Advantages of the rhombic antenna,discussed in the previous tutorial is useful over a wide frequency range. Although some changes in gain, directivity, and characteristic impedance do occur with a change in operating frequency, these changes are small enough to be neglected.
The rhombic antenna is much easier to construct and maintain than other antennas of comparable gain and directivity. Only four supporting poles of common heights from 15 to 20 meters are needed for the antenna.
The rhombic antenna also has the advantage of being noncritical as far as operation and adjustment are concerned. This is because of the broad frequency characteristics of the antenna.
Still other advantages are that the voltages present on the antenna are much lower than those produced by the same input power on a resonant antenna. This is particularly important when high transmitter powers are used or when high-altitude operation is required.
The rhombic antenna advantages are not as prevalent. 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.
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.
The figure below 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.
Formation of a rhombic antenna beam.
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 for reception. The reduction of back radiation is
perhaps of lesser importance for transmission. When an antenna is
terminated with resistance, the energy that would be radiated backward
is absorbed in the resistor.
The TURNSTILE ANTENNA is one of the many types that has been developed primarily for omnidirectional vhf communications. The basic turnstile consists of two horizontal half-wave antennas mounted at right angles to each other in the same horizontal plane. When these two antennas are excited with equal currents 90 degrees out of phase, the typical figure-eight patterns of the two antennas merge to produce the nearly circular pattern shown in the figure below, view A.
Turnstile antenna radiation pattern.
Pairs of such antennas are frequently stacked, as shown in the next figure below. Each pair is called a BAY. In the figure below two bays are used and are spaced 1/2 wavelength apart, and the corresponding elements are excited in phase. These conditions cause a part of the vertical radiation from each bay to cancel that of the other bay. Advantages of this are that it results in a decrease in energy radiated at high vertical angles and increases the energy radiated in the horizontal plane.
Stacking a number of bays can alter the vertical radiation pattern, causing a substantial gain in a horizontal direction without altering the overall horizontal directivity pattern. In the figure above, view B, compares the circular vertical radiation pattern of a single-bay turnstile with the sharp pattern of a four-bay turnstile array. A three-dimensional radiation pattern of a four-bay turnstile antenna is shown in the figure above, view C.
Stacked turnstile antennas.