which equals B/(1 – n) and the impedance facing Q is equal to (1 – n)A which equals nB. If P is a generator delivering a power Wp watts then nWp watts flow to A and nWq watts flows to B.

A similar set of relations between impedances and powers may be obtained if the latter of the above relations is satisfied.

Suppose P to be the incoming feeder and Q to be a dead load, then if *** is the succeeding feeder going to further tapping points and B is the load tapped at the point under consideration, the impedance facing A and the power delivered to A will be unaffected by the value of B provided that n(1 – n)P=Q. If P is itself a length of feeder fed from a similar bridge circuit and so on back to a generator connected to one end of the line, it is possible to fix P for all conditions of previous tapped loads and so to fix Q to obtain the balance condition. Similarly and B can be made the incoming feeder and the loading impedance, and P and Q can be the outgoing circuits. Suitable earthing conditions and impedance values can be obtained by inserting transformers, which transformers for radio frequency work may be tuned. By fixing the value of n, the fraction of the power tapped off at any point may be varied, and different values of n may be used at different tapping points. The tapped load may consist of a further length of feeder supplying branched tapping points.

Figures 2 and 2 show two simple circuits embodying this invention. In both Figures an incoming feeder 1 is coupled to two outgoing feeders 2 and 3 by means of a circuit including a suitable loading impedance 4. In both cases the coupling circuit comprises an auto-transformer 5, and a transformer 6 is provided in the input to feeder 3.

In Fig. 2 it is most convenient to make feeder 3 the connection to the tapped load and feeder 2 the continuation of the incoming feeder 1. The condensers 7, 8 shown in dotted lines may be inserted for tuning purposes. Any shunt loss in the auto-transformer 5 may be considered as part of the shunt impedance of the incoming feeder 1 and may be allowed for in fixing the magnitude of impedance 4. The capacity to earth of the primary winding of transformer 6 may be considered as a part of the tuning capacity 7 across auto-transformer 5 to which impedance 4 is connected is determined by the fraction of the incoming power required by the tapped load. By connecting the feeder 1 to point 9 such that it is across EQUAT. HERE turns of auto-transformer 5 (the total number of turns being S and the number between points 10 and 11 being nS) it is arranged that the impedances looking into feeders 1 and 2 are equal to one another. Similarly by making the turns ratio of the transformer 6 equal to EQUAT. HERE the impedance looking ton feeder 3 becomes equal to that looking into feeders 1 and 2 so that in this case 3 may be a branch feeder similar to the main feeder.

In the arrangement of Fig. 3 it will be seen that feeders 2 and 3 are both coupled by transformers to feeder 1.

It must be noted that if the impedances of the tapped load 3 and the outgoing feeder 2 are in suitable ratio (e.g. equal to the impedances feeding them), there will be no transmission from the incoming feeder 1 to the loading impedance 4, and hence no loss of power apart from that necessitated by losses in transformers etc. Thus in an ideal case with a generator feeding equal available power to m loads, the value of n for each successive tapping point, arranged as in Fig. 2, would be EQUAT. HERE and so on, the last but one tapping point having a value of an equal to 1/2 and the last load being connected directly to the end of the feeder. In practice in allowance must be made for the inevitable losses introduced at each tapping point by the transformers.

Many arrangements, other than those shown in the drawing are possible.

Dated this 12th day of August, 1935.

REDDIE & GROSE,

Agents for the Applicant,

6, Bream’s Buildings, London, E.C.4.

 

COMPLETE SPECIFICATION

Improvements in or relating to High Frequency Electric 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 high frequency electric transmission lines, and more particularly to lines which are required to feed alternating current to a number of loads distributed along them.

In such systems it is important that the connection of one load across the line shall not disturb other loads which the line is arranged to feed. At power frequencies (e.g. 50 cycles per second) this result is obtained by making the impedance of the source of current low compared with the impedance of the load and by employing a line of low inductance and resistance. At telephone and radio frequencies, however, efficient operation requires the use of loads of the same order of impedance as the source of current, and the successful working of electrically long lines requires the use of loads of the same order of impedance as the source of current, and the successful working of electrically long lines requires that they shall be properly matched to their associated sources and loads.

It is an object of the present invention to provide a novel or improved transmission system which is suitable for use at telephone and radio frequencies.

The present invention accordingly provides an electric transmission system comprising a transmission line coupled at one of it ends to a source of alternating current, and a load coupled to the line at a tapping point therein by means of a bridge circuit, the bridge circuit having two coupled inductive arms and having its remaining arms and its diagonals constituted by said load, an auxiliary impedance, and the parts of the line preceding and following the tapping point, the relative magnitudes of the impedances of the four arms of the bridge circuit, or of said inductive arm in said diagonals, being such that, in operation, the current which flows from the source to the part of the line following the tapping point is substantially independent of the magnitude of the current which flows to said load.

