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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