US2371252A - Capacity loading system - Google Patents

Description

Marsh. E3, H945, J, W, MILNQR ZELZZ CAPACITY LOADING sYsTEM Filed Dee. 1941 Oa acgi LlTcAzade o en w e, LL. :,140 05h16: jo 3 /g fr, grwQ/rvbo/b CY v Patented Mai. 13, 1945 CAPACITY LOADING SYSTEM Joseph W. Miinor, Maplewood, N. J., assignor to `The Western Union Telegraph Company, New t York, N. Y.. a corporation o! New York Application December 23, 194.1, Serial No. 42.4,208

s claims. (ci. ris- 44) This invention relates to electrical transmission systems comprising sections of cablel interspersed with open wire line sections. The principal object of my invention is the'reduction of attenuation in the transmissionat high frequencies over such mixed cable and open wire circuits by a method of loading the open wire sections.

spaced condensers effect a reduction of theimpedance of the open wire until it matches that of the cable, thereby eliminating the reflection losses which occur at the junctions of the open wire and cable.

Transmission lines for high frequencies usually consist predominantly of open wire having an impedance Ofapproximately 600 ohms with a small negative angle and characterized by relatively low attenuation. The open wire is normally interspersed with occasional short sections of cable necessitated by river crossings. city entrance purposes, and the like. These cable sections are detrimental to transmission in that they notonly have higher attenuation .ut since their 'impedance is of the order of 140 ohms, serious reflections occur at the junctions oi the two types of'conductor. To reduce this attenuation it is often necessary to load the cable sections, at the same time makingthe impedance the same as that of the open wire line in order to prevent reflections at the junctions. However, the spacing of the loading. coils grows smaller as the frequency increases so that for frequencies of the order of 30 kc., it may become as low as 900 feet. To provide loading coils at such frequent intervals becomes very expensive, particularly in the case of underground cables, where manholes must be provided. In many cases it is physically impossible to locate man holes at the proper locations.

Since a loaded cable terminating at mid secr tion presents a, resistive impedance rising with.

frequency, a further requirement of such loading systems is theA necessity of modifying this impedance by means of `special fractional sizes of coils for insertion near circuit terminals and near the junctions-of the cable and open wire. By this refinement a constant impedance is obtained for the cable closely -matching that the open wire underthe conditions of irregular cable lengths found in practice. One system for terminating such loaded cables is disclosed in Patent No. 2,096,401. l

As the frequencies employed' become higher, the cost of loading increases rapidly so that often it is more economical to omit the loading andto space the repeaters more closely instead. However, the impedance inequality at junctions of cable and open wire cause a severe reiiecticn loss which may be aggravated by multiple reflections. This loss, because it is often irregular with frequency, cannot be satisfactorily recovered by means of repeaters, and furthermore, problems of equalization and of selective crosstalk are introduced. Efforts have beenmade to reduce this reflection loss by employing transformers or other impedance transforming devices for joining the two types of conductors.

are limited in ,their frequency range` such that quenciea an excessive loss may be introducedA reduction in attenuation both to carrier andy if a, satisfactory match is provided at high freat voice frequencies or vice versa. In addition, the devices usually necessitate a direct or a condenser connection between the two conductors of the pair. This connection interferes with ytelegraph or other direct current utilization of` the conductors.

The system of loading disclosed herein differs from the one justdescribedin that it is designed for particular application to situations Where relatively short lengths of open wire are located between sections of cable. It is most effective when the cable sections -are very long but it is useful also where short cable sections are separated by short sections of open wire. By the method disclosed herein, the impedanceof the open wire is reduced by means of capacity loading of the open wire line so that it approximately matchesthat of the associated cable thus prevents the occurrences of reflections.' "TQS loading is accomplished by means of small co densers connected between the two wires of the pair at intervals analogous to those employed for loading coils in cable. For use near each end, a se'ries of fractional sizes of condensers are desirable. In certain cases a compromise -ar rangement may be used, that is, the open wire will be capacity loaded only sufiiciently to approach the impedance of the cable since the reection loss is not large for relatively small impedance discrepancies. This will permit a :eduction in the size of condensers with consequent for fa composite circuit of open wire and nonloaded cable sections showing the resistance and reactance components; the change in said components after loading and the effect of terminat- These devices, however,

ing a loaded line at mid-section, in this case midcondenserj Figure 2 is a diagram illustrating a non-loaded cable to which is connected a length of capacity loaded open wire terminating at mid-condenser and provide with a special terminating section;

Figure 3 is a diagram showing essential details in more specific form; and.

