US1687535A - Reduction of losses in insulators, pins, and cross arms - Google Patents

Description

Octal-6, 192s.

, 1,687,535 H. A. AFFEL I?! AL REDUCTIDNOF LOSSES IN INSULATORS. PINS, AND CROSS ARMS Filed July 1, 1925 2 Sheets-Sheet 7 2&4

FWMY INVENTORS ATTORNEY Oct. 16, 1928.

H. A. AFFEL ET AL REDUCTION OF LOSSES IN INSULAIORS, PINS, AND CROSS ARMS 1 i/ W rm ATTORNEY Pea-ma Oct. 16, 1928.

UNITED STATES v 1,687,535 PATENT OFFICE.

HERMAN A. AFFEL, OF MAPLEWOOD, AND EGTILL I. GREEN, OF EAST ORANGE, NEW JERSEY, ASSIGNORS 'IO AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A

CORPORATION OF NEW YORK.

REDUCTION OF LOSSES IN INSULATORS, PINS, AND CROSS Application filed July 1,

This invention relates to transmission circuits, and more particularly to means for and, methods of reducing the transmission loss in such circuits.

With the development of methods of transmitting telegraphic and telephonic signals by means, of carrier currents propagated along open wire lines new transmission problems have been introduced. Owing to the fact that the carrier currents employed are relatively high in frequency as compared with the voice currents or Morse currents utilized in'the ordinary methods of commuof the present invention. The invention may now be more, fully unnication, it has been found that the attenuation is very markedly increased, so much so, in fact, that repeaters for amplifying the transmitted currents must be separated by much shorter distances, thereby adding to the expense of the plant outside the terminal stations at which the carrier apparatus is applied. Furthermore, the attenuation is so great that it has been impractical to employ on telephone lines carrier frequencies much above 30,000 cycles persecond.

Ananalysis of the factors producing this attenuation shows that there are three rincipal factors entering into it,first the C. resistance of the line conductors themselves,

which increases with frequency because of' skin eifect; second, a leakage loss in the in sulators employed; and, third, an additional leakage loss'in'the cross-arms and pins carrying .the insulators. v

At present there areno practical methods of eliminating the losses due to the first of these factors, as this loss appears to be inherent in the conductor itself asnow constructed. The losses due to the second of these factors, sometimes referred to as the hysteresis loss of the material of the insulator, and the losses occurring in the crossarms and insulator pins may be reduced and practically eliminated by the methods and arrangements constituting the subjectmatter derstood from the following description when read inconnection with the accompany ingfldrawing in which Figure 1 illustrates the normal arrangement of the cross-armsand insulator pins of a transmission line; Fig. 2 is an equivalent electrical circuit for the arrangement of Fig. 1; Fig. 3 illustrates theimproved method of construction of the 1925. Serial No. 40,934.

small capacity to be used in connection with I the present invention.-

Referring to Fig. 1, which illustrates in simplified form a typical crossarm arrange ment such as is employed in connection with telephone lines, 1 and 2 designate a pair of conductors of a telephone line such as are commonly strung from pole to pole across the country in ordinary open-wire construction. The cross-arm 3 is usually a wooden bar having wooden pins 4 and 5 upon which are mounted insulators (Sand 7, usually of glass or other non-conductive material. The conductors 1 and 2 are secured to the insulators 6 and 7 by means of tie wires or conductors 8 and 9. e.

In order to understand how the losses arise from leakage through such a systemas that above described, it must be remembered that the wood comprising the cross-arm 3 and the pins 4 and- 5 is not a perfect non-conductor, but is in fact a relatively poor dielectric as compared with the glass of which the in sulators 6 and 7 are composed. The crossarm and the pins therefore'act as a'condenser with a shunt leakage path of high resistance. Furthermore, the metal of the con ductors 1 and 2 adjacent the v insulators and the metal of the tie wires 8 and 9 constitute a plate of a condenser of which the lass insulator itself is the dielectric and of'w ich the wooden pingis the other plate. I During I wet weather conditions the outerlplate of the condenser is, in-efli'ect, considerably en larged in area due to'the wetting of the outer surface of the glass of the insulator, so-that the'leakage effects produced by the insulator are great y augmented in wet weather.

The) action ofthe insulator and its associatedparts as a condenser involves three fio loci

factors,-.--first, the capacity C ,''betwe'en.its

plates (that is, the capacity between the line with frequency, and, being a surface leakage,

is, of course, worse in wet weather than in dry weather, but even at its worserepresents a ver small'element of the total transmission oss; third, the conductance G which represents the dielectric hysteresis in the material of the insulator itself. This conductance is a function of the capacity of the insulator and increases with the frequency, so that at high frequencies it becomes a very material factor.

