Basic Design Parameter of Yagi Uda Antenna

The Driven Element (dipole) length, F should around 0.5λ (half wavelength)

The Reflector length, R should longer than dipole approximately 5%

The Directors, Dn should shorther than dipole approximately 5%

The spacing between Reflector and DE, S_R in range of 0.1 to 0.35 λ

The spacing between Directors, S_D in range of 0.1 to 0.5 λ or more

Impedance at cable feed-point is decreasing due to the effect of additional parasitic element (Reflector and Directors) and their spacing. This problem can be overcomed by the folded dipole (Driven Element). As the dipole length, F is half of the wavelength, and then the impedance will be increase 4 times about 300Ω.