Updated the Bibliography
January 29, 2009 4:50 pmPractical Antenna Modeling - Part 20 Radials continued
January 27, 2009 1:47 amCreating the Radial Set
We’re going to take a look at a very simple vertical antenna in free space and use the features of EZNEC to add radials to the model. Note that the radiator is along the Z axis starting at the origin (0,0,0).
FIRST … I’ll add the single simplest radial I can to the model. in this case, I chose to add one along the Y axis from (0,0,0) to (0,50,0) or a 50 foot radial wire.
In the WIRES window, I can now use the ‘CREATE’ menu item and select ‘RADIALS’
This bring up the ‘CREATE RADIALS’ window. I get to specify the prototype group. In this case, I’ve selected just a single wire, but I could have had more than one. I also tell the program what the TOTAL number of radials is.
IMPORTANT: We’re entering the total number of radials, not the number of new ones we want to add. I’ve seen a number of students make this mistake thinking they are entering the number of radials to generate and not the total number that will be present when generated.
When I click OK, the program automatically creates additional radials to match the prototype spread out around the zero point. With 4 radials, this is pretty simple since each now goes along an axis:
The antenna now looks like this:
If I’d said that I wanted 8 total radials, I would have gotten this wire table:
You’ll notice that it automatically adjusts coordinates so that the radials come out properly. If you computed the length of each radial (numbers 3, 5, 7, & 9), you’ll find they’re all 50 feet long. So the radial system is now spread out around the antenna like this:
Practical Antenna Modeling - Part 19 Radials
January 26, 2009 1:24 pmAdding Radials to the Inverted L
An inverted L works against a ground circuit. You COULD put up the antenna and just pound a rod into the ground to complete the circuit, but for most of us, bare ground is a pretty poor conductor. We add a ground radial system in order to provide a good return path and minimize ground losses.
Ground losses reduce the efficiency of our antenna, in other words, power is lost to heating in the ground part of the antenna circuit. We have a problem here though. The antenna we’re modeling actually has buried radials (there are some pieces above ground, but the bulk consists of below ground wires. OOPS … NEC2 doesn’t handle wires below the surface of the ground.
NEC4 can handle buried radials and computer the results from using them properly, but NEC4 costs around $1000 to be licensed to use it, not including the front end for it. That takes it out of the range for most of us, so how can we come up with an acceptable model? Simple, we add another approximation! We’ll start with a very simple radial pattern and then experiment with it to see how sensitive our results are to changes in the radial pattern. If, for example, we change the number of radials or their length or their conductivity and we get no change in measurable parameters of the antenna, then we can be pretty sure that our model is ‘good enough’ to use.
FIRST … I added a simple radial pattern to see it’s effect. In this case, I modeled 8 50-foot wires spaced evenly around the feed point

Here is the wires table …

This is the 3D and elevation pattern which results
As you can see, results are approximately what we saw before. In the next post, we’ll take some time to look at actually creating the radials in the software.
NOTE: If you cannot read the figures, you should be able to see the larger ones by RIGHT CLICKING on the image and choosing VIEW IMAGE. If that doesn’t work, try going to the images directly:
Moving in the next week
January 25, 2009 3:07 pmTake a look at my other blogs
3:02 pmPractical Antenna Modeling - Part 18 Inverted L Takeoff Angle
January 20, 2009 1:42 pmLet’s take a look at the takeoff angle for the Inverted L. I posted the 3D pattern
You can see some of what’s going on, but the takeoff angle seems to be pretty broad. HOWEVER we’re not done with this yet. The model is far from complete, so we’re not ready to rely on the output. Right now, consider what we’re seeing a qualitative and not quantitative representation of our antenna’s performance. Are we getting the general pattern right? Once we get the actual model correct, the numbers will mean something, but again not very much. They’ll be pretty good, but unaccounted for effects might also mean that the numbers aren’t accurate.
