US1583827A - Loading transmission lines - Google Patents

Description

,May 11 1926. "1 ,583,827

E. GREEN E LOADING TRANSMISSION LINES Filed p 18, 1925 i 2 Sheets-Sheet 1' Spacing betweew wires INVENTOR .ZGKeeI I/ W ATTORNEY May 11 1926.

' E. I. GREEN 1 LOADING TRANSMISSION LINES Filed Sept. 18, 1925 2 Sheets-Sheet 2 INVENTOR E Z 6156,61 0 BY fV-fig ATTORNEY Patented May 11, 1926.

UN'ITEl) sT ESTILL I. GREEN, OF EAST ORAN QPHONE AND TELEGRAPH TES PATENT OFFICE.

an, new .n-rasnv, ASSIGNOR 'ro AMERICAN ram COMPANY, A coaroaArron or Newman. I I

LOADING TRANSMISSION LINES.

Application filed September 18, 1925. Serial No. 57,193.

This invention relates to the reduction of attenuation in transmission lines and cables, and more particularly to the reduction of attenuationi by loading circuits with condensers or with networks including condensers and inductances.

' It has long been known that, in the lower ranges of frequencies such, for example, as the ordinary telephone range, it is possible to reduce the attenuation: of a transmission line by increasing the inductance, the increase .in inductance being obtained either by inductance coils spaced at suitable intervals or by inductances circuitfalong its length.

proposed to reduce the leakage conductance,

uniformly .added to the It has also been attenuation by increasing although the latter method has never been practical.-

In these prior methods of attacking the problem of .attenua ity of the line has detriment, and the that in some other tion reduction, the capacalways been considered a fact was not recognized range of transmission the conditions might be such that an increase in inductance might be harmful, whereas an increase in the capacity of the line might be beneficial. The present invention involves the discovery that tain frequency,

in the range above a cerwhich frequency depends upon the character of the particular line concerned, the attenuation may be reduced by .increasing the capacity of the line. The invention further involves the discovery that for a given frequency the attenuation may be made a minimum by proper spacing between the line conductors.

The principles of ried out by the use of capacity loading, by

the line attenuation is made a single frequency, whilethe means of which a minimum for the invention are carattenuation for a range of frequencies above a certain critical frequency is reduced.

invention it is proposed to loading involving anti-resodesigned. as to make the attenuation a minimum for a range of one form of the use a system of nant circuits so quencies.

The invention may now be derstood from the following detailed fremore fully undescription when read in connection with the accompanying drawing-Figurea curve showing the relation between the spacing of the wires and the attenuation, Fig. 2 is a simplified diagram showing how capacity loading may be applied, Fig. 3 is 1 or which a a similar diagram showing how the anti-resonant loading method may be employed, and Fig. 4 is a curve showing how the anti-resonant loading improves the transmission over a range of-frequencies.

The usefulness of capacity loading will be apparent from a consideration of the approximate formula for the attenuation constant, which is quite accurate for the higher C elfective capacity in forads per. unit length. 1 1

G=effective leakage conductance in mhos per unit length and the sign denotes approximate equality. It is' clear that in this equation we are not concerned with the absolute values of'L and C but only with their ratio, which may be called K. The equation then becomes Differentiating this expression with re spect to K 'will give us the value of K, which makes a a minimum. This value of K is equal to g and the corresponding-minimum value of a is d j JR' G'. It is evident that for any value of K other than the value 'df a will be greater than that given by equation (3 Li The conditionfor minimum attenuation,

' then, may be expressed by the equation L R c e K Theoretically, in the ease of an open wire line as ordinarily constructed, it would be beneficial to. increasethevalue of K by increasing the inductance for transmission of all frequecies up'toabout 30,000 cycles, and in the case of an ordinary cable the limiting I frequency would be even higher. Practi- Since the resistance R of the line is fixed by 1 ..p ty-x 4o 1 p rtlc.

cally, however, in the range above 5,000

cycles, in the case of an-ordinary open' wire line,- the improvement resulting by .increa'sing the inductance is relatively small, and

as'theincrease in inductance results in increasing the resistance, it has been found I impractical to try to improve the range above 5,000 cycle's by increasing the inductance. In the rangereferred to, that 1s, the

