This is from
"Principles of Electricity applied to Telephone and Telegraph Work"
"A training course text for employees of the
Long Lines Department of American Telephone and Telegraph Company"
June, 1961 edition, pages 157-161
18.1 Classification of Wire Facilities
Transmission facilities or media employed in telephone and telegraph work, to be cataloged completely, would have to include both waveguides and free space, which is the medium of radio transmission. In this Chapter, however,' we shall confine our attention to various types of wire conductors, including coaxials. The kind of wire line facility to be used in a particular case depends upon economic considerations and the transmission requirements to be met. Ordinary wire facilities may be classified in several ways according to their uses, or on the basis of their physical or electrical characteristics.
It is customary first to make a general distinction between facilities used for toll (long distance) and for exchange area transmission. The latter facilities include the greater part of the total telephone plant since local or short haul service is naturally used more frequently than long distance service. Accordingly, it is economically desirable to design these facilities primarily on the basis of providing satisfactory transmission within the exchange area. For toll or long distance connections, of which local facilities necessarily form a part in every case, more costly types of facilities are used for the long distance links in order that the transmission shall remain satisfactory. This arrangement is in the interest of overall economy because the long distance facilities are relatively few as compared with the local facilities. It means in general that the .latter facilities do not have to meet as exacting requirements as do the. toll facilities with respect to attenuation per unit length, impedance regularity, or balance against noise and crosstalk. In exchange area cables, for example, wire conductors as fine as 22, 24, or 26-gage are widely used, whereas the minimum gage in long toll cables is 19. Generally similar distinctions as between local and toll transmission apply in the case of open wire facilities. However, it may be noted that there is a certain middle ground where exchange area trunks are of such great length in some cases that their transmission requirements are not widely different from those of the shorter toll
circuits. Loading is frequently applied to such
trunks and in some cases it may be necessary to use telephone repeaters
18.2 Open Wire Facilities
In both open wire and cable circuits, the development of the telephone art has involved the use of many different types of circuit facilities. At any given time, accordingly, the working plant may include facilities ranging from earlier types to newly developed types which are barely out of the experimental stage. Before the advent of the telephone repeater, the majority of long distance facilities were open wire and, in order to keep the attenuation down, practically all of this open wire was loaded with relatively high inductance coils spaced at intervals of about 8 miles. The conductors used were almost entirely 165, 128, or 104 hard drawn copper wire-and each group of four wires was usually arranged to carry a phantom circuit.
The wires were carried on crossarms in the manner indicated in Figure 18-1. Here each cross. arm carries 10 wires which are numbered consecutively starting with the left-hand pin of the top crossarm when looking in the direction of the pole
FIG. 18-1 WIRE CONFIGURATION FOR OPEN WIRE LINE CARRYING VOICE-FREQUENCY SIDE AND PHANTOM CIRCUITS
numbering of the line. The standard wire layout on two crossarms, shown in the Figure, provides ten side and five phantom circuits. Phantoms are derived from wires 1-4, 7-10, 11-14, 17-20, and 5-6, 15-16. The last is called a vertical or Pole pair phantom and has somewhat different electrical characteristics than the other phantoms because of the different spacing and configuration of the wires. Similarly the characteristics of the "non-pole-pair" side circuits such as 1-2 or 9-10, with 12 inch spacing between wires, are slightly different from those of the pole-pair circuits like 15-16, where the distance between wires is 18 inches.
Many open wire lines, with an arrangement of wires on poles as shown in Figure 18-1, are still in use in the long distance plant. Loading, however, is no longer used on open wire facilities. This is a result of the fact that the characteristics of open wire circuits-particularly the leakage change markedly with varying weather conditions. In dry weather, open wire loading is effective in reducing the attenuation of the circuits considerably. But, due principally to the increased
FIG. 18-2 WIRE CONFIGURATION FOR OPEN WIRE LINE ON WHICH TYPE-C CARRIER SYSTEMS ARE SUPERIMPOSED
leakage, loading may actually increase the attenuation of open wire circuits in wet weather. In order to increase the overall transmission stability of such circuits, accordingly, all loading was removed after the telephone repeater came into general use, and the resulting increase in attenuation was compensated for by the employment of additional repeaters.
The application of carrier systems to open wire lines has led to other changes in open wire facility arrangements. On account of the higher frequencies
FIG. 18-3 WIRE CONFIGURATION OF 8-INCH SPACED OPEN WIRE LINE FOR TYPE-J CARRIER OPERATION
employed in carrier systems, the probability of crosstalk is increased. Since the greatest crosstalk hazard is between the side and phantom circuits of a phantom group, it is desirable in many cases to dispense with the phantom circuit altogether. Further reduction in crosstalk possibilities is effected by spacing the two wires of each pair closer together on the crossarm, and increasing the separation between pairs. Thus, Figure 18-2 shows a wire configuration used to a considerable extent on lines carrying Type-C telephone carrier systems (frequencies up to 30 kc) in which the non-pole pairs have eight inch spacing between wires and the separation between the nearest wires of adjacent pairs is 16 inches.
