Practical_Antenna_Handbook_0071639586

388 P a r t V : H i g h - F r e q u e n c y A n t e n n a s f o r S p e c i a l i z e d U s e s
C 1
250 pF
C 2
250 pF
Xcvr
Antenna
L 1
33 H
Figure 16.8 Line flattener tuner.
designs, the inductors are not actually variable but, rather, use switch-selected taps on
the coils. The correct coil taps are selected when the operator selects a channel. This approach
is less frequently encountered today, when frequency synthesizers give the
owner a selection of channels to use. In those cases, either the antenna must be tuned
every time the frequency is changed or an automatic (or motor-driven) preselected
tuner is used.
The line flattener (Fig. 16.8) is a standard transmatch antenna tuner that reduces the
coaxial-cable transmission line VSWR. This type of tuner is especially useful for transmitters
with solid-state finals that react badly to high VSWR. Most recent transceiver
designs incorporate shutdown (foldback) circuits that progressively reduce (and ultimately
cut off) power as the VSWR increases. The line flattener tunes out the VSWR at
the transmitter. It does nothing to tune the antenna but does facilitate maximum power
transfer from the transmitter to the transmission line.