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usually not the case; operators often need to transmit at frequencies other than resonance,
resulting in a reactive antenna and a higher SWR.
Where F = Forward power (watts),
R = Reflected power (watts)
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 AT-100ProII automates this process.
No tuner will fix a bad antenna. If the antenna is far from resonance, the inefficiencies
inherent in such operation are inescapable; it’s simple physics. Much of the transmitted power
may be dissipated in the tuner as heat, never reaching the antenna at all. A tuner simply “fools”
the transmitter into behaving as though the antenna was resonant, 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 AT-100PROII
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
SWR=
1+
R
F
1−
R
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