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The diagram below shows the layout of elements in a typical Yagi

The design and analysis of Yagi antennas is very involved and is best done using antenna modeling software. However, to get insight into the basic operation of the Yagis, we will examine the basic three elements: a reflector, driver, and director.

There are three kinds of elements (or rods) mounted on a longitudinal connecting bar or rod. It doesn't matter if this connecting rod conducts, as it is orientated at right angles to the currents in the elements, and to the radiating electric fields; it supports little or no current, and does not contribute to the radiation. It does not matter what it is made of other than that it should have good structural properties. If it is made of conducting metal as are the elements, it can be connected electrically to the directors and to the reflector (but not to the driven element) without disturbing any of the properties of the antenna.

The three types of element are termed the driving element, the reflector(s) and the director(s). Only the driving element is connected directly to the feeder; the other elements couple to the transmitter power through the local electromagnetic fields which induce currents in them. The driving element is often a folded dipole, which by itself would have a driving point impedance of about 300 ohms to the feeder; but this is reduced by the shunting effect of the other elements, so a typical Yagi-Uda has driving point impedance in the range 20-90 ohms.

Behind the driven element is a single element that is approximately 5% longer. This is the reflector. It prevents radiation off the back of the array. In front of the director are a series of elements that are shorter than the driven element. These are the directors. They help focus the radiation in the forward direction. The spacing is the same between all three elements .Together the reflector and directors can reduce the radiation off the back of the antenna to 25 - 30 dB below the forward radiation. As more directors are added, the forward gain increases.



Simple 3 Element Yagi-Uda Array
BASIC STRUCTURE
A Yagi-Uda antenna is familiar as the commonest kind of terrestrial TV antenna to be found on the rooftops of houses. It is usually used at frequencies between about 30MHz and 3GHz, or a wavelength range of 10 metres to 10 cm. (There are some obsessional amateur radio enthusiasts who construct Yagi-Uda antennas for the 80 metre wavelength band. This is rather impractical as spacing them from the ground by more than half a wavelength is difficult.) The rod lengths in a Yagi-Uda are about a half wavelength each, and the spacings of the elements are about 1/3 of a wavelength. This puts the overall sizes of Yagi-Udas in the ranges

fre(hz)
length 3 elements
length 5 elements
length 15 elements
30M
100M
300M
1G
3G
5m
1.5m
50cm
15cm
5cm
6m
1.8m
18cm
6cm
13m
3.9m
1.3m
39cm
13cm
47m
14m
4.7m
1.4m
47cm
(lamda/2)
60cm