. range below about 30,000 cycles in the 'case of an ordinary open wire' line, for example, the values of R and G are such that it is desirable to increase the value of K or, inother words, to increase'the inductance of the line, asa'means of approaching the condition of minimum attenuation. As the frequency is increased, however, the value of] the ratio undergoesaveiy marked change due to thefact that G increases in more or less direct proportion to" the frequency, whereas vR increases at some fractionalpower of the frequency. Consequently, at

' these higher frequencies the optimum value of'K maybe obtained'by increasing the ca- "The ysical'significanceof the foregoing,

For convenience a logtion of frequency.

been used for the spacing.

arithmic scale has V the natureof the conductors employed and i the leakage G-is mostly due to the nature of the insulators employed, the values of the inductance and capacity of thecircuit may be ,that gives the desired relation varied by varying the spacing of the conduc I tors and in this manner, for a given circuit,

aspacing' between the wires may be found which Wlll give the relation of formula (4) for any particular fre ue'ncy. The curve of Fig.1 is plotted by cglculating the spacing L -R are .'using the standardformulas for determining the inductance and capacity. For ex arly asapplied toopen-wire lines, may be better un erstood by reference to: the curve of Fig. 1 of the drawing. Here, f [the spacing between wires that will give,

attenuation is plotted asa funcsible to set ample, the inductance in .mile may be determined from the following formula:

L= 0.64374 [2.3026 10 1, .+pa] 5 where a? is the diameter of each wire, D is henrys loop the distance between wires, 8 is the internal inductance factor, and is the permeability. Now,- the permeability in the caseof copper is 1 and 8 varies from .25 for direct current to 0 where the frequency isinfinite. Consequently, the term may be neglected.

Similarly, the capacity 1n farads p'er loop mile may be expressed quency the distance betweenthe wires which will produce the same at once obtained.

,u. 8 in the above equation e5 ratio of L to C may be The curve of Fig. l may be plotted from I values of D calculated in accordance with formula (7 In the case of anordinary open .wire line the spacing is fixed for all frequencies and hence in Fig. 1 the horizontal line A-andB represents the spacing 'em- -pl0yed in an ordinary open wire line. It .will be seen that this line crosses the curve representing the ideal spacing forall frequencies at the point Tcorresponding to the the spacing ordinarily'employed would correspond to the condition of minimum attennation at the frequency f,. then, from the curve, that at frequencies beow i, an increase in spacing, giving larger inductance and smaller capacit is desirable, while at frequencies above f a decrease in spacing is desirable. While it is possible to reduce the'attenuation to a minimum for any particular frequency-by aproper spacmg of the wires, in general it is im ractical to use thewire spacing that electrical conslderations make ance between insulators and the possibility of crossing, while the maximum is limited by reasons of economy, crosstalk considera- Further, it is manifestly impos-- tions, etc.

a spacing that will be correct for requency f,. The significance of this is that It will be clear,-

desirable, since the minimum lslimited by the necessity for clear-.

all frequencies. By applying the methods of 10a ng now about to be described, however, an electrical approximation of the corthe frequency f of Fig. 1 or,

for which the spacing is.

rect spacing can be obtained at a particular frequency or over a range of frequencies.

. As has already been pointed out, in any circuit it is desirable to increase the inductance in order to reduce the attenuation in the range below a certain frequency, while for the range above the said frequency the desired reduction in attenuation may be obtained by increasing the capacity. The frequency which separates these two ran es, hereinafter referred to as the critical, requency, is the frequency corresponding to in other words, is the frequency ideal for the given circuit in accordance with equation (7) Theincrease in capacity which is desirable at frequencies above the critical frequency may be very readily accomplished by connecting condensers across the wires at intervals, as illustrated in Fig. 2. Here, the added capacity is lumped at intervals along the line. Such an arrangement has the advantage over the method which involves inductance loading that the increase in the capacity does not result in any material increase in the resistance of the circuit, so that the full amount of the theoretically possible reduction in attenuation may be closely approximated in practice.

