Wireless Data Technologies
Yagi Antennae
Introduction
There are many different types of
antennae currently in use. Each different type is suited to a
different
purpose and works best at a particular frequency. These notes
concern
the Yagi-Uda antenna and will be concerned with its
characteristics and
uses. Generally we refer to this type of antenna as just a Yagi
antenna
or array. We will
also view the Yagi antenna from a mathematical perspective.
The Yagi antenna can be used for transmitting or receiving radio
signals.
Brief History
The Yagi-Uda antenna was invented in Japan at Tohoku
Imperial
University by Hidetsugu Yagi and Shintaro Uda in 1926 and
published his
research in English in 1928. Yagi arrays were used widely in
the Second
World War because they were simple to build and directional.
Yagi Antennae
You will very probably be familiar with the shape of a Yagi
antenna. It
is the type of antenna that is on the roof of many houses for
television reception.
A typical
Yagi
antenna
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
degrees to
the dipole in the line of the extra elements. The part that holds
the
elements of the Yagi antenna is also known as the boom or
crossbar.
This means that you have to point the crossbar towards the
direction of
the signal that you are trying to receive. If you look at the top
of a
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
its gain.
A typical Yagi may have a gain figure around 3 to 20 dBd.
A Yagi antenna is composed of several different parts. It will
have a
dipole which is connected to the coaxial cable and also several
parasitic elements which are not connected to the cabling. These
extra
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
dipole
will be 1/2 a wavelength in length or λ/2
metres.
The reflector lies behind the dipole and is generally 5% longer
than the dipole. It should be λ/10 metres behind the dipole
(driven element).
The directors are progressively shorter than the dipole
by
approximately 5% and are λ/10
metres
apart.
Other Yagi Antennae
Sometimes you may see an antenna such as that depicted below. This
is
sometimes known as a cigar antenna. This is a Yagi, but it is
encased.
Inside this may be the following components.
A useful page for the cigar antenna is http://www.pow.za.net/cigar.html
Yagi Usage
The Yagi is the most commonly used antenna in applications that
operate
above 10 MHz. They are simple to construct and have good gain
properties.
Lab Work
Our lab work will be to construct and test a simple Yagi antenna.
We
will split into small groups and build one Yagi antenna per group.
This
will then be tested in the anechoic chamber in N132. The antenna
will
be tested for several factors. Below is an image of a prototype
built
and tested in September 2008 at the University of Greenwich's
anechoic
chamber facility.
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
that we
need to measure and understand. The book, Wi-Fi Toys - 15 Cool
Wireless
Projects For Home, Office, And Entertainment (2004) chapter 2
gives
instructions on how to build this antenna using a lolly stick and
some
paperclips. An example is shown below.
A small amount of soldering will be required to connect your
antenna to
the coaxial cabling to allow it to be tested.
Conclusion
Yagi antennae are directional with high gain and can be
constructed
easily. They have been used in applications that operate over 10
MHz
since the Second World War. We will construct our own and test
them
with the University's anechoic chamber.
Online Yagi
Calculators
http://bfn.org/~bn589/antenna.html
Calculates in Imperial units for a 3 element Yagi
http://users.marktwain.net/aschmitz/antennas/calcantenna.html
Calculates dimensions for seven element Yagi
http://vk5dj.mountgambier.org/Yagi/Yagi.html
Detailed calculator
for Yagi - allows for number of directors to be chosen etc..
http://www.educypedia.be/electronics/electroniccalculatorsant.htm
page with many calculation resources for antennae
References
http://www.wade-antenna.com/Wade/images/915-10YAGI.jpg
