Why Front-to-back ratio is the difference in dB between the peak of the main lobe and the response exactly 180 degrees behind it. On Figure E9-1 the main lobe peaks at the outer 0 dB ring at the top of the plot, and the small rear lobe at the bottom (180 degrees) reaches in to the ring marked -18 dB, so the ratio is 18 dB. Read the scale rings, not the physical size of the lobes, since polar plots are logarithmic.