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Essentially, the turnstile antenna consists of two dipoles at 90 degree angles to each other. One dipole is connected to the main feedline, in this case a 50-Ohm line. Its gain is almost less 3dB than that of a single dipole in its direction of maximum radiation, because each element of the turnstile receives only one-half the transmitter power. It also has very good reception pattern. Figure below shows the basic layout of the turnstile antenna.
Another important characteristics of turnstile antenna is SWR value. Since the turnstile impedance is about 36 Ohms, a 50-Ohm feedline will show an SWR of between 1.3:1 and 1.4:1. Do not try to tune the antenna for a 1:1 SWR, since that will require shortening the elements below individual dipole resonance. The resultant pattern will no longer be omni-directional. Figure 3.2 shows the geometry of turnstile antenna and figure 3.3 shows the omni-directional radiation pattern of turnstile antennas.
Turnstile antennas are most used for FM broadcast reception by take the advantage of their reasonable performance in all directions without the need for a rotor. It also survive at harshest weather. The turnstile antenna also can be a useful antenna for net control stations. However, the turnstile antenna has limitations. For its area, gain is not high. Also beware of the temptation of folding the ends around to reduce the space required by the crossed dipoles, the omni-directional pattern will become a bi-directional, weak set of dipole lobes.Figure below show a few types of turnstile practical antenna.
Figure 4.1 - Basic layout of turnstile antenna
Figure 4.2 - Geometry of turnstile antenna
Figure 4.3 - radiation pattern of turnstile antennas