Log Periodic Antenna

Log Periodic Antenna
TE0321 - ANTENNA &
PROPAGATION LABORATORY
Laboratory Manual
DEPARTMENT OF
TELECOMMUNICATION ENGINEERING
SRM UNIVERSITY
S.R.M. NAGAR, KATTANKULATHUR – 603 203.
FOR PRIVATE CIRCULATION ONLY
ALL RIGHTS RESERVED
SRM UNIVERSITY
Faculty of Engineering & Technology
Department of Telecommunication Engineering
S. No CONTENTS
Introduction – antenna & Propagation
Arranging the trainer & Performing functional Checks Page no.
1
6
List of Experiments 1 2 3 4 5 6 7 8 9 Performance analysis of Half wave dipole antenna Performance analysis of Folded dipole antenna Performance analysis of Loop antenna Performance analysis of Yagi ‐Uda antenna Performance analysis of Helix antenna Performance analysis of Slot antenna Performance analysis of Log periodic antenna Performance analysis of Parabolic antenna Radio wave propagation path loss calculations 12
15
19
23
38
41
44
47
51
ANTENNA & PROPAGATION LAB
INTRODUCTION:
Antennas are a fundamental component of modern communications systems. By
Definition, an antenna acts as a transducer between a guided wave in a transmission line
and an electromagnetic wave in free space. Antennas demonstrate a property known as
reciprocity, that is an antenna will maintain the same characteristics regardless if it is
transmitting or receiving. When a signal is fed into an antenna, the antenna will emit
radiation distributed in space a certain way. A graphical representation of the relative
distribution of the radiated power in space is called a radiation pattern.
The following is a glossary of basic antenna concepts.
Antenna
An antenna is a device that transmits and/or receives electromagnetic waves.
Electromagnetic waves are often referred to as radio waves. Most antennas are resonant
devices, which operate efficiently over a relatively narrow frequency band. An antenna
must be tuned to the same frequency band that the radio system to which it is connected
operates in, otherwise reception and/or transmission will be impaired.
Wavelength
We often refer to antenna size relative to wavelength. For example: a half-wave
dipole, which is approximately a half-wavelength long. Wavelength is the distance a ra
dio wave will travel during one cycle. The formula for wavelength is shown on the next
page.
Note: The length of a half-wave dipole is slightly less than a half-wavelength due to end
effect. The speed of propagation in coaxial cable is slower than in air, so the wavelength
in the cable is shorter. The velocity of propagation of electromagnetic waves in coax is
usually given as a percentage of free space velocity, and is different for different types of
coax.
Impedance Matching
For efficient transfer of energy, the impedance of the radio, the antenna, and the
transmission line connecting the radio to the antenna must be the same. Radios typically
are designed for 50 ohms impedance and the coaxial cables (transmission lines) used with
them also have a 50 ohm impedance. Efficient antenna configurations often have an
impedance other than 50 ohms, some sort of impedance matching circuit is then required
to transform the antenna impedance to 50 ohms.
VSWR and Reflected Power
The Voltage Standing Wave Ratio (VSWR) is an indication of how good the
Impedance match is. VSWR is often abbreviated as SWR. A high VSWR is an indication
that the signal is reflected prior to being radiated by the antenna. VSWR and reflected
power are different ways of measuring and expressing the same thing. A VSWR of 2.0:1
or less is considered good. Most commercial antennas, however, are specified to be 1.5:1
or less over some bandwidth. Based on a 100 watt radio, a 1.5:1 VSWR equates to a
forward power of 96 watts and a reflected power of 4 watts, or the reflected power is
4.2% of the forward power.
Bandwidth
Bandwidth can be defined in terms of radiation patterns or VSWR/reflected
power. The definition used in this book is based on VSWR. Bandwidth is often expressed
in terms of percent bandwidth, because the percent bandwidth is constant relative to
frequency. If bandwidth is expressed in absolute units of frequency, for example MHz,
the bandwidth is then different depending upon whether the frequencies in question are
near 150, 450, or 825 MHz. A mathematical analysis of bandwidth is provided on the
next page.
Directivity and Gain
Directivity is the ability of an antenna to focus energy in a particular direction
when transmitting or to receive energy better from a particular direction when receiving.
The relationship between gain and directivity: Gain = efficiency/Directivity.
