cocoaNEC 2.0 Reference Manual
Output Window

Kok Chen, W7AY [w7ay (at) arrl (dot) net]
Last updated: June 18, 2012

Output Window

The cocoaNEC Output Window is used to inspect data that is created by the NEC-2 (nec2c) or NEC-4 compute engines.

The Output window is automatically opened after the NEC engine finishes processing a valid input card deck. You can also manually open the window with the Output Viewer menu item in the cocoaNEC Window menu.

The graphical information for the Output Window is drawn from the data in the "line printer" output of the NEC engine. The original raw output itself can be inspected in the NEC2 Output view (use the far right tab in the row of tabs under the window's toolbar).

The other tabs let you select the particular graphical data that you wish to inspect.


Toolbar and Color Palette

The top of the Output window includes an integrated Toolbar/Title bar. There are three tools at the right of the toolbar: a Printer icon, a Color Wheel and a gear-shaped tool.

The two colors that are used for plotting the feed point impedances in Figure 4-2 in the Smith Chart View can be customized by clicking on the color wheel in the Output window's toolbar. When you are viewing the Smith Chart, the Smith Chart's color palette window will appear when you click on the color wheel. When you are viewing an antenna pattern, the radiation pattern color palette will appear when you click on the wheel. Some views in the Output window do not have a color palette; for those cases, you will hear an alert sound when you click on the color wheel.

Figure 2-1 Smith Chart Color Palette

To change the color for a feed point, click inside the corresponding color well once. A color well is one of the framed rectangles, each containing a color value. This will bring up the Mac OS X Color Picker and you can then use the various color picker methods to select a new color. Note that if the antenna has more than 16 feed points, the colors used in the Smith Chart will repeat.

Because of an idiosyncrasy in Mac OS X, a color well becomes unselected when you double click on it. Once unselected, Color Picker changes will not be applied to the color well. If this happens, simply go back to the color well to re-click it just once to select it again.


Output options Drawer

The tool that is shaped like a gear is used to open the Output options drawer.

A Mac OS X drawer is an area of a window that is usually hidden away and made visible only when you need it. The Output options drawer will pop out from under the Output window when you press the gear shaped tool. Pressing it a second time will again hide the drawer.

Figure 3-1 shows the drawer extending out from the right side of the Output window.

Figure 3-1 Output options Drawer

Depending on the screen space available behind the Output Window, Mac OS X can decide to extend the drawer from either the left side or the right side of the wIndow.

Notice the two output options (Reference Zo and SWR circle size) that were mentioned earlier. When you change the values from the cocoaNEC default, they are saved to the plist when you exit cocoaNEC.


Smith Chart View

Figure 4-1 shows the Smith Chart view of the Output Window:

Figure 4-1 Smith Chart

The green dots in the Smith Chart are the antenna's feed point impedances. Each dot represents the impedance at a particular frequency.

The Smith Chart encompasses the entire left half of the semi-infinite complex impedance plane into a finite disc. The horizontal line that cuts the Smith Chart into two halves is the resistance axis. The leftmost point on this horizontal line represents zero resistance, while the rightmost point represents infinite resistance. The center of the circle is the reference impedance. The reference impedance is an option (see below later) that the user can select. The caption that is above and to the left of the chart shows the reference impedance (Zo) that is being used; in this case, 50 Ω.

Impedances that have the same VSWR lie on the same concentric circle in a Smith Chart. The center of the Smith Chart is the impedance that presents a VSWR of 1.0:1. The light gray circle in Figure 4-1 are points on the Smith Chart where the VSWR is 2.0:1. The size of the SWR circle is set in the Output options drawer that is described later below. cocoaNEC only draws an SWR circle when its value in the options drawer is greater than 1.05:1. The reference impedance, Zo can also be set in the same options drawer.

Points on the Smith Chart that are below the resistance axis have capacitive reactance and points above the resistance axis have inductive reactance.

The Smith Chart is therefore a very compact representation of a feed point impedance, visually displaying the resistive and reactive parts of an impedance and VSWR information with just a single point in the chart.

A disadvantage of a Smith Chart representation is that it contains only the left half of the complex impedance plane. Negative feed point resistances (which is often encountered in phased arrays) will fall outside the Smith Chart disc. For such cases, it might be better to view the feed point impedance using the Scalar charts instead of a Smith Chart.

Figure 4-1 shows the feed point impedances of an antenna at nine different frequencies as nine green dots on the Smith Chart.

