What is the front-to-back ratio of the antenna radiation pattern shown in Figure E9-1?

Front-to-back ratio is a common measurement of an antenna's directivity. It is calculated by taking the signal strength at the maximum forward gain (the 'front' of the beam) and comparing it to the signal strength exactly 180 degrees opposite (the 'back'). In the polar plot shown in Figure E9-1, the main forward lobe peaks at the 0 dB ring exactly on the 0-degree axis. To find the rearward gain, follow the straight vertical line down to the 180-degree mark and find where the rear lobe intersects the axis. At 180 degrees, the graph's rear lobe reaches the -18 dB circle. Subtracting the back value from the front peak value (0 dB minus -18 dB) results in a total front-to-back ratio of 18 dB.