Return loss or pre-back ratio is a characteristic value of directional anten nas such as Yagi anten nas and parabolic antennas. In the radiation pattern of such antennas, one recognizes a strongly pronounced main lobe and less strongly pronounced side lobes and a back lobe.
The logarithmic ratio of the received field strength from the main lobe to the back lobe is the return loss or pre-back ratio, given in decibels( dB). In dB terms, it is the difference between the dB value of the main lobe - 0 dB in the example - and that of the back lobe, -44 dB in the example. The return loss corresponds to the front-to-back ratio and is -44 dB in the example.
For Yagi antennas, the return loss is largely determined by the number of directors and reflectors.