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This example, from the ANTENNA MODEL Help Manual, takes you through the steps required to define and analyze a rhombic antenna.

Let's design a terminated rhombic antenna, intended for use on a point-to-point communications circuit. Suppose we have determined that the incoming wave on this circuit arrives at an elevation angle of twenty degrees most of the time. We’ll design this antenna for use over average soil, with the main lobe aligned at the desired elevation angle of twenty degrees.

We’ll use design formulas by E. Bruce, A. C. Beck, and L. R. Lowry, from “Horizontal Rhombic Antennas,” Proc. IRE, 23, 24-26, January 1935:

Since we always want to enter information beginning with the Options window and ending with the Comments window (left to right), we’ll start with the Options window. Press the  New Antenna toolbar button (not shown) to load a new antenna, and then click on the Options tab to bring the Options window into view.

Options Window

We’ll need a termination at the end of this rhombic, so click on “Impedance” to select impedance loads. Because this antenna operates on only one frequency, and because rhombic terminations usually are resistive only and don’t involve any reactance, we won’t have to worry about the fact that impedance loads don’t scale reactance correctly when frequency changes.

Because "Auto Elevation Angle Selection" is checked, ANTENNA MODEL will find the angle of maximum radiation and calculate the 2D azimuth plot at that elevation.

Press the Apply button, and then click on the Environment tab to bring the Environment window into view.

Environment Window

We’ll use 10 MHz as our operating frequency, so enter “10” in the Center Frequency field.

This antenna is over a “real” ground and there are no wire ends connected to ground, so we can click on the “Sommerfeld-Norton” ground model. The Number of Media radio buttons will switch to “One”. We could change that to “More Than One”, but this example uses only one real ground, extending to infinity in all directions, and the Media window will be formatted accordingly.

Press the Apply button, and then click on the Symbols tab to bring the Symbols window into view.

Symbols Window

Enter the symbols shown above. Symbol Alpha is set equal to the desired angle of maximum radiation. Height, Length, and Tilt are defined using the formulas given earlier. Height, W, L1, and L2 are the symbols we’ll use in the Wires window to define the locations of the wire ends in space.

The comments are not required, but are very helpful if you come back to this antenna definition later after not seeing it for awhile.

Press the Apply button, and then click on the Media tab to bring the Media window into view.

Media Window

The Media window has been formatted according to the kind of ground we selected in the Environment  window. “Average” ground has a conductivity of five millisiemens/meter and a dielectric constant of thirteen, so enter those numbers and press the Apply button.

If you wanted to use ground conditions closer to those in your area, you could have selected Soil Properties on the Tools menu to see some other choices that might be more appropriate for where you live.

Now click on the Wires tab to bring the Wires window into view.

Wires Window

Make the entries for the four wires as shown above. The two wires at the feed point are defined first, and then the wires at the terminated end. Common end coordinates are easily duplicated by selecting the X, Y, Z cells, and then dragging and dropping them to a new location. When two wire ends share a common point in space, the wires are connected together and current can flow from one wire to the other.

We have chosen to use #10 Copperweld for this antenna. Obviously there will be some sag between the antenna supports, and the individual legs are really catenaries instead of straight wires, but this is not modeled.

When you have finished entering the wire data, press the Apply button. The wires will appear in the Geometry window, and you can verify that they have been entered correctly. With Normal segmentation density (selected in the Options window) this model has 888 segments.

Geometry Window

Now click on the Sources tab to bring the Sources window into view.

 
Sources Window

Now we have a minor problem—we know the source will be at End 1 of Wire 1 or Wire 2, but which wire? They both have an end point where we want the source to be. We can resolve this by taking a look at the log.

Log Window

From the log, we see that only Wire 2 has a pulse located at End 1 with X and Y coordinates of zero. The source is actually attached to the center of a pulse, so we have to use the pulse that’s located where we want the source to be. So, enter “Wire 2, End 1” as the location of the source—but what we really mean is the pulse located at Wire 2, End 1.

You can enter anything you like for the amplitude and phase angle of the source, as long as the amplitude isn't zero. We have chosen 100 volts at a phase angle of zero.

Balanced 600-ohm open-wire feed line is a good choice for this antenna, so select that entry from the drop-down list in the Feed Line Z0 column.

Press the Apply button, and then click on the Loads tab to bring the Loads window into view.

Loads Window

Now we have the same problem as before—we know the load will be at End 2 of Wire 3 or Wire 4, but which wire? OK, let’s take another look at the log.

Log Window

From the log, we see that Wire 4 has a pulse at End 2, located where we want the load to be.

The only thing left to do is enter the resistance and reactance of the load. Let’s try 800 ohms for the resistance—and the reactance, of course, will be zero. Press the Apply button.

Now that all the Data Good indicators are green, items on the Calculate menu will be enabled and we can analyze this antenna. Press the  3D Pattern toolbar button (not shown), and after the interaction matrix is filled and solved, the radiation pattern of the rhombic will appear.

3D Pattern Window

The figure above has the pattern tilted so you can see the many side lobes a little better. The average gain is very low, because half the input power is dissipated in the terminating resistor. This is interesting, but what we really want to know is this: Is the major lobe aligned at twenty degrees? Press the  Azimuth / Elevation Patterns toolbar button (not shown), and look at the azimuth pattern.

Az/El Pattern Window, (Azimuth)

The  major lobe is indeed aligned at twenty degrees elevation. Now press the Source and Pattern Data toolbar button (not shown), and the log will be positioned in the Log window so that the Source and Pattern data are displayed.

Log Window

This design has the major lobe aligned at twenty degrees above the horizon (alignment design). If you wanted to change the design to a different elevation angle, say fifteen degrees above the horizon, all you would have to do is change the value assigned to symbol Alpha in the Symbols window, and then run the calculations again. The wire coordinates in the Wires window will be updated automatically when the value of symbol Alpha is changed.

Surprisingly, this alignment design doesn't give the maximum possible response at twenty degrees elevation. A longer leg length will give a slightly greater response, but the major lobe would then be aligned at an elevation angle lower than twenty degrees. This is called the "maximum E" design. To obtain the maximum E design, substitute "0.500" for "0.371" in the formula for leg length at the top of this page.

As the desired elevation angle for lobe alignment becomes smaller, the rhombic legs become longer and more segments are required. For lobe alignment at seven degrees, 6,996 segments are required. For alignment at six degrees, 9,508 segments are required.