The way to understand the working principle behind the yagi antenna theory is getting yourself familiar with the stages of its currents which are evenly distributed in the antenna through its different elements.
The yagi antenna and its parasitic elements operate by virtue of re-radiating inherent signals in what can be described best as an altered state of the element driven. This allows for signal reinforcement in particular directions but in others, it is canceled out.
While we will never qualify any additional elements to the Yagi system antenna as directly driven because their source of power is actually from the component itself, then we refer to them as parasitic elements.
One design boundary for the Yagi system of an antenna, the power intended for its additional elements is not driven. Accordingly, we don’t have any way or mechanism to implement control over the phase and the amplitude of the current induced. This would all depend on the spacing and length between them as well as the driven element or dipole.
What does this signify to us?
It is telling us that there is no possible way we can cancel it in one single direction only. But obtaining a high level of reinforcement would remain and will not be taken away. The yagi antennas can provide us relevant gain levels.
To acquire the necessary phase shirt, one element needs to be transformed as either capacitive or inductive. Each of the different kinds of reactance comes with a different kind of impact.
The moment that a parasitic element becomes inductive, the currents induced in this phase will reflect power at a safer distance away from the parasitic elements. By this measure, it renders the RF system antenna to diffuse more energy but in the opposite direction to this type of parasitic element. Elements that are displaying this kind of behavior is referred to as a reflector.
By tuning an element to go below resonance, it can become inductive. Make it possible when you add inductance using a coil. Another common technique used here is growing it longer and allow it to go beyond the length of the resonant.
Normally, it is made to reach around 5% longer compared to the element driven since it will significantly help save on costs. It also helps in mechanically keeping the element as just one single piece, consequently renders it stronger and cheaper.
It is just the reflector that will ever be used. Adding more reflectors is not likely to make any significant disparity.
Should the parasitic element becomes capacitive, the currents induced will be in a stage that will cause the power emanated by the antenna towards the parasitic element’s direction. An element that displays this kind of behavior is referred to as the director and can become capacitive by having it go above resonance. To get this done, you can try adding capacitance to the element in the shape of a capacitor.
By further adding directors, you are also strengthening the antenna’s directivity to the next level, which will reduce the beamwidth but increases the gain. As for the successive director’s length, it is slightly reduced.