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CHAPTER 6
DESIGN AND FABRICATION OF WLAN ANTENNAS
6.1 Introduction
In the installation of WLAN in any Indoor environments, the deployment of the access
point [AP] is the most important part. The security aspect is to be taken care while the AP
is installed.
When the AP is placed at any one corner of the office room, the signal from this AP can
go beyond office region. People who parked their vehicle at the parking area can get
these signals which may lead to intrusion. Some times, one AP is not enough to cover the
entire office area, so two or more APs are to be deployed. Even in this case, one AP may
be placed at any one corner can leak the signal to the adjacent road or to nearby buildings.
To avoid the leakage of the signals, AP may be placed at the center of the office.
Generally the AP is made up of Omni directional antennas. This type of antenna radiates
energy equally in all directions. The radiation pattern is of spherical shape which forms a
circular cross section. This radiation pattern may miss the corners of the office as most of
the offices are built in rectangular shape. So, Placing AP at the center of the office may
not provide coverage for the entire office which is considered as a serious disadvantage.
If the gain of the AP is to be increased to cover the entire region, the circular cross
section may exceed the required region. Once again the signal is leaked out and available
for the intruders.
Combining the above said issues, the placement of AP is a crucial and difficult one. To
fix the placement of AP, a survey with a laptop and analyzing software like NetStumbler
[75] is to be done through out the entire region. Based on the result, the number of APs
and the position of APs are to be fixed. There is another method with which the above
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said issue can be solved. The Omni directional antennas which are being used as AP may
get replaced with Directional antennas.
Zhaohui Sun et al [13] explained the importance of directional antennas to provide
security of WLAN. AP is placed at the centre and a terminal antenna is placed at a radial
distance. The terminal antenna can be considered as the receiving antenna. It is placed at
an edge of the desired space region. There are four cases considered.
i. AP: Omni Directional Antenna; Terminal: Omni Directional Antenna
ii. AP: Directional Antenna; Terminal: Omni Directional Antenna
iii. AP: Omni Directional Antenna; Terminal: Directional Antenna
iv. AP: Directional Antenna; Terminal: Directional Antenna
By measuring signal strengths in the above mentioned way, they concluded that
employing directional antennas as both AP and terminal antenna can increase the level of
security. As the intruders are possibly blocked out due to directional beam, the security of
WLAN can be enhanced. They advise to reduce the power and gain of the transmitting
antenna and to increase the gain of the receiving antenna to achieve better security for
WLAN.
Directional antennas radiate maximum energy in a particular direction. It is possible to
adjust the antenna placement and the orientation in such a way that the radiation is
maximum in the direction where it is desired. Depending on the location of the antenna
and on the shape of the office space, the direction of radiation can be modified.
In this chapter, a directional yagi antenna is designed based on Generic Algorithm [GA].
GA optimizes dimensions of yagi antenna to minimize backward radiations. With the
modern trend, a four element and five element yagi array in micro strip form are also
designed. The above mentioned antennas can provide better security for WLAN as signal
leakage in unwanted direction is reduced.
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6.2 Yagi - Uda Antenna
Yagi – Uda antenna is considered as one of the easiest and reliable directional antenna.
This antenna was invented by Professor Uda and his teacher Professor Yagi in Japan.
This is a popular antenna as it is used to receive VHF TV transmission across the world.
As directional antennas can serve better for WLAN Indoor networks, the implementation
of Yagi array is considered.
Yagi antenna is made up of a dipole and parasitic elements [17]. The dipole is given input
feed through coaxial cable and is known as driven element. Folded dipole can also be
used as driven element. Yagi antenna consists of a driven element, a reflector and one or
more directors. The reflector and the directors are known as parasitic elements which are
not connected directly to the feeding line but are electrically coupled. The driven element
is generally a half-wave dipole at the frequency of operation. The driven element and
parasitic elements are made up of metallic rods. They are arranged collinearly and close
together with one reflector and one or more directors. The length of the reflector is 5%
more than length of driven element. The length of the first director is 5% less than the
driven element. The subsequent directors are 5 % less in length compared with previous
directors. The spacing between elements is around 0.25λ.
Fig: 6.1 Yagi-Uda Antenna
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6.2.1. Working Principle of Yagi- Uda antenna
The objective of the Yagi-Uda antenna design is to make a traveling wave structure with
currents in all elements contributing in the forward direction. The contributions are
designed to add up in phase in the forward direction, and to cancel in the reverse
direction. The phase of total voltage depends on the spacing between the elements and
upon the length of the elements.
The reactance may be varied by changing the length of the parasitic element. The
directors are smaller than the driven element and present capacitive impedance. Similarly
the reflector is larger than the driven element and presents inductive impedance. Only a
small amount of power is radiated backwards. The net power radiated by the yagi antenna
is directed at forward direction.
6.2.2. Radiation Pattern of Yagi-Uda antenna
Fig: 6.2 Radiation pattern of a yagi-uda antenna
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The radiation pattern of a yagi-uda antenna consists of a major lobe in the forward
direction, minor lobes on the backward direction, and side lobes along the lateral
direction. Only the major lobe is of interest and all other lobes are expected to be as small
as possible for a good antenna performance. Fig. 6.2 shows a typical radiation pattern of a
yagi antenna.
6.2.3. Optimization of radiation pattern
As shown in Fig 6.2, yagi antenna radiates maximum of its energy in the forward
direction. This is also referred as the direction of array axis or at 00 with respect to the
axis of the elements of yagi antenna. There is some part of the energy radiated at angles
other than forward direction. They are known as side lobes. Also from Fig 6.2, it can be
viewed that small amount of energy is radiated backwards.
As some amount of energy is radiated sideways and backwards, people in unintended
directions can receive the signal. If it is possible to reduce the energy at the undesired
directions and redirect them to the direction in need of, users only at the intended region
can receive the signal. With additional gain control mechanisms, it is possible for us to
adjust the gain so that the signal can be restricted within the required region and also
covers entire eligible users. In short, by controlling gain of a directional antenna, the
signal leakage to outside world can be minimized to the extent possible. It provides ‘NO’
signal to intruders. So, the security of WLAN can be improved.
To enhance the gain at the forward direction, optimization methods are to be used. The
following are few optimization methods being used.
i. Neural Netwoks.
ii. Particle Swam Optimization.
iii. Gradient Descent Method.
iv. Genetic Algorithm.
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The optimization algorithms provide fresh dimensions for the antenna. With the design
and implementation of antenna dimensions based on the optimization results gives better
radiation patterns.
6.2.4. Reason for using Genetic Algorithm
A Genetic Algorithm is an iterative procedure that consists of a constant-size population
of individuals, each one represented by a finite string of symbols, known as the ‘genome’,
which encodes a possible solution in a given problem space. This space, referred as the
‘search space’, comprises of all possible solutions to the problem. Genetic Algorithm is
applied to spaces which are too large to be exhaustively searched. The symbol alphabet
used is often binary, though other representations such as character-based encodings,
real-valued encodings, and tree representations have also been used.
The goal of Genetic Algorithm [GA] is to find an optimized solution for a given problem.
Other optimization techniques such as gradient descent methods search a region of the
solution space around an initial guess to get the best local solution. For problems that
have a small number of parameters and small solution space, gradient descent methods
perform well because they are able to search a significant portion of the entire solution
space.
However, as the number of parameters and size of the solution space increase, the quality
of the solution depends upon the location of the initial guess. If the initial guess falls in a
region of the solution space where all the local solutions are poor, a local search can find
the best of available poor solutions.
Because of the coupling effects between the various structures in an antenna, it is often
difficult to provide good initial guesses for the design. For such problems, the GA
approach of conducting a global search for a solution that satisfies some specified
performance criterion becomes effective.
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GA differs from most traditional optimization in two ways. First, they do not operate
directly on the design parameter, but on a symbolic representation of the parameters.
Second, they simultaneously optimize entire population of designs at once, not a single
design at a time. GA is a class of search technique that mimics the metaphor of biological
evolution to conduct a global search in the given solution space.
