_______________________________________________________________________ Antennas
antenna. This is particularly important when high transmitter powers are
used or when high-altitude operation is required.
4-169. Disadvantages. The rhombic antenna is not without its
disadvantages. The principal one is that a fairly large antenna site is required
for its erection. Each leg is made at least 1 or 2 wavelengths long at the lowest
operating frequency. When increased gain and directivity are required, legs of
from 8 to 12 wavelengths are used. These requirements mean that high-
Therefore, they are used only when a large plot of land is available.
4-170. Another disadvantage is that the horizontal and vertical patterns
depend on each other. If a rhombic antenna is made to have a narrow
horizontal beam, the beam is also lower in the vertical direction. Therefore,
obtaining high vertical-angle radiation is impossible except with a very broad
horizontal pattern and low gain. Rhombic antennas are used, however, for
long-distance skywave coverage at the high frequencies. Under these
conditions low vertical angles of radiation (less than 20 degrees) are
desirable. With the rhombic antenna, a considerable amount of the input
power is dissipated uselessly in the terminating resistor. However, this
resistor is necessary to make the antenna unidirectional. The great gain of
the antenna more than makes up for this loss.
4-171. Radiation patterns. Figure 4-39 shows the individual radiation
patterns produced by the four legs of the rhombic antenna and the resultant
radiation pattern. The principle of operation is the same as for the V and the
half-rhombic antennas.
Figure 4-39. Formation of a Rhombic Antenna Beam
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