Six-wire Unbalanced Line

Author: Edmund A. Laport

This type of feeder (Fig. 4.14) resembles type VII except for its lower characteristic impedance due to the use of a second high-potential wire. Except for this difference, the same method of construction may be used for both types. One insulator per pole is all that is necessary. The grounded wires can be supported in a number of ways, using prefabricated bayonet brackets for attachment to the poles (as in Fig. 4.67) or by pole and crossarm methods similar to those suggested in Fig. 4.6. It is a very popular type, having found wide application in medium-frequency broadcasting and also in low-frequency antenna-feed systems. It was originally introduced by the Radio Corporation of America in 1938 for use with directive broadcast antenna systems, where low feeder radiation was essential, and to avoid the expense and complications of concentric feeders.

Fig. 4.14

Figures 4.68 to 4.73 show various constructional details that have been used at different broadcast stations. As constructed using wires of radius 0.064 inch, this line is capable of transmitting a peak power of the order of 600 kilowatts, and with larger conductors this rating , can be increased. Its characteristic impedance, of the order of 230 ohms in its usual form, is a very convenient value for broadcasting applications because the coupling networks usually require values of inductance and capacitance readily realizable with available components. When h >> a,

When ρ₁ = ρ₂,

A set of characteristic values is: