parabolic antenna
DESCRIPTION
microwave antennasTRANSCRIPT
Resonant Antennas
• Antennas that have dimensions near to a half-wavelength or any of its multiples.
• The impedance and bandwidth vary significantly with small changes in the operating frequency; the impedance vary from negative reactance, thru pure resistance to positive reactance as the frequency changes from below to above resonance.
• Characteristics of the antenna are strongly frequency dependent.
Non-resonant antennas
• The radiation pattern and the input impedance are not so variable with frequency.
• Referred to as broadband antennas
Microwave Antennas or Antennas with Parabolic Reflector
Terrestrial Microwave Antennas for Point-To-Point Communication
• Terrestrial microwave antennas generate a beam of RF signal to communicate between two locations.
• Point-To-Point communication depends upon a clear line of sight between two microwave antennas.
• Obstructions, such as buildings, trees or terrain interfere with the signal.
• Depending upon the location, usage and frequency, different types can be utilized.
Because a line of sight radio link is made, the radio frequencies used occupy only a narrow path between stations (with the exception of a certain radius of each station).
Antennas used must have a high directive effect; these antennas are installed in elevated locations such as large radio towers.
Parabolic Reflector Antenna
• Structure – parabolic reflector and the feed mechanism
• The feed mechanism– house the primary antenna(dipole/dipole array) which
radiates EM waves toward the reflector
• The reflector– reflects the energy into a concentrated, highly directional
emission eave
Features of a parabolic antenna
• Extreme high gain and directivity – used in μ-wave and satellite link
• Beam width (Θ ) Θ = 70λ/D
where λ = wavelength D = antenna mouth diameter Θ = beam width of half power point ( degrees )
Gain
Front to Back Ratio (F/B)
Radiation Pattern Envelope
Cross-Polar Discrimination (XPD)
Return Loss (VSWR)
Interport Isolation (IPI)
Electrical Performance Parameters
Ga (dBi) = 10 log10 h [ 4 p Aa / l2 ]
Where:
Ga = Antenna Directive Gain (Catalog spec)
h = Aperture Efficiency (50-55%)
Aa = Antenna Aperture Area
l = Wavelength (speed of light / frequency)
Parabolic Antenna Gain
Power gain of Parabolic Antenna
• Power gain for a transmit parabolic antenna (Pgain)
Pgain = η(πD/λ)2
where Pgain= power gain referring to isotropic antenna
D = mouth diameter of parabolic reflector(meters)
λ = wavelength(meters)
η = antenna efficiency (typical 55%)
• Power gain in terms of dBi (assume η = 55%)
Pgain (db) = 20 log f(MHz) + 20 log D(m) + 42.2
Illustrated Example
• A 2-m diameter parabolic reflector with 5W of power radiated by the feed mechanism operating at 5GHz with transmit efficiency of 55%
• Beam Width Θ Θ = 70λ/D = 70(3 x 108)/[(5 x 109)(2)] = 2.1o
note how narrow the beam is !
• Transmit power gainPgain(dB) = 20 log 5000 + 20 log 2 + 42.2 = 37.78 dBi
Typical ParabolicAntenna Gain in dBi
Antenna Diameter
Fre
qu
ency
2 ft(0.6m)
4 ft(1.2m)
6 ft(1.8m)
8 ft(2.4m)
10 ft(3.0m)
12 ft(3.7m)
15 ft(4.5m)
2 GHz 19.5 25.5 29.1 31.6 33.5 35.1 37
4 GHz 25.5 31.6 35.1 37.6 39.5 41.1 43.1
6 GHz 29.1 35.1 38.6 41.1 43.1 44.6 46.6
8 GHz 31.6 37.6 41.1 43.6 45.5 47.1 49.1
11 GHz 34.3 40.4 43.9 46.4 48.3 49.9 51.8
15 GHz 37 43.1 46.6 49.1 51 52.6 N A
18 GHz 38.6 44.6 48.2 50.7 N A N A N A
22 GHz 40.4 46.4 49.9 N A N A N A N A
38 GHz 45.1 51.1 N A N A N A N A N A
Radiation Pattern Concept
Antenna Under Test
Source Antenna
Antenna Test Range
Radiation Pattern(a) Polar plot (b) Rectangular plot
Radiation Patternabout Bore sight
Cut Through Radiation Pattern at Boresight
Front-to-Back Ratio
• Ratio of the signal level at beam peak to that directed behind the antenna
• Considered in intra-system interference calculation (hop overreach)
• Expressed in dB
Co-Polarization and Cross-Polarization
• Co-Polarization– Where Transmit & Receive Antennas have
the Same Polarization– Either Horizontal or Vertical (HH or VV)– In your System, The Wanted Signal
Co-polarized antenna pattern
Azimuth AngleR
ela
tiv
e P
ow
er
X-polarized patttern
• Cross-Polarization– Where Transmit & Receive Antennas
have Different Polarizations
– Either HV or VH.
