(3) The ratio of voltage to current at any point on the line is
constant and equals Zo.
(4) There is maximum transfer of power from source to load.
b. Less power is lost when the transmission line is fitted or matched
to the load.
What happens when the transmission line does not match the
impedance of the load?
Reflections occur when there is an impedance
mismatch between the transmission line and the load.
Reflections cause a
loss in power known as reflection loss.
17. Reflections occur when there is a sudden change in impedance.
a. Let's take the statement apart and see what it really means.
Suppose you're in a room and you call your buddy outside. The sound of your
voice reaches him, but some of the power of your voice is lost before it
gets to him. The loss takes place at the walls of the room. We can call
this loss of sound, reflection loss.
b. The sound of your voice does all right traveling through the air
until it hits the wall. Some of the sound continues through the wall to the
outside. Another portion of the sound is reflected or bounced back to you.
The reflection is called an echo.
How much of the sound is reflected
depends on how hard the wall is.
A hard, solid wall causes a lot of
reflection, because the sound can't penetrate the wall as well as it can
penetrate the air.
Part of the sound can't get through the wall at all.
The reflection takes place right at the point where the conductors change,
from air to wood. The reflection loss of sound, then, is caused by a change
or mismatch in the impedance of the conductors. The loss occurs right at
the point where the mismatch is located, in this case, the wall.
18. The same thing happens in RF transmission lines.
a. A reflection occurs when the RF energy meets a sudden change in
impedance. When the RF energy reaches the point where the mismatch occurs,
part of the wave is reflected back to the source. This means less energy is
available for the load. This is reflection loss.
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