What is Antenna?
An Antenna is a transducer, which converts
electrical power into electromagnetic waves
and vice versa.
An Antenna can be used either as a
transmitting antenna or a receiving antenna.
❑ A transmitting antenna is one, which
converts electrical signals into
electromagnetic waves and radiates them.
❑ A receiving antenna is one, which
converts electromagnetic waves from the
received beam into electrical signals.
In two-way communication, the same antenna
can be used for both transmission and
reception.
Definition of Antenna
A means: radiating or receiving radio
waves
In other words the antenna is the
transitional structure between
free-space and a guiding
device.
The
guiding
device
or
transmission line may take the
form of a coaxial line or a hollow
pipe (waveguide), and it is used
to transport electromagnetic
energy from the transmitting
source to the antenna, or from the
antenna to the receiver.
RL= Dielectric Loss, Conduction Loss
Rr = Antenna radiation resistance
XA= Antenna reactance
Magnetostatic
Scenerio
For the above
cases, Static E
field and Static H
Field generated,
No wave is
generated
For wave, we
need a time
varying current
For the radiation, the current should be time varying. If the
current is time varying or charges are accelerating in any
form, there is a possibility of radiation.
Can this circuit radiate??
Radiation Condition
1. If a charge is not moving, current is not
created and there is no radiation.
2. If charge is moving with a uniform
velocity:
a. There is no radiation if the wire is
straight, and infinite in extent.
b. There is radiation if the wire is curved,
bent, discontinuous, terminated, or truncated.
3. If charge is oscillating in a time-motion, it
radiates even if the wire is straight
Radiation Mechanism – Two wire
Radio Frequency Band with Wavelength
Microwave Frequency Band
Types of Antenna
Antennas have various types depending on• Physical Structure
• Frequency ranges of operation
• Mode of applications etc.
Types of antennas under common
operating principles
1. Yagi - Uda Antenna
2. Log- Periodic Antenna
3. Helix Antenna
4. Parabolic Antenna
5. Micro strip Patch Antenna
6. Loop Antenna
Mode of applications
•
•
•
•
Point-to-point communications
Broadcasting applications
Radar communications
Satellite communications.
1.Yagi - Uda Antenna
• First published in 1926.
• Yagi-Uda antenna
consist of reflector,
director and driven
element.
• It is unidirectional
antenna.
• Frequency range –
300MHz-3GHz
Yagi - Uda Antenna - Advantages
• High gain and good front to back ratio.
• It has narrow bandwidth.
• It is fixed frequency device.
• Greater directivity due to director and reflector.
• Yagi UDA antenna is mostly used in
astronomical antennas and defense antennas.
Log- Periodic Antenna
• A unidirectional antenna in
which the length and spacing
of the elements increase
logarithmically from
• one end to the other.
• It is frequency-independent
antenna.
• Frequency range – VHF and
UHF band.
Log- Periodic Antenna
ADVANTAGE:
• It is broadband antenna.
• It is unidirectional antenna.
• It is frequency independent antenna.
APPLICATION
• HF Communication.
• UHF Terrestrial TV.
• EMC Measurements.
Helix Antenna
• A helical antenna is an
antenna consisting of one or
more
conducting
wires
wound in the form of a helix.
• These helix antennas are
referred to as axial-mode
helical antennas.
• A helical antenna which is one
of type of directional antennas is
used for increasing the gain of
the antenna.
Helix Antenna - Benefit
• It has a wide bandwidth,
• It is very robust in construction,
• Has a real input impedance, and
• Produce circularly polarized fields.
Polarization of these antennas is circular in nature.
And these antennas are used in radio Astronomy.
Helix Antenna - Disadvantages
• It is large in size
• This requires more space for installation.
• For higher number of turns its efficiency
decreases.
• It is higher in cost
Parabolic Antenna
• Parabolic Antenna: A
parabolic antenna is an
antenna that uses a
parabolic reflector ,or a
curved surface
• with the cross-sectional
shape of parabola, to
direct the radio waves
Parabolic Antenna
German physicist Heinrich Hertz
constructed the world's first
parabolic reflector antenna in 1888.
Parabolic Antenna
The operating principle of a parabolic
antenna is that a point source of radio waves
at the focal point in front of a paraboloid
reflector of conductive material will be
reflected into a collimated plane wave beam
along the axis of the reflector.
Application of Parabolic
Antenna
• High-gain antennas for P2P communications.
• Microwave relay links that carry telephone and television signals
between nearby cities.
•
Wireless WAN/LAN links for data communications
•
Satellite communications and spacecraft communication
antennas.
