Ultra High Frequency Antennas

Updated on 2017/06/20 03:43

Syllabus

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

  • Biconical Antenna
  • Helical Antenna

Biconical Antenna

  • To achieve wideband(Broadband) characteristics, this is the simplest configuration.
  • Two cones of infinite extent together are used to form the Biconical antenna.
  •  This can be considered to represent a uniformly tapered transmission line. 
  •  The applied vtg (V1)at inputterminalswill produce outgoing spherical waves as shown in Fig.
  • The i/p vtg(V1) when applied to feeder input, it radiatesspherical waves and produces a current.

'I' at any point (r1, θc, ψ), along the surface of a cone and voltage 'V' between the cones.biconical-antenna.png

  • In radio systems, a biconical antenna is a broad-bandwidth antenna made of two roughly conical conductive objects, nearly touching at their points.
  • Biconical antennas are broadband dipole antennas, typically exhibiting a bandwidth of three octaves or more.
  • A common subtype is the bowtie antenna, essentially a two-dimensional version of the biconial design which is often used for short-range UHF television reception. These are also sometimes referred to as butterfly antennas.
  • Omnidirectional biconical antenna
  • The biconical antenna has a broad bandwidth because it is an example of a travelling wave structure; the analysis for a theoretical infinite antenna resembles that of a transmission line.
  • A simple conical monopole antenna is a wire approximation of the solid biconical antenna and has increased bandwidth (over a simple monopole)

Radiation pattern

Spherical Waves Radiated by Biconical Antenna
radiation-pattern-biconical-antenna.png

Applications

Biconical (or "bicon") antennas are often used in electromagnetic interference (EMI) testing either for immunity testing, or emissions testing. 

Helical Antenna

Introduction

  • Definition: Helical antenna is an example of wire antenna and itself forms the shape of a helix.
  • Operating Frequency: 30MHz to 3GHz. This antenna works in VHF and UHF ranges.

Structural Details

  • These are the antenna in which the conducting wire is wound in helical shape and connected to the ground plate with a feeder line. 
  • Helical antenna gives rise to circularly polarized waves.
    helical.png
  • It consists of a flat metal plate termed as ground plate on which helix of thick copper wire is mounted in the  form of a screw thread used as an antenna.
  • One end of the helix is connected to the center conductor of the cable and the outer conductor is connected to the ground plate.

Radiation Pattern

The radiation pattern depends on the diameter of helix, the space between turn and the pitch angle.
Pitch angle is the angle between a line tangent to the helix wire and plane normal to the helix axis. 

The parameters of the helix antenna are defined below.

  • D - Diameter of a turn on the helix antenna.
  • C - Circumference of a turn on the helix antenna (C=π*D).
  • S - Vertical separation between turns for helical antenna.
  •  - pitch angle, which controls how far the helix antenna grows in the z-direction per turn, and is given by  

helical-equation.jpg

  • N - Number of turns on the helix antenna.
  • H - Total height of helix antenna, H=NS.

Operating modes

Normal mode

In this mode the radiation field is normal to the helix axis. The radiated waves are circularly polarized. This mode of radiation is obtained if the dimensions of helix are small compared to the wavelength. The radiation pattern of this helical antenna is a combination of short dipole and loop antenna.

normal-mode-helical.png

The above figure shows the radiation pattern for normal mode of radiation in helical antenna.
It depends upon the values of diameter of helix, D and its turn spacing, S. Drawbacks of this mode of operation are low radiation efficiency and narrow bandwidth. Hence, it is hardly used.

Axial mode

In axial mode the radiation is in the direction of end-firen along the helical axis and the waves are circularly or approximately circularly polarized. This mode of operation is obtained by increasing the circumference to the order of one wavelength (λ) and spacing of approximately λ/4. The below image shows that radiation pattern is broad and directional along the axial beam producing minor lobes at oblique angles.

axial-mode-helical.png

Merits and Demerits

Merits

  • Simple design
  • Highest directivity
  • Wider bandwidth
  • Radiate circulary polarized waveforms

Demerits

  • Requires more and open space as antenna is large
  • As number of turns increases efficiency decreases

Applications

  • An individual antenna can be used to transmit and receive VHF signals
  • Sometimes used for satellite and space probe communications
  • Telemetry links with ballastic missiles and satellites at Earth stations
  • Communications between the moon and the Earth
  • Many Applications in radio astronomy

References

  • Prof. Sujit Wagh
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Created by Vishal E on 2017/05/05 18:48