For'example, suppose that it is desired-to load a given open wire line for a frequency f, and thatfthe optimum of inductance to capacity for this frequency is obtained by increasing the normal capacity C of the circuit to a value C, per unit len h. The condition for minimum atten- T uation of this circuit may be expressed a R 1 G- i value for the ratio Then the desired value for the capacity may be obtained from the equation As G, L and R are known, the ideal value for the capacity C, can readily be calculated. The capacity to be added per unit length is evidently equal to C -C. It remains to determine the distribution of the added capacity along the line. Let n equalthe number of load points per' unit length. Then the normal line capacity and line inductance per loading section are R and If we load the circuit by lumped shunt capacity the line becomes analogous to a 103W Pass filter and will 'have an upper cut-off point. Let f, be the cut-oil frequency. This cut-off frequency may then be'expressed approximately as follows:

. W 4.9 r LCI Substituting the value of equation- (11) in equation (10) we may express the'minimum 11 her of loadingpoints as follows:

'n-- 1.25f1r /LC load-points per unit length (12) The loading ,condensers to be used would generally be of approximately the same capacity, compensate forvline irregularities or differences in the length of-loading sections by making suitable allowances in the values of the condensers applied; atthe, different points, as is the-practice in ing' coils. By increasing each pole, practically uniformly distributed This might beacincreasing the capacity of the insulators. Such loading, being virtually continuous, would have an extremely complished by using a separate condenser at each pole or'by high cut-ofi-frequency.

5 The effect of loading with shunt condensers would be to lower the velocity (if-propagation for the circuit, just as is the case for 100 series inductance loading. This type of loading would lower although it might be desirable to the use of load-' the capacity at the characteristic impedance, however, whereas the series inductance loading raises the characteristic impedance. I v For the 1 65-mil open wire lines commonly used, this method of'capacity be useful for frequencies above about 30,000 cycles, as already explained. The advantage to be obtainedfby adding capacity to the circuit obviously increases with the frequency. Inloading a particular circuit for a particular frequency, tion is reduced quency, fected for all other frequencies between the critical frequency of the circuit and the cutoff frequency of the a filter. In applying this method of loading to an open wire line on whichhigh frequency channels are superposed on the ordinary voice channel, the improvement at high frequencies is, of course, obtained at the, expense 'while the attenua- I to a minimum at that fre some reductionv in attenuation is es;

' loaded circuit acting as loading would 4 of a detriment to the lower frequency transmission channels falling below the critical frequency of the circuit. The method -might therefore be most advantageously l5v used'in lines carrying high frequenciesonly,

such as long radio antennas etc. It should,

be pointed out, however, that the increase in attenuation at .the lower frequencies is in no case suflicient to make the total attenua 'tion at these frequencies as large as the minimum which has been obtained at the higher frequency.

For power lines, which have smaller wire resistance and a large value of%- dueto wide spacing, and for telephone lines specially constructed along power line principles, the method would be useful at much lower frequencies than inthe case of an open wire circuit. For cables the usefulness of the method would be restricted to extremely high frequencies because of the small value 26-.of In the case. of ordinary power line construction used for carrier frequencies, the limitation imposed by the increase in attenuation at frequencies below the critical ,frequency does notjapply, as .line attenua- 'tion' does notfigure appreciably in'60 cycle power transmission and the carrier fre-' quency channels would all be above the critical frequency of the circuit. In fact, the capacity added-to obtain the desired loading effect would be beneficial in compensating for poor power'factor. This particular application of theinvention has the disadvantage, however, that the cost of condensers capable of withstanding high voltages is very great, This disadvantage might be overcome, however, by so designing the power insulators asto increase the desired amount.

It-has already been pointed out that the.

