Very High Frequency Antennas

Updated on 2017/06/20 03:38

Syllabus

Structural details, dimensions, radiation pattern, specifications, features and applications of following Antennas:

  • Turnstile Antenna
  • Super Turnstile Antenna

Turnstile Antenna

  • Two half wave λ/2 dipoles are placed at right angles to each other in the same plane, excited 90o out of phase with each other.
  • The two λ/2 dipoles carry equal current in magnitude but in quadrature phase.

turn-stile.png

The radiation pattern of turnstile antenna shows, it is polarized in the same plane so that it were mounted in the horizontal plane, the antenna would radiate horizontally polarized wave about equally wellin all directions along the ground. 

Thus such several turnstile antennas can be stacked along the vertical axis and phased properly.

This results in improved radiation directivity along the ground (i.e. in the plane of polarization).

turn-stile-radiation-pattern.png

The main drawback of the Turnstile antenna is that it produces horizontally polarized wave.

Super-Turnsile Antenna

  • Turnstile antennahas been modified by scientist R.W.Mastersin order to obtain a very low VSWR over a considerable bandwidth.
  • In this super-turnstile antenna, the simple dipole elements are replaced by four flat sheets or their equivalent.

super-turn-stile.png
Super‐Turnstile Antenna with Four Flat Sheet

  • The four flat sheet sare attached to the metallic masts by the bars 'P', leaving a circularslot which is driven at the point QR.
  • The current flows mainly radially out from the vertical mast and shape of the sheets is such that Downward and upward radiation is cancelled out.
  • The other pair of the sheets would be driven with current at 90˚(quadrature) time phase from the first pair of the sheets, as applied in simple turnstile antenna.
  • The length of slot is usually greater than λ/2 i.e. 0.7 λ for resonance process.

super-turn-stile-ANTENNAS.png

Different methods of using super turn stile Antennas 

This method of driving the antenna, completely avoids the need of insulators.

  • Plane metal sheet is normally replaced by the grid of c onductors to avoid wind resistance (wind-age-effect).
  • In super-turnstile antenna, power density in a horizontal plane is similar to the equatorial power density of a normal dipole radiating the same total power input.
  • Also such super-turnstile antenna are stacked vertically to achieve greater gain and greater directivity in horizontal direction.

Applications

This type of stacked superturn stile antenna array is widely used in

  • FM radio broadcast
  • TV signal broadcast.

Lens Antenna

  • Operated at microwave frequencies.
  • It starts at 1 GHz & used above 3 GHz.
  • At low frequencies it becomes bulky and complex.
  •  It works as a glass lens as used in optics theory.
  • We all know the collimating action of a simple  optical lens. In this light source at focal point, along the axis is kept, then it is seen that collimated rays are obtained.
  • If light source is at left side focal point, then parallel rays are obtained at the right side of lens and vice versa.

References

  •  Constantine A Balanis, Antenna Theory; Analysis and Design, John Wiley & Sons, Inc, 2007 
  • John D Kraus, Ronald J Marhefka, Ahmad S Khan, Antennas for All Applications, Tata McGraw-Hill Publishing Company Limited, 2002 
  • G. S. N. Raju , Antennas and Wave Propagation
  • http://www.csgnetwork.com/antennahtcalc.html
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Created by Vishal E on 2017/05/05 19:05