Antenna Design - Seminar Report

Antenna Design
This project involves study of different types of antennas.
We have designed following 2 types of antenna:-
1. Circular loop : Designed for f.m. broadcasting, the antenna gives a substantially circular radiation pattern in the horizontal plane,  it is a simple mechanical structure (at least from the commercial viewpoint), and can be mounted without insulation on a grounded metal pole.
2. Array type antenna (Endfire) : An antenna array is a similar antennas oriented similarly to get greater directivity in a desired direction. It may also be defined as a radiating system consisting of several spaced and properly phased radiators.
Working frequency of both antennas = 175 MHz .
Both the types of antennas were not available in the Lab so this project is useful for the college.
Software used for design purpose :- NEC
Application: - It is used in TV and Video communication.

Problem statement:
In our college, in communication lab we have antennas working at 750MHZ which are able to transmit as well as receive only audio signals.
These antennas have negligible BW and very low gain.
Size of these high frequency antennas is also very small.
Thus we have built two antennas -  1) Circular loop antenna
             2) Endfire array antenna

Working at 175MHZ frequency.
This frequency is very less than what our college antennas have.
This will enable us to pass audio as well as video signals proving flexibility.
We have used aluminium as it has following advantages:
1) High corrosion resistivity
2) Light weight
3) Easily available
4) Economical

Introduction to antenna
Our project focuses on the hardware fabrication and software simulation of circular and endfire antennas. In order to completely understand  the above it is necessary to start off  by understanding various terms associated with antennas.
An antenna (or aerial) is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver. In transmission, a radio transmitter applies an oscillating radio frequency electric current to the antenna's terminals, and the antenna radiates the energy from the current as electromagnetic waves (radio waves). In reception, an antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage at its terminals, that is applied to a receiver to be amplified. An antenna can be used for both transmitting and receiving.

Antenna parameters 
An antenna is an electrical conductor or system of conductors
Transmitter - Radiates electromagnetic energy into space
Receiver - Collects electromagnetic energy from space
The IEEE definition of an antenna as given by Stutzman and Thiele is, “That part of a transmitting or receiving system that is designed to radiate or receive electromagnetic waves”. The major parameters associated with an antenna are defined in the following sections.

 Antenna Gain
Gain is a measure of the ability of the antenna to direct the input power into radiation in a particular direction and is measured at the peak radiation intensity. Consider the power density radiated by an isotropic antenna with input power P0 at a distance R which is given by  S  =  P0/4πR2. An isotropic antenna radiates equally in all directions, and its radiated power density S is found by dividing the radiated power by the area of the sphere 4πR2. An isotropic radiator is considered to  be 100% efficient.
The gain of an actual antenna increases the power density in the direction of the peak radiation:
          S=P0 G/4πR2      = |E|2/ŋ.

Antenna Efficiency    
The surface integral of the radiation intensity over the radiation sphere divided by the input power P0 is a measure of the relative power radiated by the antenna, or the antenna efficiency.
             Pr/P0    =  ŋe  
where Pr is the radiated power. Material losses in the antenna or reflected power due to poor impedance match reduce the radiated power.

Effective Area
Antennas capture power from passing waves and deliver some of it to the terminals. For an aperture antenna such as a horn, parabolic reflector, or flat-plate array, effective area is physical area multiplied by aperture efficiency. In general, losses due to material, distribution, and mismatch reduce the ratio of the effective area to  the physical area.

Directivity is a measure of the concentration of radiation in the direction of the maximum.
Directivity=(max radiation intensity)/(avg radiation intensity)  =Umax / U0
Directivity and gain differ only by the efficiency, but directivity is easily estimated from patterns. Gain—directivity times efficiency—must be measured.
Input Impedance
The input impedance of an antenna is defined as “the impedance presented by an antenna at its terminals or the ratio of the voltage to the current at the pair of terminals or the ratio of the appropriate components of the electric to magnetic fields at a point”. Hence the impedance of the antenna can be written as given below.  Zin = Rin + jXin    
where -Zin is the antenna impedance at the terminals
            Rin is the antenna resistance at the terminals
             Xin is the antenna reactance at the terminals      

Antenna Factor
The engineering community uses an antenna connected to a receiver such as a spectrum analyzer, a network analyzer, or an RF voltmeter to measure field strength E. Most of the time these devices have a load resistor ZL that matches the antenna impedance. The incident field strength Ei equals antenna factor AF times the received voltage Vrec. We relate this to the antenna effective height  
AF = Ei / Vref   =  2/h          
AF has units meter−1  but is often given as dB(m−1).    

Radiation Pattern
The radiation pattern of an antenna is a plot of the far-field radiation properties of an antenna as a function of the spatial co-ordinates which are specified by the elevation angle (θ) and the azimuth angle (φ). More specifically it is a plot of the power radiated from an antenna per unit solid angle which is nothing but the radiation intensity. It can be plotted as a 3D graph or as a 2D polar or  Cartesian slice of this 3D graph. It is an extremely parameter as it shows the antenna’s directivity as well as gain at various points in space. It serves as the signature of an antenna and one look at it is often enough to realize the antenna that produced it.  Because this parameter was so important to our software simulations we needed to understand it completely. For this purpose we obtained the 2D polar plots of radiation patterns for a few antennas in our lab using  a ScienTech antenna trainer kit. The transmitter of the kit was rotated through 360 degrees in 20 degree intervals and the received power was measured (in µV – converted to dB) by a receiver to plot the radiation patterns of a few antennas.  

