Yagi Antenna Theory: How does a Yagi Antenna Work

- the design of the Yagi antenna relies heavily on the theory of the operation of the antenna. A look at how does the Yagi antenna work.

Some of the Yagi antenna theory can be complicated, but a basic understanding of how a Yagi antenna works can be given sufficient for design purposes.

The different elements of the Yagi antenna react in a complex and interrelated way to provide the overall performance.

In order to be able to develop a Yagi antenna it is necessary to have at least a basic understanding of the Yagi antenna theory.

Yagi antenna theory - the basics

The key element to the Yagi theory is the phases of the currents flowing in the additional elements of the antenna.

The parasitic elements of the Yagi antenna operate by re-radiating their signals in a slightly different phase to that of the driven element. In this way the signal is reinforced in some directions and cancelled out in others. As a result these additional elements are referred to as parasitic elements.

In view of the fact that the power in these additional elements is not directly driven, the amplitude and phase of the induced current cannot be completely controlled. It is dependent upon their length and the spacing between them and the dipole or driven element.

As a result, it is not possible to obtain complete cancellation in one direction. Nevertheless it is still possible to obtain a high degree of reinforcement in one direction and have a high level of gain, and also have a high degree of cancellation in another to provide a good front to back ratio. The Yagi antenna is able to provide very useful levels of gain and front to back ratios.

Image of a typical Yagi antenna showing the reflector, driven element and the directors
Yagi Uda antenna showing element types

To obtain the required phase shift an element can be made either inductive or capacitive.

  • Inductive:   If the parasitic element is made inductive it is found that the induced currents are in such a phase that they reflect the power away from the parasitic element. This causes the RF antenna to radiate more power away from it. An element that does this is called a reflector. It can be made inductive by tuning it below resonance. This can be done by physically adding some inductance to the element in the form of a coil, or more commonly by making it longer than the resonant length. Generally it is made about 5% longer than the driven element.
  • Capacitive:   If the parasitic element is made capacitive it will be found that the induced currents are in such a phase that they direct the power radiated by the whole antenna in the direction of the parasitic element. An element which does this is called a director. It can be made capacitive tuning it above resonance. This can be done by physically adding some capacitance to the element in the form of a capacitor, or more commonly by making it about 5% shorter than the driven element.

    It is found that the addition of further directors increases the directivity of the antenna, increasing the gain and reducing the beamwidth. The addition of further reflectors makes no noticeable difference.

In summary:

Reflectors - longer than driven element = Inductive

Directors - shorter than driven element = Capacitive

Image of a typical Yagi antenna showing the direction of maximum radiation from the yagi theory
Yagi Uda antenna showing direction of maximum radiation

By Ian Poole

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