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Electronics Technician Volume 07-Antennas and Wave Propagation | Next
Vertical Monopole LPA |
Figure 2-12.—Trideco-type antenna.
Figure 2-11.—Goliath-type antenna.
HIGH FREQUENCY (HF)
LOW FREQUENCY (LF)
Antennas for lf are not quite as large as antennas
for vlf, but they still occupy a large surface area. Two
examples of If antenna design are shown in figures
2-13 and 2-14. The Pan polar antenna (fig. 2-1 3) is
an umbrella top-loaded monopole. It has three loading
loops spaced 120 degrees apart, interconnected between
the tower guy cables. Two of the loops terminate at
ground, while the other is used as a feed. The NORD
antenna (fig. 2-14), based on the the folded-unipole
principle, is a vertical tower radiator grounded at the
base and fed by one or more wires connected to the
top of the tower. The three top loading wires extend
from the top of the antenna at 120-degree intervals
to three terminating towers. Each loading wire has
a length approximately equal to the height of the main
tower plus 100 feet.
The top loading wires are
insulated from ground and their tower supports are
one-third the height of the transmitting antenna.
High-frequency (hf) radio antenna systems are used
to support many different types of circuits, including
ship-to-shore,
point-to-point,
and ground-to-air
broadcast. These diverse applications require the use
of various numbers and types of antennas that we will
review on the following pages.
Yagi
The Yagi antenna is an end-fired parasitic array.
It is constructed of parallel and coplaner dipole
elements arranged along a line perpendicular to the
axis of the dipoles, as illustrated in figure 2-15. The
most limiting characteristic of the Yagi antenna is its
extremely narrow bandwidth. Three percent of the
center frequency is considered to be an acceptable
bandwidth ratio for a Yagi antenna. The width of
the array is determined by the lengths of the elements.
The length of each element is approximately one-half
2-7
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