Genetic Algorithms are on the rise in electromagnetism as design tool and problem solver
because of their versatility and ability to optimize in complex search spaces [19]. GA acts
on an initial population of individuals and search the whole population for the best fit
individual. These individuals are encoded as strings using various alphabets. The strings
are known as ‘Chromosomes’ and the alphabets are represented in binary.
The procedure of standard genetic algorithm is given as follows [18]:
i. An initial population of individuals is generated.
ii. In every evolutionary step, known as a ‘generation’, the individuals in the
current population are decoded and evaluated according to some predefined
quality criterion, referred as the ‘fitness’ or ‘fitness function’.
iii. To form a new population at next generation, individuals are ‘selected’
according to their fitness. Individuals are selected with a probability
proportional to their relative fitness. This ensures that the expected number of
times an individual is chosen is approximately proportional to its relative
performance in the population. Thus, high-fitness individuals stand a better
chance of ‘reproducing’, while low-fitness ones are more likely to disappear.
The search process is carried out on the encoded individuals rather than on the
original individuals.
iv. The goodness of individual is evaluated through an objective function.
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Three important genetic operators are selection, cross-over and mutation. A new set of
individuals are created at every generation by selecting individuals according to a
selection rule and breeding a new generation by using the genetic operators.
Selection alone cannot introduce any new individuals into the population, i.e., it cannot
find new points in the search space. These are generated by genetically-inspired operators,
which are known as ‘crossover’ and ‘mutation’. Crossover is performed with probability
‘pcross’ between two selected individuals, called ‘parents’, by exchanging parts of their
genomes to form two new individuals, called ‘offspring’. ‘pcross’ is known as the
‘crossover probability’ or ‘crossover rate’. This operator tends to enable the evolutionary
process to move toward promising regions of the search space.
The mutation operator is introduced to prevent premature convergence to local optima by
randomly sampling new points in the search space. It is carried out by flipping bits at
random, with a probability ‘pmut’.
Genetic algorithms are iterative processes that are aimed at convergence for optimal
solution. The termination condition may be specified as number of generations or as the
attainment of an acceptable fitness level.
The standard genetic algorithm in pseudo-code format is given below:
begin GA g = 0 Initialize population P(g) Evaluate population P(g) while not done do g = g+1 Select P(g) from P(g-1) Crossover P(g) Mutate P(g) Evaluate P(g) end while end GA
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6.2.5. Flow Chart of Genetic Algorithm
Fig: 6.3. Flow Chart of Genetic Algorithm
The various parameters involved in Genetic Algorithm are listed and defined below:
i. Gene
Gene is the fundamental building element of genetic algorithms. Genes are used to
encode the solution parameters with the most suitable codification. The most common
representation for genes is the binary alphabets.
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ii. Chromosome
A chromosome is formed by concatenation of the encoded individual measurement,
elemental length and spacing. These are considered as the general initial population
which the genetic algorithm acts upon.
Fig: 6.4 Chromosomes
iii. Population
A population is a set of individuals or chromosomes which contains all the proposed
solutions generated by the algorithm.
iv. Generation
During the first part of the algorithm, a set of many populations of individuals are created
in order to sweep the entire area of the solution space. A set of population is called
generation. Later, the two terms ‘generation’ and ‘population’ means the same thing since
any generation is composed of a single population.
v. Objective Function
The quality of a design is expressed mathematically by an Objective function. The
Objective function calculates the goodness of a trial solution. The following objective
function is applicable for an antenna design.
O(x) = aG(x) - b|Z0-Re (Z(x))| - c|Im (Z(x))| --- (6.1)
0 0 0 1 11 1 1 0 0 0 01 0 1 1 1 0
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Here,
G (x) is gain of the antenna
Z (x) is input impedance of antenna
Z0 is characteristic impedance of cable which feeds antenna.
The constants ‘a’, ‘b’ and ‘c’ are weights that control the contribution from each
term to the overall objective function.
vi. Fitness Scaling
The fitness value used to rank the individual designs for the selection process is a scaled
version of the Objective function. Scaling the objective function serves two main
purposes. First, the selection phase requires that the fitness of an individual is a non-
negative value. Second, scaling used to control the competition between individuals.
By using a scaling function, the search is more localized because the population is full of
similar, highly fit individuals.
There are many approaches to scaling objective functions. One of the standard methods is
linear scaling. Linear scaling translates objective functions to fitness values in the
following manner.
F(x) = a O(x) + b ---- (6.2)
Here,
O(x) is the objective function specified in equation 6.1.
The constants ‘a’ and ‘b’ are chosen so that an individual with the average objective
value will have an average fitness value.
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vii. Cross-Over
The cross-over or recombination is the basic operator for producing new chromosomes. It
takes two individuals known as parents to generate two new ones known as children. In
this way, new individuals have some parts of both parent’s genetic material.
Cross-over takes place with a probability ‘pcross’ and, in the case of single point, a random
location in the two parent’s chromosomes is selected and subsequently the two first
portions of the two chromosomes are exchanged to create children. The aim is to produce
more fit individuals, thanks to the exploration of the search space that cross-over
encourages.
Fig: 6.5 Cross-Over operation
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viii. Mutation
The mutation operator guarantees an exhaustive possibility of exploration all over the
available solution space. Mutation has a probability to occur and corresponds to the
change of a random selected element in the string. A random variable is generated for
each bit in a sequence. That particular bit is modified based on this random variable. In
the case of binary coding, the selected bit in the chromosome is inverted.
Fig: 6.6 Mutation operation
ix. Selection
The Optimization process takes place under the influence of a selection process, which
acts based on the Objective function. Numerous selection strategies, both stochastic and
deterministic, have been proposed for the GA optimization. A very common
deterministic strategy is the population decimation. In this case, the population is ranked
according to the fitness values of the chromosomes.
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Then an arbitrary fitness threshold is chosen and each individual with a worse fitness is
discarded. This simple strategy, which allows fast convergence, has a serious drawback.
It eliminates chromosomes having positive genetic characteristics but not good fitness.
To prevent this fault, a stochastic strategy, called as ‘roulette wheel selection’ is
implemented. In this method, even unfit chromosomes may have a finite probability of
selection. Its value is:
---- (6.3)
During selection, individual is assigned a probability of selection, based on the ratio of
the individual’s fitness [fi] to the total fitness of the member population [fk].
Fig: 6.7 Roulette wheel selection
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The roulette-wheel selection allots a portion of a roulette wheel to each individual so that
every time the roulette wheel is spun, an individual has ‘Pi’ chance of being selected. If
the roulette wheel is spun ‘N’ times, an individual of average fitness expects to be
selected once and an individual with twice the average fitness expects to be selected
twice. However, the samples are usually too few to produce the expected selection
distribution. Instead, some individuals may be selected more often than they should be
while some average individuals may never be selected at all.
Another method of selecting the individuals is the binary tournament, where two pairs of
chromosomes are randomly selected from the population. Then a tournament is
conducted between them. Those two which win the tournament are considered the fit
individuals.
Fig: 6.8 Tournament Selection
Those two individuals who are known as ‘parents’ crossover to produce new ‘children’.
After this, there is a comparison between the parents and children to choose the best and
replacing the children in the place of their parents. This will continue for the number of
generation the user wants.
Depending on the requirement and user preference, any of the above mentioned methods
can be used.
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6.3 Representing Antennas in Genetic Algorithm
GA works with chromosomes which are representations of the solution to the problem.
Thus, each individual solution in a population is represented by a chromosome.
A yagi array is the antenna structure which is to be optimized. Jones et al explained the
optimization of yagi antenna based on Genetic algorithm [20].
A four-element Yagi-Uda array consists of a single driven element, one reflector element,
and two director elements. This antenna array is described by specifying the length ‘L’ of
each element and the spacing ‘S’ between adjacent elements. A half wavelength dipole is
considered as driven element.