The impedance for each frequency is shown as a green dot. The "selected point" is represented by a green dot that has a hole in it (donut). The details of that point (its frequency, impedance and VSWR) are listed under the chart.

Mouse click on any other dot in the Smith Chart to select and view its details. (If the click misses a dot, you will hear a beep.)

The nine green dots in Figure 4-1 are interconnected by a green curve. The locus of this curve is not computed by NEC-2 but is simply an estimate of the trajectory of the intermediate impedances by using splines. Unless the green dots are moderately dense, do not rely on the green curve to accurately represent the impedance values between the green dots. The Interpolate checkbox at the bottom of the Smith Chart view lets you choose whether to display the interconnecting curve. The drawing of this curve is also automatically suppressed when there are fewer than 4 data points.

When the antenna model has more than one feed point, the Feedpoint popup menu at the bottom left of the window lets you to choose which feed point to display inside the Smith Chart. You can also show all feed points at the same time. The following figure shows the output from a phased dipole with two feed points, when the Show All checkbox is selected.

Figure 4-2 Smith Chart with two feed points

If the "Smart Interpolation" checkbox at the bottom of the Smith Chart View is selected, cocoaNEC will not draw an interpolated curve in between disjoint frequency bands of a multi-band antenna. The following shows the Smith Chart View of the W1ZR 2-band sleeve dipole drawn with Smart Interpolation off (left) and Smart Interpolation on (right):

Scalar Charts

The scalar charts provide an alternate way of visualizing the feed point impedances of an antenna. Figure 5-1 shows the scalar plot displaying the impedance (R-X) plot.

Figure 5-1 Impedance (scalar) plot

The horizontal axis of a scalar plot is the frequency scale. In the case of an impedance plot, the vertical axis is an impedance value (in ohms). The real (resistive) part of the impedance is plotted with yellow dots and the imaginary (reactive) part of the impedance appears in cyan.

Like the Smith Chart, if there are 4 or more points, and if the Interpolate checkbox is selected, a curve will be drawn through the actual impedance points that are computed by the NEC engine. The imaginary curve uses a dashed line to join the points.

Also like the SmithChart case, you can choose which feed point of a multiple feed point model to plot. To avoid very confusing plots, only one feed point can be displayed at any one time.

For the Impedance plot, you can only click on the real part (yellow dots) to select a point for which to extract detailed data. This data is shown under the scalar chart and include the frequency of the computed point, its complex impedance, the magnitude of the impedance, the VSWR and the return loss.

In addition, you will find a Scale menu at the bottom right of the window. Together with the scroll knob of the scalar plot, this gives you finer control of what you would like to view. Figure 5-2 shows the same plot, with the Scale menu changed to view a larger impedance range:

Figure 5-2 Impedance (scalar) plot with a different vertical scale

You can also see a taller plot by resizing the window. The scale inside the plot is a constant number per pixel.

The menu at the bottom right of the window lets you select other scalar views to display. Figure 5-3 shows the magnitude of the impedance for the same antenna.

Figure 5-3 |Z| plot

Figure 5-4 shows the VSWR of the same antenna.

Figure 5-4 VSWR plot

2D Antenna Patterns

The Elevation tab button in the Output Window takes you to the far field elevation radiation pattern of the antenna.

Figure 6-1 Elevation Pattern

The reference gain for the outer circle ("0 dB" circle) is shown on the top left of the plot as decibels referenced to an isotropic antenna (dBi). The relative gains (in dB) for the inner circles are labeled along the horizontal axis of the chart. Note that the toolbar is hidden in Figure 6-1 (by clicking on the translucent button at the topmost right corner of the Output Window).

The directivity of the antenna is shown above and to the right of the antenna pattern. A lossless antenna will have the same directivity (in dB) as the maximum isotropic gain (in dBi). When the gain value (in dBi) is different from the directivity (in dB), it can be due to losses (including ground losses), or it can be because the azimuth and elevation angles chosen for the antenna pattern are not the angles where the antenna gain peaks.

Notice that the logarithmic scale of the plot in Figure 6-1 places the -10 dB point about halfway out from the center. This represent a scale factor of 0.89 per 2 dB, and is the standard which is used in ARRL publications. There are two other scale factors that you can choose in the options drawer. Figure 6-2 shows the same antenna as Figure 6-1, but plotted using the scale factor of 0.80 per 2 dB.

Figure 6-2 Elevation Pattern plotted at a scale of 0.80 per 2 dB

Instead of expanding the outer radii, can also expand the inner radii by choosing a scale factor of 0.92 per 2 dB in the options drawer.

