| 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
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