LDG YT-100 Operation Manual - page 19
PAGE 19
have the same impedance, the system is said to be “matched”, and maximum transfer of power from the
transmitter to the antenna occurs. While the transmitter output circuit and transmission line are of fixed,
carefully designed impedance, the antenna presents 50-ohm, non-reactive load only at its natural
resonant frequencies. At other frequencies, 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 YT-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
SWR
=
1
+
R
F
1
−
R
F