Beamwidths and Sidelobe Levels
These concepts can be easily illustrated. Consider the radiation pattern given by:
This pattern is actually fairly easy to generate using Antenna Arrays, as will be seen in that section. The 3-dimensional view of this radiation pattern is given in Figure 1.
The polar (polar angle measured off of z-axis) plot is given by:
The main beam is the region around the direction of maximum radiation (usually the region that is within 3 dB of the peak of the main beam). The main beam in Figure 2 is centered at 90 degrees.
The sidelobes are smaller beams that are away from the main beam. These sidelobes are usually radiation in undesired directions which can never be completely eliminated. The sidelobes in Figure 2 occur at roughly 45 and 135 degrees.
The Half Power Beamwidth (HPBW) is the angular separation in which the magnitude of the radiation pattern decrease by 50% (or -3 dB) from the peak of the main beam. From Figure 2, the pattern decreases to -3 dB at 77.7 and 102.3 degrees. Hence the HPBW is 102.3-77.7 = 24.6 degrees.
Another commonly quoted beamwidth is the Null to Null Beamwidth. This is the angular separation from which the magnitude of the radiation pattern decreases to zero (negative infinity dB) away from the main beam. From Figure 2, the pattern goes to zero (or minus infinity) at 60 degrees and 120 degrees. Hence, the Null-Null Beamwidth is 120-60=60 degrees.
Finally, the Sidelobe Level is another important parameter used to characterize radiation patterns. The sidelobe level is the maximum value of the sidelobes (away from the main beam). From Figure 2, the Sidelobe Level (SLL) is -14.5 dB.
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