# What is an Antenna

What is an 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.

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, which 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.

The transmitting antenna, connected to a spark gap at the secondary windings of a conduction coil, was a dipole. The receiving antenna was a loop 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. This 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 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 purposes. 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

Yagi Uda Antenna: Yagi-Uda or Yagi is a high gain antenna and it is known as after the name of Professor S. Uda & H. Yagi. This was invented and described in Japanese by the former sometime around 1928 by Professor S. Uda and afterward, it was translated or described by H. Yagi in English.

Log Periodic Antenna (LPA): This is an antenna array of the same type of dipole elements but the length of these elements increases with a common ratio. All these elements which are placed in the antenna are electrically connected. All elements are excited by a common input with the same phase.

Horn Antenna: The antenna which has the shape of horn is called ‘Horn Antenna’. Its shape is as because of a waveguide has one end of which is flared out. A waveguide is a hollow metallic tube and in this metallic tube, EM waves travel. When the waveguide is excited at one end and open at the second end, it radiates but this radiation is very poor and non-directive pattern results because of the mismatch between the waveguide and free space. For improving this radiation the mouth of the waveguide is flared out, then the radiation efficiency, directive pattern and directivity will improve.

Helical Antenna: This is an antenna that has a helix or spring shape. Its polarisation and radiation properties depend on the diameter, pitch, number of turns, wavelength, excitation and spacing between the helical loops.

Microstrip Antenna: An antenna serves as the “communicator” between the RF front-end circuitry and the radiation and propagation of electromagnetic waves in free space. In the microwave and other wireless applications, and antenna plays a very efficient job. Planar oriented antennas, such as microstrip patch and printed dipole have attracted significant attention to the community of antenna engineers because of it’s excellent performance and a lot of benefits.

Rhombic : This is an antenna that is in the shape of a rhombus. It is usually terminated in a resistance. The side of the rhombus, the angle between the sides, the elevation, termination, and height above the earth are chosen to obtain the desired radiation characteristics.

## Antenna Parameters

### Impedance

It is defined as the ratio of input voltage to input current or $Z_{a}=\frac{V_{i}}{I_{i}}$

Here,

$Z_{a}$ is a complex quantity & it is written as

$Z_{a}=R_{a}+jX_{a}$

Here, the reactive part

$X_{a}$ = results from fields surrounding the antenna.

The resistive part, $R_{a}$ is given by,

$R_{a}=R_{l}+R_{r}$

Where, $R_{l}$ is losses & $R_{r}$ is 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.

$R_{r}=\frac{Power Radiated}{I_{rms}^{2}}$

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.

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

$R_{I}=r^{2}P=\frac{r^{2}E^{2}}{\eta _{0}}$

• $\eta _{0}$ = Intrinsic Impedance of the medium ($\Omega$)
$R_{I}=R_{I}(\theta ,\phi )$ is a function of $\theta$ and $\phi$