Fig 6.9 A four-element yagi-uda antenna
Fig 6.10 Concatenation of all values together to form the chromosome
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The method with which yagi antenna is represented in terms of chromosome is shown in
Fig.6.10. The parameters are discretized and encoded as binary values so that
--- (6.4)
Here, ‘BL’ is the number of bits used to represent ‘L’. ‘BL’ is found by
--- (6.5)
Where ‘Lmax’ and ‘Lmin’ define the range over which ‘L’ can vary and ‘Lres’ is the
resolution of the discretization. If ‘L’ is not an integer, it is rounded up to the next integer
value. The binary representation of ‘L’ is decoded by
--- (6.6)
The chromosome is formed by concatenating all the encoded parameters together into a
single binary string. The size of the search space is defined by the length of the
chromosome. GA converges to a solution faster for smaller search spaces.
6.4 Functioning of Genetic Algorithm
A prefixed number of populations are randomly created initially. For example, if it is
assumed that each population is formed by 20 chromosomes and prefixed number of
population is 40, there are 800 chromosomes at the end of the first step. With the
assumption that each population is formed by 20 chromosomes, and being constant the
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number of chromosome in population, the number of chromosomes involved in each step
of the process after the generation of the initial population will be 20. Among these 800
chromosomes, we choose the best 20 to create the first generation.
The second generation is created by evaluating the objective function of chromosomes
and selecting couple of chromosomes with best fitness value and mating them. This
selection process is achieved by employing roulette-wheel selection. The main objective
of the selection process is to select the best fit individuals and contribute their positive
trait to the next generation.
After the selection process, the next genetic process is the cross-over, where there is a
transfer of genetic material from parent chromosome to child chromosome. A pair of
parents of the older generation makes a pair of children in the next consecutive
generation. This process plays a significant role in contributing the good trait to the next
generation.
The next genetic operator that comes in the line is the mutation operator. In Genetic
Algorithm, mutation is used to create variety of solution. During the selection and
crossover, many good individual with good and positive trait might be lost. Mutation may
bring back those good traits of individuals and help in obtaining a good, optimized
solution.
After performing the genetic operations, it is necessary to select the best 20 chromosome
which represent the next generation. This is done by evaluating objective function for all
chromosomes. By proceeding this way, it is possible to generate the required population
for the prescribed number of generations.
6.5 Convergence of Genetic Algorithm
The convergence of Genetic Algorithm is based on how fast the algorithm finds a
solution for the given problem. The algorithm converges faster for a problem which has a
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smaller chromosomal length and fewer solutions in the solution space. On the other hand,
if the problem has a larger chromosomal length and more solutions in the available
solution space, the convergence will be slower. The convergence also depends on the
complexity of the problem and the complexity of the objective function.
For an optimal solution, the convergence should neither be fast or slow. Thus, the
convergence is also acting as an important parameter in deciding the quality of the
solution. In Fig 6.11 shown below, each dot represents an individual solution for the
given problem. It has fewer solutions compared with the solution space represented in Fig
6.12.
Fig. 6.11 Fewer solutions in the solution space
Fig. 6.12 More solutions in the solution space
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6.6 Designing parameters of yagi array
To optimize the performance of the antenna, Genetic Algorithm is implemented. This
implementation was done using software called yagi genetic optimizer 3 Version 1.0a.
This is a freeware.
6.6.1 Yagi Genetic Optimizer
This is a self extracting program. On running the program, it self extracts all the
necessary files. The important file of all is the configuration file named YGO3.CFG. The
inputs are the number of generations, population size, number of elements, selection
methodology, cross-over methodology and weighted values for the objective function.
It is also necessary to select the objective function, named as Figure of Merit (FoM). This
software provides two FoMs. There are two possible equations available from which the
required equation can be selected.
FoM1 = [a*Gfwd - b*(SWR-c)*U (SWR-c) + d*FB + e*FR - f*MaxSLL] / (a+b+d+e+f)
--- (6.7)
FoM2 = [a*Gfwd - b*³Zo - Rin³ - c*³Xin³ + d*FB + e*FR - f*MaxSLL] / (a+b+c+d+e+f)
--- (6.8)
Where,
Gfwd - Forward gain (dBi), along +X-axis (θ =0o).
FB - Front-to-Back Ratio (dB).
FR - Front-to-Rear Ratio (dB).
Rin - Feed Point Input Resistance.
Xin - Feed Point Input Reactance.
SWR - Standing Wave Ratio.
MaxSLL - Maximum Side lobe Level
* - Multiplication.
/ - Division.
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U - Unit Step, U(x) = 0 for x < 0, and = 1 for x > 0.
And a, b, c, d, e, f are the weighted values for the objective function.
To run the program it is necessary to define the upper and the lower limits of the
elemental length, spacing between the elements, radius of the element, and the offset.
Equation 6.7 is chosen as the equation of Figure of Merit in this work.
After defining all the required parameters, the program is set to run. The program starts
with generation one and optimizes the entire chromosome in that generation. At the end
of first generation, the best chromosomes are stored and the rest are discarded.
The program runs till it reaches the maximum number of generations specified by the
user. At the end of the run, the best chromosomes can be viewed. The output of the
optimizer contains the elemental lengths and spacing for the yagi antenna. The
dimensions of antenna elements are given in terms of wavelength. The wavelength can be
calculated based on the frequency of operation. By substituting the value of wavelength,
actual dimensions of various antenna elements can be found out. In this work,
optimization is carried out for four element yagi array which consists of a driven element,
a reflector and two directors. The lengths of these elements and the spacing between two
consecutive elements are found out with this optimization procedure.
6.6.2 Result from Yagi Genetic Optimizer
Name Dimension Reflector length 0.55000 λ Reflector spacing 0.07500 λ Driven Element length 0.49538 λ Driven Element spacing 0.12500 λ Director1 length 0.45000 λ Director1 spacing 0.17500 λ Director2 length 0.37500 λ
Table 6.1 GA Optimized Yagi Array Dimensions
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In this work, ten numbers of generations are specified. The dimensions obtained from the
Yagi Genetic Optimizer at the end of tenth generation are given in Table 6.1.
From the dimensions given in the above table, the antenna can be constructed and its
performance characteristics can be checked. Here, the dimensions are given in terms of
wavelengths. So, it is possible to construct the yagi antenna for any frequency. The
frequency can be converted into the corresponding wavelength and with this value, it is
possible to construct yagi antenna for that particular frequency.
To have better operational characteristics for a wire based yagi antenna, the operating
frequency is selected as 750 MHz. The corresponding wavelength is calculated from the
equation λ = C / f, where ‘λ’ is the wavelength, ‘f’ is the frequency and ‘C’ is the velocity
of light [3 × 108 m/s]. The dimensions of antenna elements are given in table 6.2.
Name Dimension (in meter) Reflector length 0.22 Reflector spacing 0.03 Driven Element length 0.198 Driven Element spacing 0.05 Director1 length 0.18 Director1 spacing 0.07 Director2 length 0.15
Table 6.2 Yagi Antenna dimensions at 750 MHz
6.7 Simulation of yagi array
6.7.1. MMANA Version 1.77
The software used in this work is MMANA Version 1.77. This software supports the
analysis of all wired type antennas. MMANA is an antenna-analyzing tool based on the
moment method, which was introduced in MININEC version 3. The BASIC source code
of the computation engine is published in MININEC. It has been ported to C++ (Borland
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C++ Builder) and made the graphical user interface for the ease of use. An advantage of
this software is that it is user friendly, easy to understand and gives better results.
MMANA has the following functions:
i. Table-based editor for the antenna design and definition
ii. Graphical antenna viewer (it shows the segmentation and current distribution)
iii. Viewer for the horizontal and vertical beam pattern
iv. Comparator for two or more computation results
v. Antenna element editor
vi. Antenna wire editor
vii.Tools for defining the combination of pipes with different diameters
viii.Antenna optimizer tool
ix. Sheet browser of the optimized results with manual tuning capability
x. Frequency characteristics chart maker
xi. Data file generator
Yagi array based on table 6.2 is implemented in this software.
