main lobe width

Please help me. I can't decide the width of mainlobe from this picture... from 0 degree to something.

How can I define width of main lobe?

Thank you so much!

dont know

Beamwidth is commonly expressed as -3 dB width. It seems that the -3 db level is at about 15 deg. So the 3 db beamwidth would be 2 times 15 = 30 deg.

See http://en.wikipedia.org/wiki/Beamwidth.

The above answer is literally correct. I note that your pattern is from 0 to 90 degrees. What is the pattern for the other 270 degrees, and in the other plane? Are you after -3dB or -10dB beamwidths, null to null beamwidth? All are encountered but -3 dB is most common.

The full pattern might change the answer. Note also that this is probably an elevation pattern with the beam pointing straight up (variable theta at phi=1 deg)

Another way to get at the beamwidth is to estimate it based upon the aperture dimensions. BW(deg) = 65/(D/λ) The constant changes with aperture distribution but this is usually a reasonable estimate.