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that spreads out; the pattern actually shows lobes that shoot out from along the wire (but not right off the end). If
you looked at the pattern from the end of the wire, it would look something like a doughnut.
The signal emission pattern changes depending on how high above the ground it is. If
the wire is placed anywhere from a 1/2 wavelength off the ground or higher, the pattern will
remain almost perfect. As the wire gets closer to the ground from the 1/2 wavelength figure,
the pattern becomes more skewed.
Another way to change the signal pattern from the antenna is to add another wavelength to the wire, thus
making it two wavelengths long. Now the lobes are tighter, more numerous, and at an angle that is even closer to
that of the antenna wire.
Objects always appear different when you look at them from a different angle. So, now imagine that the
original one-wavelength wire is no longer horizontal, but is now vertical, with the end at the ground being fed by the
transmitter. The pattern from the antenna is the same and the angle by which the signal
leaves the antenna is the same. A figure that is related to this angle is the angle of
radiation, which is the angle that radiation leaves the antenna, with the horizontal land
plane (not the antenna) as the reference. The angle of radiation is important because of the
bending of the signal in the atmosphere. If the angle of radiation is "low," then the signal
will reach the ionosphere at a large distance from the transmitter site, the angle by which
the signal is refracted will be wide, and the distance to the first hop will be equally distant. The opposite will be true
of the angle of radiation is high. As a result, antennas with low angles of radiation are preferable if long distance is a
must and antennas with high angles of radiation are better for short-distance communications.
As you could see from the one-wavelength horizontal longwire antenna, the signal was being emitted in a
particular direction. And as the antenna grows in wavelengths, the transmission characteristics become even more
directional. As a result, antennas such as the longwire are important if you are trying to
communicate with station(s) from a particular direction. If you want to reach all directions
equally well, then an antenna (such as a vertical) would work best. The radiation pattern from
the vertical antenna (as described in the previous paragraph), is toward the sky, but from a
"doughnut" pattern around the wire. This all-sides-equally-well pattern is known as being
either omnidirectional or nondirectional.
The illustrations that have been covered so far in this section only cover signal transmission patterns. However,
you can expect that any good transmitting antenna will also be a solid antenna for receiving. Likewise, receiving
antennas will have the same general characteristics for receiving as they would for transmitting. For example, if a
directional antenna produces a particular set of lobes when transmitting, the directions that those lobes face will be
where the antenna receives best from (if you aim your antenna so that you can transmit to Europe, that antenna will
also be best poised to receive signals from Europe). As you can see, the receiving characteristics of antennas are so
Angle of Radiation:
the angle that
radiation leaves an
antenna, with the
horizontal land
plane as a
Omnidirectional or
Nondirectional
antenna: radiates
an "all-sides-
equally-well, donut-
shaped, pattern".
The Signal Emission
Pattern changes
depending upon its
height above the
land or sea.
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