In 1890s only few antennas were present in the world for experimental purposes. But nowadays every person carries one or more antennas (mobile phone, GPS, etc...). Mobile phones have multiple antennas for multiple uses. At present situation, the whole world is affected by wave phenomena. Antenna is a static device, which is used to transmit and receive electromagnetic waves or radio waves, and also it converts electrical power to electromagnetic wave. Over a relatively narrow band, antennas work effectively. Most of the antennas are resonant devices. Receiving antenna should always be tuned at the same frequency band with the connected radio system, otherwise reception and transmission gets disturbed. Wireless communication, now has become a main part of everyday life. Antenna is the center world of wireless communication system. Growth of RFID technology is the other factor for the increase use of antenna.
Wave length and frequency controls the speed of propagated waves through a medium. An example is given below; in free space, electromagnetic wave propagation is about 1.18 x 101° inches/sec. Therefore the wave length for the frequency of 1GHz is 1.182 x 101° / 1 x 109. In microwave band, approximate frequency of large-diameter antenna is about 2 to 100 GHz and the wavelength ranges from 6 inches to 1/8th of an inch. Radio/television frequencies are higher than the infrared, optical and gamma ray frequencies. Infrared, optical and gamma frequencies are lower than micro wave frequencies. Diameter of microwave antenna is ranging from 10 meters to 100 meters. Microwave antenna should have surface reflection accuracies from 1/12th to 1/15th of its wavelength. Designing of microwave antenna is more difficult for engineers.
Aperture of an antenna is nothing but the area of antenna that captures energy from the surface. Aperture of antenna otherwise known as effective area is perpendicular to the direction of incoming wave. Without any loss of energy this area collect same amount of energy that was transmitted from the transmitting antenna. This is the main function of aperture.
Effective aperture can be expressed as,
Here, p is power density of plane wave in W/m2 and Pt is defined as the power at the antenna terminal available to the antennas receiver in watts. From the above equation the effective aperture can be simply defined as the amount of power captured from a given plane wave. In the case of dish antenna, its reflector works as the aperture or effective area of antenna and the mouth of the horn will act as its aperture. For wire antenna ellipse is its aperture and for Yagi-Uda antenna the aperture is large.
Directivity of an antenna depends according to the design of an antenna. For an Omni directional antenna, it has 360o coverage area and for the directional antenna it propagates in a particular direction only. For a transmitting antenna, the directivity can be explained by the efficiency of focus energy of transmitting wave in particular direction. From the receiving end, the directivity can be explained by the ability of receiving wave in a particular direction. Directivity of an antenna is the peak value of antenna compared with the average gain of an antenna in all directions.
Directivity can be expressed as,
Gain of an antenna is the sum of directivity of antenna and its electrical efficiency. In a single word, antenna gain can be described as its performance. The gain of antenna is related with the directivity of antenna. In case of transmitting antenna, gain for a particular direction can be explained as the ability of converting input power to transmitting wave. And in case of receiving antenna, it is the ability of converting receiving radio wave from the same direction into output power. Antenna gain is directly proportional to the aperture of an antenna.
Antenna gain can be expressed as,
Reciprocity theorem is the most important for antenna propagation. Reciprocity states that the relation between the properties of transmitting receiving antenna.
Efficiency of antenna is the overall performance of antenna. It is the ability of antenna to transmit maximum amount of power and receive maximum amount of power. Efficiency can be expressed as the ratio of radiated power (Pradiated) to the input power (Pinput) of the antenna.
Types of Antenna
Short Dipole Antenna: It is the simplest form of antenna. Its efficiency depends upon the length of wire. Wavelength of this type of antenna is 1/10th of its length.
Dipole Antenna: The structure of Dipole antenna is similar to the Short Dipole Antenna. It has very thin radius.
Half-wave Dipole: Length of this antenna is half of its wavelength. Input impedance of half-wave dipole antenna can be expressed as,
Broadband Dipole Antenna: Broadband Dipole antenna is used for both short-distance, medium distance and Long distance communication. Its propagation ranges from 1.5 to 30 MHz frequencies.
Monopole Antenna: Monopole Antenna is placed perpendicular to a conducting surface. Conducting surface is known as ground plane. Length of monopole dipole antenna is one half of dipole antenna.
Folded Dipole Antenna: Folded dipole antenna is similar to the Dipole antenna, but its structure is just different from the dipole antenna. Two ends of folded dipole antenna is folded and connected to each other.
Small Loop Antenna: Loop antenna consist a loop of electrical conductors, and these conductors are connected to a balanced transmission line. It has high resistance and low radiation resistance.