6.7.2. Antenna Definition in MMANA
MMANA provides a couple of ways to define the antenna. The straightest way is to
input the antenna coordinates in the form of a table. Clicking on the Geometry tab, an
editor is available in the table format like a spreadsheet. With the table, the antenna
parameters can be defined including wire dimension, sources, and loads.
The wires which are used to construct yagi antenna is defines as follows:
X1 X-axis starting position of the wire (unit is meter or lambda)
Y1 Y-axis starting position of the wire (unit is meter or lambda)
Z1 Z-axis starting position of the wire (unit is meter or lambda)
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X2 X-axis ending position of the wire (unit is meter or lambda)
Y2 Y-axis ending position of the wire (unit is meter or lambda)
Z2 Z-axis ending position of the wire (unit is meter or lambda)
R Radius of the wire (unit is millimeter or lambda)
SEG Segmentation method
The antenna is to be first drawn at the geometry window of the MMANA editor. The
frequency is selected as 750 MHz. The dimensions in X,Y and Z axis are to be calculated
and the corresponding length of elements are to be drawn at the geometry editor window
of MMANA.
Fig: 6.13 Antenna definition – geometry tab
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Yagi array generally radiates in single direction. To have radiation in both opposite
directions, a double sided yagi array is proposed. This is an improvement over the
conventional yagi antenna shown in figure 6.1. Double sided yagi array is designed to
provide directional main beam of the antenna in both 0o and 180o with respect to the array
axis. Here two sets of yagi array based on the design of yagi genetic optimizer is placed
facing each other and both are placed in the same plane.
The antenna element’s length and spacing are entered in the geometry tab as shown in the
above diagram. Here, double sided yagi antenna is implemented based on the design done
for single sided normal yagi antenna. The mirror image like structure, which is similar to
normal yagi antenna, is formed. The antenna which is to be simulated is having two
reflectors facing opposite direction. Each of the antennas has one dipole, one reflector
and two directors. This structure is drawn and simulated using MMANA.
6.7.3 Viewing the antenna in MMANA Environment
Fig: 6.14 Viewing the antenna
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The antenna structure of a double sided yagi array can be viewed by clicking on the view
tab. Based on the dimensions specified in table 6.2, this antenna is drawn in the drawing
editor of MMANA. Sources are placed at the exact centre point of the dipole on each side.
With the horizontal rotate and vertical rotate tabs, it is possible to rotate the structure of
antenna in this view editor and antenna can be checked at various angles.
6.7.4. Computing the antenna in MMANA
After defining the antenna, it is necessary to find its performance characteristics.
Fig.6.15 Computing the antenna
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The following parameters are to be defined before computing:
The design Frequency
Ground condition
Antenna height above the ground
The wire material
6.7.5. Far field plot in MMANA
Fig 6.16 Simulated Radiation Pattern
Far field plot is shown in the above diagram. The left chart shows the horizontal pattern.
The right chart shows the vertical pattern. The vertical pattern is obtained by slicing the
radiation pattern with the vertical plane that includes the X-axis. The horizontal pattern
is obtained by slicing the radiation pattern horizontally. The front to back ratio and other
results are also calculated.
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6.8 Fabrication and testing of yagi array
6.8.1 Design of the antenna
The design of the antenna requires the elemental lengths and spacing. The following
dimensions are obtained from the output of the YAGI GENETIC OPTIMIZER 3 Version
1.0a. This output contains
i. The length of all the antenna elements.
ii. The spacing between the elements.
iii. The radius of the elements.
The output is represented in wavelength units based on the resonant frequency. The
simulated results are found out using MMANA software.
6.8.2 Components used for the fabrication of antenna
The components required for the construction of antenna consists of an antenna base
boom and antenna elements.
i. The antenna base is an important component and serves as point where the feed
cable can be connected. It is used to connect antenna with remaining electric circuitry
of communication system.
ii. The boom is the central rod that holds the antenna elements in place. The boom
forms the basic skeleton of the antenna.
iii. The ‘antenna elements’ are the functional units of the antenna. They are made up
of conducting material, and known as wired elements. They convert electrical signal
to EM wave and vice versa.
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6.8.3 Material used for antenna fabrication
The materials used the yagi antenna are:
(i) Antenna base – An antenna female pin connector is used.
(ii) Boom – the requirements for the boom are:
i. A Light-weight material
ii. A Non-conducting material
iii. Cheap and easily available
A material that could satisfy these requirements is wood. A single, thin slab of wooden
material is used.
(iii) Antenna elements – the requirements for the selection of antenna elements are
i. A Light-weight material
ii. A Conducting material
iii. Material with a desired radius
iv. Cheap and easily available
An ‘aluminium’ rod is used to construct the antenna. The constructed bi-directional yagi
antenna is shown in Fig 6.17.
Fig 6.17 Constructed Bi-directional Yagi Array
96
6.8.4 Antenna experimental setup
The antenna setup consists of a transmitting antenna, receiving antenna, transmitter box,
receiver box, a stepper motor controller and a computer with a plotting program.
i. Transmitting antenna
A folded dipole antenna is used as transmitter.
ii. Receiving antenna
The fabricated bi-directional yagi-uda antenna which is shown in fig 6.17is used as
receiving antenna.
iii. Transmitter box
It is a digital transmitter, which can generate signals ranging from 80MHz –
860MHz. The generated signal is fed to the transmitting antenna through co-axial
cable.
iv. Receiver box
It is a digital receiver that is used to receive the signals from receiving antenna.
The received signal is fed to the computer.
v. Stepper motor controller
This is used rotate antenna in steps of a predefined angle. It can rotate antenna to
the entire 360 degrees.
vi. Computer
The received signal from the receiving antenna is fed to a computer. The received
signal strength against the angle of rotation is noted on a polar chart. With all the
readings available, the radiation pattern is obtained in polar chart. It is possible to
take print out of this radiation pattern.
97
6.8.5 Measurement of radiation pattern
To measure the radiation pattern, the following steps are used.
i. Connect all the devices and their supply wires.
ii. Fix the operating frequency as 750MHz in both the transmitter and receiver box.
iii. Fix the step angle to 5 degrees in stepper motor controller box.
iv. Switch on the stepper motor controller and ‘collect data’ tab in polar chart.
v. The radiation pattern of the antenna is plotted on a polar plot with a dB scale.
6.8.6 Inference from the radiation pattern
Fig 6.18 Measured Radiation Pattern
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Figure 6.18 shows the measured radiation pattern of fabricated bi-directional yagi
antenna.
The result which is obtained from the laboratory using the fabricated bi-directional yagi
antenna is to be compared with that of simulation result from the software MMANA. The
comparison is tabulated below.
Performance
Parameters
Results from Simulation Results from the Laboratory
(without any conducting material between the
antennas) Gain 15.88 dB 12.74 dB
HPBW 62 degrees 45.25 degrees
Table: 6.3 Comparison of simulation and laboratory results
Where,
Gain = Gain of the antenna.
HPBW = Half power beam width of the antenna.
6.9 Interference between two yagi arrays
6.9.1 Reason for Interference
Section 6.3 briefly explains how an individual yagi-uda antenna works. So, any signal
radiated from the driven element is reflected back by the reflector element. But, in
practice not all the radiation is reflected by the reflector element, a part of the radiation is
available beyond the reflector element. This part of the radiation is represented as a minor
lobe in the undesired, backward direction.
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Conventionally, yagi array radiates only in the direction of its axis. This is also at the
direction opposite to the reflector. To have yagi array to radiate at either ends of array
axis, stacked yagi array is used. In this array, two yagi arrays are placed opposite
direction but not in the same plane.
In this work, a pair of yagi-uda antennas is arranged back-to-back in the same plane.
There is a possibility that radiation from both the reflector elements will interfere with
each other. The interference from the two antennas may reinforce together or cancel each
other. This is the reason for the interference occurred in the radiation pattern. Interference
is observed as minor lobes at 90o and 270o directions to the array axis when measurement
is done with fabricated antenna.
