There is no antenna with better gain/expense ratio than the 2-element Yagi antenna. The Dipole alone gives us 2.15dBi. As we can see from the radiation pattern, the maximum gain (in the direction of the x-axes) is 6.5dBi, a gain in over 4dBi by just adding a reflector or a director! By adding one additional director we will have only approximately 1dBi more.
For the lower frequencies, where the dimension of the antenna becomes quite significant, as well as its price, it is obvious that adding that additional element does not pay off.
There are two variants of the Yagi 2: with Reflector and without Directors and with one Director but without Reflector. If the value in the last column is larger than for the Radiator-Dipole, then it is the measure for the Reflector, and vice versa – Reflectors are always longer than the Dipole and the Directors are always shorter.
There is a need for an appropriate BalUn. See more in the BalUn section