4-44
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-frequency rhombic antennas have wires of several hundred feet in
length. Therefore, they are used only when a large plot of land is available.
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 sky wave 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.
Radiation Patterns
Figure 4-38 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 4-38.—Formation of a rhombic antenna beam.
Terminating Resistor
The terminating resistor plays an important part in the operation of the rhombic antenna. Upon it
depend the unidirectivity of the antenna and the lack of resonance effects. An antenna should be properly
terminated so it will have a constant impedance at its input. Terminating the antenna properly will also
allow it to be operated over a wide frequency range without the necessity for changing the coupling
adjustments at the transmitter. Discrimination against signals coming from the rear is of great importance
