LDG AT-200PROII, Руководство по эксплуатации, Страница: 23 / 27

PAGE 23
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.
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-200ProII
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
SWR
=
1
+
R
F
1
−
R
F