Lecture 3, Earth Segment

 Presentation / Lecture 3, Earth Segment Date Submitted: 06 June 2001 Written by RPC Telecommunications. Website: http://www.rpctelecom.com ExcellentGoodAveragePoorVery Poor   Rate This Article This is the third in the series of general satcom tutorial lectures submitted by RPC Telecommunications. Comment On This Article
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 -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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