This configuration which is designated 8-16-8 includes a pole-pair phantom group which ordinarily would be used only for voice frequencies. The change in spacing from 12 inches to 8 inches reduces the linear inductance of the pair and increases
FIG. 18-4 WIRE CONFIGURATION OF 6-INCH SPACED OPEN WIRE LINE FOR TYPE-J CARRIER OPERATION
its linear capacitance by about 8%. The resistance and leakage are not changed and the attenuation is slightly increased. The characteristic impedance is reduced by about 50 ohms.
Where open wire line facilities are designed to carry broad-band carrier systems (Type-J) employing frequencies up to 140 kc, 8 or 6 inch spacing between wires of a pair is employed, and the pole-pair groups are usually dispensed with. Each crossarm then carries 8 wires, with spacings and configurations as indicated in Figures 18-3 and 18-4, and no phantom circuits are provided for. These configurations are designated 8-24-8 and 6-30-6 respectively.
Open wire facilities are subject to the effects of leakage which increase attenuation losses, particularly at carrier frequencies, and which must be adequately controlled to obtain satisfactory transmission. This is done by insulating the wires from their supporting structure with glass insulators. The effectiveness of such insulators under given conditions of weather varies with their size, shape, and the kind of pin employed.
When new open wire facilities are placed on existing lines and are likely to be used for carrier operation at frequencies above 10 kc, it is necessary to take into account the manner in which the other pairs on the line are insulated. This arises from the fact that the wet weather attenuation of similar gage facilities equipped with different types of insulators is unequal and that as a result energy level differences may occur, which cause crosstalk. When this is the case, it may be
FIG. 18-5 ATTENUATION-FREQUENCY CHARACTERISTICS OF OPEN WIRE SIDE CIRCUITS OVER THE VOICE RANGE
FIG. 18-6 ATTENUATION-FREQUENCY CHARACTERISTICS OF OPEN WIRE PHANTOM CIRCUITS OVER THE VOICE RANGE
desirable that all of the open wire facilities of the same gage which are to be used for carrier operation at frequencies above 10 kc be equipped with the same type insulators.
Table IX gives the more important physical and electrical constants of the commonly used types of open wire circuits. The values given are calculated for the single frequency of 1000 cycles and they apply only under more or less ideal conditions. Caution must therefore be used in applying them to practical problems. For example, the leakage of open wire conductors depends upon weather conditions. In wet weather the values for G given in the Table may be very considerably increased, and the various constants dependent to a greater or lesser extent on this value, such as attenuation, wavelength, and characteristic impedance, would change accordingly.
The Table of course does not give information regarding any variations of the circuit constants through the voice frequency range. In practically all cases, however, the attenuation, as well as certain of the other circuit constants, changes somewhat with changing frequency. The magnitude of this attenuation change can be determined from curves in which attenuation is plotted against frequency through the working range. Figures 18-5 and 18-6 give representative attenuation-frequency curves for 104, 128, and 165 open wire, side and phantom circuits, having the wire spacing and configuration shown in Figure 18-1, over the frequency range from 0 to 5000 cycles. Separate curves are given for dry and, wet weather
conditions but the latter curves naturally represent merely an average situation since the "degree of wetness" of the weather is a rather variable quantity. From these curves, it will be noted that, in general, there is an increase of attenuation between 500 and 5000 cycles of somewhere in the order of 50%.
As would be expected, when open wire circuits are used as conductors for carrier systems, the variation in attenuation from the low-to the high frequency end of the transmission band is much greater. Thus, Figure 18-7 gives curves for 8 inch spaced, physical circuits, transposed for Type-C carrier, through the frequency range up to 50,000 cycles. Here, in the band between 5000 and 50,000 cycles, it will be seen that the attenuation more than doubles. Similarly as shown in Figure 18-8, the losses over the open wire broad-band carrier range (Type-J) increase by almost 300% in the
FIG. 18-7 ATTENUATION-FREQUENCY . CHARACTERISTICS OF OPEN WIRE PHYSICAL CIRCUITS OVER THE TYPE-C CARRIER RANGE
range from 20 to 140 kc. Moreover, in the higher carrier ranges, the loss of open wire circuits may be increased to values very much larger than those indicated in this latter Figure by unusual weather conditions, such as ice, sleet or snow
FIG. 18-8 ATTENUATION-FREQUENCY CHARACTERISTICS OF OPEN WIRE PHYSICAL CIRCUITS OVER THE TYPE-J CARRIER RANGE
accumulating on the wires. Thus, Figure 18-9 gives a representative example of the measured effect of melting glaze of an estimated diameter of 1/2 inch on an 8-inch spaced pair of 165-gage wires. Here, the attenuation at 140 kc is some four times the normal wet weather attenuation.
FIG. 18-9 CURVE SHOWING THE EFFECT OF SLEET DEPOSIT ON ATTENUATION OF OPEN WIRE CIRCUIT
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Written Friday, February 1, 2002