- condenser method of loading gives the optimum value of for but a single frequency. This difiiculty may be to some extent overi come by employing a loading system such as illustrated in Fig. 3, in which anti-resonant units comprising an inductance in arallel with a capacity conductors."

abovelthe resonant point an anti-resonant circuit resembles a capacity which increases with increasing frequency; approaching a constant value; at the resonant point it pre sents an infinite reactance (or zero capacity), while belowthe' resonant point it resembles an inductance which decreases with decreasing frequency likewise approaching a constant value. By using anti-resonant cir cuits shunted across the line conductors to the capacity are shunted across t e'lin'e It,is-a well known fact that at frequencies close approach to the optimum value of can be' obtained for a considerable. range of.

frequencies. This is illustrated by the curves shown in Fig. 4, in which curve 1 is a plot of the hawforthe actual line. Curve 2 is aplot of the ratio for the line before it is loaded, and curve 3 is a plot of the 6 the anti-resonant l'oading'applied. It will be observed by comparing curves -1' and 2 that these curves coincide for a critical freequivalent ratio quency the line as it is normally constructed for thesame line with v quency f and that at thatparticular frehas'a minimum attenuation, for at that par-' ticular frequency the relation stated the ratio of the equivalent inductance of the circuit to the equivalent capacity thereof, which is represented by the curve 3, practically coincides withthe ideal curve 1 for. a considerable range above the critical frequency f By properly choosing the values of the inductance and capacityv of the anti-resonant circuits, this range may be made of considerable width. The anti resonant circuit loading which produces curve Sevidently' may have advantages over loading with shunt condensers or series inductances, whlch would merely move curve 2 a ing its slope.

certain. distance down or. up without changtype, however, account should be taken-of the fact thattheline thus loaded s in the nature of a band filter having an upper and lower cut-off, and the loadingshould be so designed that the upper cut-off point falls well above the upper limit of the range of frequencies to be transmitted, while the holds. After loading'the line in the manner- In the design of a loading system-of this I lower cut-off point falls well below the lower limit of said range. It is clear that some decrease in attenuation over the non-loaded condition would be obtained by this method of loading at all cal frequency frequencies above the critiand below. the upper cut-off v frequency of the circuitwacting as a filter.

This is for the reason that capacity would be added at all frequencies in this range.

It will be obvious that the general principles herein disclosed may be embodied in many other organizations widely different.

from those illustrated Without departing from the -spirit ofthe inventionas defined in the appended claims.

. ductors and loading units:

a circuit for said conductors, ing a capacity, proportioned as to bring the effective capacity of the circuit for a given-desired freq quency to a value such that the ratio of inductance to capacity will be substantially equal to the ratio of resistance to leakage ,ratio of resistance conductance.

3. In a circuit whose ratio. of inductance to capacity is substantially equal to its ratio of resistance to leakage conductance at a the method of reducing the attenuation of the circuit at frequencies above said critical frequency which consists in increasing the capacity of the circuit.

4. In a circuit having a ratio. of inductance to capacity substantially equal to the to leakage conductance at a. critical frequency, the method of reducing the attenuation at said critical frequency which consists vinadding capacity to the circuit until the condition that the ratio of inductance to capacity equals the ratioof resistance to leakage conductance holds true at a higher frequency -than the normal critical frequency.

' at the frequency at given frequency related as tomake .the ratio of the loading units being so ratio of resistance to tion of a circuit a mi -frequenc es frequencies above 7 5. A loading system comprising a pair of conductors and-loading units bridged across said conductors at intervals, said loading units including inductance andcapacity so related to each other as to be anti-resonant which the ratio of inductance. to capacity of the conductors themselves is equal to the ratio of the re-.

sistance of the conductors to the leakage conductance thereof. 4 I

6. A loading system comprising line conductors having such electrical characteris-' ties that the ratio of inductance to capacity is substantially equal to the ratio of resistance to leakage conductance at a critical frequency and loading units bridged across "said conductors at intervals, each loading unit comprising inductance and capacity so ductance to effective capacity 'equal to the ratio of resistance to leakage conductance over a range'of frequencies.

7. Aloading system comprising a pair of conductors having electrical characteristics such that for a critical frequency the ratio of inductance to capacity is equal to the I leakage conductance and loading units bridged across said cirprising inductance and capacity of ues as to be anti-resonant at the critical frequency of-the line and so designed as to' make the ratio of effective inductance to effective capacity equal to the ratio of resistance to leakage conductance over a range of frequencies."

8. The method of in king the attenuaumfor a range 0 by making the ratio of effective inductance to efiecti've capacity equal to the ratio of resistance to leakage conductance for thesaid range of frequencies.

In testimony whereof, I have signed my name to this specification this 17th day of September, 1925.

16 cuit at intervals, said loading units comsuch val-

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