With all of our updates, we can now rerun the model ( which remember is still incomplete) and get this for the maximum azimuthal pattern:
As you can see, it shows the main lobe centered on 20 degrees of elevation. That’s consistent with my own experience for vertical radiators like this, so I’m sure it’s somewhere close to the actual take off angle.
Next, let’s take a look at what happens when we add a radial system to this model.
Practical Antenna Modeling - Part 17 Inverted L Source
January 19, 2009 1:21 pmThe source is what drives the antenna. Think of it as the feed point of the antenna. It’s where we apply either a SOURCE of voltage or current to drive the antenna.
In EZNEC, sources are configured in the source window
Here is the source I specified for the Inverted L antenna
FIRST … I specified only 1 source
SECOND … I specified the wire position as Wire #1, at 0% from END 1 (E1), in other words, at the bottom of the antenna. The program put the source as close to what I specified as possible. NEC always puts the source in the middle of a segment, so the ‘Actual Pos.’ or actual position is 12.5% from E1 in Segment #1.
THIRD … I specified an amplitude of ‘1’ and phase ‘0’ degrees. One is always a good starting point for an investigation. Phase is useful when there is more than one source in the model and you need to establish the relative phase between them.
FOURTH … The type of source is ‘I’ a current source. This means that a CONSTANT 1 amp is applied to the feed point and everything else works from there. If you’re applying a constant voltage, you specify it as a ‘V’ voltage source. When the source is applied across a junction, you specific a ‘SPLIT’ source (SI or SV).
Practical Antenna Modeling - Part 16 Delta Loop another look
January 18, 2009 12:28 pmJust for fun, another look at the Delta Loop with the Antenna Model program is interesting because it uses MiniNEC instead of NEC as a computational engine:
Compare this to the NEC model and see what you think.
Practical Antenna Modeling Part 15 - Back to the Inverted L
January 17, 2009 12:25 pmAfter some discussion, we refined the description of the antenna to this:
—————– Message ——————————–
|
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But there is also a problem with this description. Do you see it? Let’s look at the height above ground segment by segment:
—————– Message ——————————–
| 1. Starts 6 inches from the ground … Z = .5
2. Goes up 4 feet along the post … Z = 4.5 at the top of the pole 3. Goes at a 45 degree angle for 12 feet … Z = 8.49’ + 4.5’ = 12.99’ at the top of this section You have a 12 foot hypotenuse (the length of the wire) at 45 degrees, so the other legs of the triangle satisfy the equation Hypotenuse^2 = horizontal^2 + vertical^2 Where ‘^2’ means squared. Since this is at a 45 degree angle, horizontal = vertical, so solving for vertical, Vertical = SQRT(( Hypotenuse^2 ) / 2) Where SQRT is the square root. This gives 8.49 feet, so the Z at this point is 12.99’ 4. Now, going up 74 feet vertically from there takes us to Z = 12.99’ + 74’ = 86.99’ You’ve got 90 feet of wire, but the actual vertical location is only 86.99 feet because of the section which goes up at an angle. |
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Let me step back and clarify the point “every bit of information is critical to get a good model”. Especially at this stage it’s true, but we might find that we can compromise later. The reason is very simple. Radiation is caused by current flow in the antenna wire. The important parts of the wire for the remote pattern will be where current is highest. A section where current is low or zero will have little or no impact on the pattern and so inaccuracies at this point will not have a measurable impact on the result. Once you’ve been modeling for a while, you’ll begin to have a feeling for where these places are, but any time you’re working with an unfamiliar antenna or an unusual setup for antenna that you may have worked with in a different environment you’ll need to do some preliminary modeling to understand where the current is high and where it’s not.
