The half-power beam width (HPBW) is used to define the footprint of the antenna of a microwave radiometer or radar because it represents a critical characteristic that quantifies the antenna's angular resolution and coverage area.
The half-power beam width is the angle between the points on the antenna's radiation pattern where the power (or signal strength) is half of the maximum. In other words, it defines the angular span over which the antenna radiates a significant portion of its power. For example, if the maximum power is 100%, then the half-power points would correspond to 50% power.
Here's why the HPBW is important for microwave radiometers and radars:
Angular Resolution: The HPBW provides a measure of the antenna's ability to distinguish between closely spaced objects or features in the target area. A smaller HPBW indicates higher angular resolution, allowing the radar or radiometer to detect and distinguish smaller objects or variations in the received signals.
Coverage Area: The HPBW defines the extent of the area covered by the antenna's main lobe. The main lobe is the central region of the antenna's radiation pattern with the highest signal strength. Knowing the HPBW helps determine the size and shape of the "footprint" on the ground or target area where the antenna is sensitive to signals.
Sensitivity: The HPBW affects the antenna's sensitivity to signals originating from different directions. A narrower HPBW concentrates the antenna's sensitivity into a smaller angular range, making it more selective in detecting signals from specific directions.
Side Lobes: Alongside the main lobe, antennas can also produce side lobes, which are smaller lobes in the radiation pattern. The HPBW helps identify the angular range where these side lobes may occur, and it's essential to minimize their impact as they can lead to unwanted reflections and interference.
In summary, the half-power beam width is a crucial parameter for microwave radiometers and radars because it directly influences the antenna's angular resolution, coverage area, sensitivity, and sidelobe performance. By understanding and controlling the HPBW, engineers and scientists can optimize the antenna's performance to suit the specific application and improve the accuracy of the data collected by the instrument.