6.9.2 Mitigation of interference
The interference of the signals between the antennas is due to the radiation that is found
beyond the two reflectors. To get better results, it is necessary to reduce these
interferences. There are two ways of reducing these interferences:
i. By placing a microwave absorbing material between the two antennas.
ii. By placing a material with high conductivity between the two antennas.
The former idea of placing a microwave absorbing material like polyurethane foam is not
preferred because they are costly. But the idea of placing a material with high
conductivity between the antennas is preferable, because the material is easily available
and cheap.
By applying this idea, we can reflect the signals traveling in the opposite direction and
make them concentrate in the desired direction. A material with a better conductivity and
also cheap is Aluminium. A thin sheet of aluminium is to be used in between two yagi
arrays which are placed opposite each other.
100
Fig 6.19 Measured Radiation Pattern with reduced Interference
Fig 6.19 shows the measured radiation pattern of bidirectional yagi antenna with
aluminium sheet placed in between two yagi structures. By comparing fig 6.18 with the
figure shown above, it can be concluded that minor lobes are reduced a lot. It shows the
aluminium sheet reflects the signal and avoids the interference in bi-directional yagi
antenna. The width of the aluminum sheet may be increased to get better interference
reduction.
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6.10 Micro Strip Yagi Array Micro strip antennas are preferred now over many other antennas. Micro strip antennas
offer several advantages and also few disadvantages. These antennas are very small and
lightweight. This comes at the cost of not being able to handle as much output power as
other antennas. Initially, Micro strip antennas are not well suited for wideband
communications systems. But with few techniques, wide band micro strip antennas are
used in practice.
Table 6.4 Comparison of different types of antennas
6.10.1. Requirements of Micro strip Yagi Array
Yagi antennas which are directional are constructed with wires. Basically, it is considered
as a wire antenna. But with the advent of micro strip antennas, all other types of antennas
are fabricated in the micro strip form. In this form, those antennas enjoy their own
advantages and also they can avail the advantages provided by the micro strip structures
such as smaller size, light weight, easy to handle and easier fabrication.
As micro strip antenna is planar and compact, conventional antennas are fabricated in
micro strip form. The yagi antenna is designed and tested as wire antenna in sections 6.7
and 6.8. It can also be designed, fabricated and tested as the micro strip yagi antenna.
The following parameters are to be selected prior to design the antenna.
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i. Selection of Frequency Band
The antenna is designed to operate at WLAN band. So the resonant frequency of the
antenna is selected as 2.4 GHz.
ii. Selection of the Feeding Technique
The coaxial feed, micro strip feed, proximity coupling are few feeding techniques
available for micro strip type antennas. Coaxial feed is selected for micro strip yagi array
which is designed with conventional yagi antenna dimensions. The micro strip feed is
used for gain optimized micro strip yagi antenna.
iii. Selection of Substrate
Before the construction of an antenna, a suitable substrate must be chosen. The selection
of a substrate is guided by the following factors:
i. A low value for ‘εr‘increases the fringing field and thus the radiated power.
ii. A high loss tangent increases the dielectric loss and reduces the antenna efficiency.
iii. A low ‘εr‘ decreases the quality factor and in turn increases the bandwidth.
iv. A high ‘εr‘ increases the quality factor and in turn decreases the size of the
substrate thickness.
v. The substrate thickness is to be kept as thin as possible to prevent multiple surface
waves.
vi. To keep the overall attenuation as low as possible.
vii. Environmental limitations, such as temperature, humidity, and aging effects.
viii.Physical limitations, such as conformability, stability and weight
ix. Availability
x. Cost
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Due to the low cost and the ease of availability, FR4 is preferred over other substrates. It
stands for ‘fire resistant’. It is basically epoxy substrate with glass enforcement compliant
with electroplating. It is used for micro wave frequency applications due to its low cost,
easy availability, ease of fabrication and its strength.
6.10.2 Simulation of conventional Micro strip yagi Antenna
The basic antenna to be designed is micro strip yagi array. Korinek et al proposed a
planar micro strip yagi array with reflector as partial ground plane [27]. In a planar
antenna it is better to have all the elements in the same plane. The micro strip yagi array
is to be simulated and tested with reflector in the same plane as that of other elements. To
connect the dipole with the feed, a gap which is small compared to the length, is provided
at reflector. The software used for the simulation is Ansoft HFSS. The operating
frequency is 2.4 GHz which is the standard frequency for WLAN operations. The
dimensions of the substrate are 165×82(mm). The substrate used is FR4, having relative
permittivity of 4. The wavelength to be used is the ratio of free space wavelength to the
square root of relative permittivity of the substrate.
Length of the driven element is 0.5 λ, which is 29.9 cm.
Length of the director1 is 5% less than driven element, which is 28.4 cm
Length of the director2 is 5% less than director1 which is 26.98 cm.
Length of director3 is 5% less than director 2, which is 25.63 cm.
Length of the reflector is 5% more than driven element which is 31.4 cm.
Spacing between the elements is 0.3λ for normal Yagi array. Using the above specifications which are the standard design for a normal yagi array, the
antenna structure is constructed in the HFSS environment. The top view of the structure
is shown in Fig. 6.20. This is a five element yagi array, constructed in x-y plane.
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Once the yagi array is simulated, return loss Vs Frequency graph can be found out. The
graph, Obtained from HFSS is given in Fig 6.21. Similarly VSWR Vs Frequency and the
Gain Vs Frequency can also be found out which are shown as Fig 6.22 and 6.23
respectively.
Fig 6.20 Normal Yagi Array
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Fig 6.21 Return Loss Vs Frequency for Normal Yagi array
Fig 6.22 VSWR for Normal Yagi array
106
From the Fig 6.21, the return loss is -25.5 dB at 2.4 GHz. So, it is evident that the antenna
is resonating at 2.4 GHz, which is the frequency intended. It shows a good return loss
which means that only small amount of power is reflected and the maximum of power is
radiated from the antenna.
The Voltage Standing Wave Ratio [VSWR] curve in Fig 6.22 shows that VSWR value of
1.7. VSWR value less than ‘2’ is also a satisfying result.
6.10.3 Simulation of optimized Micro Strip yagi antenna
The dimension of optimized yagi antenna is given in section 6.6. The lengths and spacing
of the various elements for the gain optimized yagi structure are to be used to draw
optimized yagi antenna in HFSS.
Here the operating frequency is 2.4 GHz and the wavelength is calculated for this
frequency by considering the relative permittivity of the substrate as 4.4. To have better
gain, one more antenna element is added in this structure. A five element micro strip yagi
structure is designed based on yagi genetic optimizer. The design is given below.
Name Dimension Reflector length 0.5625 λ Reflector spacing 0.116 λ DE length 0.498 λ DE spacing 0.105 λ D1 length 0.465 λ D1 spacing 0.167 λ D2 length 0.382 λ D2 spacing 0.155 λ D3 length 0.357 λ
Table 6.5 Optimized Yagi array design values
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Table 6.5 gives the dimension of various parts of five element yagi array. Compared with
the wire yagi antenna, an additional element is added with the given length and spacing.
The substrate on which the micro strip yagi array is to be fabricated is FR4. The dielectric
constant of the substrate is 4.4.
The calculation for the wavelength is given as
λ = r
Cε
. f1 ------ (6.9)
In equation (6.9),
‘εr’ is the permittivity of the substrate material used,
‘C’ is the velocity of light which is 3 × 108 m/s
‘f’ is the resonant frequency at which the antenna is to be designed.
The length of various elements of yagi array and their spacing are calculated based on the
table 6.5. For the operating frequency of 2.4 GHz, value of ‘λ‘ is calculated as per
equation 6.8.
The value of ‘λ‘ is calculates as 0.598 meter. The lengths and spacing of various elements
specified in table 6.5 are calculated with this ‘λ’ value. These dimensions are used to
construct optimized micro strip yagi array in the design environment of HFSS. The return
loss, VSWR can be found out from simulation results.
