The Yagi beam antenna (more correctly, the Yagi–Uda antenna, after both
of the designers of Tohoku University in Japan 1926) is unidirectional. It can
be vertically polarized or horizontally polarized with little difference in
performance (other than the polarization!). The Yagi antenna can be rotated
into position with little effort. Yet the Yagi antenna shows power gain (so it
puts out and receives a stronger signal), reduces the interfering signals from
other directions, and is relatively compact.
Here are some definitions:
The name of axis of an antenna call the BOOM. Directors increase the directivity of the antenna (sense of the arrow) but as the gain, it is because of them that with 4 elements the antenna to a Gain of 6dB and 5 elements procures a Gain of 8dB. The reflector return the bundle of wave ” collected ” by directors toward the dipole, but his major role is to protect the antenna against the parasitic signals coming from the rear
The minimalist two element beam antenna may be composed of either a
driven element and a reflector, or a driven element and a director. The
reflector and directors are known as parasitic elements. The parasitic elements are not connected to the
driven element, but rather receive their power from the driven element by
indirect means. The indirect means is that they intercept the signal, and then
re-radiate them.
The parasitic reflector is three to five per cent longer than the half wavelength
driven element. It provides power gain in the direction away from
itself. It is inductive in reactance and lagging in phase.
The parasitic director is three to five per cent shorter than the half wavelength
driven element. It provides power gain in its own direction. It is
capacitive in reactance and leading in phase.

The factors that affect the phase difference between the direct and reradiated
signals is determined principally by the element length and the
spacing between the elements. Proper adjustment of these factors determines
the gain and the front-to-back ratio that is available.

Two-element Yagi array antenna.

Two-element Yagi array antenna uses a driven element and a director, so the direction of maximum signal is
in the direction of the director. The gain of a two-element Yagi is about
5.5 dBd (gain above a dipole) for spacing less than 0.1λ and the parasitic
element is a director. For the case where a reflector is used, the gain peak is
4.7 dBd at about 0.2λ spacing.

The difference between reflector and director usage is quite profound.
The usual curve shows the director with higher gain, but it is more responsive
to element spacing. The reflector has less gain, but is more tolerant of
spacing errors.
The front-to-back ratio of the beam antenna is poor for two-element
antennas. A compromise spacing of 0.15λ provides front-to-back ratios of
5 to 12 dB.
The feedpoint resistance of the antenna is clearly not 73 ohms as
would be implied by the use of a half wavelength dipole for a driven
element. The feedpoint impedance will vary roughly linearly from about 5
ohms at a spacing of 0.05λ to about 30 ohms for spacings of about
0.15λ. Above 0.15λ the differences between director and reflector implementations
takes place. A reflector two-element beam feedpoint
impedance will increase roughly linearly from 30 ohms at 0.15λ to
about 45 ohms at 0.25λ spacing. The director implementation is a little
less linearly related to spacing, but varies from about 30 ohms at 0.15λ to
about 37 ohms at 0.25λ spacing.
Element lengths:
The element lengths for a two-element Yagi beam are given below:
Director: Director ¼
138:6 FMHz
Driven element: D:E: ¼
146 FMHz
Spacing: Spacing ¼
44:98 FMHz
Where:
Director is the length of the director
D.E. is the length of the driven element in meters (m)
Spacing is the spacing between the elements in meters (m)
FMHz is the frequency in megahertz.
These element lengths will result in 0.15λ spacing, which is considered
about ideal.