The diagonals of the bridge circuit may be constituted by the source and the auxiliary impedance, or by the load and the part of the line following the tapping point is substantially independent of the magnitude of the current which flows to said load.

The diagonals of the bridge circuit may be constituted by the source and the auxiliary impedance, or by the load and the part of the line following the tapping point. It is preferably arranged that the impedance of the load, and of the part of the line following the tapping point respectively, are such that, in operation, substantially no power is dissipated in the auxiliary impedance.

The load may be associated with the bridge circuit by means of a transformer, and transformer may also be provided between the parts of the line preceding and following the tapping point and the bridge circuit; account must, of course, be taken of the impedance-transformations effected by these transformers in the design and arrangement of the bridge circuit. If desired, the characteristic impedances of the parts of the line preceding and following the tapping point may be different. The load may comprise a current-utilising device connected to the bridge circuit through a length of transmission line, and a number of further current-utilising devices may be coupled to this line at points along its length by means of bridge circuits so as to form further systems according to the invention.

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

Fig. 1 is an explanatory diagram illustrative of the principles of the present invention and

Figs. 2 and 3 are circuit diagrams of two embodiments of the present invention.

Referring to Fig. 1, four impedance elements A, B, P and Q and a tapped choke coil 1 are connected as shown. The choke coil comprises S turns of wire and it is assumed that its inductance is so high and that the coupling between its turns is so tight that it operates as a perfect auto-transformer.

The number of turns in the part of the auto-transformer 1 between points 2 and 3 is nS where n has a suitable value less than unity. The number of turns between points 3 and 4 is therefore (1 – n)S. If A, B, P and Q represent the magnitudes of the impedances denote by these references, and if n(1 – n)P=Q, then if P or Q represents a source of current, the magnitude of the current flowing to A or B is independent of the current which flows to B or A respectively. Similarly, if (1 – n)A=nB, and A or B is a source of current, the magnitudes of the currents which flow to P and Q are independent of each other.

When the relation (1 – n)A=nB is satisfied, the impedance facing P is equal to A/n (which equals B/(1 – n)) and the impedance facing Q is equal to (1 – n)A (which equals nB). If P is a generator delivering a power equal to Wp watts then nWp watts flow to A and (l – n)Wp watts flow to B. If Q is a generator delivering a power equal to Wq watts then (1 – n)Wq watts flow to A and nWq watts flow to B.

A similar set of relations between impedances an powers may be obtained of the relation n(1 – n)P=Q is satisfied.

Suppose P to be the impedance of a source of current such as an incoming feeder and Q to be the impedance of an auxiliary impedance, then if n(l – n)P=Q, it may be stated generally that the impedance A or B may be changed or removed without affecting the current fed from source P to the impedance B or A respectively; thus if B is the impedance of a succeeding feeder which passes to a further load, or further loads, not shown, and A is the impedance of a load such as a branch feeder tapped into the incoming feeder at the point under consideration, the impedance facing B and the power delivered to B will be unaffected by the value of A provided that n(1 – n)P=Q. Thus with all our impedances connected as shown, it may be arranged that P is correctly matched to the total impedance which it faces; disconnection of either A or B, although destroying the matching, does not, however, effect the power delivered to B or A respectively. If the impedance P is that of a length of feeder fed from a bridge circuit similar to that described above, it is possible to determine the value of P for all conditions of previous tapped loads and so to determine the value of Q which is required for the balanced condition.

Similarly the impedances A and B can represent the impedance of an incoming feeder and the auxiliary impedance respectively, P and Q then being the impedances of the outgoing circuits. Suitable earthing conditions and suitable impedance relationships can be obtained by the use of transformer; these transformers may be tuned if desired when the currents to be handled are at radio frequencies.

By suitable choice of the value of n, the fraction of the power tapped off at any point may be given any desired value, and different values of n may be used at different tapping points in the line. The tapped load associated with any tapping point may consist of a further length of feeder supplying a current-utilising devices coupled to it at points along its length by means of bridge circuits so as to form further systems according to the invention.

Figs. 2 and 3 of the drawings filed with the provisional specification show two simple circuits embodying the present invention. In both figures an incoming feeder 1 is fed with current from a generator (not shown) having an impedance of the same order of magnitude as the characteristic impedance of the feeder 1; the latter is coupled to two outgoing feeders 2 and 3 by means of a circuit including a suitable auxiliary impedance 4 represented diagrammatically as a resistance. In both cases the coupling circuit comprises an auto-transformer 5, and transformer 5 is provided in the input to feeder 3.