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Fig 6.23 Optimized Micro Strip Yagi Antenna
The above diagram gives the structure of five element Micro strip yagi array based on
Genetic Algorithm optimization. The feed is micro strip feed bisecting the reflector
element. The feed is connected at the center of the dipole with the help of matching
network which is a quarter wave transformer [27]. The entire region on the other side is
considered as the ground plane. The reflector is placed in the same plane where
remaining elements are placed.
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Fig 6.24 Return loss graph for antenna with GA
The above diagram shows that the maximum return loss of -28 dB is obtained at 2.32
GHz. Though the resonant frequency is slightly shifted, the return loss is good.
Fig 6.25 VSWR for Micro Strip Yagi Antenna with GA
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From Fig.6.25, the VSWR is 1.25 at 2.32 GHz which shows maximum of the power is
reflected in that frequency.
The current distribution of Micro strip yagi is shown in the diagram shown below. The
maximum current is shown in ‘Red’. The dipole attracts maximum current so that
maximum power is trasmitted. The ‘balun’ acts as a good impedance matching unit so
that maximum of the feed current reaches the dipole which is the active elemet in the
above structure.
Fig 6.26 Current distribution for Micro Strip Yagi Antenna with GA
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6.11 Four Element Micro strip yagi array Section 6.6 resulted in a four element wired yagi array whose elemental lengths and
spacings are optimized for forward gain as per equation 6.7. The dimension of gain
optimized yagi antenna was given in table 6.1. In section 6.7, gain optimized wired
yagi antenna was simulated using MMANA. In section 6.8, the fabricated antenna is
tested and it was compared with the simulated result. This antenna operates at 750
MHz which makes the design and testing of wired yagi antenna simple.
The dimensions given in table 6.1 are used to construct micro strip based gain
optimized four element yagi array. This antenna operates at WLAN frequency of 2.4
GHz. A conventional yagi antenna is also simulated and results are compared.
The conventional micro strip yagi antenna with four elements has dimensions same
as that of first four elements of the conventional yagi antenna discussed section
6.10.2. FR4 is used as the substrate material.
Fig 6.27 Conventional four element micro strip yagi antenna
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This antenna is simulated with CST Microwave studio, a three dimensional
microwave simulator. The structure of conventional yagi antenna is given in fig
6.27. The dipole is fed with discrete edge port.
Fig 6.28 Return Loss Vs Frequency
Fig 6.29 VSWR Vs Frequency
Fig 6.28 shows the return loss Vs frequency characteristics of conventional four element
yagi antenna. The return loss is negative maximum at 16.894 dB at 2.8 GHz. This is a
good result. Fig 6.29 shoes the VSWR Vs Frequency. The VSWR is 1.334 which is also a
desired result.
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Fig 6.30 Gain Characteristics of conventional yagi antenna
The gain of the conventional four element yagi antenna is shown in the fig 6.30. The gain
is specified as 5.235 dB in the forward direction.
Fig 6.31 Gain optimized Four Element Micro strip Yagi Antenna
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As per table 6.1, the dimensions of gain optimized four element micro strip yagi antenna
is constructed in CST Microwave studio. The antenna is given in fig 6.31. The return loss
and VSWR are also given in the following figures.
Fig 6.32 Return Loss Vs Frequency
Fig 6.33 VSWR Vs Frequency From fig 6.32, the return loss of -34.11 dB is achieved at 2.9 GHz. Fig 6.33 gives VSWR
of 1.04405.
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Fig 6.34. Gain Characterists of optimized array Fig 6.34 shows the gain characteristics of optimized yagi antenna. It gives gain of 7.996
dB in forward direction.
6.12 Enhancing Security of WLAN with Optimized yagi Antenna 6.12.1 Five Element Micro strip yagi array
From section 6.10, five element Micro strip yagi antenna with the conventional method
and in gain optimized method are designed. The corresponding return loss and VSWR of
those two antennas are given below.
Type Return Loss [in dB] VSWR [in dB]
Conventional Micro strip
yagi
-22.5 1.7
Proposed Optimized Micro
strip yagi array
-28 1.25
Table 6.6 Comparison of conventional and proposed five element Micro strip yagi array
116
By comparing the results of conventional antenna and the optimized antenna structure, it
is clear that -5.5 dB of more return loss is obtained with optimized micro strip yagi
antenna. The above fact tells that more power is radiated in the required direction. This
shows that this antenna has more gain and directivity.
6.12.2 Four Element Micro strip Yagi array
Conventional Micro strip Yagi antenna
proposed Micro strip yagi antenna
Return loss [dB] -16.894 -34.11
VSWR 1.334 1.04405
Gain [dB] 5.235 7.996db
Table 6.7 Comparison of conventional and proposed four element Micro strip yagi antenna
Table 6.7 gives comparison on various characteristics of conventional micro strip yagi
antenna and gain optimized micro strip yagi structure with four elements. It is evident
that optimized structure gives better return loss, VSWR and forward gain. More than
double the gain is achieved with the optimized yagi antenna design. The VSWR of
1.04405 is almost equal to the ideal case. It can be concluded that by optimizing yagi
antenna, better characteristics can be obtained. With the increase in gain, it is known that
optimized structure reduces the radiation in unwanted direction and directed at array axis
with enhanced gain.
6.12.3 Security enhancement with optimized antennas
The gain of the optimized micro strip yagi antenna is increased compared with the gain of
the normal micro strip yagi antenna. As directivity of an antenna is proportional to the
gain, the directivity is also increased for the optimized micro strip yagi antenna.
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The directivity and the beam width are inversely proportional as per the basic antenna
theory. So, the beam width of the optimized micro strip yagi antenna is smaller than that
of the normal antenna.
As the beam width is reduced, it is possible to concentrate radiated energy in a particular
direction. The narrow beam can be radiated towards the required or intended region. The
region where the intruders may exist can be avoided. Similarly, depending on the shape
of the indoor office buildings, the beam can be radiated based on the beam width and the
region where the eligible users exist.
As it is possible to control the beam width and based on that the region at which the
eligible persons exist, the possibility of the signal reaching intruders can be avoided. In
omni directional antennas which are used as APs for WLAN, the signal is radiated
uniformly through out 360 degrees. As most of the indoor offices are in rectangular shape,
it is a possibility that the signal can reach people who are outside the office. So, it offers
the possibility of intruders to interfere the signal easily.
With the gain optimized Micro strip yagi antennas, this possibility is reduced as the beam
direction may orient with the shape of the office and the location of AP. The signal
leakage is largely avoided. So, gain optimized micro strip yagi antennas can be employed
to enhance security of WLAN.
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6.13 Wearable WLAN Antenna 6.13.1 Requirement of Wearable Antenna
Antennas which are used in WLAN are to be placed at an appropriate place so that entire
desired region can be well covered. As WLAN antennas are Omni directional antennas,
they are placed at the centre of the region in which the coverage is desired.
It is proved that only with directional antennas, it is possible to restrict the signal within
the required region [9]. Directional antennas can be placed at the roof top, curtains of
windows and doors. The cloth materials used as curtains are used to fabricate antenna
[24].
Wearable antennas are micro strip type antennas. The cloth materials are used as the
substrate for the antenna. These antennas are also known as ‘Textile Antennas’. They are
flexible and can be integrated with any type of surface. Barring the substrate material,
design procedure for a micro strip antenna is maintained to design wearable antenna.
Wearable antennas are developed for the military purpose. When the soldiers are
traveling long distances, they have to carry arms, ammunitions, food, water and
Communication Equipments. The wearable antenna is fabricated with the shirt or hat of
the soldier. There is no need for the soldier to carry any separate antenna for any
communication equipments he is using. It is possible to create ad-hoc network within the
group of soldiers in a short range. This is also known as ‘Body area networks’ [31].