Gain is given in reference to a standard antenna. The two most common reference
Antennas are the isotropic antenna and the resonant half-wave dipole antenna. The
isotropic antenna radiates equally well in "all" directions. Real isotropic antennas do not
exist, but they provide useful and simple theoretical antenna patterns with which to
compare real antennas. An antenna gain of 2 (3 dB) compared to an isotropic antenna
would be written as 3 dBi. The resonant half-wave dipole can be a useful standard for
comparing to other antennas at one frequency or over a very narrow band of frequencies.
To compare the dipole to an antenna over a range of frequencies requires an adjustable
dipole or a number of dipoles of different lengths.
Gain Measurement
One method of measuring gain is by comparing the antenna under test against a
known standard antenna. This is technically known as a gain transfer technique. At lower
frequencies, it is convenient to use a 1/2-wave dipole as the standard. At higher
frequencies, it is common to use a calibrated gain horn as a gain standard, with gain
typically expressed in dBi. Another method for measuring gain is the 3 antenna method.
Transmitted and received power at the antenna terminals is measured between three
arbitrary antennas at a known fixed distance. The Friis transmission formula is used to
develop three equations and three unknowns. The equations are solved to find the gain
expressed in dBi of all three antennas.
Antenna Placement
Correct antenna placement is critical to the performance of an antenna. An
antenna mounted on the roof will function better than the same antenna installed on the
hood or trunk of a car. Knowledge of the vehicle may also be an important factor in
determining what type of antenna to use. You do not want to install a glass mount
antenna on the rear window of a vehicle in which metal has been used to tint the glass.
The metal tinting will
work as a shield and not allow signals to pass through the glass. When installing antennas
at a base station, a stainless steel mast should be used to properly pass stray RF current
away from the antenna and provide proper support.
Radiation Patterns
The radiation or antenna pattern describes the relative strength of the radiated field
in various directions from the antenna, at a fixed or constant distance. The radiation
pattern is a "reception pattern" as well, since it also describes the receiving properties of
the antenna. The radiation pattern is three-dimensional, but it is difficult to display the
three dimensional radiation pattern in a meaningful manner, it is also time consuming to
measure a three-dimensional radiation pattern. Often radiation patterns are measured that
are a slice of the three-dimensional pattern, which is of course a two-dimensional
radiation pattern which can be displayed easily on a screen or piece of paper. These
pattern measurements are presented in either a rectangular or a polar format.
Near-Field and Far-Field Patterns
The radiation pattern in the region close to the antenna is not exactly the same as
the pattern at large distances. The term near-field refers to the field pattern that exists
close to the antenna; the term far-field refers to the field pattern at large distances. The
far-field is also called the radiation field, and is what is most commonly of interest. The
near-field is called the induction field (although it also has a radiation
component).Ordinarily, it is the radiated power that is of interest, and so antenna patterns
are usually measured in the far-field region. For pattern measurement it is important to
choose a distance sufficiently large to be in the far-field, well out of the near-field. The
minimum permissible distance depends on the dimensions of the antenna in relation to
the wavelength. The accepted formula for this distance is:
When extremely high power is being radiated (as from some modern radar antennas), the
near-field pattern is needed to determine what regions near the antenna, if any, are
hazardous to human beings.
Beamwidth
Depending on the radio system in which an antenna is being employed there can
be many definitions of beamwidth. A common definition is the half power beamwidth.
The peak radiation intensity is found and then the points on either side of the peak
represent half the power of the peak intensity are located. The angular distance between
the half power points traveling through the peak is the beamwidth. Half the power is —
3dB, so the half power beamwidth is sometimes referred to as the 3dB beamwidth.
Antenna Polarization
Polarization is defined as the orientation of the electric field of an electromagnetic
wave. Polarization is in general described by an ellipse. Two often used special cases of
elliptical polarization are linear polarization and circular polarization. The initial
polarization of a radio wave is determined by the antenna that launches the waves into
space. The environment through which the radio wave passes on its way from the
transmit antenna to the receive antenna may cause a change in polarization.
With linear polarization the electric field vector stays in the same plane. In circular
polarization the electric field vector appears to be rotating with circular motion about the
direction of propagation, making one full turn for each RF cycle. The rotation may be
righthand or left-hand.
Choice of polarization is one of the design choices available to the RF system
designer. For example, low frequency (< 1 MHz) vertically polarized radio waves
propagate much more successfully near the earth than horizontally polarized radio waves,
because horizontally polarized waves will be canceled out by reflections from the earth.
