Yagi Modeler

This java applet/application implements a simple yagi modeling code. The initial input is a really trivial 2 elmenent yagi. 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.

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.

I use a completely straightforward method of moments solution using a piecewise sinusoidal current and the thinwire approximation. It is not based on either Mininec or NEC sources. 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 my note, IEEE Trans. on Antennas and Propagation, 44, 1298 (1996), also at http://fermi.la.asu.edu/w9cf/articles/mutual). Given a driving voltage across the feedpoint, 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.

This applet can be used to scale the example yagis or any others to new frequencies. The naive way to do this is:
1. Model the yagi at the design frequency
2. Change the units to radians (or wavelengths). This will convert all dimensions to radians or wavelengths.
3. Change the frequency to the new desired frequency.
4. Change the units to Centimeters, Millimeters, Meters, Inches, or feet. This will scale all the values to the new frequency.

The only problem with this approach is that the element diameters also get scaled, often to values that are mechanically inconvenient or structurally unsound. You need to change the element diameter to one you really want. If the frequency change has been large, this will completely ruin the pattern, etc. A quick and dirty fix is to adjust the frequency and recalculate until the gain, pattern, etc. are reasonable. This new frequency should be near the desired frequency. Simply repeat the steps 1-4 above to scale from this nearby frequency to your final desired frequency. Then again change the diameter to the one you want and see if the results are acceptable. Repeat until they are.

Here is an example you can try. To scale the K1FO 12 element 2 meter yagi to 432.1 MHz with 4.7625 mm diameter elements, first select the 12 element K1FO 2 meter yagi from the Examples menu it has 6.35 mm diameter elements. It has a gain of about 14.5 dB and f/b 22.9 dB with a nice pattern at 144.1 MHz. Select Radians from the Units menu. Then change the frequency from 144.1 to 432.1. Then select Millimeters from the Units menu. Clicking calculate, I get the same results at 432.1 MHz that I got at 144.1 MHz originally but the element diameter is now 2.1179 mm for the 432.1 MHz yagi.

Now change the element diameter to the desired 4.7625, and click calculate again. The gain is 14.5 dB, the f/b is 16.5 dB, but the pattern is ugly. Since you have increased the diameter of the elements, they are a bit too long. You could now shorten each of them, but this isn't built into the code. The simple thing to do is to lower the frequency and see if the pattern cleans up. Lower the frequency a few MHz at a time, click calculate, and look at the pattern. I found that I get a pretty nice pattern at 420 MHz. I now scale the antenna from this frequency as before. That is I select Radians from the Units menu, change the frequency to 432.1, and then select Millimeters from the Units menu. This gives me a 432.1 MHz antenna with element diameters of 4.62890. I change this element diameter to my desired 4.7625 mm, and note that when I click calculate the pattern, gain, and front to back don't change substantially. This is my final scaled design. Comparing with the K1FO 12 element 432 MHz design, I have about .2 dB more gain, 0.4dB more front to back, but the pattern is a little worse. The rear lobe of the scaled yagi is not as small as the designed at frequency one, but it isn't really bad either. The scaled yagi is about 6 cm longer. Considering that I scaled the original yagi by a factor of 3 in frequency the results are pretty good.

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

You can download the complete applet/application and source code in the gzipped tar file http://fermi.la.asu.edu/w9cf/yagi.tar.gz. Gunzip and Untar this file on your system, and point your browser at the file yagipub/index.html, to run within your Web browser.

For full functionality of all IO operations the Applet requires JDK1.1 support. This is available in the Netscape 4.05 preview 1 (AWT 1.1.5) version. This preview version is only available from Netscape at its developers download location at the time of this writing. Internet Explorer 4.0 should also work. I believe that everything but file IO should work on all netscape versions 2.0 or higher and Internet Explorer 3.0, but I haven't tested this.

  • Security Issues
    The Applet requires permissions to read and write files. These considerations only apply if you have downloaded the applet to your own computer and are running from your own filesystem (not over the net).
  • To run as an application (my preferred way) you need a Java run time environment or a Java Development kit. In the classes subdirectory type
    java YagiModel
    or equivalent to pop up the modeler.

    To build the applet from source you need a Java Development Kit from Sun Microsystems at http://java.sun.com/, or Linux users may prefer the most recent from the Java-Linux porting project at http://www.blackdown.org or you may find other related development kits. The Makefile I use on my unix/linux systems is in the javasrc directory. The first few lines are variables that point to the directories where the java compiler etc are. These may need to be changed for your system.

    W9CF Home

    73 Kevin w9cf

    Copyright (C) 1998, Kevin Schmidt

    This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

    This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

    You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.