The wearable antenna can be used for the purpose of WLAN also [32]. With the
development of directional antennas as the Access Point [AP] for WLAN, it is preferable
to place the AP at the window or at the side walls of the office facing inwards. As the
directional antennas are highly isolated at backward direction, the people who may be
intruders outside the building cannot get any signal. The antenna which is fabricated with
cloth as the substrate material can be used as curtains or the interior designs. With the
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above mentioned antennas, the visibility of the antennas is restricted. It protects antennas
from physical damage caused knowingly or unknowingly.
Patch antennas offer effective low-profile designs for a wide range of wireless
applications. They are inexpensive to fabricate, light in weight, and can be made
conformable with planar and non-planar surfaces. The antennas are compact and
compatible with microwave integrated circuits (MIC’s) for high-frequency applications.
Unfortunately, they have some shortcoming such as low gain, narrow bandwidth, and
sensitivity to fabrication errors.
Locher et al [30] provide design and fabrication of textile patch antennas for Bluetooth
applications in wearable computing at the frequency range of 2.4 GHz. The antennas can
withstand clothing bends down to a radius of 37.5 mm without violating specifications.
P.Salonen et al [29] discuss the performance of a dual-band coplanar patch antenna
integrated with an electromagnetic band gap substrate. The antenna structure is made up
of clothing fabrics and operates at the 2.45 and 5 GHz wireless bands. The design of the
coplanar antenna, band gap substrate, and their integration is presented. The band gap
array consists of just 3 × 3 elements but reduces radiation into the body by over 10 dB
and improves the antenna gain by 3 dB. The performance of the antenna under bending
conditions and when placed on the human body is also presented.
6.13.2 Dual-band antenna in communication media
Antennas which operate at two tuned frequencies are called as ‘Dual Band Antennas’. As
WLAN operates in 2.4 GHz and 5 GHz, WLAN antennas are designed to operate as
‘dual’ in these frequencies. Dual band antennas can be fabricated based on micro strip
design procedure. Dual band wearable antennas can also be fabricated with cloth material
as substrate.
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Mobile and WLAN are two important communication techniques available in the world.
Mobile phones are operating around 900 MHz range and WLAN is operating at 2.4 GHz.
If it is possible for an antenna which can work at those two different frequencies, it is
highly desirable one. In this work, such an antenna is developed.
6.13.3 Design of dual band wearable antenna
To achieve a dual-band characteristic for a patch antenna, the introduction of slots in the
radiating patch is one of the basic methods. Two patches which differ in size can resonate
to be constructed. A patch resonates at a frequency inversely proportional to its
dimensions. The bigger patch resonates at smaller frequency and vice-versa. So, the
logical conclusion is to have patches of two different sizes that would resonate at their
respective frequencies. The shape of the patch is not having any prime importance.
Anyhow, for simplicity and ease of construction, it was decided to have patches of square
shape. They are connected together with connectors which ‘Bridge’ the gap between the
two slots. The smaller patch is placed inside the larger one.
Chang et al proposed a dual band antenna with two rectangular patches of different sizes
to operate at 2.4 GHz and 5 GHz WLAN frequencies [28]. This antenna was simulated
with conventional substrate material. The inner patch is given the feed and it is connected
with the outer patch with four arms one at each side. A similar dual band antenna is
constructed with jeans cloth material as the substrate but for mobile and WLAN
frequencies. The number of arms between two slots is increased. The characteristics due
to change in width of the arms and in width of the slots are also studied.
6.13.4 Methodology Used for the design of wearable antenna
The design approach adopted to design dual band wearable antenna is given below:
i. First a single rectangular patch of freq 900 MHz is to be designed
ii. Then a rectangular patch for 2.4 GHz is designed and both the patches are placed
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with a common feed point.
iii. Both the patches were connected with 5 connectors on each side.
iv. Ground plane dimension is 140mm ×155mm and Jeans cloth material which has
dielectric constant of 1.6 is used as the substrate.
Inner rectangular patch and outer rectangular patch are separated by a Jeans substrate of
thickness 1.5mm. The inner patch is fed by a 50 Ω coaxial feed probe from the bottom of
the substrate. The exact centre of patch is placed at the origin. Feed point to the inner
patch is placed at (5,-19).
6.13.5 Design of dual patch antenna
The essential parameters for the design of a rectangular Micro strip Patch Antenna are:
(i) Frequency of operation (fo): This antenna is expected to operate at both mobile and
WLAN frequencies. The dual resonant frequencies selected for this design is 0.9 GHz
and 2.4 GHz.
(ii) Dielectric constant of the substrate (εr): The dielectric material selected for this design
is Jeans cloth which has a dielectric constant of 1.6.
(iii) Height of dielectric substrate (h): For the micro strip patch antenna to be used in
cellular phones, it is essential that the antenna is not bulky. Hence, the height of the
dielectric substrate is selected as 1.5 mm.
Top view of a micro strip patch Antenna is given in Fig 6.27. As per the basic design
equations the width and length of a patch antenna are calculated. The resonant frequency,
height of the substrate and the dielectric constant of the substrate are the basic parameters
with which the design is carried out.
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Fig 6.35 Basic Patch Antenna
The following Steps are to be followed in designing a rectangular patch antenna.
Step1: Calculation of the width (W):
1
22 +
=rr
o
fv
Wε
----- (6.10)
Step2: Calculation of Effective dielectric constant (e reff):
21
]121[2
)1(2
1 −+
−+
+=
Whrr
reffεε
ε ---- (6.11)
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Step 3: Calculation of Effective length ( Leff ):
Leff = reff
o
fvε02
---- (6.12)
Step 4: Calculation of Length extension (ΔL):
)8.0)(258.0(
)264.0)(3.0(412.0
+−
++=
Δ
hW
hW
hL
reff
reff
ε
ε ------ (6.13)
Step 5: Calculation of actual length of the patch (L):
L=Leff-2ΔL ------ (6.14)
Step 6: Calculation of ground plane dimensions (gL and gW):
The transmission line model is applicable to infinite ground planes only. However,
for practical considerations, it is essential to have a finite ground plane. Similar results for
finite and infinite ground plane can be obtained if the size of the ground plane is greater
than the patch dimensions by approximately six times the substrate thickness all around
the periphery. Hence, for this design, the ground plane dimensions are given as:
Lg=6h+L -------- (6.15)
Wg=6h+w -------- (6.16)
Step 7: Determination of feed point location f(x,y) :
A coaxial probe type feed is to be used in this design. The center of the patch is taken as
the origin and the feed point location is given by the co-ordinates f(x,y) from the origin.
The feed point must be located at the point, where the input impedance is 50 Ω for the
resonant frequency. Hence, a trial and error method is used to locate the feed point. For
different locations of the feed point, the return loss (RL) is compared and that feed point
124
is selected where the RL is most negative. The most negative RL value indicates
maximum of the fed signal is transmitted using the antenna which is the desired one.
For the equations (6.10) to (6.16) which are stated above,
‘L’ is the length of patch
‘W’ is the width of the patch
‘h’ is the height of the substrate
‘εr’ is the relative permittivity of substrate
‘εreff’ is the effective relative permittivity of patch
‘vo’ is the velocity of EM wave
‘Leff’ is the effective length of patch
‘fo’ is the resonant frequency.
The above design equations are used to calculate dimensions for 2.4 GHz and 900 MHz
one by one. The calculated dimensions are to be implemented in any one of the
microwave simulation software. In this work, these patches are implemented in Ie3D
which is a product of Zeland Corporation Ltd.,
6.13.6 Implementation of the Antenna using IE3D
The dual band wearable antenna is to be implemented in a microwave antenna simulator.
IE3D is a 2.5 dimension antenna simulator tool which is being used world wide. The dual
band antenna is to be implemented in IE3D as per the design parameters specified in the
previous section.
First, a single patch antenna of frequency 2.4 GHz on the jeans cloth substrate of
thickness 1.5 mm is implemented. The substrate’s dielectric constant is 1.6.The square
patch has dimension of 50mm × 50mm.Ground plane dimension is of length 134mm and
breadth 150mm.
