Simple Yagi Antenna

Description of 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.

Design Curves

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
* Gain
* Reflector length
* The ratio of reflector length to director length
* Bandwidth

Figure 1 indicates the optimum number of elements for an antenna of specific boom length
The gain resulting from using the optimum number of elements is shown in figure 2 as a function of boom length.
This curve can be used to obtain the gain that will be achieved from a given boomlength in a boom length limited design, or vice versa. Figure 3 shows the reflector length required to achieve optimum gain as a function of boom length.
The director length can be obtained using the known reflector length, and the curve of the ratio of reflector length to director length shown in Figure 4.
Finally the bandwidth for the particular antenna can be obtained using figure 5. If a greater bandwidth is required, the reflector to director ratio can be adjusted to give an increased bandwidth as shown by the red and magenta plots.

Example

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 physical characteristics:

* A boom length of 1.12m
* reflector length of 336mm
* director length of 287mm

and the following electrical characteristics:

* gain of 12.5 dBi
* percentage bandwidth of 14%

Home Price List Contact Us Download News Info Applications

Top of Page

Note: 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.

Reference:
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.