– In your System, The Unwanted Signal
XPD
Inter-port Isolation (IPI)
• Leakage of Signal Between Antenna Ports
• Internal Noise
• Expressed in dBTowards Reflector
Transmit Signal
Leakage Signal
Feed Horn
Voltage Standing Wave Ratio (VSWR)
• VSWR : 1.30 Reflection Coefficient : 13% (0.13)• VSWR : 1.20 Reflection Coefficient : 9.1% (0.091)• VSWR : 1.10 Reflection Coefficient : 4.7% (0.047)• VSWR : 1.08 Reflection Coefficient : 3.8% (0.038)• VSWR : 1.06 Reflection Coefficient : 2.9% (0.029)
VSWR = 1 + (Reflection Coefficient) 1 - (Reflection Coefficient)
Reflected Signal
Incident Signal
Return Loss The Amount of Energy Lost due to Reflected (Returned) Signal
• RL : 17.8dB Reflection : 13% (0.13) VSWR : 1.30• RL : 20.8dB Reflection : 9.1% (0.091) VSWR : 1.20• RL : 26.7dB Reflection : 4.7% (0.047) VSWR : 1.10• RL : 28.4dB Reflection : 3.8% (0.038) VSWR : 1.08• RL : 30.7dB Reflection : 2.9% (0.029) VSWR : 1.06
RL = -20 x log (Reflection Coefficient)
Basic Antenna Types
Standard Parabolic Antenna
Shielded Antenna
GRIDPAK® Antenna
Focal Plane Antenna
GRIDPAK® Antenna
• Grid Reflector
• Low Wind load
• Single Polarized
• Below 2.7GHz
• Shipped in Flat, Lightweight Package
Standard Parabolic Antenna
• Basic Antenna
• Comprised of
– Reflector
– Feed Assembly
– Mount
Focal Plane Antenna
• Deeper Reflector
• Edge Geometry
• Improved F/B Ratio
• Slightly Lower Gain
Shielded Antenna
• Absorber-Lined Shield
• Improved Feed System
• Planar Radome
• Improved RPE
Antenna Efficiency
Well-designed antennas have efficiency ratings of 45 - 65%
Efficiency Factor Affected By :
• Feed Illumination
• Aperture Blockage
• Reflector Surface Tolerance
Efficiency can never be 100%
f
D
f/D = 0.250
Antenna f/D Ratio
f
D
f/D = 0.333
Standard & Shielded Antennas
Focal Plane Antennas
Unwanted Signals
Scattering
Diffraction
Spillover
Front to Back Ratio
Direction of Signal
Direction of Signal
Direction of Signal
Shielded Antenna
Focal Plane Antenna
Standard Parabolic Antenna
Parabolic Reflector Beamwidth
0.3 m 0.6 m 1.2 m 1.8 m 2.4 m 3 m 3.7 m 4.5 m
2 GHz 35 17.5 8.75 5.83 4.38 3.5 2.84 2.33
6 GHz 11.67 5.83 2.92 1.94 1.46 1.17 0.95 0.78
8 GHz 8.75 4.38 2.19 1.46 1 0.88 0.71 0.58
11 GHz 6.36 3.18 1.59 1 0.8 0.64 0.52 0.42
14 GHz 5 2.5 1.25 0.83 0.63 0.5 0.41 0.33
18 GHz 3.89 1.94 0.97 0.65 0.49 0.39 0.32 0.26
23 GHz 3 1.52 0.76 0.51 0.38 0.3 0.25 0.2
38 GHz 1.84 0.92 0.46 0.31 0.23 0.18 0.15 0.12
Beamwidth in Degrees
Diameter
Fre
qu
en
cy
3dB
0
10
20
30
40
50
60
70
80
90
100
0 10 15 205 4060 80 100 120 140 160 180
Azimuth - Degrees from Main Lobe
An
ten
na
Dir
ecti
vity
dB
do
wn
fro
m M
ain
Lo
be
Radiation Pattern Envelope
Radomes
• Reduce Windloading on Tower
• Protection Against Ice, Snow and Dirt
Parabolic dish
antennas
Parabolic dish antenna with Cassegrain feed
Penzias&Wilson
New Jersey - Bell Laboratories
Horn
antennas