What does “front-to-back ratio” mean in reference to a Yagi antenna?

The "front-to-back ratio" of a directional antenna like a Yagi compares the signal strength radiated in the favored direction (the front, or major lobe) to the signal strength radiated in the exact opposite direction (the back). It is typically expressed in decibels (dB). A high front-to-back ratio means the antenna is highly effective at rejecting signals from behind it, which is excellent for minimizing interference. This term describes the antenna's radiation pattern, not its physical construction. Therefore, it has nothing to do with the physical count of directors and reflectors, nor does it refer to the placement of the driven element. Furthermore, front-to-back ratio should not be confused with forward gain. Forward gain compares the main lobe's strength to an external reference antenna (like a dipole), whereas the front-to-back ratio compares the main lobe to the antenna's own rearward radiation.