1 gain and power gain an isotropic antenna simply beams its energy evenly in all directions the gain...
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1
Gain and Power Gain
• An isotropic antenna simply beams its energy evenly in all directions
• The gain is the ratio of the maximum power received from a parabolic reflector to that from an isotropic antenna emitting the same total amount of energy
• The antenna or power Gain (in dB) is defined as: 10 log10 (gain)
2
Antenna (Power) Gain of NRP Radars
• Typical weather antenna gains range from 20 to about 45 dB (Rinehart 1997)
• The antenna gain of operational weather radars is quite high…...
• CWSR 98-A : 46 dB
• CWSR 98-E : 43 dB
• WSR88D: 45 dB
3
Antenna-beam width relationship
• Beam width (deg) = 70 / (Antenna diameter)
• The diameter of the reflector of the new systems is 6.1m, compared to 3.7m for the retrofit systems
• Thus, the angular beam width (to half power) of the new systems is narrower than in the retrofit cases (0.65 deg vs 1.1 deg)
• This improves resolution and useful range
• A small beam width minimizes the effects of partial beam filling and beam blockage, the main reasons for poor performance with range
4
Gain and Beam width
• For circular reflectors, the expression relating gain and beam width is:
g=
where is the beam width in radians (Rhinehart 1997 quoting Battan, 1973)
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Antenna Gain vs Beam width
44.0
45.0
46.0
47.0
48.0
49.0
50.0
51.0
52.00.5
0.55 0.6
0.65 0.7
0.75 0.8
0.85 0.9
0.95 1
1.05 1.1
Beamwidth
An
ten
na
Gai
n (
dB
)
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Gain vs Beam width
• Thus, the smaller the beam width, the better the gain
• Improved gain means higher signal strength for distant objects or small targets– e.g. light precipitation may be detected at
greater distances
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