Dθ,φ 10 log104 πPθ,φ PO dBi
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans
2.15
Antenna Gain
G G0,0 10 log104 πP0,0 PT dBi
SatCommsA - Part 2 - Antennas - B G Evans
2.11
INTELSAT V coverage diagrams
Shaped zone beams
Shaped hemi beams
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans
S – SPILLOVER EFFICIENCY ratio of the total power in the antenna aperture to the total power radiated by the primary feedhorn
I – BLOCKAGE FACTOR incomplete utilisation of the antenna aperture due to the blocking effects of subreflector, supports, etc.
• Assume –4dB contour at E.O.C. area, then 4dB beamwidth (4) of antenna should be,
4 = N Degrees • Relationship between 4dB and 3dB beamwidth
• From tracking considerations we have
2.12
4 GHz and 6 GHz antennas on INTELSAT VI
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans
2.13
4 GHz and 6 GHz antennas on INTELSAT VI (cont.)
Antenna Radiation Pattern
Antenna radiation pattern in polar coordinates
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans
2.2
2
Loss (dB) =
12
θP θH
dB
• This is only a simple equation for the antenna main beam, therefore we
could find 4dB beamwidth relationship by putting
θP
1 2
θ4
and loss = -4
Typical Example: Global Coverage Beam (17.0° Beamwidth) General Requirement is to maximise edge of coverage gain. Occurs when the E.O.C. gain contour is approximately –4dB from the peak.
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans
LOSS (dB) 0.06 0.54 0.18 0.33 0.34 0.2
G
10
log10
4π λ2
. η .
dBi
where, = operating wavelength = physical aperture area of the antenna = antenna efficiency factor
For circular aperture antennas,
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans
2.9
Relationship between coverage area and antenna diameter
• Circular Coverage area diameter = N degrees
= antenna efficiency factor (less than or equal to unity)
100 x = antenna efficiency expressed as a percentage
= I x S x B x E x L x …
I – ILLUMINATION EFFICIENCY accounts for the non-uniformity of the illumination and phase distributions in the antenna aperture
Computed isogain contours at 6 GHz Using multimode feed
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans
2.4
Antenna Radiation Pattern
2 D2 R
λ Near-Field Region
λ Phase difference 16
Far-Field Region
Autumn2004 (c) University of Surrey
SatCo Evans
2.3
Multimode Feed
SatCommsA - Part 2 - Antennas - B G Evans
2.17
Typical efficiency factors for a large Cassegrain antenna
FACTOR
EFFICIENCY (%)
ILLUMINATION EFF.
98.7
SUBREFLECTOR S/O
SatCommsA - Part 2 - Antennas - B G Evans
2.8
Simple Satellite Antennas
SPOT ATLANTIC
SPOT EAST
SPOT ATLANTIC
EUROPEAN
Coverage patterns for ECS (Circular and Elliptic Beams)
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans
2.14
Antenna Radiation Characteristics
PT – Total power supplied to the antenna PO – Total power radiated by the antenna P(,) – Radiated power in the angular director (,)
Satellite Communications A Part 2
Antenna Basics -Professor Barry G Evans-
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans
2.1
2
4
12
1 2
θ
4
θH
dB
θ4
4 3
θH
1.15θH
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans
2.10
Contoured beam coverage
Contoured beam coverage
Autumn2004 (c) University of Surrey
SatCommsA - Part 2 - Antennas - B G Evans
2.5
Passive Reflecting surface
Autumn2004 (c) University of Surrey
Radiating Source (Feedhorn)
2π π
PO Pθ,φsinθ dθ dφ
00
Antenna radiation pattern or polar diagram
Eθ,φ 10 log10Pθ,φ P0,0 dB
Antenna gain function
Gθ,φ 10 log104 πPθ,φ PT dBi
Antenna directivity function
Main reflector 5.5m diameter
Dual –offset Gregorian antenna for satellite communication services
Antenna is shown At 30°angle of elevation
Autumn2004 (c) University of Surrey
reflector
1000m
scale
0m
34.28 °