a. Impedance is the combined opposition of resistance, capacitive

reactance, and inductive reactance. In a transmission line, the impedance

to RF is caused by the distributed properties of resistance, capacitance,

and inductance.

This impedance is called characteristic impedance.

The

symbol for characteristic impedance is Zo. It is measured in ohms.

b. The characteristic impedance of a transmission line is determined by

the following:

(1) Size of the wire use.

(2) Spacing between the wires.

(3) Insulation used to separate the wires.

c. The characteristic impedance is not affected by the length of the

line.

14. Why is characteristic impedance important?

a. Imagine that you have a generator coupled to a transmission line

that is infinite in length as in Part A of Figure 8.

That is, the

transmission line has no end--it just goes on and on to infinity. Now, put

an ammeter at the input end and apply a voltage to the line. The ammeter,

surprisingly enough, indicates that current is flowing in the line. You're

probably asking, "How can current flow when we don't have a complete

circuit?"

But we do have a complete circuit--through the distributed

properties of the line.

The amount of current that flows on the line

depends upon the applied voltage and the distributed properties of

resistance, capacitance, and inductance.

The impedance at the input, Zo,

then, is equal to the applied voltage divided by the line current.

b. The RF energy, and by this we mean current and voltage, travels down

the infinite line in phase. The amplitude drops off somewhat because of the

resistance of the line.

The RF signal never reaches the load at the far

end, so none of it ever comes back.

The input impedance that the energy

meets at point 2 in Part A of Figure 8 is the same as the input impedance at

point 1. This is so because the ratio of applied voltage to line current

remains constant at any point on an infinite line. The RF energy goes past

point 2 because the impedance ahead is exactly like the impedance it has

just passed through. The same thing holds for point 3. Since none of the

energy ever reaches the end of the line, then the load impedance has no

effect on the input impedance of the line.

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