Beamwidths
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Beamwidth

Beamwidth is the measurement of how broad or narrow the focus of an antenna is.

• measured both horizontally and vertically
• measurement from the center, or strongest point, of the antenna signal to each of the points along the horizontal and vertical axes where the signal decreases by half power (– 3 dB)
  • as seen below. These –3 dB points are often referred to as half-power points.
  • The distance between the two half-power points on the horizontal axis is measured in degrees, giving the horizontal beamwidth measurement.
  • The distance between the two half-power points on the vertical axis is also measured in degrees, giving the vertical beamwidth measurement.

Beamwidth

Manufacturers typically includes the numerical values for the horizontal and vertical beamwidths of the antenna. It is important for you to understand how these numbers are calculated.

Beamwidth on Polarization chart

Beamwidth Calculation

1. First determine the scale of the polar chart.On this chart, you can see that the solid circles represent the –10 dB, –20 dB, and –30 dB lines and the dotted circles therefore represent the –5 dB, –15 dB, and –25 dB lines. These represent the dB decrease from the peak signal.
2. To determine the beamwidth of this antenna, first locate the point on the chart where the antenna signal is the strongest.In this example, the signal is strongest where the number 1 arrow is pointing.
3. Move along the antenna pattern away from the peak signal (as shown by the two number 2 arrows) until you reach the point where the antenna pattern is 3 dB closer to the center of the diagram (as shown by the two number 3 arrows).This is why you needed to know the scale of the chart first.
4. Draw a line from each of these points to the middle of the polar chart (as shown by the dark dotted lines).
5. Measure the distance in degrees between these lines to calculate the beamwidth of the antenna.In this example, the beamwidth of this antenna is about 28 degrees.

 

• It is important to realize that even though the majority of the RF signal that is generated is focused within the beamwidth of the antenna, a significant amount of signal can still radiate from outside the beamwidth, from what is known as the antenna’s side or rear lobes.
• As you look at the azimuth charts of different antennas, you will notice that some of these side and rear lobes are fairly significant.
• Although the signal of these lobes is drastically less than the signal of the main beamwidth, they are dependable, and in certain implementations very functional.

Beamwidths of various antenna types

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Reference:
Coleman, David D.,Westcott, David A. CWNA: Certified Wireless Network Administrator Official Study Guide: Exam CWNA-106 Wiley.