Complex sentence: Therefore it works best when fed by a balanced transmission line, such as a ladder line, because in that case the symmetry (one aspect of the impedance complex, which is a complex number) matches and therefore the power transfer is extremal.
Split into simpler sentences: Therefore it works best when fed by a balanced transmission line, such as a ladder line. This is because in that case the symmetry (one aspect of the impedance complex, which is a complex number) matches and therefore the power transfer is extremal.