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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