Mobile radio systems waves generally are vertically polarized. TV broadcasting has
adopted horizontal polarization as a standard. This choice was made to maximize signalto-noise ratios. At frequencies above 1 GHz, there is little basis for a choice of horizontal
or vertical polarization, although in specific applications, there may be some possible
advantage in one or the other. Circular polarization has also been found to be of
advantage in some microwave radar applications to minimize the "clutter" echoes
received from raindrops, in relation to the echoes from larger targets such as aircraft.
Circular polarization can also be
used to reduce multipath. The majority of the antennas utilized in this experiment are
vertically polarized because of their predominance in antenna applications.
Pre Lab Questions:
1.
2.
3.
4.
5.
6.
Define Antenna
Give antenna parameters
Define Impedance Matching.
Define VSWR.
Relationship bandwidth Directivity & gain
What is need for radiation pattern?
1. Half Wave Dipole
THEORY
The half wave dipole is perhaps the simplest and most fundamental antenna design
possible. Hertz used a dipole antenna during his initial radio experimentation. This is why
a dipole is often referred to as the “hertz dipole” antenna. The dipole is so practical that it
is utilized (in some form) in at least half of all antenna systems used today. Here are some
key principles of the dipole antenna:
1.) A dipole antenna is a wire or conducting element whose length is half the transmitting
wavelength.
To calculate the length of a half wave dipole in free space, one may use the following
equation:
length (ft) = 492 / frequency (MHz)
2.) A dipole antenna is fed in the center.
By using a piece of coaxial cable transmission line, one may feed the center conductor of
a transmission line to a ¼ wavelength piece of wire. The outer shield or ground of the
cable may be connected to the remaining ¼ wavelength piece of wire. Thus, you have a
dipole antenna, fed in the center, with an overall length of ½ wavelength. The total ½
wavelength of wire is to be stretched out evenly, being perpendicular to the transmission
line. How exactly does the signal come out of the cable and emanate from the wires into
space? The ¼ wavelength wire which is fed by the center conductor of the transmission
line is known as the hot portion. One quarter of the wave leaks from the attached wire,
and the remaining quarter of the wave “hops” over to the grounded second ¼ wavelength
wire. Since these two pieces of ¼ wavelength wire work together to emit the wave, we
often refer to a dipole as a perfect resonant antenna. Why is this
important? If an antenna is resonant, it will be matched to the transmission line and/or
transmitter and the bulk of the signal will actually be transmitted, not reflected back and
wasted as heat ( i.e. Standing Wave Ratio SWR ). It should be noted that a dipole has an
impedance of 75 ohms, not 50 ohms. Ordinarily a mismatch could cause a problem, but
the mismatch of 50 ohm cable feeding a 75 ohm antenna is minimal with a resultant SWR
of 1.5:1. This corresponds to roughly a 5% waste of power.
3.) The dipole antenna has a unique radiation pattern.
The radiation pattern of a dipole antenna in free space is strongest at right angles to the
wire. This pattern, when the antenna is positioned horizontally over the ground,
resembles a figure eight. This figure eight pattern will be verified during the experiment.
Let’s assume we shift the antenna around and make it vertical (perpendicular to the
ground). The ends of the wire which emit the least amount of energy are now directed
towards both the earth and the sky. This results in a vertically polarized signal which is
focused quite evenly across the reception zone. This brings up an important concept:
antenna radiation patterns can be quite different horizontally and vertically. This concept
will be verified when the dipoles are tested. Also, it is important to note that for a signal
to be received effectively, the receiving antenna must be in the same plane as the
transmitting antenna. If these are mismatched, a large portion of the signal will be lost or
distorted. This concept will be verified as well.
4.) The dipole antenna is extremely flexible.
What changes can one make to a dipole to subsequently alter its radiation pattern? There
are limitless modifications that can be made. For instance: Instead of keeping the ½
wavelength elements perpendicular to the transmission line, let’s bend them or “slope”
them by 45 degrees. This simple change will modify the radiation pattern. What happens
when we stack two dipoles on top of each other, separated by one full wavelength of
space, and feed them in phase? This is known as a stackable phased array. This focuses
more of the radiated power towards the horizon, where it is most useful. Stacking
antennas for this purpose produces gain. Gain is useful because it improves the strength
of the signal that is transmitting or receiving. For instance: if a signal is fed into an
antenna with 3db (decibels) of gain. The transmitted signal will appear on the receiving
end twice as strong as it would have been if the transmitting antenna had no gain. This
can be quite beneficial to a communications engineer. It is very costly to produce high
powered transmitters. Gain offers a good compromise. A 10 watt signal fed into an
antenna with 3dB of gain will result in an effective radiated power (ERP) of 20 watts. By
introducing an antenna with gain, an engineer can avoid having to use a 20 watt signal
and an antenna with no gain. Examples of gain will be demonstrated in the lab. Gain and
radiation patterns go hand in hand. If we were to place three dipoles in a row, the
radiation pattern would be projected into a forward direction. This is known as forward
gain.
