The basic Yagi antenna design consists of a dipole and elements. It is frequently referred to as a beam because a rotor is used to direct the antenna toward the needed signaling area. This popular style of antenna was originally created by Shintaro Uda and Hidetsugo Yagi in 1926. The Yagi is still used today in a variety of ways as the design for this antenna is quite versatile.
The pair that invented this design realized that by adding numerous elements to a dipole the antenna would be more effective. The elements are of varying lengths and have different spacing between them. They are attached in front of and behind the dipole. This technique creates more power for the transmitter as the antenna can concentrate its focus on a specific target area.
The dipole is the driven element in this antenna system. It is resonant fed once the electrical length is half of the frequency wavelength. The feed line or coax runs from the transmitting device to the Yagi antenna. This enables the energy to move from the transmitter to the antenna and essentially to the person receiving the signal.
The element that is pointed toward the receiving signal is called the director. It is the shortest in length of all the elements on the system as it should be merely five percent of the length of the dipole. The length of this parasitic element can be altered if there is more than one director on the antenna. The directors should be spaced between .1 and .5 wavelength.
The element behind the dipole is called the reflector. It is usually about five percent longer than the dipole but the true length may vary considerably. This variance depends upon the diameter of the element and the spacing between reflectors. The spacing can be anywhere from .1 to .25 wavelengths.
The general performance of the Yagi antenna is determined by the radiation pattern. The directional gain, beam width, side lobes and front to back ratio are the elements that create the radiation pattern or polar plot. The consistent pattern of the range of frequencies over and under the design frequency causes the radiation pattern bandwidth.
Changes to the Yagi antenna design will alter the performance drastically. This could potentially increase the antenna’s SWRs (standing wave ratio). If the SWR is too high there is a change that the transmitter will be destroyed. The SWRs must be checked after any antenna design alteration.