The patterns in Figures 6-1 and 6-2 each shows two antenna patterns. This is because NEC-2 was asked to model the antenna at two different frequencies. The color captions under the antenna patterns show the red curve is the elevation pattern for 14 MHz, computed at an azimuth angle of 90 degrees, and the green curve is the elevation pattern for 15 MHz, taken at the same azimuth angle.

When you specify multiple elevation angles in addition to multiple frequencies, you will see yet more plots, as seen in Figure 6-3.

Figure 6-3 Elevation pattern with multiple frequencies and multiple azimuth angles

The Azimuth tab button in the Output Window takes you to the far field azimuth radiation pattern of the antenna. In the case of the Azimuth pattern, the color captions under the patterns will show the azimuth angle of the corresponding pattern. Figure 6-4 shows the Azimuth plot at two frequencies and two different elevation ("take-off") angles:

Figure 6-4 Azimuth plots

As in the Smith Chart plots, the colors used in the Azimuth and Elevation antenna patterns are user customizable. The antenna patterns (including the antenna patterns in the Summary view discussed further below) share a color palette, but is distinct from the color palette used for the Smith Chart.


Reference Plots

The Output Window discards the data from a previous run when you rerun a model through NEC.

However, when you run more than one model during a cocoaNEC session, the data from each model is saved into a different context. You can quickly switch between the data from the different contexts by using the popup menu that is in toolbar of the Output window:

Figure 7-1 Context selection

When you no longer need a context, select it as the current context and use the minus button on the right of the menu to remove it.

NEC-2 and NEC-4 runs from the same model will create different contexts. You can therefore compare NEC-2 outputs with NEC-4 outputs. NEC-4 contexts will have a "(NEC-4)" label in the context name.

Any context can be used as a "reference antenna." To do that, first select the context and then go to the Output Menu in the menu bar to select Use As Reference:

Figure 7-2 Setting a context as the reference antenna

A black square is shown at the left of the reference context.

Figure 7-3 Reference context indication

Figure 7-2 also shows a "Use Previous Run as Reference" menu item. Instead of using a different antenna model as the reference, you can use the most recent run from the same model as the reference. By selecting "Use Previous Run as Reference," you can observe your progress when you make changes to a model.

Once you choose a reference antenna, its plot will be superimposed on the plots of the other antennas. Figure 7-4 shows a reference antenna (a dipole) superimposed as a black dashed line on top of the plot (red) of a three element Yagi-Uda array.

Figure 7-4 Elevation plot of an antenna and a reference

If the reference antenna produces multiple antenna patterns, the first pattern is plotted as the reference pattern.

The Smith Chart draws the feed point of the reference antenna as a gray disc, shown below:

Figure 7-5 Reference context in the Smith Chart

If the reference antenna has multiple feed points, the first feed point is displayed as the reference in the Smith Chart.


3D Antenna Pattern

The 3D tab button in the Output Window takes you to the antenna's 3D radiation pattern.

Figure 8-1 3D Radiation Pattern Shape

The pattern can be rotated in the azimuth by changing the Azimuth field at the bottom left of the window, or by using the stepper arrows next to the azimuth field.

If you are using an older computer, the use of the stepper is not recommended since the drawing can be very slow. However, any Intel based Macintosh with a good graphics card should be able step through the azimuth angles quite fluidly. You can also disable 3D drawing completely on slower computers by disabling the Enable 3D Radiation Pattern menu item in the cocoaNEC Options Menu (in the main menu bar). The state of this menu item is not saved to the plist.

The Contrast slider on the bottom right of the window lets you adjust the contrast of the image.

Figure 8-1 is drawn as a "shape" of the gain of the radiation pattern. The brightness of a surface patch, shaded using Phong shading, is proportional to the surface normal of an imaginary light beam in the direction of the antenna pattern.

Figure 8-2 below is drawn by using the gain of the antenna pattern itself to control the brightness.

Figure 8-2 3D Radiation Pattern Gain

In Figure 8-2, the brightness of a surface patch on the 3D surface is simply how far that point protrudes from the center of the 3D pattern. Notice that the sidelobes of the antenna is very dim (low gain) compared to the sidelobes in Figure 8-1. Figure 8-2 ("Gain") is more useful for locating the high gain directions of the antenna while Figure 8-1 ("shape") is more useful at showing the 3 dimensional shape of the radiation pattern.