a picture of the ideal radiation pattern of a half-wave dipole in free space. This is the
radiation pattern with the antenna mounted horizontally. Observe the figure
eight pattern. Notice the dotted lines. The pattern is a little distorted because of the
antenna mast and ground distort the pattern. It is inevitable that external factors will make
the real world radiation patterns less than perfect, however, the antenna radiation patterns
will still resemble their theoretical counterparts.
Specific procedures for testing the dipole antennas:
You will be testing the dipoles initially. You should have two dipoles connected.
The one on the transmitting antenna apparatus is made of PVC pipe and will be shifted
between horizontal and vertical. The dipole on the receiving antenna apparatus is a
professional grade dipole made of metal. This is the one whose radiation pattern is being
plotted. The radiation pattern of the dipoles mounted vertically is to be tested first. Make
sure the transmitting PVC dipole antenna is vertical (with the arrow pointing towards the
sky). Mount the metal dipole in the vertical position as well, as seen in the background
section of the lab manual. Make sure that the extra metal bar (gamma match) on the
antenna is on the top side of the antenna, facing the sky.
Once you have both of the antennas mounted vertically, the cables connected to
them and the remaining systems in place, it is time to take the readings. Follow the
general procedure when doing this. Make sure the antenna is facing forward towards the
transmitting antenna at your 0 degree position. Once complete, it is time to measure the
dipoles radiation pattern when it is mounted horizontally. To do so, you must rotate the
PVC dipole antenna so that it is positioned horizontally, as seen in the background
section of the lab. Once this is done, you will be required to mount the metal receiving
dipole horizontally as well. Simply duplicate the mounting picture in the background
section. Once everything is re-mounted, follow the general procedure and take your
readings. You should notice that these readings are slightly different from the last set.
This is to be expected.
Experimental Set up
PROCEDURE
1. Arrange the setup as given in the block diagram
2. Mount Half wave dipole antenna on the transmitter mask
3. Bring the detector assembly near to main and adjust the height of both
transmitting and receiving antenna
4. Keep Detector assembly away from the main unit approximately 1.5 meter and
align both of them .Ensure that there is no reflector sort things in the vicinity of
the experiment such as a steel structure ,pipes, cables etc.
5. Keep the RF level and FS adjust to minimum and unidirectional coupler switch to
FWD(Forward adjustment knob).
6. Keep detector level control in the centre approximately
7. Increase RF level gradually and see that there is deflection in the detector meter
8. Adjust RF level and detector level, so that the deflection in detector meter is
approximately 30-35mA.
9. Align arrow mark on the disk with zero of the goniometer scale
10. Start taking the reading at the interval of 10 degree, and note the deflection on the
detector assembly.
11. Using conversion chart convert mA readings into db.
12. Plot the polar graph in degrees of rotation of antenna against level in the detector
in dBs
Tabulation:
S.No
Angle in Degrees
Detector
reading(mA)
Gain in dB
Post Lab Questions:
1.
2.
3.
4.
Define half wave dipole.
Draw the radiation pattern of half wave dipole antenna.
Give the application of half wave dipole antenna.
Write the frequency range of RF signal
Pre Lab Questions:
1.
2.
3.
4.
5.
6.
Define Folded Dipole Antenna
Give antenna parameters
Define Impedance Matching.
Define VSWR.
Relationship bandwidth Directivity & gain
What is need for radiation pattern?
1. FOLDED DIPOLE ANTENNAS
A folded dipole is a dipole antenna, with the ends folded back around and connected to
each other, forming a loop as shown in Figure 1.
Figure . Folded dipole of length L.
Typically, the width d of the folded dipole is much smaller than the length L.
Because the dipole is a closed loop, one would expect the input impedance to depend on
the input impedance of a short-circuited transmission line of length L (although
unfortunately it depends on a transmission line of length L/2, which doesn't quite make
intuitive sense to me). Also, because the dipole is folded back on itself, the currents can
reinforce each other instead of cancelling each other out, so the input impedance will also
depend on the impedance of a dipole antenna of length L.