Travelling wave Antennas
Helical Antenna: Helical Antenna or otherwise known as Helix Antenna. Its shape is like a helix, and it is molded over a ground plane.
Yagi-Uda Antenna: It is very easy to construct. Its gain ranges from 10db with small bandwidth. It consists of dipole, directors, reflectors and a transmission line. Operation ranges from 3MHz to 3GHz.This antenna is very familiar for everyone.
Spiral Antennas: Its shape resembles to a spiral like structure. Operational frequency range is very wide.
Corner Reflector: Reflector is used to collect radio waves by placing it perpendicularly. These type of antenna helps to collect more amount of radio waves. Angle between these two reflectors is about 90o.
Dish Antenna: Aperture area of Dish antenna is very high. It is otherwise known as Parabolic Reflector Antenna or Satellite Dish Antenna. Gain ranges from 30-40Gb.Operational frequency ranges from150MHz to 1.5GHz.
Microstrip antennas are narrow band wide beam antennas. The structure of microstrip antenna consists of a metal patch over a grounded dielectric substrate. Other name of Microchip Antenna is Patch antenna. Microchip Antenna are two types, they are, Rectangular Micro strip (Patch) Antenna and Planar Inverted-F Antenna (PIFA).
Log- Periodic Antenna
Log periodic antenna, otherwise known as log-periodic array or aerial have an operational frequency range from 200MHz to 2GHz. And also it has high gain. Log periodic antennas are multi element antenna. Structure of Log periodic antenna consist of a series of dipoles arranged in an axis at a particular frequency. Log- periodic antennas are two types. They are, Bow Tie Antennas and Log Periodic Antenna
Construction of aperture antenna consists of dielectric and metal walls. In aperture antenna waveguide ports are used as excitation.Several types of aperture antennas are, Slot Antenna, Cavity- Backed Slot Antenna, Inverted-F Antenna, Slotted Waveguide Antenna, Horn Antenna,Vivaldi Antenna and Telescopes.
Types of Propogation
Ground Wave Propagation
In this type of transmission, propagation of frequency takes place over the surface of earth that is data transmission takes place in the ionosphere. Ground wave propagation is commonly used in local radio communication. This type of propagation is more helpful for short range communication. But one drawback is that, distance for communication is very less.
Signal from transmitter will spread over the earth. Usually transmitted radio signal travels in a straight line, but in the case of Ground wave propagation, these radio waves are spread over the surface. Frequency range of ground wave propagation is up to 2MHz. AM radio is best example for ground wave propagation.
Sky wave Propagation
Transmitted radio waves are reflected back to earth from the ionized layer of earth. These signals can travel anywhere between earth surface and ionosphere. Amateur radio, CB radio, International broadcast are the examples for Sky wave propagation. When radio signals with high frequencies enter in to the ionosphere indirectly, it will scatter back into the earth’s surface in the form of scatter waves. Altitude of ionosphere ranges from 80km to 1000km. In ionosphere reflection effect is caused by refraction process.
Line of sight propagation is nothing but the transmission and reception of the data takes place only when transmitting and receiving stations are in view with each other without any disturbance between them. Satellite communication, microwave communication and FM radio are the best examples for Line of sight propagation.
Expression for maximum distance between the two antennas in Line-of-sight propagation is,
D = 3.57 (√Kh1+√Kh2)
h1 = height of first antenna
h2 = height of second antenna.
Polarization of antenna is refers to the orientation of electric field of the electromagnetic wave with respect to the ground plane. It depends on the physical structure of antenna and its orientation. Antenna radiates either in linear, circular or elliptical polarization.
In linear polarization, antenna radiates in a linear manner along the direction of propagation. It may vertical or horizontal. Vertically polarized antennas have their electric field perpendicular to ground plane. Horizontally polarized antennas orientation of electric field is parallel to the ground plane. In this case, electric field components are in phase.
In circular polarization, the electric field of electromagnetic radiation traces a circular path. In this case, the electric field components are 90 degrees out of phase and are equal in magnitude. If the electric field traces the circular path in a clockwise direction, then it is called right hand circular polarization. While, if it is in anti-clockwise direction then it is called left hand circular polarization.
In an elliptically polarized antenna, electric field traces an elliptical path and the electric field components are 90 degrees out of phase and are not equal in magnitude. If the electric field traces the elliptical path in a clockwise direction, then it is called right hand elliptical polarization. While, if it is in anti-clockwise direction then it is called left hand elliptical polarization.