CHAPTER 3 - Foundations of RF & Microwave Circuit Characterisation
136
We can see the total voltage along the line as the sum (or superposition) of incident and
reflected voltages of amplitudes A and B respectively. Since the overall voltage established on the
line is higher than the source voltage, we can infer that A and B have the same polarity and add to
increase the voltage along the line. To work out how much the voltage rises by, we just use equation
(3.2-6).
We can also look at things from a circuital point of view by using an equivalent electrical
model for our lossless line (Figure 3.4-2). As we get to the end of the line we encounter a load which
opposes greater resistance to current flow than the transmission line hence some current I
r
is
reflected. I
r
flows back into the last capacitor thereby increasing its charge and its voltage up
to
⁄
E. This process continues in a similar fashion to the open-circuited case illustrated in section
2.5.1.1.1 and allows the higher voltage to propagate back to the source. Whist the current I
r
acts to
increase the overall voltage along the line, it also acts decrease the overall current, since it has
opposite sign to the incident current. This is shown in Figure 3.4-2.
Figure 3.4-2 Reflected voltage travels back towards the generator and adds to the incident one
To determine the overall current, we need to work out the current reflection coefficient. The
expression for it is very similar to that for the voltage reflection coefficient (eq. (3.2-3)), but has
opposite sign. From eqns. (3.1-8) and (3.1-9)
( )
( )
I
E
2E
E
E
E →
E
3
4
E
3
1
E
E
3
4
Z
L
I I I
3
2
3
1
-
I
r
=
I
3
1
Conquer Radio Frequency
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