These. elements entering into the action of the insulator are illustrated in diagrammatic form in Fig. 2. Over and above these factors there are two other factors, viz, the

equivalent capacity C of the cross-arm and pins, and the equivalent conductance G representing the sum of the true conductance between the inner surface of one insulator and that of the other, and an additional conductance representing the dielectric losses in the cross-arm and pins. These factors are also represented schematically in Fig. 2. In order to eliminate the equivalent con- 'ductance of the pins and the cross-arm, the

pins are either made of conductive metal as shown at 4' and 5' in Fig. 3, or, where wood is used, the wood is sheathed by a .metallic conductor such asv a metallic foil wrapped about the pin or by a thin metal jacket of copper or other suitable material. In addition, the metallic pins in the one'case or the metallic sheaths in the other case are directly connected by a metallic conductor 10 of Fig. 3, said conductor being of substantially zero resistance, so that practically a dead short-circuit connection exists between the inner surface of the insulator 6 and the inner surface of the insulator 7. The resultant equivalent electrical circuit is illusi 'trated diagrammatically in Fig. 4.

The present invention thus far in the description has been limited to a circuit of a single pair of wires. It is obviously'applicable to circuits consisting of several wires. For example the common phantom circuit employs '2 wires in each side of the circuit. For such a circuit the metallic pins in one case or the metallic sheaths in the.

other case are all joined by a metallic conductor so that a substantially short circuit connection exists between the inner surface of all insulators of the one side of the circuit a and the inner surface of all insulators of the other side.

In order to understand how this construction" results in reducing the leakage loss, in-

volving, as it does, an arrangement which at first thought would seem to provide a better. leakage path than the ori nal construction, a br1ef discussion of the t eory of transmission will now be considered. Referring to Fig. 5, any transmission system of the usual type herein discussed may be thought of as a line made up of a large number of sections, each section comprising series inductalnce L, due to the line wires themselves, series resistance R, which is also inherent in the line wires themselves, a shunt capacity C, and a shunt conductance G. The propagation constant of such a conductive system may be expressed by the well known formula in which% is the propagation constant per unit len and R, L, G and C are, respectively, t e resistance, inductance, conductance, and capacity per unit length. m is 211' times the frequency; 7' is the operator 1; a is the attenuation constant, and ,8 is a term representing a mere change in the phase of the current transmitted. Now the value of' a in the above equation is given by the expression.

When L w is large compared to R and C w is large compared with G which is the case for the frequencies employed for carrier transmission, this expression reduces to:

asR 2 o T+G 2,/L 0 It therefore follows that any reduction of either the resistance R or the conductance G.

will result in a decrease in the attenuation of the circuit.

, As has already been stated, the resistance B being an inherent characteristic of the line conductors themselves, cannot be eliminated by any practical physical means. It is possible, however, by utilizing the principle of the present invention, to materially I reduce the conductance G.

line conductors, and the capacities C and C,

which correspond to the capacities due to the action of an insulator as a condenser, as already described. The action of the airas a dielectric involves no leakage loss, or, at any rate, the. leakage loss is so small that it may be neglected. The capacities C, and

. C however, have associated with them diecircuit, the conductance Gof Fig. is proportionately reduced, and, as will be apparent from the equations above given, this reduction in the value of Gr results in a decrease of the energy loss.

A demonstration of this fact may be given as follows: Assume for the moment that the conductances associated with the insulators are zero, a condition which, as already noted, can be approximated in practice. For this condition it is clear that the current flowing through the conductance G will produce an energy loss which of course must be subtracted from the energy transmitted along the circuit. If this conductance is short-circuited, however, the current flowing between wires is a pure capacity current which produces no loss. In this connection it may be noted that the result which applicant attains by short-circuiting the .pins and cross-arm could theoretically be obtained by making their equivalent conductance zero, but it is impossible to realize this condition in practice.

An idea of the magnitude of the reduction in leakageloss thus effected may be obtained from consideration of the curves of Fig. 7. These curves represent the attenuation at different frequencies of a transmission /line, each of the curves representinga different condition of the circuit. The curve A, for example, represents the variation of attenuation with frequency where the transmission line involves series resistance R (the skin effect), series inductance L, and shunt capacity C, but no leakage conductance G. It will be observed that the attenuation increases as the frequency becomes higher, and this is due to the fact that the series resistance R is involved. If there were no skin effect, the attenuation would be uniform at all frequencies.

