LDG IT-100 Operation Manual page 15

it will exhibit capacitive or inductive reactance, causing it to have an impedance other than 50
ohms.
When the impedance of the antenna is different from that of the transmitter and transmission
line, a "mismatch" is said to exist. In this case, some of the RF energy from the transmitter is
reflected from the antenna back down the transmission line and into the transmitter. If this
reflected energy is strong enough, it can damage the transmitter's output circuits.
The ratio of transmitted to reflected energy is called the "standing wave ratio", or SWR. An
SWR of 1 (sometimes written 1:1) indicates a perfect match. As more energy is reflected, the
SWR increases to 2, 3, or higher. As a general rule, modern solid state transmitters must operate
with an SWR of 2 or less. Tube exciters are somewhat more tolerant of high SWR. If a 50 ohm
antenna is resonant at the operating frequency, it will show an SWR close to 1. However, this is
usually not the case; operators often need to transmit at frequencies other than resonance,
resulting in a reactive antenna and a higher SWR.
F = Forward power
R
1
F
SWR
R = Reflected power
R
1
F
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 IT-100 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 were 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.
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