Letting Zd represent the impedance of a dipole antenna and Zt represent the transmission
line impedance given by:
The input impedance ZA of the folded dipole is given by:
The folded dipole is resonant and radiates well at odd integer multiples of a halfwavelength (0.5
dipole.
, 1.5
, ...). The input impedance is higher than that for a regular
The antenna impedance for a half-wavelength folded dipole antenna can be found from
the above equation for ZA; the result is ZA=4*Zd. At resonance, the impedance of a halfwave dipole antenna is approximately 70 Ohms, so that the input impedance for a halfwave folded dipole is roughly 280 Ohms.
Because the characteristic impedance of twin-lead transmission lines are roughly 300
Ohms, this dipole is often used when connecting to this type of line, for optimal power
transfer.
The radiation pattern of half-wavelength folded dipoles have the same form as that of
half-wavelength dipoles.
PROCEDURE
1. Arrange the setup as given in the block diagram
2. Mount folded dipole antenna on the transmitter mask
3. Bring the detector assembly near to main and adjust the height of both
transmitting and receiving antenna
4. Keep Detector assembly away from the main unit approximately 1.5 meter and
align both of them .Ensure that there is no reflector sort things in the vicinity of
the experiment such as a steel structure ,pipes, cables etc.
5. Keep the RF level and FS adjust to minimum and unidirectional coupler switch to
FWD(Forward adjustment knob).
6. Keep detector level control in the centre approximately
7. Increase RF level gradually and see that there is deflection in the detector meter
8. Adjust RF level and detector level, so that the deflection in detector meter is
approximately 30-35mA.
9. Align arrow mark on the disk with zero of the goniometer scale
10. Start taking the reading at the interval of 10 degree, and note the deflection on the
detector assembly.
11. Using conversion chart convert mA readings into db.
12. Plot the polar graph in degrees of rotation of antenna against level in the detector
in dBs
Tabulation:
S.No
Angle in Degrees
Detector
reading(mA)
Post Lab Questions:
1.
2.
3.
4.
Define folded dipole.
Draw the radiation pattern of folded dipole antenna.
Give the application of folded dipole antenna.
Write the frequency range of RF signal
Gain in dB
Pre Lab Questions:
1.
2.
3.
4.
5.
6.
Define Antenna
Give antenna parameters
Define Impedance Matching.
Define VSWR.
Relationship bandwidth Directivity & gain
What is need for radiation pattern?
3. YAGI-UDA ANTENNA
The Yagi antenna is used frequently because it offers gain an directivity. The
Yagi antenna was developed by a Japanese engineer Yagi-Uda. Its design is based exclu
sively on dipoles. A quick glance at a standard TV antenna will show a series of dipoles
in parallel to each other with fixed spacing between the elements. The number of
elements used will depend on the gain desired and the limits of the supporting structure.
A three element Yagi consists of a director, a driven element, and a reflector. Below is a
picture of how these elements are configured:
Notice that the driven element is in the center and is nothing more than a center fed
dipole. To the right of the driven element is the reflector. The reflector is slightly longer
than the driven element to allow for proper tuning. The reflector is simply a passive piece
of metal slightly longer than ½ wavelength. At the front of the antenna is the director. It
is electrically and physically shorter than the driven element. These elements work
together to pr oject a radiation pattern in the forward direction. This forward radiation
pattern has gain. This type of system is ideal for television broadcasts. Let’s assume you
want to receive CBS’s channel 2 signal from Mount Wilson. You could simply use a pair
of rabbit ears (which is a dipole) to pick up the signal, but it probably would come in
snowy. This is where a Yagi can come into play. A three element Yagi in free space can
have a maximum gain of around 9dB. This means the signal will be amplified by about
16 times by the antenna. Also, the radiation pattern of the antenna tends to transmit or
receive the bulk of the signal from the forward direction. Thus, aiming a Yagi at Mount
Wilson would receive a strong focused signal, resulting in a much better picture. Like a
dipole, a Yagi can be placed either vertically or horizontally. Although this could shift the
radiation pattern slightly, the concepts of gain and directivity still remain.
