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This is the ratio of gain in the desired direction to the gain 180 degrees away. It is dependent on which elevation angle the two gains are measured at.
F/B (Front to Back Ratio) - This ratio tells how good the antenna is at rejecting signals from the rear. It is seldom truly important because interference seldom comes from the rear, but it can happen. This ratio is the gain factor in the forward direction divided by the gain factor from the rearward direction. But since gains and F/B ratios are usually given in dB, you normally get the F/B figure by subtracting the rearward gain from the forward gain (both in dB).
The F/B ratio is mostly a marketing gimmick, and is not a very useful number. Respectable antenna textbooks don’t define it. The true ratio often varies dramatically from channel to channel. This author has seen two common definitions for the F/B ratio. One uses the gain in the 180-degrees direction as the rearward gain. The other uses the X-degrees direction, where X is the rearward direction (from 90 to 270 degrees) with the most gain. It is usually impossible to tell which is being used if a radiation pattern is not available. (Of course, if you have the radiation pattern then you don’t need the F/B number.)
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