What is an Antenna

Anteena

What is antenna? – An antenna is simply called a transducer, which converts radio frequency (RF) electrical current into an electromagnetic (EM) wave at the same frequency and vice versa. It can works as a transmitter of signals as well as receivers them.

An antenna is simply called a transducer, which converts radio frequency (RF) electrical current into an electromagnetic (EM) wave at the same frequency and vice versa. It can works as a transmitter of signals as well as receivers them.

Anteena
What is an antenna?

An antenna is also known as Ariel. It can be understood in the following different ways

  1. It may be a piece of a connecting material in the form of wire, rod or any other shape which can be excited.
  2. This is a transmitter or radiator of RF electromagnetic waves.
  3. It is an impedance matching device which matches the impedance of one medium to another medium.
  4. It acts as a coupler, which couple a generator/transmission line and space or vice versa.

The dictionary meaning of antenna is defined as ” a usually metallic device (as a rod of tubing or wire) for radiating or receiving electromagnetic or radio waves“.)

Early History of Antennas

When James Clerk Maxwell, in the 1860s, united electricity, and magnetism into electromagnetism, he described light as and proved it to be an electromagnetic phenomenon. He predicted the existence of electromagnetic waves at radio frequencies, that is at much lower frequencies than light. In 1886, Maxwell was proven right by Heinrich Rudolf Hertz who without realizing it himself created the first-ever radio system, consisting of a transmitter and a receiver.

Hertz’s radio system

The transmitting antenna, connected to a spark gap at the secondary windings of a conduction coil, was a dipole antenna. The receiving antenna was a loop antenna ending in a second spark gap. Hertz, who conducted his experiments at frequencies around 50 MHz, was able to create electromagnetic waves and to transmit and receive these waves by using antennas.

Modern History of Antennas

Guglielmo Marconi grasped the potential of Hertz’s equipment and started experimenting with wireless telegraphy. In 1895 he hit upon a new arrangement of his equipment that suddenly allowed him to transmit and receive over distances that progressively increased up to and beyond 1.5 km. Marconi had enlarged the antenna. His monopole antenna was resonant at a wavelength much larger than any that had been studied before was this creation of long-wavelength electromagnetic waves that turned out to be the key to his success. It was also Marconi who, in 1909, introduced the term antenna for the device that was formerly referred to as an areal or an elevated wire.

Basic Antenna Elements

The basic antenna elements are

  • Alternating current element or Hertzian dipole
  • Short dipole
  • Short monopole
  • Half-wave dipole
  • Quarter-wave monopole

Alternating current element or Hertzian dipole It is a short linear antenna in which the current along its length is assumed to be constant.

Short Dipole    It is a linear antenna whose length is less than and the current distribution is assumed to be triangular.

Short Monopole    It is a linear antenna whose length is less than and the current distribution is assumed to be triangular.

Half-wave Dipole    It is a linear antenna whose length is and the current distribution is assumed to be sinusoidal. It is usually centre-fed.

Quarter-wave Monopole    It is a linear antenna whose length is and the current distribution is assumed to be sinusoidal. It is fed at one end with respect to each.

Properties of antenna

  1. It has identical impedance when used for transmitting and receiving purpose. This property is called equality of impedance.
  2. It has identical directional characteristics/patterns when it is used for transmitting & receiving purpose. This property is called equality of directional pattern.
  3. It has the same effective length when it is used for transmitting & receiving purpose. This property is called equality of effective length.

Types of Antennas

Antenna Parameters

Impedance

It is defined as the ratio of input voltage to input current or

 Here,         

is a complex quantity & it is written as

Here, the reactive part

results from fields surrounding the antenna. The resistive part,
is given by
= Losses
= Radiation Resistance

Radiation Resistance ( Rr )

It is defined as fictitious or hypothetical resistance that would dissipate an amount of power equal to the radiated power.

Radiation pattern

An antenna radiation pattern is a three-dimensional variation field. It is a pattern drawn as a function of θ and  Ø. The pattern consists of one main lobe & a number of minor/side lobes.  

Radiation Intensity

It is defined as the power radiated in a specific direction per unit solid angle.

= Intrinsic Impedance of the medium (Ω) R = radius of sphere, (m) P = Power radiated instantaneous E = Electric field strength (V/m)
is a function of θ and Ø.

 Directional Gain   (Gd)

The directiveGain is defined as the ratio of radiation intensity in a specified direction to the average radiation intensity.

Directivity

Directivity is defined as the ratio of the maximum radiation intensity to the average radiation intensity.   It is also called as the maximum directive gain of an antenna. It is denoted by D.

Power Gain

Power Gain is defined as the ratio of the radiation intensity in a given direction to the average total input power. It is denoted by 

  It is also defined as the ratio of 4 π times of radiation intensity to the total input power. 

= ohmic losses in the antenna

Efficiency

It is defined as the ratio of the radiated power to input power. It is denoted by η.

Effective Area

It is defined as
Or

Where, 

= received power (Watt) P = Power flow per square meter (Watt/m2)

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