Practical_Antenna_Handbook_0071639586

C h a p t e r 2 4 : a n t e n n a T u n e r s ( A T U s ) 537
Because of the limitations on the range of load impedances it can match when either
the source or the load has a reactive component, an L network located at the transmitter
end of a feedline should be configurable to accommodate the different topologies
shown in Fig. 24.1. Alternatively, an L network located at the antenna feedpoint can be
a fixed topology once the nature of the feedpoint impedance of the antenna is known.
Pi Networks
The pi network (or pi-Âsection network) shown in Fig. 24.2 is used to match a high source
impedance to a low load impedance. These circuits are typically used in vacuum tube
RF power amplifiers that need to match output impedances of a few thousand ohms to
much lower system transmission line impedances—typically 50 or 75 W. The name of
the circuit comes from its resemblance to the Greek letter pi (p). The equations for the pi
network are:
R1 > R2
and 5 < Q < 15
(24.13)
X
C 2
R1
Q > – 1
R
2
R2
= ⎛ R ⎞
2
2
⎜ ⎟ (1+ Q ) – 1
⎝ R ⎠
1
(24.14)
(24.15)
X
C1
= R 1 (24.16)
Q
X
L
R1 {( Q + ( R2 / X
C 2)}
=
2
Q + 1
(24.17)
Of course, if R 1 < R 2 , the pi network can be flipped left to right; that is, R 1 can become
the load and R 2 the source. Because the pi network typically has a higher Q than an
L-Âsection network, its bandwidth will generally be narrower. If bandwidth is an important
issue, a pair of cascaded L sections may be a better solution to a particular matching task.
L
R1
C1
C2
R2
R 1 R 2
Figure 24.2 Pi network.