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Antenna Noise Temperature
Antenna Noise Temperature
In telecommunication, antenna noise temperature is the
temperature of a hypothetical resistor at the i/p of an
ideal noise-free receiver that would generate the
same o/p Noise power per unit BandWidth as that at
the antenna output at a specified frequency.
In other words, antenna noise temperature is a
parameter that describes how much noise an
antenna produces in a given environment.
This temperature is not the physical temperature of the
antenna.
Antenna Noise Temperature
The temperature depends on its gain pattern and the thermal
environment that it is placed in. Antenna temperature is also
sometimes referred to as Antenna Noise Temperature.
Temperature distribution (T(θ,Ф)(temperature in every direction
away from the antenna in spherical coordinates)
An antenna's temperature will vary depending on whether it is
directional and pointed into space or staring into the sun.
For an antenna with a Radiation Pattern R(θ,Ф), The Antenna
noise temperature is mathematically defined as:
The Noise Power received from an antenna at
temperature (TA ) can be expressed in terms of the
bandwidth (B) the antenna (and its receiver) are operating
over:
PTA =K TA B
k is Boltzmann’s constant (1.38×10−23 J/K).
Any object whose temperature is above the absolute zero
radiates EM energy. Thus, each antenna is surrounded by noise
sources, which create noise power at the antenna terminals.
Two types of Antenna Noise:
- Noise due to the loss resistance of the antenna itself;
- Noise, which the antenna picks up from the
surrounding environment.
Sources of Antenna noise temperature:
•Galactic radiation
•Earth heating
•The sun
•Electrical devices
•The antenna itself
Galactic noise is high below 1000 MHz. At around
150 MHz, it is approximately 1000K. At 2500 MHz, it has
leveled off to around 10K.
Earth has an accepted standard temperature of 290˚K.
The level of the sun's contribution depends on the solar
flux. It is given by
F is Solar flux
Antenna Polarization
The polarization of the radio wave can be defined
by direction in which the electric vector E is aligned
during the passage of at least one full cycle.
Also polarization can also be defined the physical
orientation of the radiated electromagnetic waves in
space.
The polarization are 3 types.
Elliptical polarization
circular polarization
Linear polarization.
also,
Co-Polarization(Tx & Rx Antenna polarized in same orientation)
Cross Polarization
Polarisation of EM wave
circular
vertical
Electrical field, E
horizontal
Basics of “Radiation Phenomena”
What is field?
If at each point in a region is a corresponding value of
some physical quantity such as Potential, Temperature,
E, H etc , the region is called Field.
“Field is spatial distribution of Physical Quantity”
What is field theory ?
Field theory is describing the existence and
variations of the “field” quantities such as E and H in
free space
How EM radiation is related to Energy
of the EM signal?
E=hf ;
h = plank’s constant = 6.624x10-34 J-sec
Say the freq of operation is 3 Ghz;
E = 6.624x10-34 X 3x109 J;
E = 6.624x10-25 X 3 J
But , 1 eV = 1.6x10-19 J
or
1 J = 1/ (1.6x10-19 ) eV
So , E = 6.624x10-25 x 3 x 1/ (1.6x10-19 ) eV
E = 1.2x10-5 eV
So , for different freq, EM signal would have the following Energy
Inference:
?......
EM spectrum
CHARGE , ELECTRON , WAVE
Ions are identified as Charged particls since they are electrically positive or
negative ( +Ve Charge, -Ve Charge):
 Atom is electrically neutral:
Charge is conserved, implies “ A disappearance of charge at one place
always follows by a reappearance of charge at another place.
Charge is quantized; it exhists in integer multiple of +e or –e
The value of charge of one electron is 1.6x10-19 C
1C = 6,250,000,000,000,000,000 electrons i.e, (6.250x10-18)
Electron is a packet of Energy
Electron is a source of Force
Electron is a quantification of Electricity
Electron is a Particle
Electron is a Wave
•Group of Electron is Volt
•Flow of electron is Current
•Transition of Electron leads to Absorbtion or Emission
•Oscillation of Electron leads to “Radiation”
RADIATION FROM OSCILLATING DIPOLE
A device that uses an oscillating distribution to produce
electromagnetic radiation is called an antenna.
A simple example of an antenna is an oscillating electric dipole,

“A pair of electric charges that vary sinusoidally with
time such that at any instant the two charges have equal
magnitude but opposite sign”.
One charge could be equal to Q sin vt and the other to −Q sin vt
.
An oscillating dipole antenna can be
constructed in various ways, depending
on frequency.
One technique that works well for radio
frequencies is to connect two straight
conductors to the terminals of an ac
source
A key feature of the radiation in the far(Fraunhofer) region is
that it is not a plane wave, but a wave that travels out radially in
all directions from the source.
The wave fronts are not planes; in the far region they are
expanding concentric spheres centered at the source.
