LDG KT-100 Operation Manual page 17

SWR is measured using a device called an "SWR bridge", inserted in the transmission line
between the transmitter and the antenna. This circuit measures forward and reflected power from
which SWR may be calculated (some meters calculate SWR for you). More advanced units can
measure forward and reflected power simultaneously, and show these values and SWR at the
same time.
An antenna tuner is a device used to cancel out the effects of antenna reactance. Tuners add
capacitance to cancel out inductive reactance in the antenna, and vice versa. Simple tuners use
variable capacitors and inductors; the operator adjusts them by hand while observing reflected
power on the SWR meter until a minimum
SWR is reached. The LDG Electronics
KT-100 automates this process.
No tuner will fix a bad antenna. If the
antenna is far from resonance,
inefficiencies inherent in such operation
are inescapable; it's simple physics. Much
of the transmitted
dissipated in the tuner as heat, never
reaching the antenna at all. A tuner simply
"fools" the transmitter into behaving as
though the antenna
avoiding any damage that might otherwise
be caused by high reflected power. For
best performance, the antenna used should
always be as close to resonance as is
practical.
THE LDG KT-100
In 1995, LDG Electronics pioneered a
new type of automatic antenna tuner. The
LDG design uses banks of fixed capacitors
and inductors, switched in and out of the
circuit by relays under microprocessor
control. An additional relay switches between high and low impedance ranges. A built-in SWR
sensor provides feedback; the microprocessor searches the capacitor and inductor banks, seeking
the lowest possible SWR. The tuner is a "Switched L" network, consisting of series inductors
and parallel capacitors. LDG chose the L network for its minimum number of parts and its ability
to tune unbalanced loads, such as coax-fed dipoles, verticals, Yagis, and, in fact, virtually any
coax-fed antenna.
The series inductors are switched in and out of the circuit, and the parallel capacitors are
switched to ground under microprocessor control. The high/low impedance relay capacitor bank
either to the transmitter side of the inductor bank, or to the antenna side. This allows the KT-100
to handle loads that are either greater than or less than 50 ohms. All relays are sized to carry 125
watts continuously.
the
power may be
were resonant,
PAGE 17