Beamwidth of an antenna is easily determined from its 2D radiation pattern and is also a very important parameter. Beamwidth is the angular separation of the half-power points of the radiated pattern.  

Circular loop antenna
Loop antenna is one of the simple, inexpensive and very versatile antenna type. Loop antennas take many different forms such as a rectangular, square, triangle, ellipse, circle and many other configurations. Because of the simplicity in analysis and construction, the circular loop is the most popular and has received the widest attention. The single-turn loop antenna is a metallic 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
Loop antennas are usually classified into two categories, electrically small and electrically large. Electrically small antennas are those whose overall length(circumference) is usually less than about one tenth of a wavelength (C< l /10). However, electrically large loops are those whose circumference is about a free- space wavelength
(C ≈ l).
Loop antennas can be used as single element whose plane of its area is perpendicular to the ground. The relative orientation of the loop can be in other directions, including its plane being parallel relative to the ground. Thus its mounting orientation will determine its radiation characteristics relative to the ground.

1) For finding directions
2) Aircraft receiver
3) As UHF transmitter


As the electrical size of the loop antenna is increased, the current distribution in the loop departs from the simple uniform distribution of the electrically small loop. For single-turn loops, this departure has a significant effect on performance when the circumference is greater than about 0.1l.
The theoretical model for the circular-loop antenna generally assumes a point-source gen­erator of voltage V at the position f = 0, making the input impedance of the loop Z = R + jX = V/I(f = 0). In practical applications, the full-loop antenna is usually driven from a balanced source, such as a parallel-wire transmission line

Far-field pattern:
The far-zone field patterns for the resonant loop shown in Figure 5-15a–c are also simi­lar to those for the pair of dipoles; they have little resemblance to the figure-eight pattern of the electrically small loop

Endfire array antenna
Introduction to Array Antennas:
The field radiated by ay small linear antenna is un-uniformly distributed in the plane perpendicular to the axis of antenna. For example, the radiation pattern of an elementary dipole in which maximum radiation takes place at right angle to the axis and decreases slowly to minimum, as the polar angle decreases towards the axis of dipole. Thus a non-uniform type of radiation pattern may be preferred in many broadcast services but not at all desirable for point to point communication and preferred-coverage services i.e. services in which it is desired to radiate most of the energy in particular direction. The field strength can be increased in preferred directions by properly exciting group or array of antennas simultaneously in an arrangement known as array of antennas or simply antenna arrays. Array of antennas is an arrangement, of several individual antennas so spaced and phased that their individual contribution coming in one preferred direction and cancel in all other directions, to get greater directive gain or directivity.
Thus an antenna array is a system of similar antennas oriented similarly to get greater directivity in a desired direction. It may also be defined as, “A radiating system consisting of several spaced and properly phased radiators.”

Array Configurations :
In general, arrays are found in many geometrical configurations. Some of these are :
(1) Linear Array : In this configuration, the array element centers lie along a straight line. The elements may be equally or unequally spaced.
(2) Planar Array : Here array element centers are located in a plane. Examples of planar arrays are rectangular and circular array.
(3) Conformal Array : In this, array element locations must conform to some non-planar surface such as found on air craft or missile.

End-Fire Array Antennas :
The endfire array is nothing but broadside array except that individual elements are fed in, out of phase. Thus in the endfire array, a number of identical antennas are spaced equally along a line and individual elements are fed with currents of equal magnitude but their phases varies progressively along the line in such a way as to make the entire arrangement substantially unidirectional. In other words, individual elements are excited in such a manner that a progressive phase difference between adjacent elements becomes equal to the spacing (in wavelength) between the elements.
In end fire array the maximum radiation can be directed along the axis of the uniform array by allowing the progressive phase, shift between element to ± fid. This produces a maximum field in the direction 0=0 but does not give the  maximum directivity.

Antenna array is one of the common method of combining the radiations from a group or array of similar antenna in which the phenomena of wave influence is involved. The total field produced by an array is the vector sum of the fields produced by the individual antenna of the array. The antenna array is used in services in which it is desired to radiate most of the energy is one particular direction like highly populated area etc. The field strength can be increased in preferred direction by properly exciting the array of antenna simultaneously. End fire Arrays have individual elements fed in out of phase. Thus in end fire array, a no. of identical antennas are spaced equally along a line and individual element are fed with current of equal magnitude but their phase varies progressively along the line in such a way as to make the entire arrangement substantially unidirectional.

The overall working of antennas was understood. The major parameters (such as Return Loss curves, Radiation Patterns, Directivity and Beamwidth) that affect design and applications were studied and their implications are understood.
We have first simulated our antenna in NEC software. The results which we got for simulation were matched with the actual results using the antennas which we have designed.
For loop antenna radiation resistance is improved and for endfire antenna directivity is improved.

Future scope
Using these antenna we are going to transmit and receive audio as well as video signals. We will try to use these antennas for own purpose and we will try to improve its directivity for future use.
Also we will try to use these antennas for longer distance communication.
As these antennas are kept in the college laboratory, they can be used to perform regular practical by the students.

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