Patch Driven Yagi Antenna

Description of Antenna

This antenna design yagi-style directors to enhance the performance of a standard patch. The configuration is shown below in the diagram.

Design Curves

The design curves included here enable the dimensions of the patch and the director boom to be obtained as a function of gain and bandwidth.

Figure 1 shows the gain obtained for specific boom lengths of between 1.2 lambda and 15 lambda, for a patch width of 0.45 lambda. Patch height is between . Director spacing is 0.3 lambda.
	Figure 1: Gain vs Boom Length

The optimum director length for the range of boomlengths (all dimesions in wavelengths) is shown in the table below:

Once the boom length is known the bandwidth can be adjusted by varying the patch width. The patch height is 0.037 lambdaand the the gain is not significantly affected by varying the patch width.
	Figure 2: Bandwidth vs Patch Width
If an even further increase in bandwidth is needed then the patch height may be increased resulting in the bandwidth being multiplied by a factor determined from figure 3.
	Figure 3: Bandwidth Multipication Factor vs Patch Width

Finally the patch length may be obtained from the equation 1, the discreet values for the patch heights of 0.0876 lambda, 0.07586 lambda, 0.06353 lambda are 0.3371 lambda, 0.3448 lambda, 0.3529 lambda respectively.
Patch Length = -0.658425(Patch Height) + 0.39475....1

Example

An antenna of the following specification is required:

* maximum boomlength of 8.4 lambda
* mimimum bandwidth of 40%
* maximum possible gain.

From figure 1 the gain for this antenna will be approximately 16.5 dBi. The directors will have a length of 0.422 lambda (from table) which are spaced 0.3 lambda apart. The maximum bandwidth (with a patch width of 0.8 lambda) is approximately 23.5% (from figure 2). This can be further increased to 42.3% (from figure 3) by increasing the patch height to 0.0876 lambda. The corresponding patch length is then 0.3371 lambda.

The final characteristics of the antenna are therefore as follows:

* total boomlength of 8.4 lambda
* bandwith of 42.3%
* directors of 0.422 lambda long and 0.3 lambda apart from each other
* patch of 0.8 lambda wide, 0.3371 lambda long and 0.0876 lambda high

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Reference:
Fourth Year Design Report, "Investigate the Design of a Patch Driven Yagi Antenna" prepared by Tobin Devasia for the School of Electrical and Information Engineering, University of the Witwatersrand, August 2003.