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Characteristics of Some Typical Antennas

Antennas come in different shapes and sizes to suit different types of wireless applications. The characteristics of an antenna are very much determined by its shape, size and the type of material that it is made of. Some of the commonly used antennas are briefly described here.
  1. Hertzian dipole antenna, or short dipole antenna
  2. Dipole antenna
  3. Half-wave dipole antenna,
  4. Folded dipole antenna
  5. Quarter-wave monopole antenna, or rod antenna
  6. Loop antenna or magnetic dipole
  7. Helical antenna
  8. Horn antenna
  9. Yagi-Uda antenna
  10. Parabolic dish antenna
  11. Cassegrain feed antenna
  12. Frequency independent antenna: spiral antenna and log periodic antenna

Dipole Antenna

A dipole antenna can be formed by bending apart the conductors of an open-circuited two-wire transmission line.
In a two-wire transmission line, where the two currents in the conductors are of sinusoidal distribution and equal in amplitude, but opposite in direction, due to cancelling effects, no radiation occurs from the transmission line. However, when the conductors are bent to construct a dipole antenna as shown in Figure the currents in the arms of the dipole are in the same direction and they produce radiation in the horizontal direction. Thus, for a vertical orientation, the dipole radiates in the horizontal direction. For this reason, the alignment of the current on the two arms of the dipole antenna enhances the radiation properties over that of the two-wire line which has closely spaced conductors carrying equal currents in opposite directions.
Construction of a dipole antenna

Folded Dipole Antenna

Advantages of Folded Dipole Antenna
  1. Increase in Impedance When higher impedance feeders need to be used, or when the impedance of the dipole is reduced by factors such as parasitic elements, a folded dipole provides a significant increase in impedance level that enables the antenna to be matched more easily to the feeder available.
  2. Wide Bandwidth The folded dipole has a flatter frequency response; this enables it to be used over a wider bandwidth.
Applications of Folded Dipole Antenna
Folded dipole antennas are used in wide band applications, such as TV antennas. For example, the driven element in a Yagi–Uda antenna is a folded dipole antenna.

Quarter-Wave Monopole Antenna, or Rod Antenna

A monopole antenna is another simple and efficient wire antenna which is formed by driving a wire with a voltage between the wire and a conducting ground plane.
Description: 49373.png
Monopole antenna

The operating principle of rod antennas (or monopoles) is based on the fact that on an antenna of only a quarter of a wavelength, the same current distribution is obtained as on a half-wave dipole.

Loop Antenna (Magnetic Dipole)

The loop antenna is a conductor bent into the shape of a closed curve such as a circle or a square with a gap in the conductor to form the terminals as shown in the figure.

Description: 49498.png
Loop antenna
There are two types of loop antennas:
  1. Electrically small-loop antennas
  2. Electrically large-loop antennas
If the total loop circumference is very small as compared to the wavelength (L << λ) then the loop antenna is said to be electrically small.
If the perimeter or circumference of the loop antenna is close to a wavelength then the antenna is said to be an electrically large loop antenna.

Helical Antenna

This is one of the simplest antennas used in extraterrestrial communications. A helical antenna, or helix, is one in which a conductor connected to a ground plane, is wound into a helical shape. Figure  illustrates a helix antenna.
The following symbols are used to describe the geometry of a helical antenna:
D = Diameter of helix
C = Circumference of helix = (πD)
d = Diameter of helix conductor
S = Spacing between turns
A = Axial length
L = Length of one turn
N = Number of turns
α = Pitch angle = Description: 19062.png
Helical Antenna

Yagi-Uda Antenna

A Yagi–Uda antenna, commonly known simply as a Yagi antenna or Yagi, is a directional antenna system developed by Japanese scientists in the 1930s.
A typical Yagi antenna consists of the following parts:
  1. Active Element, or Driven Element
The driven, or active, element of a Yagi is the equivalent of a centre-fed, half-wave dipole antenna. A basic Yagi consists of a certain number of driven or active elements.
  1. Passive Elements, or Parasitic Elements
Parallel to the driven element, and approximately 0.2 to 0.5 wavelengths on either side of it, are straight rods or wires called reflectors and directors, collectively referred as the passive elements, or parasitic elements.
Description: 25479.png
Yagi antenna

Parabolic Dish Antenna

Antennas based on parabolic reflectors are the most common type of directive antennas when a high gain is required at microwave frequencies. These antennas consist of a reflective dish in the shape of a parabola which collects and concentrates an incoming parallel beam of radio waves and focuses them onto the actual antenna placed at its focalpoint or focus. The actual antenna at the focus is sometimes referred to as the antenna feed.
Description: 25498.png
 Geometry of parabolic dish antenna

Cassegrain Feed Antenna

A Cassegrain antenna is a parabolic antenna in which the feed radiator is mounted at or behind the surface of the concave main parabolic reflector dish (i.e., the vertex of the paraboloid). The feed radiator is aimed at a smaller convex secondary hyperboloid reflector, known as secondary reflector or subreflector, suspended in front of the primary reflector. The beam of radio waves from the feed radiator illuminates the secondary reflector, which reflects it back to the main reflector dish, which reflects it forward again to form the desired beam.

This design is an alternative to the most common parabolic antenna design, called front feed, in which the feed antenna itself is mounted and suspended in front of the dish at the focus.

Frequency-Independent Antennas

Frequency-independent antennas are antennas whose radiation pattern, impedance and polarisation remain virtually unchanged over a large bandwidth. Their electrical dimensions, however, scale with frequency. Ideally, the electrical size of such antennas would remain constant over the entire electromagnetic spectrum.

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