consisting of four coupling units, with four SWR meters
at the top (one for each coupler).
To achieve a perfect standing wave ratio of 1:1
would mean that we have succeeded in tuning out all
other impedances and that the antenna is matched
perfectly to the transmitted frequency. With such a low
SWR, the antenna would now offer only its
characteristic impedance. A 1:1 SWR is rarely
achieved, of course. There will always be some power
loss between the transmitter and the antenna because of
natural impedances that exist between the two.
Nevertheless, the objective is to achieve the lowest
SWR possible. In other words, we want only the
characteristic impedance of the antenna remaining.
Various factors in the antenna circuit affect the
radiation of RF energy. When we energize or feed an
antenna with an alternating current (ac) signal, waves of
energy are created along the length of the antenna.
These waves, which travel from a transmitter to the end
of the antenna, are the incident waves.
Lets look at figure 2-17. If we feed an ac signal at
point A, energy waves will travel along the antenna
until they reach the end (point B). Since the B end is
free, an open circuit exists and the waves cannot travel
farther. This is the point of high impedance. The
energy waves bounce back (reflect) from this point of
high impedance and travel toward the feed point, where
they are again reflected.
We call the energy reflected back to the feed point
the reflected wave. The resistance of the wire
gradually decreases the energy of the waves in this
back-and-forth motion (oscillation). However, each
time the waves reach the feed point (point A of figure
2-17), they are reinforced by enough power to replace
Figure 2-17.Incident and reflected waves on an antenna.
the lost energy. This results in continuous oscillations
of energy along the wire and a high voltage at point A on
the end of the wire. These oscillations are applied to the
antenna at a rate equal to the frequency of the RF
In a perfect antenna system, all the energy supplied
to the antenna would be radiated into space. In an
imperfect system, which we use, some portion of the
energy is reflected back to the source with a resultant
decrease in radiated energy. The more energy reflected
back, the more inefficient the antenna. The condition of
most antennas can be determined by measuring the
power being supplied to the antenna (forward power)
and the power being reflected back to the source
(reflected power). These two measurements determine
the voltage standing wave ratio (VSWR), which
indicates antenna performance.
If an antenna is resonant to the frequency supplied
by the transmitter, the reflected waves and the incident
waves are in phase along the length of the antenna and
tend to reinforce each other. It is at this point that
radiation is maximum, and the SWR is best. When the
antenna is not resonant at the frequency supplied by the
transmitter, the incident and reflected waves are out of
phase along the length of the antenna and tend to cancel
out each other. These cancellations are called power
losses and occur when the SWR is poor, such as 6:1 or
Most transmitters have a long productive life and
require only periodic adjustment and routine
maintenance to provide maximum operating efficiency
and reliable communications. Experience has shown
that many of the problems associated with unreliable
radio communication and transmitter failures can be
attributed to high antenna VSWR.
Under radio silence conditions, placing a carrier on
the air during transmitter tuning would give an enemy
the opportunity to take direction-finding bearings and
determine the location of the ship. Even during normal
periods of operation, transmitters should be tuned by
methods that do not require radiation from the antenna.
This precaution minimizes interference with other
stations using the circuit.
A dummy load (also called dummy antenna) can be
used to tune a transmitter without causing unwanted
radiation. Dummy loads have resistors that dissipate
the RF energy in the form of heat and prevent radiation
by the transmitter during the tuning operation. The