Output Summary and Average Gain Test

The Summary tab displays the azimuth plot, elevation plot and distilled NEC-2 output all in the same view:

Figure 9-1 Output Summary

The antenna patterns at the top of the Output Summary are abbreviated copies of the Azimuth and Elevation plots. As with the larger originals, the captions at the left of the circles are the gains for the outer circle, and the captions on the right of the circles show the directivity of the antenna. In addition, the elevation angle for azimuth plot is shown in the azimuth view, while the azimuth angle doe the elevation plot is shown in the elevation view.

Other data are shown in the scroll view under the antenna patterns. Here, you can find the ground used for the model together with the azimuth and elevation angles for the peak gain of the antenna. All feed point currents of a phased array are also listed in the summary.

There are two front-to-back values in the output summary, together with a front-to-rear number.

The azimuth of antenna lobe with the largest gain is considered the "front" of the antenna. The "back" of the antenna is 180 degrees from this azimuth. One of the front-to-back numbers in the output summary refers to the response in the "back" direction which has the same elevation angle as the front lobe.

The second front-to-back number compares the front lobe to the "largest" value in the back lobe over all elevation angles. These two front-to-back ratios are not always the same, with the latter one being more pessimistic.

The azimuth angles that are more than 90 degrees away from the "front" lobe is considered by cocoaNEC to be the "rear" of the antenna. The front-to-rear number is computed by looking for the largest lobe on the rear of the antenna. The front-to-rear number can be a useful for evaluating antennas that have cardiod patterns (very typical of two element vertical phased arrays) where the front-to-back ratio can return an infinite number, but obviously not reflecting the true performance of an antenna in real world use.

The antenna model's Average Gain is listed in the output summary. This number can be used to judge if the model of a lossless antenna has converged (the so called Average Gain Test or AGT).

Regardless of the directive gain, the average gain of any lossless antenna should be close to 1.0 (0 dB). This fact can be used to judge if the NEC model of an antenna has converged. Although a model's accuracy is not completely guaranteed, an antenna model that yields an average gain which is within 0.2 dB of unity can be considered to be moderately reliable. On the other hand, an antenna model whose average gain number is more than 1 dB from unity (i.e, an average gain factor that is smaller than 0.8 or greater than 1.25) is almost certain to be a poor model of a real antenna.

The Average Gain Test number in cocoaNEC is not useful when modeling an antenna that is over lossy grounds, or when modeling antennas with lossy elements. To make use the Average Gain Test, you should model the antenna over a Perfect Ground or in Free Space.


Polarization of Antenna Patterns

In addition to total power gain, the NEC output also provides power gains for horizontal and vertical polarizations. cocoaNEC computes the left hand and right hand circular polarization responses from the axial ratio and predominant polarization values.

When cocoaNEC is launched, its output window defaults to plotting total power.

You can select which polarization to plot either by selecting one of the Polarization radio buttons in the Options drawer (see Figure 3-1) or by selecting one of the Radiation Pattern Polarization menu items in the Output menu in the menu bar.

You can draw both Horizontal and Vertical polarization responses on the same plot by choosing "Horizontal+Vertical." Likewise, both RHCP and LHCP responses can be drawn on the same plot. The figure below shows the Output Summary azimuth and elevation patterns for a quadrature fed Inverted Vee Turnstile antenna when "RHCP+LHCP" is selected:

Figure 10-1 Composite RHCP pattern (solid line) and LHCP pattern (dashed line) in the Output Summary

The solid line is the RHCP pattern and the dashed line is the LHCP pattern. When horizontal and vertical polarizations are combined, the horizontal polarization pattern is drawn with a solid line and the vertical polarization pattern is drawn with a dashed line.

Please note that you need not rerun the antenna model when you change polarization. This is an output post processing task.

The Polarization selection also affects 3D plots. Figure 10-2 shows the 3D patterns for the same antenna that is shown in Figure 8-1. The left side of Figure 10-2 is the pattern for horizontal polarization and the right side is the pattern for vertical polarization.

Figure 10-2 3D Horizontal Polarization (left) and Vertical Polarization (right)

Geometry and Currents

The Geometry tab takes you to the panel that shows the geometry of wire antennas. cocoaNEC may not draw geometries such as arcs, helices and surface patches that are created by the NEC card deck. Complex wire shapes that are programmatically generated by NC should draw correctly.

Figure 11-1 shows what a six element Yagi-Uda looks like in the Geometry view.

Figure 11-1 - Antenna Geometry and Currents

The two fields at the bottom left of the window control the viewing angle relative to the centroid of the antenna. An elevation angle of 0 places the eye at the same height as the centroid. An elevation angle of 90 degrees corresponds to placing the eye straight above the centroid and looking back at the antenna from the +z axis. An elevation angle of -90 degrees corresponds to placing the eye below the centroid and looking up at the antenna from the -z axis.

