Table of Contents
Antenna
What is an antenna? – An antenna is simply called a transducer, which converts radio frequency (RF) electrical current into electromagnetic (EM) waves at the same frequency and vice versa. It can work as a transmitter of signals as well as a receiver of 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 work as a transmitter of signals as well as a receiver of them.
An antenna is also known as Ariel. It can be understood in the following different ways
- It may be a piece of a connecting material in the form of wire, rod, or any other shape which can be excited.
- This is a transmitter or radiator of RF electromagnetic waves.
- It is an impedance matching device that matches the impedance of one medium to another medium.
- It acts as a coupler, which couples 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 are 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 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 this creation of long-wavelength electromagnetic waves 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
- It has identical impedance when used for transmitting and receiving purposes. This property is called equality of impedance.
- It has identical directional characteristics/patterns when it is used for transmitting & receiving purpose. This property is called equality of directional pattern.
- 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 a horn is called ‘Horn Antenna’. Its shape is as because 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: It serves as the “communicator” between the RF front-end circuitry and the radiation and propagation of electromagnetic waves in free space. In microwave and other wireless applications, an antenna plays a very efficient job. Planar-oriented antennas, such as microstrip patches and printed dipoles have attracted significant attention from the community of antenna engineers because of their 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.
Folded Dipole Antenna: A folded dipole antenna is a modified λ/2 dipole with an additional wire connecting its two ends. The folded dipole antenna is an extremely practical wire antenna and is also called an ultra closed spaced array. It consists of two parallel, closely spaced λ/2 dipoles joined together at the outer ends forming a narrow wire loop (d << L and d << λ). The antenna is fed at the center of one dipole, i.e., the dipoles have the same voltage at their ends. As well as radiation fields are concerned it is the same as the λ/2 dipole antenna, but input impedance differs and is equal to 300 Ω.
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}}
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.
R_{I}=r^{2}P=\frac{r^{2}E^{2}}{\eta _{0}}
- \eta _{0} = Intrinsic Impedance of the medium (\Omega)
- R=Radius of shphere (m),
- P= Power Radiatedinstantaneous
- E= Electric field strength (V/m)
R_{I}=R_{I}(\theta ,\phi ) is a function of \theta and \phi
Directional Gain (Gd)
The directive gain is defined as the ratio of radiation intensity in a specified direction to the average radiation intensity.
G_d= \frac{Radiation Intensity in a specific direction}{ Avrege pardiated power}=\frac{R_{I}}{R_{I_{av}}} G_d= \frac{4\pi (R_{I})}{W_{r}}Directivity
Directivity is defined as the ratio of the maximum radiation intensity to the average radiation intensity. It is also called the maximum directive gain of an antenna. It is denoted by D.
D=\frac{Maximum Radiation Intensity of antenna}{Average Radiation Intensity of antenna} D=(G_d) max D_{dB} = 10 \log_{10} (G_d) maxPower 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
G_P=\frac{Radiation intensity in a given direction}{ Average total input power}It is also defined as the ratio of 4 π times of radiation intensity to the total input power.
G_P=\frac{4\pi (R_{I})}{W_{t}} W_t=W_r+W_lW_{l} = ohmic losses in the antenna
Efficiency of antenna
It is defined as the ratio of the radiated power to input power. It is denoted by η.
\eta =\frac{W_{r}}{W_{t}}=\frac{W_{r}}{W_{r}+W_{l}}=\frac{G_{P}}{G_{d}}=\frac{Power Gain}{Directive Gain}Effective Area
It is defined as
A_{e}= \frac{\lambda ^{2}}{4\pi }G_{d}Or
A_{e}= \frac{^{W_{r}}}{P}Where,
{W_{r}}= received power (Watt), P = Power flow per square meter (Watt/m2)