Production of Electromagnetic Waves
Oscillating charges will produce electromagnetic
waves:
Production of Electromagnetic Waves
Far from the source, the waves are plane waves:
CROSS SECTION OF THE RADIATION PATTERN AT ONE INSTANT
Representation of the electric field (red lines) and the magnetic
field (blue dots and crosses) in a plane containing an oscillating
electric dipole. During one period the loop of E shown closest to
the source moves out and expands to become the loop shown
farthest from the source.
The Magnitudes in the Far region are :
Magnitudes are proportional to 1/r. (contrast to the E field of a stationary
point charge or the B field of a point charge moving with constant velocity, both of
which are proportional of 1/r2), Since 1/r2 terms become negligible at Far
region.
The field magnitudes are greatest in the directions
Perpendicular to the dipole, where θ = π /2;
There is no radiation along the axis of the dipole, where θ = 0˚ or π
At points very far from the oscillating dipole, E and B are
perpendicular to each other, and the direction of the Poynting
vector S = (E x B)/µo is radially outward from the source.
Oscillating magnetic dipoles also act as radiation sources;
An example is a circular loop antenna that uses a sinusoidal
current. At sufficiently high frequencies a magnetic dipole antenna
is more efficient at radiating energy than is an electric dipole
antenna of the same overall size.
Cont.,
E-field (E) & M-field (B) used to determine radiation pattern
• E goes through antenna ends & spreads out in increasing loops
• B is a series of concentric circles centered at midpoint gap
E
B
Cont.,
3-dimensional field pattern is donut shaped
-antenna is shaft through donut center
radiation pattern determined by taking slice of donut
- if antenna is horizontal  slice reveals figure 8
- maximum radiation is broadside to antenna’s arms
Azimuth Pattern
Elevation Pattern
Polar Radiation Pattern
HALF WAVE DIPOLE ANTENNA
• The half-wave dipole antenna is just a special case of the
dipole antenna.
• Half-wave term means that the length of this dipole antenna
is equal to a half-wavelength at the frequency of operation.
• The dipole antenna, is the basis for most antenna designs, is
a balanced component, with equal but opposite voltages and
currents applied at its two terminals through a balanced
transmission line.
• To make it crystal clear, if the antenna is to radiate at 600
MHz, what size should the half-wavelength dipole be?
• One wavelength at 600 MHz is = c / f = 0.5 meters.
Hence, the half-wavelength dipole antenna's length is
0.25 meters.
• The half-wave dipole antenna is as you may expect, a
simple half-wavelength wire fed at the center as shown in
Figure
λ= c / f
λ= (3x108 ) / (600x106)
λ= 0.5 mts
λ= 0.25 mts
Electric Current on a half-wave dipole
antenna
The fields from the half-wave dipole antenna are given
by:
It can be noted that by reducing the length slightly the antenna can
become resonant. If the dipole's length is reduced to 0.48 , the input impedance of
the antenna becomes Zin = 70 Ω, with no reactive component. This is a desirable
property, and hence is often done in practice. The radiation pattern remains virtually
the same.
The above length is valid if the dipole is very thin. In practice, dipoles are often
made with fatter or thicker material, which tends to increase the bandwidth of the
antenna.
The voltage and current levels vary along the length of the radiating section of the
antenna. This occurs because standing waves are set up along the length of the
radiating element.
The centre point is where the current is a maximum and the voltage is a minimum,
this makes a convenient point to feed the antenna as it present a low impedance.
For a dipole antenna that is an electrical half wavelength long, the inductive and
capacitive reactance's cancel each other and the antenna becomes resonant. With
the inductive and capacitive reactance levels cancelling each other out, the load
becomes purely resistive and this makes feeding the half wave dipole antenna far
easier. Coaxial feeder can easily be used as standing waves are not present
RADIATION PATTERN OF HALFWAVE DIPOLE ANTENNA
RADIATION FROM HALF WAVE DIPOLE
The current distribution on a thin half-wave dipole antenna is closely
approximated by a sinusoidal standing wave of the form:
Ko = 2π/λ
The far-zone radiated field from the half-wave dipole antenna:
the unit vector
The Electric field is given by:
Note that :
The magnetic field is given by:
The power flux per unit area is given by:
The total radiated power is obtained by integrating this expression over the
surface of a sphere of radius r; thus
The result of carrying out the integration is:
When we equate this expression to P= (1/2)I2R, we find that the radiation
resistance of the half-wave dipole is 2x36.56, or 73.13 Ω .
Intuitive Picture of Radiation
When the size of the system is approaching half wavelength (c=f x λ) ,
then the system becomes an efficient radiator
FOLDED DIPOLE ANTENNA
• The folded dipole is the same
length as a standard dipole, but
is made with two parallel
conductors, joined at both ends
and separated by a distance
that is short compared with the
length of the antenna.
• The folded dipole differs in that
it has wider bandwidth and has
approximately four times the
feed point impedance (≈300Ω)
of a standard dipole
FOLDED DIPOLE ANTENNA
• Folded antenna is a single antenna but it
consists of two elements.
• First element is fed directly while second
one is coupled inductively at its end.
• Radiation pattern of folded dipole is
same as that of dipole antenna i.e figure
of eight (8).
