Lecture 3, Earth Segment

Presentation / Lecture 3, Earth Segment

Date Submitted: 06 June 2001

Written by RPC Telecommunications. Website: http://www.rpctelecom.com

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This is the third in the series of general satcom tutorial lectures submitted by RPC Telecommunications.

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 Printable Version
-Section 1
Types and components of an Earth Station
-Section 2
General Construction
-Section 3
Antenna Theory
-Section 4
Radiation Patterns
-Section 5
Polarisation
-Section 6
Noise
-Section 7
Low-noise Amplifiers
-Section 8
Power Amplifiers
-Section 9
Tracking

Radiation Patterns

ITU-R Recommendation S.465 defines a radiation mask for use in coordination and interference assessment:

For antennas with D >= 100*l:

G = 32 - 25 log q dBi for 1 <= q < 48
  = -10 dBi  for 48 <= q <= 180

For antennas with D < 100*l:

G = 52 - 10 log(D/l) - 25 log q dBi  for 1 <= q < 48
   = -10 - 10 log(D/l) dBi  for 48 <= q <= 180

ITU-R Recommendation S.580 defines a radiation mask design objective for new earth stations:

For antennas with D > 150*l:

G = 29 - 25 log q dBi 1 <= q < qr

For antennas with 50*l < D <= 150*l installed before 1995:

G = 32 - 25 log q dBi for 1 <= q < qr

For antennas with 50*l < D <= 150*l installed after 1995:

G = 29 - 25 log q dBi 1 <= q < qr

(Where qr is the maximum of 20 and (100*l/D)

The main influences on sidelobes are:

  • Aperture illumination:
    - Sidelobes are an integral part of the radiation pattern of an aperture
    - The larger an aperture the more rapidly the sidelobes decrease
  • Scattering and blockage:
    - Any object which blocks part of the radiation from an aperture disturbs the wavefront and causes additional sidelobes
    - There is a lower practical limit to the subreflector size so for small antennas the front fed configuration may give better sidelobes
    - Struts supporting the feed or subreflector also cause blockage and scattering
  • Spillover:
    - Spillover past the subreflector makes a significant contribution to the near-in radiation pattern of an axisymmetric dual reflector antenna
    - Spillover also occurs past the main reflector but this is at a lower level
  • Reflector profile errors:
    - Sidelobes caused by reflector profile errors are influenced by two factors, the rms value of the errors and the correlation distance of the errors
    - A repeated pattern of errors in the formation pr assembly of a series of panels can cause a serious increase in the sidelobes.
    - Profile accuracies are usually better than 1 mm rms

Next: Section 5 - Polarisation

 
 

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