Prof. Girish Kumar - nptel.ac.innptel.ac.in/...1-Antenna-Fundamentals-6Sep2016.pdf · Prof. Girish...
Transcript of Prof. Girish Kumar - nptel.ac.innptel.ac.in/...1-Antenna-Fundamentals-6Sep2016.pdf · Prof. Girish...
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Prof. Girish KumarElectrical Engineering Department, IIT Bombay
[email protected](022) 2576 7436
Antenna Fundamentals
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`3-D Radiation Pattern of Antenna
Omni-Directional Radiation Pattern of λ/2 Dipole Antenna
D = 1.64 = 2.1dB
Isotropic Radiation PatternD = 1 = 0dB
Directional Radiation Pattern of Microstrip Antenna Array
D = 500 = 27dB
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`2-D Radiation Pattern of Antenna
Beamwidth between first nulls (FNBW) ~ 2.25 x HPBW (Half Power Beamwidth)
Side Lobe Level (SLL) < 20 dB for satellite and high power applications
Front to Back Ratio (F/B) > 20 dB
Back Lobe
Minor Lobes
(HPBW)(FNBW)
y
x
Major Lobe
Side Lobe
z
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`Directivity of Antenna
oU
m oU DU
Directivity of an antenna is the ratio of radiation density in the direction of maximum radiation to the radiation density averaged over all the directions.
Example: For Infinitesimal Dipole
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`Directivity and Gain of Antenna
Gain = η D
Directivity of Large Antenna
Practice Problem: Find the gain in dB of a parabolic reflector antenna at 15 GHzhaving diameter of 1m. Assume efficiency is 0.6. What will be its gain at 36 GHz?Hint: Aperture Area of parabolic reflector antenna = π r2
where η is Efficiency of Antenna
Directivity is proportional to the Effective Aperture Area of Antenna
41253
E H
D
Directivity of Small Antenna
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Polarization of Antenna
Orientation of radiated electric field vector in the main beam of the antenna
Wave is Linearly Polarized
Wave is Circularly Polarized
Wave is Elliptically Polarized
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Axial Ratio of Antenna
Axial Ratio Bandwidth: Frequency range over which AR < 3 dB
Axial Ratio Plot of Circularly Polarized MSABandwidth for AR < 3dB = 380MHz (13%)
, circular polarization, elliptical polarization, linear polarization
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Input Impedance and VSWR of Antenna
Input ImpedanceRA represents power lossfrom the antenna and XA
gives the power stored inthe near field of theantenna
A r LR R R
r rr
A r L
R Re
R R R
Radiation Efficiency
0
0
A
A
Z Z
Z Z
max
min
1VVSWR
V 1
Reflection Coefficient and VSWR
Practice Problem: Calculate Reflection Coefficient and VSWR for impedance ZA = 10, 30, 50,100Ω
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Example: If antenna impedance , calculate Γ and VSWR.
Input Impedance Plot on Smith Chart
Normalized Input Impedance Plot on Smith Chart gives Γ and VSWR
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`Microstrip Antenna at 5.8 GHz
Return Loss Plot BW for |Γ| ≤ -10 dB
is 85MHz (1.5%)
Input Impedance Plot on Smith Chart normalized
with 50 ohm
MSA Design at 5.8GHz for RT Duroid
5880 Substrate height = 0.8mm
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`Microstrip Antenna Radiation Pattern and Gain
Antenna Gain PlotBW for 1dB Gain Variation
= 126MHz
Radiation PatternHPBW (H-plane) = 88°
HPBW (E-plane) = 80°
Antenna Efficiency Plot
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`Microstrip Antenna Array – Millimeter Wave
8x8 EMCP MSA Array at Millimeter Wave Gain PlotGain = 250 = 24 dB
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`Radiation Pattern of 8x8 MSA Array
Side Lobes
Main Beam
Cross Polar
Cartesian Plot Polar Plot
HPBW= 8.8°, FNBW=20°
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Link Budget
Receiving AntennaTransmitting
Antenna
r
Transmitter Receiver
Aet Aer
Friis Transmission Equation
Power Density
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Example: A GSM1800 cell tower antenna is transmitting 20W of power inthe frequency range of 1840 to 1845MHz. The gain of the antenna is 17dB.Find the power density at a distance of (a) 50m and (b) 300m in thedirection of maximum radiation.
Power Density:
(a) r = 50m:
(b) r = 300m:
Power Density