Archive for January, 2009
We had a question posted to the Antenna Modeling group which presents an interesting modeling problem.
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I am using EZNEC+ 4.0 and I am trying to model the effect of
increasing the spacing between the inside ends of a simple split dipole.
I can’t figure out how to place the source so it feeds the first
segment of each of the inside ends of the two wires that constitute
the dipole. I thought this was what a split source was, but after
trying that and reading the help file, I see I had that all wrong.
Is there a way to do this?
I replied (with some editing):
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|With the NEC2 engine, there are only approximations for this instance. You can do four things:
- Put a source on each segment - which puts it always in the middle of the segment
- Use a split source - which does the same for you except that it assumes that the two legs are connected.
- Put a short wire between the two dipole wires and put the source on that wire. The source will be in the middle of this added wire.
- Make the number of segments high enough that the sources are very close to the actual wire end. You DO have to make sure that you don’t make them TOO short. Some recommendations suggest that segments should be at least 4 times the radius of the wire used.
NONE of the options does precisely what you want, but you should be able to get some feeling for the impact. The biggest problem to me (beyond NEC2’s placement of the source) is that the individual segments are likely to be large enough to mask the impact of separating the two legs of the dipole.
Remember, the whole thing is an approximation to the solution of the
integral equations governing the creation of the radiation field. The
assumptions are good in general for most large scale antenna problems, but the effect of changing the gap might very well differ from the calculation results because the scale of the change is too small.
One other possibility is to model the antenna fed by a balanced feed line from a source at a distance from the antenna. Even that’s not guaranteed to give you good answers, but they should be comparable.
What you might try is use several of these options and compare the results. If you try option 4, try increasing the number of segments and running the model at smaller and smaller segment sizes and see what happens to the solution. Does it converge? At some point does making the segments smaller lead to a divergence in the results?
Let’s take some time and play with the model segmentation and see what impact it has on the model. We’ll also try some of the sourcing options as well.
I’ve updated my BIBLIOGRAPHY page and included a link for each book to Amazon where the book can be found. Some of the links are to used books and some can be obtained new.
Creating 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:
Adding 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:
Antenna & Radials
I’ll be moving in the next week, so there will likely be some interruption in my posting schedule. I’ll try to get some posts ready to go, but no promises.
If you find this interesting, try these
- Writing for Techies - my collected notes and thoughts on writing, speaking, and communicating in general
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Let’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.
The 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).
January 18, 2009 12:28 pm
Just 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.
January 17, 2009 12:25 pm
After some discussion, we refined the description of the antenna to this:
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- The L wire starts out about 6 inches from the ground which is the feed point; it goes straight up about 4 feet
thru two glass insulators on a 2 x 2 wooden support post.———that’s only 4 feet so far
- From the glass insulator on top of the 2 x 2 support post, it angles at a 45 degree for about 12 feet towards
the pine tree and branches————–that’s a total of 16 feet so far (12 + 4 = 16)
- at the 16 foot point, the wire then goes straight up thru the limbs of the tree to the top for 74 feet,where it folds over the top——————that’s a total of 90 feet so far (16 + 74 = 90)
- After folding over at the top, the wire lays horizontally for 48 feet where it terminates to an insulator and tied off thru another 90 foot tall pine tree about 85 feet away ———-that’s a total of 138 feet (90 + 48 = 138)
- 138 feet is the total length of the inv-L wire, 90 feet is the total Height of the antenna and 48 feet is the horizontal leg of the antenna; so 90 foot vertical leg plus 48 foot horizontal leg equals 138 feet; but it takes a 4 foot rise and a 12 foot angle of 45 degrees to get the wire to run vertically up the tree to the top to obtain the 90 foot height.
But there is also a problem with this description. Do you see it? Let’s look at the height above ground segment by segment:
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|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.
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.