Antenna Theory Turnstile Antenna - Antenna Theory

What is Antenna Theory Turnstile Antenna?

The Turnstile antenna is one more kind of array antenna. The shape of this array represents the turnstile, which is used at the appearances of few places. This antenna has a wide range of military applications.

Frequency range

The frequency range in which the turnstile antennas operate is around 30 MHz to 3GHz which belong to the VHF and UHF bands.

Construction & Working of Turnstile Antenna

Two equal half-wave dipoles are located at right angles to each other and are fed inphase. These dipoles are excited 90° out of phase with each other. Turnstile array can also be termed as crossed dipoles array.

turnstile_antennas


The beyond images explain turnstile antennas.

To offer high directivity, several turnstiles may be stacked along a vertical axis, and are phased as shown in the figure given beyond. The polarization of these turnstile antennas be contingent upon their mode of operation.

The pair of such dipoles frequently stacked, is known as BAY. In the figures revealed above, two bays are spaced half wavelength (λ/2) apart and the corresponding elements are fed in phase. The radiation yield by the combination of bays results in better directivity.

Modes of Operation

The resulting are the modes of operation of a Turnstile antenna.

Normal mode

In Normal mode of operation, the antenna radiates horizontally polarizedwaves which are perpendicular to its axis.

Axial mode

In Axial mode of operation, the antenna emits circularly polarized waves beside its axis i.e. parallel to its axis.

For circular polarization, the transmitter radiating with right-circular polarization must have a receiver with same right-circular polarization and vice versa. If it is left-circular polarized one, unlike the transmitter, there will be a severe loss of gain.

Super Turnstile Antenna

For a turnstile antenna, the radiation power is 3dB below the maximum radiation of a halfwave dipole radiating the same power. Hence, to overcome this drawback, the Super-turnstile antenna is built.

The modest dipole elements in turnstile are swapped by four flat sheets in Super-turnstile. The design of Super-turnstile array is such that 1 to 8 bays can be constructed on a single mast. The other name for Super-turnstile antenna is the Batwing Antenna.


batwing_antenna

The beyond images show super-turnstile antenna. Figure 1 displays the planning of superturnstile array with the red dots being the feed points. Figure 2 shows the stacked turnstile array used in satellite communications.

Radiation Pattern

The radiation pattern will be parallel to the radiation pattern of two super imposed dipoles. Though it is close to omni-directional pattern, it leaves a cloveleaf shaped pattern.

radiation_pattern_turnstile


The beyond figure shows the radiational pattern of a turnstile array. The characteristic figure-ofeight patterns were united to yield a closely circular pattern.

  • Figure A shows the individual patterns being combined.
  • Figure B shows the vertical pattern of single bay and also the combined pattern of four bays.
  • Figure C shows the resultant combined pattern of four bays showing better directivity.

Advantages

The resulting are the advantages of Turnstile antennas −

  • High-gain is attained by stacking
  • Super-turnstile produces high-gain output
  • Better directivity is achieved

Disadvantage

The resulting is the disadvantage of Turnstile antennas −

  • The radiation power is 3dB below the maximum radiation of a half wave dipole radiating the same power.

Applications

The resulting are the applications of Turnstile antennas −

  • Used for VHF communications
  • Used for FM and TV broadcasting
  • Used in military communications
  • Used in satellite communications

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