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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.
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-600Pro 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.
SWR
=
1
+
R
F
1
−
R
F
Summary of Contents for AT-600Pro
Page 8: ...PAGE 8...