Below is a picture of the radiation pattern of a three element Yagi in free space. Is it
what you would expect from the information provided about Yagi antennas? Notice the
principles of forward gain and directivity? Note: the forward facing pattern is known as
the forward lobe. The backward facing pattern is known as the backward lobe. This is an
ideal radiation pattern measured in a special chamber. When the Yagi antenna is tested in
the experiment, the radiation pattern will not be perfectly aligned with the theoretical
model, but the concepts of gain and directivity should be evident from the plot.
A Yagi antenna often has an impedance of 200 ohms and needs to be matched down to
standard 50 ohm cable. A method used to correct this mismatch is to insert a gamma
match between the feedline and the antenna. A picture of this match is shown below.
Specific procedures for testing the Yagi antenna:
The Yagi antenna is to be tested after the dipole because its design is based heavily on
the basic Hertz dipole. It will also expose you to a very different set of readings. You
will notice some rapid changes in your measurements. This is normal, because the Yagi
offers a very focused radiation pattern. The Yagi is a vertical antenna. When you install
it on the receiving apparatus, be sure to follow the mounting picture in the background
section of the lab. Also, make sure that the metal bar (gamma match) on the antenna is
facing upwards.
Make sure the PVC dipole on the transmitting antenna apparatus is also in the vertical
position, with the arrow pointed to the sky. You will want the Yagi to be facing forward
towards the transmitting antenna when the rotator is at its 0 degree position. Follow the
general procedure and take the readings.
PROCEDURE
1. Arrange the setup as given in the block diagram
2. Mount Yagi Uda antenna on the transmitter mask
3. Bring the detector assembly near to main and adjust the height of both
transmitting and receiving antenna
4. Keep Detector assembly away from the main unit approximately 1.5 meter and
align both of them .Ensure that there is no reflector sort things in the vicinity of
the experiment such as a steel structure ,pipes, cables etc.
5. Keep the RF level and FS adjust to minimum and unidirectional coupler switch to
FWD(Forward adjustment knob).
6. Keep detector level control in the centre approximately
7. Increase RF level gradually and see that there is deflection in the detector meter
8. Adjust RF level and detector level, so that the deflection in detector meter is
approximately 30-35mA.
9. Align arrow mark on the disk with zero of the goniometer scale
10. Start taking the reading at the interval of 10 degree, and note the deflection on the
detector assembly.
11. Using conversion chart convert mA readings into db.
12. Plot the polar graph in degrees of rotation of antenna against level in the detector
in dBs
Tabulation:
S.No
Angle in Degrees
Detector
reading(mA)
Post Lab Questions:
1.
2.
3.
4.
Define Yagi Uda Antenna.
Draw the radiation pattern of Yagi Uda antenna.
Give the application of Yagi Uda antenna.
Write the frequency range of RF signal
Gain in dB
Pre Lab Questions:
1.
2.
3.
4.
5.
6.
Define Antenna
Give antenna parameters
Define Impedance Matching.
Define VSWR.
Relationship bandwidth Directivity & gain
What is need for radiation pattern?
4. HELIX ANTENNA
A helical antenna is an antenna consisting of a conducting wire wound in the form of a
helix. In most cases, helical antennas are mounted over a ground plane. Helical antennas
can operate in one of two principal modes: normal (broadside) mode or axial (or end-fire)
mode.
B: Central Support,
C: Coaxial Cable,
E: Spacers/Supports for the Helix,
R: Reflector/Base,
S: Helical Aerial Element
In the normal mode, the dimensions of the helix are small compared with the wavelength.
The far field radiation pattern is similar to an electrically short dipole or monopole. A
Tesla coil as a secondary coil is also an example.
Post Lab Questions:
1.
2.
3.
4.
Define HELIX Antenna.
Draw the radiation pattern of HELIX antenna.
Give the application of HELIX antenna.
Write the frequency range of RF signal
Pre Lab Questions:
1.
2.
3.
4.
5.
6.
Define Antenna
Give antenna parameters
Define Impedance Matching.
Define VSWR.
Relationship bandwidth Directivity & gain
What is need for radiation pattern?
4. SLOT ANTENNA
Slot antennas are used typically at frequencies between 300 MHz and 24 GHz.
These antennas are popular because they can be cut out of whatever surface they are to be
mounted on, and have radiation patterns that are roughly omnidirectional (similar to a
linear wire antenna, as we'll see). The polarization is linear. The slot size, shape and what
is behind it (the cavity) offer design variables that can be used to tune performance.
