User's Manual
Antenna Evaluation
February 2004 17
3.3.2.2 Half Power Points (—3 dB Points)
The most important part of an antenna’s signal pattern is that portion where the signal is at least at the 50
percent power level. This level is commonly referred to as the half power point, or the —3 dB point.
The antenna pattern in Figure 6 has its half power points (—3 dB points) at 330 degrees and 30 degrees,
giving this antenna a usable horizontal pattern of 60 degrees.
Figure 6 Half Power Points
This angle between the two half power points is also commonly referred to as the antenna’s beamwidth.
Figure 7 shows sample patterns for three antennas where the half power points are marked by dark lines
extending from the center of the pattern, and the primary part of the pattern is shaded in dark gray.
The antennas have the following horizontal and vertical components:
• Antenna A 180 degrees Horizontal 15 degrees Vertical
• Antenna B 60 degrees Horizontal 30 degrees Vertical
• Antenna C 30 degrees Horizontal 60 degrees Vertical
Note that as the horizontal component decreases, the vertical component increases. This relationship
always exists as long as the antenna’s signal amplification properties, or gain, remains the same.