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circuit. At any given RF frequency, each of these can exhibit resistance, and impedance in the form
of capacitive or inductive “reactance”.
Transmitters, transmission lines, antennas, and impedance
The output circuits of a transmitter, the transmission line, and the antenna, all have a
characteristic impedance. For reasons beyond the scope of this document, the standard impedance
is nominally 50 ohms resistive, with zero capacitive and zero inductive components. When all
three parts of the system 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.
F = Forward power
R = Reflected power
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 Z-100Plus
