Looking at the graph below, i can see that HPBW of the main lobe is around 45 degrees, but when it comes to identifying the FNBW i have trouble understanding it. Reading various articles online i've seen it mentioned that FNBW is around twice HPBW.
My approach was to look for a point in which the radiation power starts increasing after the main lobe and that would mean i've found a side lobe, thus having a null between main lobe and side lobe. For the diagram below, this would mean ~ -145 degrees and +120 degrees (power graph is on the right, amplitude is on the left) and obtain a FNBW of about 100 degrees that would coincide with about twice the width of the HPBW. Is this correct?
I am unsure whether this is correct as the power for those points is not 0. The only points with 0 power are at around +/- 25 degrees.