• The folded dipole antenna
is resonant and radiates well at odd
integer multiples of a half-wavelength
(0.5λ , 1.5λ , ...), when the antenna is fed
in the center.
Advantages of folded dipole antenna
• Input impedance of folded dipole is
four times higher than that of
straight dipole.
• Typically the input impedance of
half wavelength folded dipole
antenna is 288 ohm.
• Bandwidth of folded dipole is
higher than that of straight dipole.
Find F , if total length of
antenna is 2 mts
Radiation analysis of folded dipole
The folded dipole operates as an unbalanced transmission line.
The current on the folded dipole can be decomposed into two distinct modes:
-An antenna mode (currents flowing in the same direction
yielding significant radiation) and
-Transmission line mode (currents flowing in opposite
directions yielding little radiation).
The total folded dipole input current can then be defined as the
sum of the transmission line and antenna currents such that:
so that the folded dipole input impedance may be written as
W.k.t, The general equation for the input impedance of a
transmission line of characteristic impedance Zo and length l
terminated with an load impedance ZL is
But for a special case,…
For the shorted line, ZL = 0 and the length is l/2 so that
For the special case of a folded dipole of length l = /2,
The impedance of the half-wave folded dipole becomes:
Zd is Impedance of λ/2
antenna
The half-wave folded dipole can be made resonant with an
impedance of approximately 300 which matches a common
transmission line impedance (twin-lead). Thus, the half-wave folded
dipole can be connected directly to a twin-lead line without any
matching network necessary.
Used in domestic television and VHF FM broadcast antennas
YAGI-UDA ANTENNA
YAGI-UDA ANTENNA

In the 1926, Dr. Shintaro UDA and Dr.
Hidetsugu YAGI of the tohoku imperial university
invented a directional antenna system consisting
of an array of coupled parallel dipoles. This is
commonly known as yagi-uda or simply yagi
antenna.
 Yagi-uda
antenna
is
familiar as the
commonest kind of terrestrial TV antenna to
be found on the rooftops of houses.

It is usually used at frequencies between
30Mhz and 3Ghz and covers 40 to 60 km.
construction
• A Yagi-Uda array consists of 2 or more simple
antennas (elements) arranged in a line.
• The RF power is fed into only one of the
antennas (elements), called the driver.
• Other elements get their RF power from the
driver through mutual impedance.
• The largest element in the array is called the
reflector.
• There may be one or more elements located
on the opposite side of the driver from the
reflector. These are directors.
FIVE ELEMENT YAGI-UDA
DRIVER
REFLECTOR
Ele
Gain
ment dBi
s
Gai
n
dBd
3
7.5
5.5
4
8.5
6.5
5
10
8
6
11.5
9.5
7
12.5
10.5
8
13.5
11.5
•
•
•
•
This type of Yagi-Uda array uses dipole elements
The reflector is ~ 5% longer than the driver.
The driver is ~ 0.5l long
The first director ~ 5% shorter than the driver, and subsequent
directors are progressively shorter
• Interelement spacings are 0.1 to 0.2 λ
Radiation pattern formed
by the YAGI-UDA directional antenna
Radiation pattern formed
by the YAGI-UDA directional antenna
Typical yagis (6 m and 10m)
The 2 element Yagi
• The parasitic element in a 2- element yagi
may be a reflector or director
• Designs using a reflector have lower gain
(~6.2 dBi) and poor FB(~10 dB), but higher
input Z (32+j49 W)
• Designs using a director have higher gain (6.7
dBi) and good FB(~20 dB) but very low input Z
(10 W)
• It is not possible simultaneously to have good
Zin, G and FB
The 3 element Yagi
• High gain designs (G~ 8 dBi) have narrow BW and
low input Z
• Designs having good input Z have lower gain (~ 7
dBi), larger BW, and a longer boom.
• Either design can have FB > 20 dB over a limited
frequency range
• It is possible to optimize any pair of of the
parameters Zin, G and FB
Larger yagis (N > 3)
• There are no simple yagi designs, beyond 2 or 3
element arrays.
• Given the large number of degrees of freedom, it is
possible to optimize BW, FBR, gain and sometimes
control side lobes through proper design.
Yagi Array of Loops (quad array)
•
•
•
•
This Yagi-Uda array uses rectangular loops as elements.
The reflector’s perimeter is ~ 3% larger than the driver’s.
The driver’s perimeter is ~ 1l
The first director’s perimeter is ~ 3% smaller than the
driver’s, and additional directors are progressively smaller.
• Interelement spacings are 0.1 to 0.2 λ.
Advantages:
IT HAS A MODERATE GAIN OF ABOUT 7 (DB).
IT IS A DIRECTIONAL ANTENNA.
CAN BE USED AT HIGH FREQUENCY.
ADJUSTABLE FRONT TO BACK RATIO.
Disadvantages:
THE GAIN IS NOT VERY HIGH.
NEEDS A LARGE NUMBER OF ELEMENTS TO BE
USED.