Curve B represents the variation in attenuation with frequency, as observed inan actual transmission-line under dry weather conditions.v Here, of course, we have leakage conductance G 'duegto the hysteresis loss in the insulatorsand. due to the leakage through the cross-arms and pins. Curve C is a similar curve for the same circuit under wet weather conditions. It will be observed that the attenuation has now been enormously increased, due primarily to the wetting of the surface of the insulators, thereby increasing the capacity with a consequent increase of the dielectric losses represented by G and G of Fig. 6.

Unfortunately, the plant must be engineered for the wet weather condition when the attenuation is enormously increased. Not only must the circuit be so arranged that the transmission will be commercial under this condition, but special arrangements must be provided for maintaining the transmission constant under all weather conditions. It becomes obvious, therefore, that if the enormous loss represented by the curve C can be so substantially reduced that we have a condition more or less approaching the curve A, an immense saving in the lant will be effected not only by reason of t e reduction in the number of repeaters necessary but also by reason of the fact that the transmission regulators for maintaining the transmission constant under different weather conditions will not be necessary.

The curve D represents a variation of the attenuation in frequency under wet weather conditions where the pins and crosssarms have been short-circuited in accordance with the present invention. 'It will be observed that the transmission loss due to the conductance G of Fig. 5 has been reduced to almost half its original value.

Suitable methods of insulator construction will now be described whereby the transmission loss represented by the curve D may be materially reduced even during wet weather conditions.

The loss represented by the difference between curves A and D is in a large measure due to what is known as the hysteresis loss of the dielectric constituting the insulator. This dielectric hysteresis is somewhat analogous to the hysteresis loss encountered in magnetizing iron. While the phenomenon isnot wholly understood, it apparently produces an effect equivalent to the loss introduced by a conductance in the insulator path. This conductance increases with frequency and is proportional to the capacity of the insulator acting as a condenser.

It follows, therefore, that if the insulator is so designed mechanically that its capacity is reduced, there will be a corresponding reduction in the hysteresis loss due to the dielectric of the insulator.

As has already been stated, the capacityjof the insulator is due to the surface of the insulator pin actingas one plate of a condenser, the tie wire and. line conductor acting as the other plate. The external conducting surface comprising the line conductor and tie wire is, in effect, greatly increased in area during wet weather due to the moistureon the external surface of the insulator.

Referring to Fig. 8, for example, the effect is that of a condenser comprising a dielectric in the form of an insulator, as shown, with an internal, cylindrical, conductive area corresponding to its inner screw-threaded surface at 25, and an external, cylindrical, conductive surface extending from the top of the insulator to the lower margin of the petticoat at 26. Owing to the fact, however, that air, whose dielectric constant is much smaller than that of insulating materials, occupies part of the space between the lower part of the pin and the outer surface of the insulator, the effective capacity may be considered as between a conducting surface in contact with the pin cavity of the glass, and that part of the exterior surface which is immediately opposite. Neglecting the top of the insulator, this may be taken as representing approximately a condenser comprising two concentric cylindrical conductors separated by a dielectric. For such a condenser, the diameter of the outer cylinder'being R and the diameter of the inner cylinder being R the capacity will be I 10 I (see page 236, Circular No. 740i the Bureau of Standards). In the foregoing ex pression K is the dielectric constant and L is the length of the cylinder. From this equation it is clear that the capacity will be reduced if we increase the ratio of R, to R Consequently, in an insulator of the type shown in Fig-8,. the capacity will be reduced and consequently the dielectric loss will be reduced if we increase the ratio of the outer diameter to the inner diameter.

It will be at once apparent, however, that this ratio cannot be increased indefinitely, for obvious mechanical reasons. Furthermore, the decrease in capacity is not directly proportional to the increase in this ratio but is proportional to the increase in the longarithm of the ratio. Let us assume, as is indeed the case in practice, that the ordinary telephone insulator is so designed that the ratio of R, to R is about 2, (the external diameter of the insulator at the tie-wire. groove being approximately two inches and the diameter of'the one inch). Then Y logy i =0 .3010

pin cavity being about In order to obtaina 2 to 1. reduction of the capacity the diameters must be so chosen that log gi =.6021

or, in other words, the ratio of the outer to the inner diameter must be 4. This change in the ratio may be readily accomplished Without changing the external shape of the insulator by using a'metallic insulator pin so that the diameter of the inner screwthreaded opening will only be about onehalf inch, as shown in Fig. 9. At the same time, in order to reduce the capacity between the top of the pin and the top of the insulator, the top of the insulator has been raised. As already stated, it is desirable, in any event, to use a metal supporting pin in order to readily short-circuit the conductance of the cross-arm. Such a design will reduce by about one half' the loss due to dielectric hysteresis or,in other words, the loss represented by the difference between the curve A and the'curve D of Fig. 7. The same result may also be obtained by a de- Sign such as that shown in Fig. 10, in which the inner diameter is the same as that of Fig.

