The practical gain of a Yagi is 6 to 20 dBi, depending on the number of elements. The Yagi—Uda array in its basic form has very narrow bandwidth, 2—3 percent of the centre frequency. The "reflector" and "director" elements increase the strength and narrow the antenna beam in the direction in front of the antenna.
This so-called trap has the effect of truncating the element at the higher frequency band, making it approximately a half wavelength in length. However these elements are not driven as such but receive their energy from the field created by the driven element, so we will find almost the opposite to be true.
The center rod like structure on which the elements are mounted is called as boom. The elements in front of the dipole are called director. The antenna propagates electromagnetic field energy in the direction running from the driven element toward the director sand is most sensitive to incoming electromagnetic field energy in this same direction.
The director elements those to the right of the feed in Figure 1 will be shorter than resonant, making them capacitive, so that the current leads the voltage. As alluded to in the previous paragraph, the lengths of the directors are typically less than the resonant length, which encourages wave propagation in the direction of the directors.
Isotropic antenna radiates uniformly in all direction. This leads to the array being designated as a travelling wave antenna.
Gain versus separation for 2-element Yagi antenna.