Antenna Field Zones, Radiation Resistance, Link budget/Friis Transmission Formula, Antenna Apertures, Antenna Temperature/ G/T ratio, Antenna Impedance/Matching

1. Dr. S.MUTHUMANICKAM
Associate Professor
Department of ECE
1
EC8701 ANTENNAS
AND
MICROWAVE ENGINEERING PART 2

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ANTENNA FIELD ZONES
(NEAR AND FAR FIELD REGION)
RADIATION RESISTANCE
LINK BUDGET AND FRIIS TRANSMISSION
FORMULA
ANTENNA APERTURES
ANTENNA NOISE TEMPERATURE AND G/T RATIO
ANTENNA IMPEDANCE AND IMPEDANCE
MATCHING
T
O
P
I
C
S

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Five-hundred-meter Aperture Spherical
Telescope (FAST)
size of 30 football fields
CHINA
LARGEST APERTURE TELESCOPE IN THE WORLD

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NARL ISRO CHITOOR

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ISTRAC BENGALURU

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GMRT PUNE

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ANTENNA SETUP IN SAMSUNG MOBILE

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iPhone Antenna Setup

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ANTENNA FIELD ZONES
Types
1. Reactive Near Field Region
2. Radiating Near Field Region
3. Far Field Region
Antenna
D is diameter of parabolic dish
L is length of a wire antenna
Area surrounding the antenna, where EM field is produced

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Reactive Near Field Region Radiation Pattern
 This region is immediately surrounding the
antenna
 For most of the antenna the outer
boundary of this region is
R < 0.62 𝑫³/𝝀
 For short dipole radiator
R < 𝝀/2п
 Objects with in this region will result
coupling with the antenna and distortion of
the ultimate far field antenna pattern
 Large conductor within this region will
couple with the antenna and ‘detune’ it.
This affects the following antenna
parameters
 resonant frequency
 radiation resistance
 radiation pattern

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Radiating Near Field Region
 The field region of the antenna between
the reactive near and far field region, the
distance from the antenna R is
0.62 𝑫³/𝝀 < R < 2D²/ 𝝀
 The region is also called as Transition
region(antenna holds its radiation shape)
 Properties:
 The antenna pattern is taking shape
but is not truly formed
 The radiation field predominates the
reactive field
 The radiated wave front is still clearly
curved
 The E and H field vectors are not
orthogonal
Radiation Pattern

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Far Field Region
 The angular field distance is essentially
independent of the distance from the
antenna
R > 2D²/ 𝝀
 Properties of this region are
 The wave front becomes
approximately planar
 The radiation pattern is completely
formed and does vary with distance
 E and H field vectors are
orthogonal to each other
Radiation Pattern

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Comparison of the Radiation patterns
Reactive near field patteRadiative near field pattern
Far field pattern

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Radiation Resistance
The value of resistance that would
dissipate the same amount of power when
an input current passing through it

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RADIATION RESISTANCE Rr
Antenna radiation is associated with
radiation resistance.
If we supply I current to antenna, then
power dissipated by antenna
P = I² x R
The energy supplied to antenna is
dissipated in two ways
1. Radiated power Prad = I² Rr
2. Due to ohmic loss Ploss = I² RL
Total power P = Prad + Ploss
= I² Rr + I² RL
= I² (Rr + RL)
Because of Rr the antenna radiates
power in free space
P ∞ Rr
when Rr increases P increases
Rr decreases, P decreases
HOW?
Radiation Efficiency ϵrad =
Rr
Rr + RL
For example
Case i:
If Rr is 50 Ω and RL is 10 Ω
Then ϵrad =
50
50 + 10
=
50
60
= 83%
Case ii:
If Rr is 10 Ω and RL is 10 Ω
Then ϵrad =
10
10 + 10
=
10
20
= 50%

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Radiation Resistance Depends upon
Configuration of
antenna
It depends upon
corona discharge
Ratio of length to
diameter of
conductor used
in antenna
Location of
antenna with
respect to ground
and other objects
Depends upon
point where
radiation
resistance is
considered
Rr

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When High Voltage applied
When High Frequency Applied

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LINK BUDGET
Accounting of power gain and losses

