b. What is a sudden change in impedance? Well, for one thing, an open
circuit is a sudden impedance change.
The RF energy does all right
traveling along the transmission line because the impedance is uniform
throughout the line. Then all at once the RF energy hits the open circuit
which has an infinite impedance.
See Part A of Figure 9.
The change in
impedance is so sudden and so large that all of the RF energy is reflected.
c.
A short circuit is a sudden impedance change too. The impedance of
a short
circuit is practically zero. So, when the RF energy hits the short
(Part B
of Figure 9), all of the energy is reflected. No energy gets to the
load at
all.
Figure 9. Reflections are Due to Impedance Mismatches.
d. The same thing happens if we connect one transmission line to
another having a different characteristic impedance (Part C of Figure 9).
Also, if a transmission line is connected to any load other than a resistive
load that is equal to the characteristic impedance of the line, reflections
occur as in Part D of Figure 9. However, only a part of the RF energy is
reflected.
The amount reflected depends on how much of an impedance
mismatch exists. The closer the impedances are matched, the less the amount
of energy reflected.
19. Reflections cause standing waves.
a. When RF energy meets a sudden impedance change, all or part of the
energy travels back to the source. Look at Part A of Figure 10 showing an
RF generator coupled to a parallel-pair type transmission line. The end of
the line is open and, therefore, presents a sudden change in impedance to
the RF energy. The RF energy placed on the transmission line by the source
is called the incident wave.
12
Previous Page