8 but the outer diameter is twice as great as that of Fig. 8 and the top of the insulator has again been raised; in other words, we may reduce the capacity, and hence the hysteresis loss, one-half by doubling the outer diameter or by making the inner diameter one-half as great. The capacity between the outer surface'and the pin cavity for such an insulator would be about 17 mmf. as compared with 35 mmf. for the present insulator.

It is not practical, however, to proceed much further along.v these lines of design by reason of the fact that the reduction in capacity is proportional to the logarithm of the ratio of the outer to the inner diameter.- For example, if it were desired to effeet a 5 to 1 reduction it would be necessary that log R2 Hence, the ratio of the outer to the inner diameter-must be 32; in other words, al-

though it happens that for the case in which ,we double the ratio the loss can be reduced by oneehalf it is necessary to raise the ratio to the fifth power in order to reduce the loss one-fifth,

Some additional reduction in capacity may be obtained, however, if the he1ght of the inner opening for the supporting pin be reduced. It is, of course, apparent that the capacity of the condenser depends upon the area. of the plates. If we decrease the height of the opening for the supporting pin we correspondingly reduce the area of the effective inner plate of the condenser. For example, in Fig. 11 an insulator is shown in whichthe ratio of the outer to the inner diameter is 4, this effectbeing obtained by reducing the inner opening to one-half the usual diameter and at the same t1me the height of the opening is made only onehalf that of Fig. 1, so that the area of the inner plate surface is reduced one-hal This, of course, produces a further material decrease in the capacity and hence a decrease in the dielectric loss.

Referring to the curves of Fig. 7, if the loss be reduced from that shown by the curve C to that shown by the curve D by shortcircuiting the conductance of the cross-arm. and pins, as already described, it will be apparent that the loss under wet weather conditions, represented by the curve D, it reduced onehalf, Will not be materially greater than the loss under dry Weather conditions, indicated by the curve B, and such. a reduction in loss may be ellected by the novel insulator designs just described. In any event the loss due to hysteresis in wet weather, may by this construction, be reduced to a Value not materially greater than one fourth of the losses due to the line wires at 80,000 cycles. Not only will there be a very considerable reduction in the maximum loss, but the variation in loss with changes in the condition of the weather Will become much less, so that arrangements for regulating the transmission may be dis pensed with and a lesser number of repeaters ma be used.

t will be obvious that the general principles herein disclosed may be embodied in many other organizations widely dififerent from those illustrated without departing from the spirit of the invention as defined in the following claims.

What is claimed is:

1. A pole line construction comprising conductors strung along a pole line in pairs, the conductors being mounted upon insulators supported by pins carried by cross arms upon the poles, one conductor of each pair acting as a return for the other, and means to'reduce the transmission loss over the conductors, said means comprising in-' sulators the ratio of whose external diameter to pin diameter is not much less than 4, in combination with a non-magnetic conductor for substantially short-circuiting'the conductance of the cross-arms and supporting pins.

2. A pole line construction comprising conductors strung along a pole line in pairs, the conductors being mounted upon insulators supported by pins carried by crossarms on the poles, one conductor'of each pair acting as a return for the other, and means to reduce the transmission loss over the conductors, said means comprising insulators the ratio of Whose external diameter to pin diameter is approximately 4:, in combination with a non-magnetic conductor for substantially short-circuiting the conductance 01 the cross-arms and supporting plIlS.

3. A pole line construction comprising conductors strung along a pole line in pairs, the conductors being mounted upon insulators supported by pins carried by crossarms upon the poles, one conductor of each pair acting as a return for the other, and means to reduce the transmission loss over the conductors, said means comprising insulators the ratio of whose external diameter to pin diameter is. approximately four, and the thickness of whose dielectric above the top of the pin is substantially the same as the thickness of the dielectric around the pin, in combination with a non-magnetic conductor for substantially short-circuiting the conductance of the cross-arms and supporting pins.

In testimony whereof, we have signed our names to this specification this 29th day of June, 1925..

HERMAN A. AFFEL. ESTILL L GREEN,

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