Simple Yagi Antenna
The Yagi (or Yagi-Uda) antenna
is a linear array of parallel dipoles. One element is energised
directly by a feed transmission line with the others acting
as parasitic radiators. The geometry of the simple Yagi antenna
is shown below in the diagram.
The following design curves
enable the optimum selection of the following variables/characteristics
(all as a function of boomlength) related to the design of
a simple Yagi antenna:
* Number of elements
* Reflector length
* The ratio of reflector length to director length
A Yagi antenna with the following
operating characteristics is required:
* Operating frequency: 400
~ 450 Mhz
* Maximum gain for a maximum boomlength of 1.12m
A boom length of 1.12m corresponds
to an electrical length of 1.6?, and using figure 1 the number
of elements in the array is 7. The gain of the antenna can
be determined with figure 2, and is approximately 12.5 dBi.
The reflector length is obtained from figure 3, and is 0.48?
or 336mm. The reflector to director ratio is 1.17 (from figure
4) and so the director length can be calculated as 287mm.
The bandwidth of the designed antenna is 14%, which is greater
than the required bandwidth of 11%.
The antenna thus has the following
* A boom length of 1.12m
* reflector length of 336mm
* director length of 287mm
and the following electrical
* gain of 12.5 dBi
* percentage bandwidth of 14%
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these design curves provide only a rough estimate of a particular
antenna's dimensions and performance. In order to fully specify
the antenna it should be simulated, using the values obtained
from the curves if so desired.
Fourth Year Design Report, "Antenna Design Wizards: Design
Curves for a Simple Yagi Antenna" prepared by Simon Mahlaba
for the School of Electrical and Information Engineering,
University of the Witwatersrand, August 2003.