A triad of unit vectors appear at the top right hand corner of the view. The red, green and blue (RGB) colors correspond to the x, y and z directions, respectively.

An azimuth angle of 0 corresponds to placing the eye on the +x axis and looking back at the antenna. An azimuth angle of 90 degrees corresponds to placing the eye on the +y axis.

You can set the angle by either typing directly into the text fields, or by using the up and down arrow steppers. The buttons autorepeat, so you can hold down the button and see an animation of the model. The elevation angle has hard stops at -90 degrees and +90 degrees. The azimuth angle wraps around the circle, with 360 degrees wrapping back to 0.

When you control click (or right mouse click) on the Geometry view, a green dot is drawn at the wire segment that is closest to the cursor. This is shown in the figure below:

Information for the selected segment is shown at the bottom right corner of the Geometry view. The first row has the x, y and z coordinates of the center of the segment. The second line of text has the vector current, and the third line shows the current magnitude and phase angle.

In addition, a smaller Wire Current window is drawn to show the distribution of currents on the wire of the selected segment.

You can select either a Magnitude/Phase plot or a Real/Imaginary plot.

The currents are normalized to the largest current in the entire geometry. Phase angles are drawn from -180 degrees (bottom) to +180 degrees (top) in the dashed yellow line as shown above. A light green bar shows the segment location within its wire.

Real and imaginary currents are centered to the middle of the plot, with negative currents below the center line and positive currents above the center line.

Use shift-control-click (or hold down the shift key with a right mouse click) anywhere in the view to remove the information (and green dot).

The Currents menu at the bottom right of the window can be set to None, Scaled Magnitude, Magnitude, Magnitude and Phase, and Magnitude and Relative Phase.

With the Currents menu set to None, antenna current information is not plotted. When the Currents menu is set to Magnitude, the colors of the antenna segments correspond to the magnitudes of the current. Maximum current appears as a bright yellow and zero current appears as dark gray. A scale is shown on the bottom left corner of the view. The Scaled Magnitude selection is similar to Magnitude selection except the low current portions are stretched to better see low currents.

When the menu is set to Magnitude and Phase, the currents appear as colors in the HSV color space. The phase angle of a current corresponds to the hue of the color, and the magnitude of a current corresponds to the value of the HSV color (brighter colors carry larger currents). The colors that correspond to the various phase angles for the maximum current are shown in the color wheel on the bottom left corner of the view.

The slider at the top left of the view magnifies the structure geometry from the original 1x continuously up to a scale factor of 16x. You can also "pan" the drawing up and down and left to right by holding down the mouse in the view and dragging the cursor while the mouse button is held down. While the mouse is held down inside the Geometry view, the cursor turns from an arrow to an open hand.

When the Geometry view is panned, a re-center button will appear and you can reset the panning action with the button:

The following shows the Magnitude and Phase view of a Half Square antenna with a reflector (note the scale slider has also been moved to magnify the image slightly):

Figure 11-2 Currents in Magnitude and Phase (HSV Color)

The Magnitude and Relative Phase setting is similar to the Magnitude and Phase setting except all phase angles are referenced to the phase of the current in the segment with the largest current.


Sources and Loads in the Geometry View

Voltage sources (seen in Figure 11-2) are drawn as open circles in the Geometry view. Current sources (shown in Figure 11-1) are drawn with a double circle.

Loads such as impedance and RLC loads are displayed in the Geometry view as small crosses.

Distributed loads such as wire conductances is only drawn when the "Draw Distributed Loads" checkbox is selected in the options drawer.



Radials in the Geometry View

Both the spreadsheet interface and NC in cocoaNEC have provisions for adding radial wires. In addition to the convenience factor, wires that are added with the special radials mechanism are specially tagged so that their drawing can be omitted in the Geometry view.

The default state of the Geometry view is to not draw the radials, but you can force cocoaNEC to draw them by checking the Draw Radials box in the Options Drawer (see Figure 3-1). Figure 11-3 shows a dipole on top of a set of radials with 19 spokes.

Please note that you have to use the radials() function in NC to view the radials. The function necRadials() generates radials that are internal to NEC-2 and don't appear as wires in the NEC output.

Figure 11-3 Current distribution in Radials

Notice that the Scaled Magnitude menu is chosen in the above figure. The currents in the radials for this case are very low and the slight differences of the currents for the individual radials would not have shown up if Magnitude were selected.

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