used to extract energy from a passing radio wave,
maximum signal pickup results when the antenna is
placed physically in the same direction as the electric
field component. For this reason, a vertical antenna is
used to receive vertically polarized waves, and a
horizontal antenna is used to receive horizontally
polarized waves.
At lower frequencies, wave polarization remains
fairly constant as it travels through space. At higher
frequencies, the polarization usually varies, sometimes
quite rapidly. This is because the wave front splits into
several components, and these components follow
different propagation paths.
When antennas are close to the ground, vertically
polarized radio waves yield a stronger signal close to the
Earth than do those that are horizontally polarized.
When the transmitting and receiving antennas are at
least one wavelength above the surface, the two types of
polarization are approximately the same in field
intensity near the surface of the Earth. When the
transmitting antenna is several wavelengths above the
surface, horizontally polarized waves result in a
stronger signal close to the Earth than is possible with
vertical polarization.
Most shipboard communication antennas are
vertically polarized. This type of polarization allows
the antenna configuration to be more easily
accommodated in the limited space allocated to
shipboard communications installations. Vertical
antenna installations often make use of the topside
structure to support the antenna elements. In some
cases, to obtain the required impedance match between
the antenna base terminal and transmission line, the
structure acts as part of the antenna.
VHF and UHF antennas used for ship-to-aircraft
communications use both vertical and circular
polarization. Because aircraft maneuvers cause cross-
polarization effects, circularly polarized shipboard
antennas frequently offer considerable signal
improvements over vertically polarized antennas.
Circularly polarized antennas are also used for ship-
to-satellite communications because these antenntas
offer the same improvement as VHF/UHF ship-to-
aircraft communications operations. Except for the
higher altitudes, satellite antenna problems are similar
to those experienced with aircraft antenna operations.
Standing Wave Ratio
Another term used in antenna tuning is standing
wave ratio (SWR), also called voltage standing wave
ratio (VSWR). A simple definition could be the
“relative degree of resonance” achieved with antenna
tuning. When tuning an antenna, you must understand
the SWR when expressed numerically.
You will hear SWR expressed numerically in nearly
every tuning procedure. For example, you will hear
such terms as “three-to-one,” or “two-to-one.” You will
see them written 3:1 SWR, 2:1 SWR, or 1:1 SWR. The
lower the number ratio is, the better the match between
the antenna and the transmitter for transmitting RF
signals. For example, a 2:1 SWR is better than a 3:1
SWR.
As you approach resonance, you will notice that
your SWR figure on the front panel meters will begin to
drop to a lower numerical value. A good SWR is
considered to be 3 or below, such as 3:1 or 2:1.
Anything over 3, such as 4:1, 5:1, or 6:1 is
unsatisfactory. The SWR becomes increasingly critical
as transmitter output is increased. Where a 3:1 SWR is
satisfactory with a 500-watt transmitter, a 2:1 SWR may
be considered satisfactory with a 10-kilowatt
transmitter.
Most antenna couplers have front panel meters that
show a readout of the relative SWR achieved via
antenna tuning. Figure 2-16 shows a multicoupler,
Figure 2-16.—AN/SRA-33 antenna multicoupler.
2-16