• They are also used in radio telescopes.
Micro-strip Patch Antenna
• A patch antenna isradio antenna with a low profile,
which can be mounted on a flat
surface.
• A micro strip antenna usually
means an antenna fabricated
using micro strip techniques on
a printed circuit board (PCB).
Micro strip Antenna - Application
➢ GSM
➢ RFID
➢ WiMAX
➢ Radar
➢ Telemedicine
➢ Mobile and satellite communication
Loop Antenna
A loop antenna is a radio antenna consisting of a loop or
coil of wire, tubing, or other electrical conductor usually fed
by a balanced source or feeding a balanced load.
Side Effects Of Antenna
➢ Cell Phone Tower
High levels of RF waves - warming of body tissues
Cell tower microwaves are travel up to 45 miles
Health Effects
•
•
•
•
•
•
Headaches
Memory loss
Cardiovascular stress
Low sperm count
Birth defects
Cancer
Antenna Parameters
Radiation Pattern
●
A mathematical function or a graphical representation of the
radiation properties of the antenna as a function of space
coordinates.
●
In most cases, the radiation pattern is determined in the far-field
region and is represented as a function of the directional
coordinates.
●
Radiation properties include power flux density, radiation intensity,
field
strength,
directivity,
phase
or
polarization.
The beamwidth of a pattern is defined as the angular separation between two
identical points on opposite side of the pattern maximum.
Half-Power Beamwidth (HPBW ) is defined as: “In a plane containing the direction
of the maximum of a beam, the angle between the two directions in which the
radiation intensity is one-half value of the beam.”
The angular separation between the first nulls of the pattern is referred to as the
First-Null Beamwidth (FNBW )
Field pattern (in linear scale) typically
represents a plot of the magnitude of the
electric or magnetic field as a function of the
angular space
● Power
pattern (in linear scale) typically
represents a plot of the square of the
magnitude of the electric or magnetic field as
a function of the angular space.
● Power
pattern (in dB) represents the
magnitude of the electric or magnetic field, in
decibels, as a function of the angular space.
●
In this and subsequent
patterns, the plus (+) and
minus (-) signs in the lobes
indicate
the
relative
polarization (positive or
negative) of the amplitude
between the various lobes,
which changes (alternates) as
the nulls are crossed.
Radiation Lobe
A radiation lobe is a “portion of the radiation pattern
bounded by regions of relatively weak radiation intensity.
● Radiation lobe can be different type
●
○
Major lobe
○
Minor Lobe
○
Side Lobe
○
Back Lobe
●
●
●
●
A major lobe (also called main beam) is defined as “the
radiation lobe containing the direction of maximum radiation.”
A side lobe is “a radiation lobe in any direction other than the
intended lobe.” (Usually a side lobe is adjacent to the main lobe
and occupies the hemisphere in the direction of the main beam.)
A back lobe is “a radiation lobe whose axis makes an angle of
approximately 180◦ with respect to the beam of an antenna.”
Minor lobes usually represent radiation in undesired directions,
and they should be minimized. Side lobes are normally the
largest of the minor lobes. The level of minor lobes is usually
expressed as a ratio of the power density in the lobe in question
to that of the major lobe. This ratio is often termed the side lobe
ratio or side lobe level. Side lobe levels of -20 dB or smaller are
usually
not
desirable
in
most
applications.
E Plane = XZ
H Plane = YX
Different type of Radiation Pattern
Radiation
Pattern
Isotropic
Omnidirectional
Directional
Isotropic Radiator
●
A hypothetical lossless antenna having equal radiation in all directions.
●
Although it is ideal and not physically realizable, it is often taken as a
reference for expressing the directive properties of actual antennas.
Directional Radiator
●
A directional antenna is one
“having the property of radiating
or receiving electromagnetic
waves more effectively in some
directions than in others.
●
This term is usually applied to an
antenna
whose
maximum
directivity is significantly greater
than that of a half-wave dipole
Omnidirectional Radiator
Having an essentially nondirectional pattern in a given plane (in this case in azimuth)
and a directional pattern in any orthogonal plane (in this case in elevation)
Field Region
Near Field
Reactive Near
Field
Radiative Near
Field
Radiation Field
Far Field
PNF, radiated=Re(E x H*)=|E||H|Cos(90o) Watts=0
PNF, stored=Im(E x H*)=|E||H|Sin(90o) Watts=Max
PRNF, radiated=Re(E x H*)=|E||H|Cos(0≤theta≤90o) Watts>0
PRNF, stored=Im(E x H*)=|E||H|Sin(0≤theta≤90o) Watts<Max
PFF, radiated=Re(E x H*)=|E||H|Cos(0o) Watts=Max
PFF, stored=Im(E x H*)=|E||H|Sin(0o) Watts=0
Reactive Near Field
●
Portion of the near-field region
immediately
surrounding
the
antenna wherein the reactive field
predominates.
