438 BELL SYSTEM TECHNICAL JOURNAL 



regularities spaced at intervals often greater than one wave-length. 

 The effect of one such irregularity may be small. The total effect in a 

 long line, however, is sometimes appreciable. 



The body of the insulator, since it has a dielectric constant ap- 

 preciably different from air and since its dimensions are comparable 

 with the line spacing, is in itself a line irregularity. Tie wires or 

 conductor clamps augment this effect. Cross arms and pins employed 

 for mounting pin-type insulators also add to the effect, particularly 

 during wet weather. 



From the standpoint of line irregularities suspension-type insulators 

 are more desirable than pin-type insulators. The latter construction, 

 however, appears to be more practical because the lines are more rigid, 

 sway less during wind storms, and because no intermediate spreaders 

 are required to maintain the desired line spacing. 



One other difficulty with open wire lines is the drift in velocity of 

 propagation and surge impedance during rain and sleet storms.'® 

 Since a similar effect occurs in the elements of the antenna there is a 

 decided drop in the efficiency of the combined antenna and line during 

 rain and sleet storms. The effects of sleet may be reduced by heating 

 the wires with sleet melting currents. The conductor size may be 

 increased to reduce the effects of wet weather but this makes sleet 

 melting more difficult. 



There is an appreciable pick-up between balanced open wire lines 

 on common supports. It appears desirable to separate lines to a com- 

 mon transmitter by at least 10 times the conductor spacing. Spacings 

 greater than this may be required if two lines are to be operated simul- 

 taneously and in some cases it is more desirable to employ separate 

 line supports in order to reduce the possibility of cross-talk difficulties. 

 Of course any current unbalances in two parallel lines greatly increase 

 the danger of cross-talk. 



Concentric tube line construction is not as simple as open wire con- 

 struction. Considering the transmitting case, there is a smaller safety 

 factor for voltage overloads. Insulators are required to withstand high 

 voltage gradients. Temperature changes with ensuing line expansions 

 and contractions must be given consideration. It is these factors in 

 addition to the added expenditure for copper which make concentric 

 line construction more costly than open wire construction. 



The first consideration in the design of a concentric line is the 

 weight of the outer sheath. If the line is to be employed for high 

 power transmitting purposes the voltage safety factor may be so low 

 that accidental dents in the sheath may lead to breakdown. Ob- 

 viously, there is a choice between a large diameter, lightweight sheath 



