TC 9-64 _________________________________________________________________________
4-172. 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 constant impedance at its input.
Terminating the antenna properly will also allow it to be operated over a
wide frequency range without needing to change the coupling adjustments at
the transmitter. 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.
Turnstile Antenna
4-173. 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 figure 4-40, view A. Pairs of such antennas are frequently
stacked, as shown in figure 4-41. Each pair is called a bay. In figure 4-41 two
bays are spaced one-half wavelength apart. Their 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. This cancellation 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. Figure 4-40, 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 figure 4-40, view C.
4-52
Previous Page
