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I am not yet super familiar with the way beamwidth are calculated, but from what I've read, it's based on the point where the strength of the signal reaches -3dB.

Here is an excerpt from CCNA Wireless 200-355 Official Cert Guide.

enter image description here

I don't know if I'm reading the diagram incorrectly but it seems clear to me that the lines from which the angle is calculated are crossing the signal at around -6 ~ -7dB and definitely not -3dB.

Why is this beamwidth measured from below -5dB ? Should it be -6dB because it's twice -3dB for reasons I'm not aware of ?

Arthur Attout
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    suggest you migrate this question onto the amateur radio stack exchange. plenty of antenna experts there. – niels nielsen May 31 '18 at 17:15
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    I think you might be giving too much credit to the accuracy of the plot. For the figure on the left, the angle on the right clearly crosses below 5 dB, and I could believe that 3 dB is intended. For the figure on the right, it seems clear with little doubt that the angles are crossing at 3 dB. – garyp May 31 '18 at 17:23
  • What confuses me is the dBm (milliwatts)--why is it relative to 1 mW? – JEB May 31 '18 at 17:29
  • While putting away for a while the accuracy of the plot, is it correct to say that the beamwidth should be measured by tracing two lines from the center, crossing a circle at -3dB ? I really started to think I was not reading the plot properly – Arthur Attout May 31 '18 at 17:44
  • If someone could migrate the question, that would help aswell because I don't seem to have enough rep to move my own :) – Arthur Attout May 31 '18 at 17:59
  • @JEB "dBm" is a typo, it should be "dB" – hyportnex May 31 '18 at 18:31
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    The scale is in steps of 5dB, and if you look carefully the dashed line defining beamwidth is down about -3dB (should be dB and not dBm) relative to the peak. – hyportnex May 31 '18 at 18:34

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The labeling on the diagram is a little confusing. The outermost ring is 0 dBm, then each ring represents an amplitude decrease by 5 dBm. The dashed lines cross the solid lines between the 0 dBm and 5 dBm circles. Your way of reading it is very tempting, however --- threw me for a minute, too.

rob
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