Yagi antennas are also known by another name, Yagi Uda antennas. They are the same directional antenna technology consisting of several parallel elements in a line.

The yagi type of antenna is named after its two Japanese national inventors,  Hidetsugu Yagi and  Shintaro Uda. It usually comes with a reflector element and normally this is slightly longer when compared to that of a driven dipole. Now, as for the directors, they are a bit shorter. 

The radio waves, which can be traced as coming from the driven element, are being absorbed and reradiated by the parasitic element with a different phase. This will help modify the radiation pattern of the dipole.

The waves that are coming from the multiple elements will create interference, paving way for the coming of enhanced single direction radiation. This helps a lot in increasing the antenna gain as opposed to just having a simple dipole.

Basics of the Yagi Antenna Theory

If you are keen to know and want to understand the Yagi theory and what it is like, good background info on the phases of flowing currents is essential here.

The yagi type of antenna and its parasitic elements work by re-radiating the signals they have in a different phase to help in matching that of the driven element. This makes it possible to have that signal reinforcement occur in some directions but would be canceled out in others, instead.

While there are a few antenna Yagi elements that are not directly driven but are rather picking up the driven element’s power, all these extra elements are qualified as just parasitic elements.

The design in which the Yagi type of antenna comes in has its own set of limitations — the additional element’s power is not driven directly. There is no guaranteed way that the induced current phase and the amplitude can be controlled or managed in any way. They are dependent upon their spacing and the length between the dipole or the driven element comes in.

What does this signify then? This only goes to show that canceling in one direction is not at all possible. Nevertheless, one can still possibly obtain an elevated degree of reinforcement in one direction and this should earn you still a high level of gain.

Additionally, this also brings a high cancellation degree in another, providing you with a good front to back ratio. Yagi antennas are highly capable of providing useful gain levels.

To have that required phase shift, it’s essential to come up with an element that can be made, falling as either capacitive or inductive. The type of reactance that it should come with is likely bringing along as well a different effect.

• Inductive:  

Once a parasitic element is transformed so they become inductive, the tendency of the induced phase will reflect the power but it will not be in close range to the parasitic element. This will make the RF antenna radiate an increased level of power, only that it is in the opposite direction of the parasitic element. An element behaving this way is referred to as the reflector.

• Capacitive:  

If you make a parasitic element as capacitive, the induced currents will be in a phase where the power of the whole antenna radiated will be towards the parasitic element’s direction. The element displaying this kind of behavior is known as the director.

You can make it capacitive, but you will have to tune it above resonance level. It is made possible when you add capacitance to the element, which has to be in a capacitor form. Or by having it 5% shorter when compared to that of the driven element.

By deliberately adding more directors, it will tend to increase the antenna’s directivity. This will increase the gain but will be inversely proportional to the beamwidth which is reduced. The successive director’s length will be reduced slightly.