Figure 4 illustrates diagrammatically the normal attenuations per mile in No. 9`Cu open wire and for No. 13 gage cable and to illustrate the, benefits of the loading system according to the present invention the two top curves show the attenuation loss caused by the insertion of a length of open wire in a long section of nonloaded cable both before and after capacity loading of the open wire section, while the irregular curve shows in particular the serious loss caused by multiple reflections.

The manner of designing a capacity loading system for open wire so that its impedance will be the same as that of an associated cable section will now be illustrated. For this example No. 9 Cu open wire will be loaded to matchl the impedance of No. 13 gauge cable although the same development will apply to other sizes and types of conductors. The characteristics of these conductors per mile are:

No. 13 ga. cable C=.062 mf., L=.0012 h. No. 9 Cu open wire (12' spacing) C=.0084 mf., L=.0036 h.

For convenience the capacity of the open wire will, be neglected in the following calculation of condenser s i ze s.

-Since \/L/C' for the open wire must be made approximately equal to -\/L/C for the cable:

whence Cw=.186 mf./mi.; that is, .186 mf. should e, be added to each mile of open wire to make its loaded open wire equal to l/f. VE, examples for 7,000, 30,000 and 50,000 cycles have been computed, as follows: v

-Nominnl loading section X/d F. i mi equency en Wire im mi. Lelilith, Concllsers, ductm,

a. 52 1.380 .2st I .00497 1. 29 322 000 00116 78 195 036 00072 That is, as an example, to load a length of open wire situated between lengths of No. 13 ga. cable so as to have the same impedance as the cable and to transmit frequencies up to 30,000 cycles, condensers of .060 mf. should be connected between the two wires of the pair at intervals -of .322 mile.

The nominal impedance of No. 9 Cu open wire and of No. 13 gauge non-loaded cable are indicated by the solid and the dashed curvesl respec- 2,096,401, previously referred to.

tively of Fig. 1 which gives both the resistance and reactance components. After loading, the relatively high resistance component of the open wire becomes nominally the same as that of the cable. However, when a loaded line is terminated at mid-section, or mid-shunt, that is in the case of the capacity loaded open wire a full section of line and a 1/2 size loadingl condenser, while the reactance component is small, the resistance increases with frequency, as indicated by the long dash curve. It becomes desirable, therefore, to provide a special terminating arrangement for both ends of the capacity loaded line section which will render this resistance' approximately constant with frequency and of the desired nominal value. An analogous system of terminating sections adapted for terminating a loaded cable to provide an impedance matching that of open wire is described in the Patent For the present capacity loading system, a terminating section can be conveniently derived from a fractional length of open wire in combination with one or two fractional sized condensers, to be located at one end or at both ends of thevfractional section.

Figure 2 illustrates a long non-loaded cable to which is connected a length of capacity loaded open wire which terminates at mld-condenser at I. Between points i and 4 lies a terminating section composed of the fractional line section a and the fractional sized condensers b and d. 'I'he manner of determining the size of the two condensers corresponding to the fractional line section va will now be derived. Several combinations will be developed, final choice of any partlcular condition depending upon the fractional length of line conveniently available at the end of the loaded section and also upon the sizes of available standardized condensers.

The impedance of a long loaded line terminatedat mid-shunt, and regarded as a series of 1r' sections having series impedances Zr. and shunt impedances 2Zc, may be stated: (see K. S. Johnson, Telephone Transmission Circuits, p 125) 4 l 4 ZC This is the impedance of the loadedopen wire line' measured at point I. The impedance of the condenser d is l/wdC, and the impedance measured f at the pointzisthen:

' i JU J I w Jud?! whichreducesto:

and substituting in The impedance of the condenser b is l/wbC, so that the impedance measured at point 4 is:

It'is desired that Z4 shall be equal as nearly as practicable to the nominalirnpedance of the capacity loaded line for all valuesof s, i. e.

z.=\/L/c The above expression then becomes:

Expanding the radical V 1+sf4:

\/1+s/-i=1+s2/8-s4/128-l-sl1024 the preceding expression we have: l

Equating the algebraic sum of the coefllcients of like powers of s to zero for the first two powers:

From s coeillclents a=bld From sz coefficients d-b=1/8a llironl these equations the values of b and Anl in tcrmsofaare foundtobe: o

8c2-1 d sail-1 16a 16a' l 00 If we assume a range of fractional loading sections of open wire a and solve for the corresponding values of b and d, we have:

sizes which may be used to load terminal vfractional lengths a of open wire ranging from .354

' to .8 oi' a section length. Below these values the element b becomes negative while above this range the values become inaccurate. The greatest accuracy occurs at the value a=.73.