Referring now particularly to Fig. 2, the feeder 3 will be considered to be the connection to the tapped load and feeder 2 the continuation of the incoming feeder 1. The condensers 7, 8 shown in dotted lines may be inserted for tuning purposes. Any shunt loss in the auto-transformer 5 may be considered as part of the shunt impedance of the incoming feeder 1 and may be allowed for in fixing the magnitude of impedance 4. The capacity to earth of the primary winding of transformer 6 may be considered as a part of the tuning capacity 7 across auto-transformer 5. In what follows, feeders 2 and 3 will be regarded as corresponding to impedances B and A respectively of Fig. 1, feeder 1 and impedance 4 corresponding respectively to impedances P and Q.

Suppose that feeders 1, 2 and 3 all have the same characteristic impedance z. The tapping point on auto-transformer 5 to which impedance 4 is connected to determine by the fraction of the incoming power required by the tapped load (not shown) which fed from feeder 3. The fraction in question is designated n, and resistance 4 is connected to a tapping point in auto-transformer 5 such that a fraction n of the total number of turns is included between points 10 and 11. Now, in order that the relation (1 – n)A=nB shall be satisfied, the effective value of A in these circumstances must be EQUAT. HERE. The transformer 6 is accordingly given a turns ratio of EQUAT. HERE and serves to transform the impedance z of feeder 3 by the factor EQUAT. HERE.

In these circumstances, the impedance across the whole auto-transformer 5 is equal to EQUAT. HERE; feeder 1 is accordingly connected to a tapping 9 in winding 5 such S is the total number of turns, and in that feeder 1 is across S EQUAT. HERE turns where order that the relationship n(1 – n)P=Q shall be satisfied, impedance 4 is given the value nz. The arrangement is then such that feeder 1 is terminated by an impedance equal to its characteristic impedance and the impedance facing the primary winding of transformer 6 is equal to EQUAT. HERE which is equal to the impedance looking into the primary winding thereof towards the load. In other words, all three feeders 1, 2 and 3 are terminated by their matched impedances.

If the characteristic impedances of the feeders are not all the same, matched conditions can nevertheless be attained, as will be apparent to those versed in the art, by suitable choice of the position of tapping point 9, of the turns ratio of transformer 6, and of the magnitude of impedance 4.

In the arrangement of Fig. 3 it will be seen that feeders 2 and 3 are both coupled by transformers to feeder 1, the transformers being given such turns ratios that the desired balanced condition obtains.

It will be noted that in the arrangements of both Figs. 2 and 3, if the impedance of the tapped load seen looking into feeder 3 and the impedance seen looking into the outgoing feeder 2 are in the ratio EQUAT. HERE to one another, there will be no transmission from the incoming feeder 1 to the impedance 4, and hence no loss of power apart from that due to losses in transformers etc. When either feeder is fed from a transformer, the impedance in question is, of course, that seen looking into the primary winding of the transformer. Thus in an ideal case with a generator feeding equal available power to m loads, the value of n for each successive tapping point, the tapped loads being arranged as in Fig. 2 for example, would be EQUAT. HERE and so on, the last but one tapping point having a value of n equal to ½ and the last load being connected directly to the end of the feeder. In practice an allowance must be made for the inevitable losses introduced at each tapping point by the transformers.

Other embodiments of the invention, within the scope of the appended claims, will occur to those versed in the art.

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. An electric transmission system comprising a transmission line coupled at one of its ends to a source of alternating current, and a load coupled to the line at a tapping point therein by means of a bridge circuit, the bridge circuit having two coupled inductive arms and having its remaining arms and its diagonals constituted by said load, an auxiliary impedance, and the parts of the line preceding and following the tapping point, the relative magnitudes of the impedances of the four arms of the bridge circuit, or of said inductive arms and said diagonals, being such that in operation, the current which flows from the source to the part of the line following the tapping point is substantially independent of the magnitude of the current which flows to said load.
  2. A transmission system according to claim 1, wherein said source and said auxiliary impedance form the opposite diagonals of said bridge circuit.
  3. A transmission system according to claim 1, wherein said load and the part of said line following the tapping point from the opposite diagonals of said bridge circuit.
  4. A transmission system according to any of the preceding claims, wherein the effective impedances in said bridge circuit of the load, and of the part of the line following said tapping point respectively, are such that, in operation, substantially no power is dissipated in said auxiliary impedance.
  5. A transmission system according to any of the preceding claims, wherein said load, or the part of said line following said tapping point, or both, are connected into said bridge circuit through a transformer.
  6. A transmission system according to any of the preceding claims, wherein said coupled inductive arms constitute one of the windings of a transformer.
  7. A transmission system according to any of the preceding claims, wherein said load comprises a length of transmission line having a plurality of current-utilising devices coupled thereto at points along its length.
  8. Electric transmission systems substantially as herein described with reference to the drawing filed with the provisional specification.

Dated this 27th day of July, 1936.

REDDIE & GROSE,

Agents for the Applicant,

6, Bream’s Buildings, London, E.C.4.

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