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1. Then the power density p (in Watts per
square meter) of the plane wave incident on the
receive antenna a distance R from the transmit
antenna is given by:
2. If the transmit antenna has an antenna gain
in the direction of the receive antenna given
by GT, then the power density equation above
becomes:
3. Assume now that the receive antenna has
an effective aperture given by AER. Then the
power received by this antenna PR is given
by:
4. Since the effective aperture for any
antenna can also be expressed as:
5. The resulting received power can be
written as:
6. Since wavelength and frequency f are
related by the speed of light c , we have
the Friis Transmission Formula in terms
of frequency f :
Derivation of FRIIS Transmission Formula
7. If the antennas are not polarization
matched, the above received power could
be multiplied by the Polarization Loss
Factor (PLF)
If we consider Tx antenna as isotropic source,
Then power density at Rx antenna is
P = PT/A = Transmitted power/Area
PR = P x AER
Gain of Tx antenna, GT =
4п 𝐴𝑒𝑇
λ²
Then PR = PT
4п𝐴𝑒𝑇
4п𝑅²λ²
x AER
slly. gain of Rx antenna GR =
4п 𝐴ᴇ𝑅
λ²

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Power Density
https://www.pasternack.com/t-calculator-
friis.aspx

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ANTENNA APERTURE (Ap)
The area or part of the antenna which extracts power
from the wave
Describes the power capturing
characteristics of antenna
Types:
1. Physical aperture
2.Effective aperture
3.Scattering aperture
4.Loss aperture
5.Collecting aperture

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Rectangular Horn antenna with dimensions a and b is given
Ap = a x b(m²)
The area of opening is called as physical aperture
If the incident wave has power density W, then the received power
P = W x Ap (watts)
where P is power received
W is power density of the plane wave (watts per m²)
Ap is physical aperture in m²
Physical Aperture (Ap)

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EFFECTIVE APERTURE (Ae)
The effective area simply represents how much power is
captured from the plane wave and delivered by the antenna.
This area factors in the losses intrinsic to the antenna (ohmic
losses, dielectric losses, etc.).
A general relation for the effective aperture in terms of the
peak antenna gain (G) of any antenna is given by: Ae =
λ²
4п
G

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Antenna Apertures
Scattering Aperture
Loss Aperture

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Capture Area/Collective Aperture

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Antenna Effective Height/Length
lef =
𝑉𝑜𝑐
𝐸

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ANTENNA NOISE TEMPERATURE
AND
G/T CALCULATION
Antenna gain in decibels
Noise temperatureFigure of Merit

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 Antenna noise temperature is a parameter that
describes how much noise an antenna produces
in a given environment
 The temperature is not the physical
temperature of the antenna

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The temperature appearing at the terminals of antenna is given by
TA = antenna noise temperature at output terminals of antenna (K)
To = physical temperature of transmission line (K)
Ta = antenna temperature at receiver terminals (K)
TA = antenna noise temperature at the antenna terminals (K)
TAP = antenna temperature at the antenna terminals due to physical temperature(K)
G = gain pattern of antenna(dB), Tm = molecular temperature (K)
Γ= reflection coefficient of surface, ϵ = emissivity (dimensionless)
Brightness temperature (K)

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Tp = Antenna physical temperature (K)
α = attenuation coefficient of transmission line
l = length of transmission line
eA = Thermal efficiency of antenna(dimension less)
Antenna Noise Temperature (TA)c
Antenna temperature at receiver terminals
Antenna temperature at the antenna terminals due to physical
temperature

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Antenna Noise Temperature (TA)c
TB = brightness temperature (K)
TA = antenna noise temperature at output terminals of antenna(K)
Ps = system noise power (W)
Ta = antenna noise temperature at receiver terminals (K)
Tr = receiving noise temperature at receiver terminals (K)
Δf = bandwidth (Hz)
k = Boltzmann’s constant (k = 1.38x10‾²³ J/K)

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ANTENNA IMPEDANCE (ZA)
Transmitting antenna Receiving antenna

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ANTENNA IMPEDANCE
Transmitting antenna and its Thevenin equivalent circuit

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Transmitting antenna and its Norton equivalent circuit
To find the amount of power
delivered to Rr for radiation and
amount dissipated RL as heat

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(16)
(17)
(18)
(19)

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IMPEDANCE MATCHING
WHY?
Maximize the power transfer to load
or
Minimize signal reflection from the load

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Pair of AC&E 120 Ω twisted pair (Krone IDC)
to 75 Ω coaxial cable balun transformers.
Actual length is about 3 cm
A 75-to-300-Ω balun built into the
antenna plug
Impedance matching device

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BALUNS
Balanced To Unbalanced

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PROBLEM

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