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)

5

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

)

6

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