A slot antenna consists of a metal surface, usually a flat plate, with a hole or slot
cut out. When the plate is driven as an antenna by a driving frequency, the slot radiates
electromagnetic waves in similar way to a dipole antenna. The shape and size of the slot,
as well as the driving frequency, determine the radiation distribution pattern. Slot
antennas are often used instead of line antennas when greater control of the radiation
pattern is required. Slot antennas are often found in standard desktop microwave sources
used for research purposes.
Figure . Two-dimensional radiation plots
A slot antenna's main advantages are its size, design simplicity, robustness, and
convenient adaptation to mass production using PC board technology.
Post Lab Questions:
1.
2.
3.
4.
Define SLOT Antenna.
Draw the radiation pattern of SLOT antenna.
Give the application of SLOT antenna.
Write the frequency range of RF signal
Pre Lab Questions:
1.
2.
3.
4.
5.
6.
Define Antenna
Give antenna parameters
Define Impedance Matching.
Define VSWR.
Relationship bandwidth Directivity & gain
What is need for radiation pattern?
ANTENNA ARRAYS
An antenna array (often called a 'phased array') is a set of 2 or more antennas. The signals
from the antennas are combined or processed in order to achieve improved performance
over that of a single antenna. The antenna array can be used to:
¾ increase the overall gain
¾ provide diversity reception
¾ cancel out interference from a particular set of directions
¾ "steer" the array so that it is most sensitive in a particular direction
¾ determine the direction of arrival of the incoming signals
¾ to maximize the Signal to Interference Plus Noise Ratio (SINR)
PROCEDURE
1. Arrange the setup as given in the block diagram
2. Mount two element antenna array on the transmitter mask
3. Bring the detector assembly near to main and adjust the height of both
transmitting and receiving antenna
4. Keep Detector assembly away from the main unit approximately 1.5 meter and
align both of them .Ensure that there is no reflector sort things in the vicinity of
the experiment such as a steel structure ,pipes, cables etc.
5. Keep the RF level and FS adjust to minimum and unidirectional coupler switch to
FWD(Forward adjustment knob).
6. Keep detector level control in the centre approximately
7. Increase RF level gradually and see that there is deflection in the detector meter
8. Adjust RF level and detector level, so that the deflection in detector meter is
approximately 30-35mA.
9. Align arrow mark on the disk with zero of the goniometer scale
10. Start taking the reading at the interval of 10 degree, and note the deflection on the
detector assembly.
11. Using conversion chart convert mA readings into db.
12. Plot the polar graph in degrees of rotation of antenna against level in the detector
in dBs
Post Lab Questions:
1.
2.
3.
4.
Define Antenna Arrays.
Draw the radiation pattern of antenna Arrays.
Give the application of antenna Arrays.
Write the frequency range of RF signal
Pre Lab Questions:
1.
2.
3.
4.
5.
6.
Define Antenna
Give antenna parameters
Define Impedance Matching.
Define VSWR.
Relationship bandwidth Directivity & gain
What is need for radiation pattern?
6. Log Periodic Antenna:
In telecommunication, a log-periodic antenna (LP, also known as a log-periodic array)
is a broadband, multi element, unidirectional, narrow-beam antenna that has impedance
and radiation characteristics that are regularly repetitive as a logarithmic function of the
excitation frequency. The individual components are often dipoles, as in a log-periodic
dipole array (LPDA). Log-periodic antennas are designed to be self-similar and are thus
also fractal antenna arrays.
It is normal to drive alternating elements with 180° (π radians) of phase shift from one
another. This is normally done by connecting individual elements to alternating wires of a
balanced transmission line.
The length and spacing of the elements of a log-periodic antenna increase logarithmically
from one end to the other. A plot of the input impedance as a function of logarithm of the
excitation frequency shows a periodic variation.
Fig. Log.-Periodic Antenna, 250–2400 MHz
PROCEDURE
13. Arrange the setup as given in the block diagram
14. Mount Log Periodic antenna on the transmitter mask
15. Bring the detector assembly near to main and adjust the height of both
transmitting and receiving antenna
16. Keep Detector assembly away from the main unit approximately 1.5 meter and
align both of them .Ensure that there is no reflector sort things in the vicinity of
the experiment such as a steel structure ,pipes, cables etc.
17. Keep the RF level and FS adjust to minimum and unidirectional coupler switch to
FWD(Forward adjustment knob).
18. Keep detector level control in the centre approximately
19. Increase RF level gradually and see that there is deflection in the detector meter
20. Adjust RF level and detector level, so that the deflection in detector meter is
approximately 30-35mA.
