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.
Yagi Antenna
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.
Parts of Yagi-Udi
        antenna
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.

Cigar Yagi

Inside this may be the following components.

Inside a cigar
        Yagi


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.

Testing paperclip
        Yagi

Results and Yagi are shown below.

Yagi and results

The results are shown below - click image to enlarge.

Test
          results

Detailed Test Results

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.

lolly-stick yagi

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
http://www.changpuak.ch/electronics/yagi_uda_antenna.php Working!
http://www.vk5dj.com/yagi.html Download software

http://www.electroschematics.com/6481/antenna-calculator-and-design-software/ Download software



References
http://www.wade-antenna.com/Wade/images/915-10YAGI.jpg