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Yagi Antenna Modeler

Open the Yagi Antenna Modeling Applet in a separate window with the button below. To close the Applet, use exit from its file menu.
This java applet implements a simple yagi antenna modeling code. The initial input is the trivial two-element yagi antenna. Click on calculate to have the applet calculate the driven element input impedance, gain, front to back ratio. The pattern is displayed on the graph. The upper part of the graph is the azimuthal pattern (E-plane) for a horizontally polarized yagi. The lower part of the graph is the elevation pattern (H-plane).

The yagi is pictured in the upper panel. The boom is blue, the elements are red except for the driven element which is green.

You can choose some example yagis from the example input menu. You must click calculate to calculate their properties.

Panel Functionality

You can change the frequency, element diameter, and number of segments used to solve the equations by typing in the input fields provided.

You can change the units by using the "Units" menu. You can change the element conductivity by using the "Conductivity" menu. If the units are radians or wave lengths, the conductivity is assumed perfect.

You can edit the element positions and lengths by either moving the mouse on top of an element and clicking to select it, or you can select "List Elements" in the File menu to show the element positions and lengths. Double clicking on one of these entries will select that element for editing. The edit panel allows you to type in the element length, and the element position. If you check the driven element checkbox, this element becomes the driven element. Clicking cancel will leave the values as displayed when the edit panel first came up. Clicking OK will change the values to the values now displayed. Clicking delete will delete this element. To add a new element, select Add Element in the Edit menu.

You can save the results to a file by selecting "Write File" in the File menu. Because of applet security, you can only do this if you are running as a Java application from your own computer, or if you are running with your browser pointed to a file on your own computer (i.e. not over the net). You can read these files back into the modeler using the Read File item in the File menu. Again, you can only read files when running the applet on your own computer.

The solution method uses a straightforward method of moments solution using a piecewise sinusoidal current and the thinwire approximation. Since only a single yagi is modeled, the current in each element is symmetrical about the center of the element. The half element is divided into equal sized segments, and the current in each segment is assumed sinusoidal with the free space wave length. The mutual impedance between two such thinwire sinusoidal currents can be calculated analytically (see any antenna text, or for a general orientation, you can look at K. E. Schmidt IEEE Trans. on Antennas and Propagation, 44, 1298 (1996)). Given a driving voltage across the feed point, and zero voltage across the junctions of other segments, I solve the resulting linear equations for all the currents using straightforward LU decomposition. The ratio of the feed voltage to feed current gives the input impedance in this delta gap feed model, and once the currents in all the elements are known, it is easy to calculate the far field and therefore the pattern, gain, and front to back ratio.

The plotting is implemented using ptplot by Edward A. Lee and Christopher Hylands, copyright University of California.


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Michael A. Lee and Kevin E. Schmidt
July 1, 2000