I always try to be careful with initial setup, looking for ways to confirm my setup and getting the numbers right. Once I can create a model that accurately reflects what I find, then I know I’ve got something to experiment with. When I’m working with someone else on a model, I try to not impose an interpretation without confirmation because as you can see from this simple example, it’s VERY easy to get the numbers wrong, and until I know just where the current is high, I don’t know which errors will be significant to the result.
| NOTE: There’s an important assumption here about my goals in this model. I am ASSUMING that the most important thing to get from the model is the far-field pattern. That’s often an important thing in your model, but it might not be the most important part. For example, if you’re calculating to verify that your antenna is not a hazard to people nearby, then other considerations may be important. Be sure you understand your GOALS when building a model, what really IS important as an output from the model |
From the reconstruction above … I entered the wires like this:

From which I get this antenna:

Now comes one of the important parts … What are the currents? I apply a source to the bottom and get an initial look at the currents. This ISN’T accurate without the radials, but it gives me some initial information to work with:
You can see from this where the current is high and where it’s lower. This will change when I add the radials, but it’s a good start.
You’ll find that modelers differ on what they consider important at various stages and the order of their steps varies with the person. I’m sure Cebik would look at it differently if he were able to comment. However, no matter how you actually use the outputs and what order to do the steps, you still face the same problems and will eventually get to the same results.
Practical Antenna Modeling - Part 14 Delta Loop Example - More
January 16, 2009 1:05 pmOur next step with the Delta loop is to get the geometry correct. A bit more clarification got this out:
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| The Delta Loop will be an equilateral triangle with the feed point at the bottom apex, 16.4′ above ground. The support trees are 40′ apart, but each of the 3 legs is 11.8′ in length for a center frequency of 28.4 MHz With a feed point at 16.4′, this will put the top horizontal leg of the antenna at 26.6′ above the ground When I translate this into the three vertices you calculated previously, (0, 0, 16.4) (0, -5.9, 26.6) (0, 5.9, 26.6) |
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With these figures in hand, we could rebuild the model
—————– Message ——————————–
| I’ve taken your figures (which work out correctly) and reworked the preliminary model to get the 3D pattern here:
The Elevation pattern looks like this:
You can see from this so far just how far off you can be when you start to make assumptions. If you want to create useful models, you need to get the numbers right. |
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The Delta Loop is an interesting case to model because we ordinarily put the source in the middle of a wire for a dipole (exactly what we’ve been doing on our 40 Meter Dipole model), but here we have a case where we’re modeling it at a vertex which introduces some potential problems.
We have three ways we can model this:
- Put the source on a wire next to the vertex and as close to the vertex as possible
- Put the source across the vertex, partly on one side and partly on the other (EZNEC has a convenient way to do this when describing the type of source)
- Put a short wire where the vertex is to connect the two wires that come to the vertex and put the source on this short wire.
For the purpose of this model, I chose to use the 2nd option and model it with what EZNEC calls a ’split source’. So here’s EZNEC’s summary of the model description at this point:
EZNEC+ ver. 4.0
10 m Delta Loop Array () 1/13/2009 7:31:10 PM
--------------- ANTENNA DESCRIPTION ---------------
Frequency = 28 MHz
Wire Loss: Copper -- Resistivity = 1.74E-08 ohm-m, Rel. Perm. = 1
--------------- WIRES ---------------
No. End 1 Coord. (ft) End 2 Coord. (ft) Dia (in) Segs Insulation
Conn. X Y Z Conn. X Y Z Diel C Thk(in)
1 W3E2 0, 0, 16.4 W2E1 0, -5.9, 26.6 #12 40 1 0
2 W1E2 0, -5.9, 26.6 W3E1 0, 5.9, 26.6 #12 40 1 0
3 W2E2 0, 5.9, 26.6 W1E1 0, 0, 16.4 #12 40 1 0
Total Segments: 120
-------------- SOURCES --------------
No. Specified Pos. Actual Pos. Amplitude Phase Type
Wire # % From E1 % From E1 Seg (V/A) (deg.)
1 1 0.00 1.25 1 1 0 SV
No loads specified
No transmission lines specified
Ground type is Real, High-Accuracy
--------------- MEDIA ---------------
No. Cond. Diel. Const. Height R Coord.
(S/m) (ft) (ft)
1 0.005 13 0 0
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