21. Align arrow mark on the disk with zero of the goniometer scale
22. Start taking the reading at the interval of 10 degree, and note the deflection on the
detector assembly.
23. Using conversion chart convert mA readings into db.
24. Plot the polar graph in degrees of rotation of antenna against level in the detector
in dBs
Post Lab Questions:
1.
2.
3.
4.
Define Logic period Antenna.
Draw the radiation pattern of Logic period antenna .
Give the application of Logic period antenna .
Write the frequency range of RF signal
Pre Lab Questions:
1.
2.
3.
4.
5.
6.
Define Antenna
Give antenna parameters
Define Impedance Matching.
Define VSWR.
Relationship bandwidth Directivity & gain
What is need for radiation pattern?
7. PARABOLIC ANTENNA
A typical parabolic antenna consists of a parabolic reflector with a small feed
antenna at its focus.The reflector is a metallic surface formed into a paraboloid of
revolution and (usually) truncated in a circular rim that forms the diameter of the antenna.
This paraboloid possesses a distinct focal point by virtue of having the reflective property
of parabolas in that a point light source at this focus produces a parallel light beam
aligned with the axis of revolution.
Fig .Parabolic Antenna
The feed antenna at the reflector's focus is typically a low-gain type such as a
half-wave dipole or a small waveguide horn. In more complex designs, such as the
Cassegrain antenna, a sub-reflector is used to direct the energy into the parabolic reflector
from a feed antenna located away from the primary focal point. The feed antenna is
connected to the associated radio-frequency (RF) transmitting or receiving equipment by
means of a coaxial cable transmission line or hollow waveguide.
A parabolic antenna is a high-gain reflector antenna used for radio, television
and data communications, and also for radiolocation (radar), on the UHF and SHF parts
of the electromagnetic spectrum. The relatively short wavelength of electromagnetic
(radio) energy at these frequencies allows reasonably sized reflectors to exhibit the very
desirable highly directional response for both receiving and transmitting.
With the advent of TVRO and DBS satellite television, the parabolic antenna
became a ubiquitous feature of urban, suburban, and even rural, landscapes. Extensive
terrestrial microwave links, such as those between cell phone base stations, and wireless
WAN/LAN applications have also proliferated this antenna type. Earlier applications
included ground-based and airborne radar and radio astronomy.
However a term dish antenna is often used for a parabolic antenna instead, it connote a
spheric antenna as well, which has a portion of spherical surface as the reflector shape.
Considering the parabolic antenna as a circular aperture gives the following
approximation for the maximum gain:
G = ( π 2 D2 ) / λ2
(OR )
G = ( 9.87 D2 ) / λ2
where:
G is power gain over isotropic
D is reflector diameter in same units as wavelength
λ is wavelength
Practical considerations of antenna effective area and sidelobe suppression reduce the
actual gain obtained to between 35 and 55 percent of this theoretical value. For
theoretical considerations of mutual interference (at frequencies between 2 and c. 30 GHz
- typically in the Fixed Satellite Service) where specific antenna performance has not
been defined, a reference antenna based on Recommendation ITU-R S.465 is used to
calculate the interference, which will include the likely sidelobes for off-axis effects.
PROCEDURE
1. Arrange the setup as given in the block diagram
2. Mount cut paraboloid antenna on the transmitter mask
3. Bring the detector assembly near to main and adjust the height of both
transmitting and receiving antenna
4. Keep Detector assembly away from the main unit approximately 1.5 meter and
align both of them .Ensure that there is no reflector sort things in the vicinity of
the experiment such as a steel structure ,pipes, cables etc.
5. Keep the RF level and FS adjust to minimum and unidirectional coupler switch to
FWD(Forward adjustment knob).
6. Keep detector level control in the centre approximately
7. Increase RF level gradually and see that there is deflection in the detector meter
8. Adjust RF level and detector level, so that the deflection in detector meter is
approximately 30-35mA.
9. Align arrow mark on the disk with zero of the goniometer scale
10. Start taking the reading at the interval of 10 degree, and note the deflection on the
detector assembly.
11. Using conversion chart convert mA readings into db.
12. Plot the polar graph in degrees of rotation of antenna against level in the detector
in dBs
Post Lab Questions:
1.
2.
3.
4.
Define Parabolic Antenna.
Draw the radiation pattern of Parabolic antenna .
Give the application of Parabolic antenna .
Write the frequency range of RF signal
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