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By randomly varying across patch we get feed point as (3,-10). At this point the patch
resonates at 2.4 GHz and the return loss is -12.2. The diagram of the single patch
resonating at 2.4 GHz is given below.
Fig 6.36 Inner Patch at 2.4 GHz
The outer Patch is to resonate at 0.9 GHz. This patch is also constructed on Jeans cloth
material. It has length of 130.6 mm and width of 146 mm on the same substrate. The
outer rectangular patch is shown in Fig 6.37.
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Fig 6.37 Outer Patch at 0.9 GHz
As both patches are to be fed at single point, the smaller patch is to be placed inside the
larger one. This can be done by cutting a slot from the outer patch and then place the
inner patch. In other words, embed the smaller patch into the larger one as shown in the
figure below. We create a rectangular slot of length 90.6mm and width 106mm from the
outer patch. In this slot, the smaller patch is placed.
The selection of feed point is a crucial one. For a dual patch antennas, he feed point is to
be selected within the inner patch. The inner patch is fed with the maximum of current
and outer patch with connected with the ‘bridges’ which exist between inner and outer
conductors.
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Fig 6.38 Inner patch is embedded with the outer patch
The two patches are arranged as per Fig 6.38. The gap between the patches is to be
bridged by connectors. The feed point is selected at the inner patch. The signal from the
inner patch is to be well connected to the outer patch to have better radiation from outer
patch. To increase the signal at the outer patch, more number of connectors is to be
employed between the patches. At each side, five connectors of width 4mm are used to
bridge the gap. These connectors are equally spaced at 6mm symmetrically across the
origin. The resulted patch is given in Fig 6.39.
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Fig. 6.39 Dual Patch Antenna with connectors
The return loss is a vital factor in the design of a micro strip antenna. The point at which
the probe is fed decides how much the return loss or ‘S11‘would be. ‘S11’ is known as an
element of scattering matrix which measures the amount of signal reflected at a port at
which the signal is fed in. Thus, the objective is to fix a feed point so that the return loss
at that point is the negative maximum for the given patch antenna. A coaxial probe feed
is used for this dual band antenna. The feed point is fixed randomly across the inner patch.
As two patches are available and only inner patch is fed, depending on the feed point the
return loss at both resonant frequencies change. Based on the magnitude of the return
loss, it is found that at the position of (5,-10mm), the antenna radiates best for both the
resonant frequencies.
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6.13.7 Simulated Result
The figure 6.40 shown below is the proposed dual band patch antenna on a wearable
material. All dimensions are given in mm. This is simulated using the IE3D software of
Zeland Inc., USA and the return loss and the VSWR are measured.
Fig.6.40 Antenna design simulated in IE3D software
Fig.6.41 Return Loss in dB Vs Frequency
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In fig 6.41, return loss has two distinct downward peaks which indicate that the antenna
is resonating at two frequencies, 2.5 GHz and 1.3 GHz. It is also seen that the ‘peaks’ are
unequal in their values of Return Loss. At 2.5 GHz, the value is close to -8.5 dB, while at
1.3 GHz, it is exactly touching -10 dB.
Fig 6.42 VSWR Vs Frequency
The standing wave ratio is dependent on the return loss. As the return loss increases
negatively, the SWR decreases. It is observed from Fig. 6.42, which is shown above that
at 2.5 GHz, VSWR has the value of 2.1. At 1.3 GHz, the VSWR is 1.9.
6.13.8 Effect of changing the number and width of arms
By increasing the number of connectors and the width of the connectors, a significant
change in the return loss for both the patches can be observed. Given below is a set of
values of return loss for both the patches due to the above mentioned changes. When the
width of the slot is 20 mm, better return loss of -23 dB is achieved at 2.4 GHz for five
connectors at four sides but at 900 MHz, the return loss is only -7 dB.
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Table 6.8 Variations with number of connectors and their width
The width of connectors for the above measurement is 20 mm.
Better results are obtained for number of connectors ‘9’ on all the four sides. The return
loss at 2.5 GHz is at -23 dB.
6.13.9 Effect of changing the width of the slot
By increasing the width of the slot, a significant change in the return loss for both the
patches can be witnessed. The table 6.9 given below lists the changes in return loss due to
changes in the width of the slot.
With the number of connectors five and the width of the arms 4 mm, the width of the slot
is varied. At 40 mm, the return lose at 2.4 GHz is -23 dB and at 900 MHz, it is -7 dB.
No. of Connectors
Width of connectors (mm)
Return loss for 2.4 GHz (in dB)
Return loss for 900 MHz (in dB)
Resonant frequency for 2.4 GHz
Resonant frequency for 0.9 GHz
1*4 20 -1.1 -0.1 2.4 1.2 7*4 2 -2 -1.6 2.7 1.2 5*4 4 -23 -7 2.5 1.4 9*4 1 -5 -9 2.3 1.3 7*4 4 -11 -1 2.3 1.3 3*4 5 -10 -0.5 2.7 1
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Number of Arms: 5 Width of the arms: 4 mm Width of the slot (mm)
Return loss for 2.4 GHz (dB)
Return loss for 900 MHz (dB)
Resonant Frequency for 2.4 GHz
Resonant Frequency for 900 MHz
60 -2.1 -1.14 2.5 1.5 54 -1.8 -0.3 2.4 1.4 46 -6 -1 2.4 1.3 40 -23 -7 2.5 1.4 36 -2 -0.1 2.1 1.2 30 -0.5 -7 2.3 1.4 20 -1.4 -0.1 2.1 1.2
Table 6.9 Variations with width of the slot
6.14 WLAN Security with Wearable antenna
The wearable antenna designed as specified above can be used as an effective WLAN
antenna. Micro strip patch antennas are radiating maximum of the energy in the
perpendicular direction to the plane of the antenna. The back lobe from a micro strip
patch antenna is almost zero. So, patch antenna can be used as directional antenna. As the
placement is easy and visually difficult to notice, patch antennas can be fabricated on
cloth materials. For a WLAN environment, the curtains of windows or doors are perfect
places where these antennas can be placed.
The gain optimized micro strip yagi antenna which is discussed in section 6.10 can also
be fabricated with this wearable structure. The gain optimized structure radiates in the
forward direction. As the gain is optimized, the back lobes and the side lobes are reduced
to the extent possible. By placing this antenna at the curtains facing inside the coverage
region, radiated signal is available only in that region. The signal is not available outside
the specified region. As intruders cannot get these signals, wearable antennas can also
help in providing security for WLAN networks.
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6.15 Conclusion
To provide better security for WLAN, directional antennas are preferred. As directional
antennas radiate signal is a pre-defined directions and beam width, the region at which
the signal is required can only be wirelessly connected through EM waves. Yagi antenna
is a simple and efficient directional antenna in which gain can be controlled by number of
directors. To reduce the radiation in unwanted direction and to improve gain in forward
direction, yagi array is optimized using gain optimized yagi array. To have compact size,
gain optimized yagi array is fabricated in micro strip form. Four Element and Five
element micro strip yagi arrays are simulated and fabricated. The results provide better
return loss and VSWR compared with conventional yagi array. It indicates the forward
gain is improved with gain optimized micro strip yagi array at the expense of back lobes.
Wearable antenna is a special type of micro strip antenna which is fabricated on cloth
material. A dual band antenna on Jeans cloth is fabricated. Wearable antennas can be
placed at walls, curtains of walls or windows. The gain optimized micro strip yagi
antenna can also be fabricated using jeans cloth as the substrates.
As the gain optimized structures radiate only in the forward direction, the leakage signal
in other directional are reduced. Intruders placed themselves in uncovered regions cannot
make use of these signals. Thus, the gain optimized micro strip yagi antennas can
improve the security of WLAN environment.
With the help of gain optimized yagi antenna, few techniques are proposed to enhance
security of WLAN. Those techniques are presented in the next chapter.
