Transcript
SATELLITE
EARTH STATION ANTENNA
By Naveen Jakhar, ITS
FREQUENCY BANDS
1. VLF 10-30 KHz World wide Telegraphy
2. LF 30-300 KHz Marine & Navigation
3. MF 300-3000 KHz MW & SW Broad-
4. HF 3-30 MHz Casting
5. VHF 30-300 MHz MARR, TV Radar,
Aeroplane, Navigation
6. UHF 300-3000 MHz TV, UHF
7. SHF 3-30 GHz Microwave & Satellite
(Super High Freq.)
8. EHF 30-300 GHz Experimental
(Extremey High Freq.)
High Gain
Wide Band width
Low Noise Temp.
Massive size.
Complete Steer ability in X-Y plane.
Narrow beam width,
Low side Lobes &
High polarization discriminations.
Satellite Earth Station Antenna characteristics.
Noise Figure, F = SNRin / SNRout
F = Si /Ni = No = No …….So/No (So/Si) Ni G Ni
F = GNi + Na = 1 + Na GNi GNi GNi is the out put noise power of a noise free network.
The value of F is standardized by fixing the input noise power as that produced by a matched source at a standard temperature of 290 K (17 C).
A noise less network has a noise figure of unity. Real networks always have a noise figure greater than
unity.
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Si, NiSo, No
F
G
The total noise in the receive chain (at the input of
demodulator) may be expressed in the form of noise
temperature.
Pn = k . Te . B
Where,
Pn = Generated noise power
k = Boltzman’s constant
= 1.38x 10exp(-23) Joules/K
B = Bandwidth over which noise is generated.
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Noise Figure, F = 1 + Na / G Ni
Na= Noise added by the Amplifier
F = 1 + G k Te B / G k To B
To is 290 K (17 degree C)
Or, F = 1 + Te / To
Or, Te / To = F – 1
Or, Te = To ( F – 1)
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Noise Figure in db = 10 log10 ( 1 + Tn /290
)
For an amplifier of Tn = 865 K
Noise Figure = 10 log10 ( 1 + 865/ 290)
= 10 (0.6) = 6 db
Noise Figure = 10 log10 ( Noise Factor)
6 db = 10 log10 ( Noise Factor)
Noise Factor = 4
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Low Noise Temperature :
High performance is required because signal levels to be
handled by the antenna are very-very low .
C/N requirement (Noise is very important factor)
Noise added by the antenna should be as low as possible.
Noise sources :
Sky Noise Collected by the antenna aperture.
Feed (Most important Noise Contributor) even 0.1 dB noise
addition affects noise temperature appreciably.
Wide Band Characteristics :
Antenna System should have sufficient band width.
So that performance is not affected.
Polarizer & Duplexer should be designed specifically.
Narrow beam width
Mechanical Characteristics :
High mechanical accuracy.
Tracking accuracy within 1/10th of beam width.
High surface Accuracy.
Steer-ability (should be track-able to all possible satellite
locations).
Classification of Antenna
(a) Radiation System. & (b) Mount Structure.
(a) Radiation System :-
(i) Horn Reflector Antenna
- Developed by Bell labs USA for satellite communication
- High Radiation efficiency.
- Good Noise Temperature performance.
It is very expensive because of its too huge structure in
proportion to its effective antenna aperture.
(ii) Parabolic Antenna with Prime Focal Feed :
• Widely used with terrestrial microwave radio relay
links.
• It has high noise temperature characteristics. So
can not be . considered as standard antenna for
Satellite com. (high
noise temp. because of spillover power from main
reflector).
• Long wave guide to connect prime radiator with
LNA.
• LNA can not be mount with the focal feed.
• Mechanical Complexity with the rotating
structure.
• Maintenance problems.
(iii) Cassegrain Antenna
- Standard antenna for satellite communication & used
universally.
- Main reflector – parabolic
Sub reflector – hyperboloid
- One of the focii of hyperboloid coincides with focal point
of parabola.
- Other focal point of hyperboloid coincides with phase
centre of primary radiator which is at vertex of paraboloid.
Paraboloid primary
reflector
Feed antenna
(horn)
Waveguide
Obstructed ray
Focus of paraboloid
and hyperboloid
Hyperboloid
secondary reflector
•
Geometry of the Cassegrain feed
Cassegrain Feed Antenna
Advantages. :-
- Reduction in feed length.
- Ease in Azimuth & elevation rotation along with
antenna structure.
- LNA can be mount very near to primary radiator.
- Low noise temperature.
- High directivity.
- Narrow beam width.
- Feed horn is covered with radomes of glass re-inforced
Teflon sheet.
- Pressurized internally by dry air to prevent moisture.
•
Parabolic
SectionCollimate
rays
Axis
Horn
feedFocus
Offset paraboloid reflector
Classification based upon
mount structure :
(a) Azimuth Elevation Mount :-
- Primary axis (Azimuth Axis)
is set vertical to the ground.
- Secondary axis (Elevation
Axis) is set Horizontal to the
ground.
- AZ-EL mounting is used
almost universally.
(b) X-Y Mount :-
- Primary Axis (X-Axis) is
kept parallel to ground such
that antenna dish centre
moves back & forth while
rotation around X-Axis.
- Rotation around Y-Axis
produces both Azimuth &
Elevation Change.
- This mounting is suitable
for small size antenna.
(c) Polar Mount :
- Here one axis i.e. Ha axis of
the structure is parallel to
the earth’s axis of rotation.
While other axis (Dec.) is
perpendicular to it.
- Tracking is achieved by
rotation around Ha-axis only.
(d)Tripod Mount :-
- Suitable for small earth stations.
- Antenna is fixed to a support by three legs.
- Length of one leg is fixed while the other two are variable.
- Limited variations of pointing are possible.
Antenna being used :
Following type of antenna with Cassegrain structure
are being used.
(i) 11m dia antenna.
(ii) 7.5 m dia antenna.
(iii) 4.5 m dia antenna.
(iv) 3.3 m dia antenna.
(v) 2.4 m dia antenna.
(vi) 8.1 Meter Ku-Band Hub Antenna
(i) 11 m dia antenna :-
- Used at Main earth stations,
G/T is 31.7 db/K & 40K LNA.
- Mount structure is Azimuth Elevation type.
- LNAs are mounted inside the centre Hub.
(ii) 7.5 m dia Antenna :-
- Used at primary E/S.
- G/T is 25.7 dB/0K and 900K LNA.
- Limited steerability.
(iii) 4.5 m dia antenna :-
- Used at remote E/S.
- G/T is 19.7 dB/0K & 1200K LNA.
- X-Y type mount.
- 3.3 m dia Antenna are used for ECTs.
- 2.4 m dia Antenna are used for MCPC-VSAT
stations.
- 14 m dia Antenna is used at MCF Hasan.
- 30 m dia Antenna at VSNL E/S Dehradun.
8.1 Meter Ku-Band Hub Antenna
• Antenna
– Parabolic
• Size
– 8.1 Meter
• Feed
– Cassegrene (4 port KU-Band Horn Feed)
• Gain
– 57 dB (Rx)
– 59 dB (Tx)
• Movement
– AZ/EL
• Tracking Mode
– Auto or Manual
• LNA Gain
– 50 dB
• Band Width
– 2050 Mhz
1.2 Meter Antenna
Thanks.
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