Yagi Antenna / Yagi-Uda Antenna

- the Yagi antenna sometimes called the Yagi-Uda RF antenna is widely used where gain and directivity are required from an RF antenna design.

The Yagi antenna or Yagi-Uda antenna / aerial is one of the most successful RF antenna designs for directive antenna applications.

The Yagi or Yagi-Uda antenna is used in a wide variety of applications where an RF antenna design with gain and directivity is required.

The Yagi has become particularly popular for television reception, but it is also used in very many other domestic and commercial applications where an RF antenna is needed that has gain and directivity.

Not only is the gain of the Yagi antenna important as it enables better levels of signal to noise ratio to be achieved, but also the directivity can be used to reduce interference levels by focussing the transmitted power on areas where it is needed, or receiving signals best from where the emanate.

Image of a typical Yagi antenna used for television reception showing the reflector, driven element and multiple directors.
Typical Yagi Uda antenna used for television reception

Yagi antenna history

The full name for the antenna is the Yagi-Uda antenna. The Yagi antenna derives its name from its two Japanese inventors Hidetsugu Yagi and Shintaro Uda. The RF antenna design concept was first outlined in a paper that Yagi presented in 1928. Since then its use has grown rapidly to the stage where today a television antenna is synonymous with an RF antenna having a central boom with lots of elements attached.

The design for the Yagi antenna appears to have been initially developed not by Yagi who was a student, but his colleague Professor Shintaro Uda. However all the original papers were all in Japanese and accordingly the design was not publicised outside Japan.

It was Hidetsugu Yagi who wrote papers in English and as a result the design is often incorrectly only attributed only to Yagi.

Yagi himself did not aim to steal the publicity, in view of his English papers, and as a result the design now bears the names of both men and is known as the Yagi-Uda antenna.

Yagi antenna - the basics

The Yagi antenna design has a dipole as the main radiating or driven element. Further 'parasitic' elements are added which are not directly connected to the driven element.

These parasitic elements within the Yagi antenna pick up power from the dipole and re-radiate it. The phase is in such a manner that it affects the properties of the RF antenna as a whole, causing power to be focussed in one particular direction and removed from others.

Diagram showing the basic concept behind the Yagi-Uda antenna showing the driven element, reflector and directors..
basic concept of Yagi Uda 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. It is found that the amplitude and phase of the current that is induced in the parasitic elements is dependent upon their length and the spacing between them and the dipole or driven element.

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

There are three types of element within a Yagi antenna:

  • Driven element:   The driven element is the Yagi antenna element to which power is applied. It is normally a half wave dipole or often a folded dipole.
  • Reflector :   The Yagi antenna will generally only have one reflector. This is behind the main driven element, i.e. the side away from the direction of maximum sensitivity.

    Further reflectors behind the first one add little to the performance. However many designs use reflectors consisting of a reflecting plate, or a series of parallel rods simulating a reflecting plate. This gives a slight improvement in performance, reducing the level of radiation or pick-up from behind the antenna, i.e. in the backwards direction.

    Typically a reflector will add around 4 or 5 dB of gain in the forward direction.
  • Director:   There may be none, one of more reflectors in the Yagi antenna. The director or directors are placed in front of the driven element, i.e. in the direction of maximum sensitivity. Typically each director will add around 1 dB of gain in the forward direction, although this level reduces as the number of directors increases.

The antenna exhibits a directional pattern consisting of a main forward lobe and a number of spurious side lobes. The main one of these is the reverse lobe caused by radiation in the direction of the reflector. The antenna can be optimised to either reduce this or produce the maximum level of forward gain. Unfortunately the two do not coincide exactly and a compromise on the performance has to be made depending upon the application.

The radiation pattern of a Yagi antenna as shown on a polar diagram with the major front lobe, the reverse lobe and side lobes
Yagi antenna radiation pattern

Yagi antenna advantages

The Yagi antenna offers many advantages for its use. The antenna provides many advantages in a number of applications:

  • Antenna has gain allowing lower strength signals to be received.
  • Yagi antenna has directivity enabling interference levels to be minimised.
  • Straightforward construction. - the Yagi antenna allows all constructional elements to be made from rods simplifying construction.
  • The construction enables the antenna to be mounted easily on vertical and other poles with standard mechanical fixings

The Yagi antenna also has a number of disadvantages that need to be considered.

  • For high gain levels the antenna becomes very long
  • Gain limited to around 20dB or so for a single antenna

Image of a typical Yagi antenna used for television reception showing the folded elements used within the antenna array.
Typical Yagi Uda antenna used for television reception

The Yagi antenna is a particularly useful form of RF antenna design. It is widely used in applications where an RF antenna design is required to provide gain and directivity. In this way the optimum transmission and reception conditions can be obtained.

By Ian Poole

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