How can the approximate beamwidth of a beam antenna be determined?

In radio communications, the beamwidth of a directional antenna is conventionally defined as the 'half-power beamwidth'. Because a 3 dB decrease represents a 50% drop in power, the beamwidth is measured between the two points on the main lobe where the signal is exactly 3 dB weaker than the maximum peak. To measure this practically, you point the antenna directly at a receiving source to find the maximum signal strength. You then rotate the antenna in one direction until the signal drops by 3 dB, note the heading, and repeat in the opposite direction. The angle between these two headings is your beamwidth. Measuring the ratio between the front and rear (or side) of the antenna calculates the front-to-back or front-to-side ratio. These measure the antenna's ability to reject unwanted signals, not the angular width of its main radiating lobe.