Referring again to Fig. 1, the long dash curve represents the impedance of the open wire loaded for 50 kc. and terminated at mid-shunt, as obtained from the expression- V 4 After the addition o! a terminating section, the loaded open wire will have an7impedance las represented bythe dot-dash resistance and reactance curves. These curv'es, computed from the expression for Zi for purposes of illustration, cor' l respond to the case where a=.73.

It will be noted that the curve for the corrected impedance of the open wire has a satisfactory correspondence to the curve for the cable resistance throughout the working range. The reactance of the open wire is reduced to substantially zero throughout this range. The magnitude of the discrepancy between the reactances of the two conductors is small except at the lower end of the frequency range.

Fig. 3 shows the lengths of 4section and the condenser sizes which should be used to load the lopen wire for 60 kc., along with the terminating section corresponding to a' .73 which gives the dot-dash impedance curves of Fig. 1. In practice, some small modifications of the values b and d are necessary in order to allow for the effect of the vresistance and capacity of the open wire which were neglected in the original derivations for the terminating section.

In Fig. 4 there is illustrated the normal attenuations `per mile for No.v 9 Cu open wire and for No. 13 gauge cable and also for the loaded open wire. The attenuation for the loaded case is derived from the well known expression:

Attenuation in db.==8.68 (R/Z-I-LG/ZC) \/C'/L, using the following constants per mile:

1.-.0036 henry. C :.188 microfarad.

In addition, to illustrate the benefits of the loading system herein described, the two top curves show the attenuation loss caused by the insertion of a 5mi1e length of open wire in a long section of non-loaded cable, both before and after capacity loading of the open wire section. The irregular curve includes the normal attenuation for 5 miles of open wire and also the more serious loss caused by multiple reilections which Ain addition to itsl severity is irregular with frequen'cy. After loading, this irregularity disappears' and the attenuation. is vreduced 'I db. or more from themaximum before encountered. A similar improvement is realized with open line sections of-both shorter and longer lengths, but the increased attenuation of the loaded open wire introduces a limit to the length which may be practically loaded. However, the elimination of vthe reiiecti'on peaks with the consequent reduction `in crosstalk, equalization, echo, and .tele/- phone repeater balance problems, may make this method of stabilization of impedance desirable for lengths oi' included open wire much longer I ohm cable. If thelmpedance of the open wire' were, for example, made 200 ohms instead, the size of the condensers would be approximately halved, the added attenuation would be reduced by about one-third, and the resulting redection losses would not be unacceptable for many less critical situations.

As is'common in loading systems, the fractional section of open wire may be built out to a desired length by means of small inductance coils. Single condensers connected between the two wires of the pair have been shown. Such condensers if too large tend to produce `interference between grounded telegraph circuits Worked on the individual conductors. This interference can be prevented if two'condensers of double size with their junction grounded are employed instead. A portion of the desired capacity increase can be obtained if desired through closer spacing of the wires or through higher capacity insulators. k

The foregoing ldescription discloses a method of capacity loading of open wire line sections designed to impart to the open wire the same impedance characteristics as the associated long plication to relatively short lengths of open wirev `contained within long sections oi non-loaded 1 cable, other applications of this organization will I claim:

1. In a signaling transmission system comprising in sequence along its length, non-loaded cable sections and openvwire line pairs of conductors provided with unit condensers of auit- Y able capacity in shunt to the pairs of open wire conductors at regular predetermined unit intervals to thereby reduce the relative impedance characteristics 'of said open wire pairs to substantially equal the lower impedance characteristics of said cable sections connected thereto, one oi said open line sections terminating at a midunit interval and means supplementing said midunit condenser for maintaining the impedance of the open Wire section approximately constant under varying frequency conditions, said means comprising the interposition of a fractional length of a conductor pair shunted by suitable fractional sized condensers.

2. The combination of a signalling systemior a range of frequencies comprising a pair of nonloaded cable conductors joined to a pair of open wire conductors of higher impedance than the cable conductors, unit condensers in shunt to said pair of open wire conductors at regular predetermined intervals and of such value that the impedance of the pair of open wire conductors at the lower end o said range of frequencies y is thereby reduced to substantially equal that of the associated cable conductors, said open Wire conductors terminating at a full length interval,

and a supplemental terminating section for interposition between said two pairs of conductors, comprising a fractional section length of open wire pair 'shunted by a fractional sized condenser at one or both ends said fractional elements having such values that the impedance of the open wire rpairs is rendered substantially equal to that of the cable over the remaining upper end of the range of frequencies.

3. A capacity loading system for transmission lines comprising a series oi unit condensers spaced vat regular intervals in shunt to said circuit, the final condenser of said series being approximately one-half unit size to produce an impedance irregularity, and a terminating section for said system comprising a fractional interval of line in combination with one or more fractional sized condensers for correcting said impedance irregularity.

. JOSEPH W. MILNOR.

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