c. Impedance measurements. The characteristic impedance of an open-wire line or cable can be determined by

making two impedance measurements. First, the impedance at the sending (near) end is measured with the receiving

(distant) end open-circuited. This gives the open-circuit impedance, ZOC. Then the impedance at the sending end is

measured with the distant end short-circuited. This gives the short-circuited impedance, ZSC. The characteristic

impedance can be calculated from these two measured impedances by using the following formula:

ZO = ZOCZSC

In A, ZOC for the single section is 1,000 ohms, and ZSC is 200 ohms and 800 ohms in parallel, or 360 ohms. Substituting

these two values in the formula, ZO is found to be 1,000 times 360, or 600 ohms. In other words, for a uniform line, the

characteristic impedance of the entire line is the same as that of a single section of the line.

60.

Transfer of Power to Transmission Line.

Since a transmission line may cause

considerable power loss, it is important in telephone

communications that lines be designed in such a way

that maximum power is transferred from the transmitter

to the receiver.

a. Maximum power transfer.

(1) The condition for maximum power

transfer from a source to a load can be developed

with the aid of figure 57. In A, a 12-volt generator

with an internal resistance, RG, of 600 ohms

is connected to a load resistance, RL, of 400 ohms.

FIGURE 57. Power Transfer to Variable

Load.

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