Marconi demonstrates how currents work using Eznec

[Marconi]

Model #1, referred to as "correct."

Bob and others that work with antenna modeling software, or would like to understand some issues considered with modeling and currents as I see the issue.

I've posted this idea before, but this time I will try to show simply what I claimed regarding to how Eznec handles currents badly, correctly, and incorrectly. In this demonstration I will not address CMC on the feed lines specifically, because I have not added a mast, feed line, or added wires to the model in order that we might see such currents flowing on the model.

At some point I will add the mast and then a feed line to these 3 models below, and try and give some simple understanding, not about currents, but about how I see Eznec handling currents...two different topics IMO.

I will post each model below as an individual attachment, and then try and explain what I see. I have reduced the number of segments on the each of the model's two wire descriptions. Hopefully this will help to produces less data for us to consider in the examples for this simple dipole. Hopefully this will also demonstrate how current distribution work in Eznec, in addition to the red lines indicated in the Antenna View for each Eznec model.

All models are done in Eznec idea for Free Space (FS)...where there are no ground losses to be noted. I will reference a FS model as having a reported gain of 2.14 dbi, at a "0" angle to the horizon. The feed point match (source) impedance is reported to be very close to R=73 ohms of resistance with X= -j 0.00 reactance. Compare these details to my models attached below and you will see they are very close to identical to what theory predicts for a simple dipole in Free Space.

Model #1
Is a horizontal dipole that is noted as correct. I use a feature in Eznec to split the feed point at the center of this dipole and this allows the model to be almost perfectly balanced...just like a real dipole should be.
You will note in this model that the gain is 2.13 dbi at 0.0* degrees. This very small deviation from the reported gain for a dipole noted above is simply due to the model not having enough segments for each wire. See note above on segments.

I've added the wire definitions in case folks with Eznec Demo wish to try this model. The wires screen in Eznec is the point of data entry

The primary thing to notice in these results is the Current Data report that shows us the current distribution per segment for the model. Each wire has a number that the data describes the balance or the lack of balance in each wire and the antenna. This data also shows the phase of each wire, and in the case of this model each wire is perfectly balance, in phase (-) sign, and thus will radiate into the far field. The value in (Amps/segment) noted under magnitude is the strength of the currents for each wire in the model. Note how these currents are all equal in both wires and they are in phase noted by the (- sign).

I won't get mad if you ask questions. The other two models will follow in this thread.

Hopefully this model will show how and Eznec model should show the currents. Also note that the gain, angle, and SWR will not change from model to model in this 3 model series for 3 different applications using Eznec.

You will see a little difference in the currents distribution area, magnitude, the sign (+-) of the currents, and the complex feed point impedance as we might expect...but the performance results do not change.

If any one would like me to add some notes to the model below...let me know.

 

[Marconi]

Model #2 is described as, "incorrect."

This model is the same as model #1, but the spit feed point (FP) has been removed. The feed point is set instead at one end of one of the 2 wires...near the center of the antenna. This change in FP location will make some difference in the current distribution, suggesting some lack of symmetry.

This difference is likely to develop a small CMC's effects at the feed point. We don't physically see these effects, because there is no wire for them to flow on, so we can see the the currents for the model. This is the only thing the article I posted from the Eznec manual talks about...the feed line...not some other element attached to the radiating part of the antenna, like DB was trying to demonstrate.

If I add a mast to the earth...we can see some currents indicated on the mast due to the feed point being place as noted above...in a bad position.

That said, the gain and angel remain the same as model #1. If I add an Eznec feed line to the model we would see the same thing. The added wire discussed in the manual only applies to feed lines folks.

BTW, a free space model with a mast attached will noticeably change any model, so I remove my physical mast when I set the model in FS. All Eznec does if we set the model to FS, and we have a mast attached, or we don't remove one that is already attached...will be for Eznec to automatically disconnect the very bottom of the physical mast from the Earth. However in such cases, the added mast is still present in the model, and does still change the model noticeably.

See the model below.

 

[Marconi]

Model #3, is referred to as "bad description."

This model is the same as model #1, with the exception that I change the wire definitions to a configuration that will work showing the same results, while the current distribution log will show the two elements are out of phase and the magnitudes equal...just like model #1.

According to my understanding these currents on this dipole that are not in phase will appear as not to radiate, because the currents are not in phase. Again however, we see the performance results are the same.

Does this help explain how and why Roy designed his interface with the NEC engine to be convenient for the user...so we do not have to follow the strict rules for data entry that NEC requires...in order for currents to be produced correctly in the Currents Log?

 

[firerunner]

Marconi I wish I knew a quarter of what you know about antennas lol I'm just truing to learn but this is way over my head. I'm sure someone will post up that understands this.

 

[The DB]

Marconi I wish I knew a quarter of what you know about antennas lol I'm just truing to learn but this is way over my head. I'm sure someone will post up that understands this.

What he is talking about strictly has to do with modeling. If your not learning modeling, or trying to understand models it really doesn't matter... well... as long as the person making the models you pay attention to makes them properly...

To explain a bit further, when modeling there is a right way to put the various elements together, and a wrong way. Some results from modeling don't change simply because you put something together wrong, but other things do.


The DB

 

[Marconi]

Marconi I wish I knew a quarter of what you know about antennas lol I'm just truing to learn but this is way over my head. I'm sure someone will post up that understands this.

Have you uploaded Eznec Demo version to your computer?

It is free and it is limited to its use, but the models I posted can work using the Demo version. This limitation in the Demo model has to do with segments...a factor that models require the user to defined for each wire (element) in you model. The limit in the Demo version is 20 segments.

My models above have only 6 segments per wire, so you can work with these models in the Demo version. This low number of segments does result in a very small difference in the performance details for the model...but it is not a big difference.

Like DB says. What I am posting is pretty simple stuff based on what my idea is. So, if you have some understanding for how modeling works and for sure Eznec, I am only addressing how Eznec handles the topic (currents).

I think I give enough details to duplicate this model, but you have to get past the big problem, which IMO is figuring out the basic idea how Ezenc works. And that includes simply how the Wire Descriptions in the data entry screen are made. Once you get this concept in you mind...the experience was for me like a light switch going off and on, and then you are on your way. For me, I looked at this product several times and always gave up trying. Then the last time...it all clicked with a friend sending me a small simple model like I've posted here.

What I'm addressing here, however is not quite so easy to understand (currents). That said, one can still easily develop an understanding how to model simple antennas, and never even consider currents.

If you have more questions...just ask.

BTW, I'm not an expert at modeling, but I have been at it for several years. Also, don't worry about making models correctly right away...in time you will learn much every day and at some point you will become more confident and informed about antennas and you will start to see the little things that might matter. Modeling is not hard, so don't convince yourself that it is impossible until you give it a fair tryout.