The Yagi antenna or Yagi-Uda antenna or aerial is a particularly popular form of antenna where directivity and gain are required.
Although the Yagi has become particularly popular for television reception, it is also used in many other applications, both domestic and commercial or professional.
The gain and directivity of the Yagi antenna enable improved reception by enabling better levels of signal to noise ratio to be achieved, and by reducing interference levels by only picking up signals from a given direction. For transmitting much better use of the available power is made because it is possible to focus the transmitted power on areas where it is needed. Similarly levels of general interference can be reduced to other users because the signal is not transmitted to areas where it is not needed.
Yagi antenna development
Although the Yagi antenna is now widely used, it was only in the late 1920s and early 1930s when it started to be used.
Although the name, Yagi is normally used, its full name is the Yagi-Uda antenna. The name is derived 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 Professor Shintaro Uda. However all the original papers were all in Japanese and accordingly the design was not publicised outside Japan. Uda did not speak English, but his student did. Accordingly it was Hidetsugu Yagi who wrote papers in English. As a result the design is often incorrectly only attributed to Yagi.
It is apparent that Yagi did not aim to steal the publicity at all and as a result the design now bears the names of both men and is formally known as the Yagi-Uda antenna.
Yagi antenna - the basics
The Yagi antenna design has a dipole as the main radiating or driven element to which power is applied directly from a feeder.
Further 'parasitic' elements are added which are not directly connected to the driven element but pick up power from the driven dipole element and re-radiate it. The phase is in such a manner that it affects the properties of the whole Yagi antenna as a whole, causing power to be focussed in one particular direction and removed from others.
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. If an element is longer than the resonant length, i.e. that of the driven element, then it becomes inductive, and shorter t becomes capacitive. In this way the phase of the currents in elements that are shorter or longer are different.
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 reflector element is made to be about 5% longer than the driven element. 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 make no noticeable different to the antenna 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. Tis can help in reducing the levels of interference received.
Typically a reflector will add around 4 or 5 dB of gain in the forward direction.
- Director: The director or directors are made to be shorter than the driven element. 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 Yagi antenna exhibits a directional pattern consisting of a main forward lobe and a number of spurious lobes to the rear and the side.
The main spurious lobe is the reverse one caused by radiation in the direction of the reflector.
The antenna can be optimised to either reduce radiation in the reverse direction by altering the length and spacing of the reflector or it can be optimised to produce the maximum level of forward gain. Unfortunately the two conditions do not coincide exactly and a compromise on the performance has to be made depending upon the application. It is necessary to choose either maximum front to back ratio or maximum forward gain.
Yagi antenna advantages
The Yagi antenna offers many advantages over other types of antenna in many applications, yet both advantages and disadvantages need to be weighed up to ensure the correct type of antenna is chosen for any particular use.:
- Directivity: The Yagi antenna is directional enabling interference levels to be minimised for receiving and transmitting.
- Gain: The Yagi antenna has gain allowing lower strength signals to be received.
- Straightforward construction: The Yagi antenna is mechanically relatively straightforward when compared to other designs. It can be constructed using straight rods which are simple to use and robust for most instances.
- Polarisation: 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 also need to be considered.
- Max gain ~20dB Gain is limited to around 20dB or so for a single antenna otherwise it becomes too large and beamwidth narrows. For low frequency antennas the physical size means that the maximum number of elements and hence the gain is much lower than 20dB.
- Long for high gain: For high gain levels the antenna becomes very long.
The Yagi antenna is a very practical form of RF antenna design that is suited for applications where gain and directivity are needed. Although the cost is higher than more basic antennas, the Yagi often provides the most cost effective option for gain and directivity .
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