Constants of Open-Wire Lines at Radio Frequencies

Open-wire lines^{15, 16, 17} are used in radio for connecting the radio transmitter to

Figure 18. Typical losses on a 165-mil open-wire copper telephone line at various frequencies. (From Bell System Tech, J., January, 1939, Vol. 18, No. 1.).

the sending antenna and for similar purposes. It is assumed that the conductors are of hard-drawn copper.

Series Resistance R. The resistance at radio frequencies of both wires of a transmission line is

where f is the frequency in cycles per second, d is the diameter of the wires in centimeters, and R is in microhms per meter of line. The resistance can also be found by the method of page 215.

Series Inductance L. The series self-inductance at radio frequencies of both wires of a transmission line is

where D is the distance between the wires, d is the diameter of the wires in the same units as D, and L is in microhenrys per meter of line. Shunt Capacitance C. The shunt capacitance between the two wires of a radio-frequency transmission line is

where C is the capacitance in microfarads per meter of transmission line and D and d are as previously explained. Equations 73 and 74 do not apply for twisted pairs and are accurate only when D/d is about 10 or more.

Shunt Conductance G. Shunt conductance G was discussed on page 219 for an open-wire line at low frequencies. A similar discussion applies at high frequencies, but this constant is usually neglected in radio.