Propagation of Electromagnetic (E-M) Energy and Pulse Volume

The WSR-88D radar transmits a stream or "beam" of energy in discrete pulses which propagate away from the radar antenna at approximately the speed of light (~3 X 108ms-1). The volume of each pulse of energy will determine how many targets are illuminated. This directly determines how much energy (power) is returned to the radar. The shape of the radar antenna, the wavelength, greek symbol lambda , of the energy transmitted, and the length of time the radar transmits determine the shape and volume of each radar pulse.

The Weather Service Radar-1988 Doppler (WSR-88D) transmits a narrow, conical-shaped beam of pulses with each pulse resembling a truncated cone. The radar pulse volume is illustrated in Figure 1 . The angular width of the radar beam is defined as that region of transmitted energy that is bounded by one-half (-3 dB) the maximum power. The maximum power lies along the beam centerline and decreases outward.

Figure 1.  Illustration of Radar Pulse Volume.  Click for a larger graphic

These "half-power" points for the WSR-88D result in an angular width of less than 1. However, the actual physical width increases with increasing range (the physical length remains constant) such that the pulse volume increases with increasing range (Figure 2). Since the amount of transmitted power is fixed, a radar pulse's power density decreases with increasing range. Pulsed transmission also allows for obtaining target range information.
Figure 2.  Radar Pulse Volume Increasing with increasing range.  Click for Larger View.