Wireless Data Technologies
There are many different types of
antennae currently in use. Each different type is suited to a
purpose and works best at a particular frequency. These notes
the Yagi-Uda antenna and will be concerned with its
uses. Generally we refer to this type of antenna as just a Yagi
or array. We will
also view the Yagi antenna from a mathematical perspective.
The Yagi antenna can be used for transmitting or receiving radio
The Yagi-Uda antenna was invented in Japan at Tohoku
University by Hidetsugu Yagi and Shintaro Uda in 1926 and
research in English in 1928. Yagi arrays were used widely in
World War because they were simple to build and directional.
You will very probably be familiar with the shape of a Yagi
is the type of antenna that is on the roof of many houses for
A standard dipole
antenna picks up signals equally well in all directions. The Yagi
antenna is different from a standard dipole antenna because it is
directional. It is directional along the axis that is at 90
the dipole in the line of the extra elements. The part that holds
elements of the Yagi antenna is also known as the boom or
This means that you have to point the crossbar towards the
the signal that you are trying to receive. If you look at the top
group of houses that have television antennae installed, you will
generally see that the antennae booms are pointing in the same
direction. This tells you the direction in which the television
transmitter is located.
The directionality of a Yagi antenna can be measured in terms of
A typical Yagi may have a gain figure around 3 to 20 dBd.
A Yagi antenna is composed of several different parts. It will
dipole which is connected to the coaxial cable and also several
parasitic elements which are not connected to the cabling. These
parts will be a reflector and at least one director.
These can be seen in the diagram below.
The dipole is the driven element of this type of antenna. The
will be 1/2 a wavelength in length or λ/2
The reflector lies behind the dipole and is generally 5% longer
than the dipole. It should be λ/10 metres behind the dipole
The directors are progressively shorter than the dipole
approximately 5% and are λ/10
Other Yagi Antennae
Sometimes you may see an antenna such as that depicted below. This
sometimes known as a cigar antenna. This is a Yagi, but it is
Inside this may be the following components.
A useful page for the cigar antenna is http://www.pow.za.net/cigar.html
The Yagi is the most commonly used antenna in applications that
above 10 MHz. They are simple to construct and have good gain
Our lab work will be to construct and test a simple Yagi antenna.
will split into small groups and build one Yagi antenna per group.
will then be tested in the anechoic chamber in N132. The antenna
be tested for several factors. Below is an image of a prototype
and tested in September 2008 at the University of Greenwich's
Results and Yagi are shown below.
The results are shown below - click image to enlarge.
Detailed Test Results
For now, we will concentrate on some of the factors of the Yagi
need to measure and understand. The book, Wi-Fi Toys - 15 Cool
Projects For Home, Office, And Entertainment (2004) chapter 2
instructions on how to build this antenna using a lolly stick and
paperclips. An example is shown below.
A small amount of soldering will be required to connect your
the coaxial cabling to allow it to be tested.
Yagi antennae are directional with high gain and can be
easily. They have been used in applications that operate over 10
since the Second World War. We will construct our own and test
with the University's anechoic chamber.
Calculates in Imperial units for a 3 element Yagi
Calculates dimensions for seven element Yagi
for Yagi - allows for number of directors to be chosen etc..
page with many calculation resources for antennae