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.