●
For most antennas, the outer
boundary of this region is
commonly taken to exist at a
distance R < 0.62√D3∕λ from the
antenna surface, where λ is the
wavelength and D is the largest
dimension of the antenna.
●
For a very short dipole, or
equivalent radiator, the outer
boundary is commonly taken to
exist at a distance λ∕2𝜋
from the antenna surface.
Radiative Near Field
●
It is the region of the field of an
antenna between the reactive
near-field region and the farfield region wherein radiation
fields
predominate
and
wherein the angular field
distribution is dependent upon
the distance from the antenna.
●
If the antenna has a maximum
dimension that is not large
compared to the wavelength,
this region may not exist.
●
The inner boundary is taken to
be the distance R ≥ 0.62√D3∕λ
and the outer boundary the
distance R < 2D2∕λ where D is
the largest dimension of the
antenna.
Far Field
●
Region of the field of an
antenna where the angular field
distribution
is
essentially
independent of the distance
from the antenna.
●
If the antenna has a maximum
overall dimension D, the farfield region is commonly taken
to exist at distances greater
than 2D2∕λ from the antenna, λ
being
the
wavelength
.
Largest
dimension of
the antenna
Pattern is
well formed
with dominant
Main lobe
Patten is flat (no
main lobes are
formed)
Main lobe
begins to
develop
→One radian is defined as the plane angle with its vertex at the center of a circle of
radius r that is subtended by an arc whose length is r.
One steradian is defined as the solid angle with its vertex at the center of a sphere of
radius r that is subtended by a spherical surface area equal to that of a square with
each side of length r.
Radiation Power Density
The power radiated per unit surface area from the antenna surface (in spherical
coordinates system) is called Radiation Power Density (in W/m2).
Instantaneous
Poynting vector
Radiation Power Density
The poynting vector can also be expressed as:
Average poynting
vector or Average
power density→
Real (calculated
over one time
period):
Imaginary part is
eliminated
Peak Values (not
RMS)
Analogous to Ohm’s law :
P=1/2 VI*
Instanatious Total Power = Integration of normal component of poynting
vector (power density) over the entire surface
Average (total) radiated power
RADIATION INTENSITY
Radiation intensity (W/unit solid angle) in a given direction is defined as “the
power radiated from an antenna per unit solid angle.” Obtained by multiplying
radiation density (in W/m2) with square of distance r.
RADIATION INTENSITY
●
●
The power radiated from an antenna per unit solid angle
Directivity of the Antenna
●
The ratio of the radiation intensity in a given
direction from the antenna to the radiation
intensity averaged over all directions
If the direction is not specified, it implies the direction of maximum
radiation intensity (maximum directivity) expressed as
●
●
The total directivity is the sum of the partial directivities
for any two orthogonal polarizations.
(Radial component of radiated
power density)→ Given
Directivity, D
•Directivity of an antenna defined as “the ratio of the radiation intensity in a given
direction from the antenna to the radiation intensity averaged over all directions.
•The average radiation intensity is equal to the total power radiated by the antenna
divided by 4π.
•If the direction is not specified, the direction of maximum radiation intensity is
implied.”
→ directivity of a nonisotropic source is equal to the ratio of its radiation intensity
in a given direction over that of an isotropic source.
Radiation power density of infinitesimal
linear dipole of length l <<λ.
The total antenna efficiency e0 is used to take into account losses at
the input terminals and within the structure of the antenna.
1. reflections because of the mismatch between the transmission line and the antenna
2. I 2R losses (conduction and dielectric)
Gain
•Gain of an antenna (in a given direction) is defined as “the ratio of the intensity, in a
given direction, to the radiation intensity that would be obtained if the power accepted
by the antenna were radiated isotropically.
•The radiation intensity corresponding to the isotropically radiated power is equal to
the power accepted (input) by the antenna
divided by 4π.”
•gain of the antenna is closely related to the directivity, it is a measure that takes into
account the efficiency of the antenna as well as its directional capabilities.
•directivity is a measure that describes only the directional properties of the antenna,
and it is therefore controlled only by the pattern.
Relative Gain
•It is the ratio of the power gain in a given direction to the power gain of a reference
antenna (a lossless isotropic source).” The power input must be the same for both
antennas.
Partial Gains
Summary of Important Parameters and Associated Formulas