Base Eznec model of a Sigma 4 with a gamma match

[Marconi]

This model shows several notable issues in modeling.

1. Shows the Eznec Control Center (CS) screen and it also shows the Free Space model that I start with when modeling the Real Earth model I hope to make accurately. This CS image shows the Average Gain Test results = 1.001, a perfect result, and it indicates accuracy for the model. If I start with this model and I don't mess with the dimensions, then the Real Earth new model should also be accurate. Average Gain located on the bottom line in the image.

This test indicates several technical things, but in General, it shows the modeler that the structure of the model and the location of the source is accurate.

2. Here is the new S4 model with a gamma match that effectively corrects this model's mismatch.

The ground type characteristic is set to Average Soil.

3. The DB reports that the gamma match on his Vector model has to have a short extension located above the dog bond connection. See wire #9 in my new model above. I modeled my S4 with this extension included, but I wanted to see if I could remove this part of the gamma and the antenna still work. So, I did another model and removed wire #9. This model shows this does not prevent me tuning the model or getting a good match.

Note: this said however, DB's Vector model is a bit different, so that has to be considered. I will have to check this with one of my New Vector 4000 models too.

4. This last image is my Eznec notes suggesting DB re-check his model

 

[bob85]

The models say 318" which is shorter than a stock sigma or vector but the current on the radiator looks like its longer than 3/4wave, what's going on there Eddie ?

 

[Marconi]

The models say 318" which is shorter than a stock sigma or vector but the current on the radiator looks like its longer than 3/4wave, what's going on there Eddie ?

Very good observation Bob.

Go back to the model, go down to image #3...the close-up image of the feed point area. If you want the details in the wire descriptions also check the "Wires" report. The radiator and the mount are made up of wires 1,2,3, that being the mount, the gamma section, and the radiator. Modeling does things this way because wires can only connect at the ends.

In the wires report for 1,2,3, the dimensions can be easily determined. Check the Z column for End 1, wire 1, this is the elevation for the bottom of the mount 415.5".

Then do the same for the Z column End 2, wire 1, that is the elevation for the top of the mount 432" with the mount = 16.5" inches.

By the way, this is the feed point elevation at 36' feet for this model. Not the tap point which is 28" inches higher up the radiator.

Do the same for wire #2...432" and 460" = 28" inches.

Do the same for wire #3...460" and 778" = 318" inches.

So, the radiator is overall = 346" inches which is about 0.80% instead of 0.75%. I wasn't intending for that to happen, but I probably forgot to undo an iteration or two, trying to get this model tuned. Some of this difference from specs could be in part due to differences in tubing diameters I used during my tuning iterations. Again, it was not intended...I perfer to keep to specifications if possible.

My real S4 is overall = 329.5" = 27.46 ' feet and my antenna has dimples in the tubing that sets tube lengths from bottom to top as "fixed." Like you've suggested before, the gamma will pretty much tune, within reason, for whatever length you have set.

Tuning this model is tedious and what you might start with for length might change a bit. I also find when the gamma is getting pretty close to a really good match...that changing the length might do little to make changes unless the change was really small. IMO, at this point, the gamma controls the match at the dimensions it has at the time.

I will do more to test these ideas regarding making the S4/NV4K longer.

I hope I was clear on this.

 

[bob85]

The extra length probably explains the high angle lobe dominating the low angle lobe Eddie,

something else i noticed while zooming to look at your feed-point, the segment lengths in the radiator are different to the radials,
is that just how the graphics work or do you have significantly different segment lengths ?

i thought segment lengths in models with parallel wires should be equal in length and as close to opposite each other / same height as possible,

you can't get it perfect unless the radials are parallel to the radiator,

am i wrong or looking at the model graphics wrong?

 

[Marconi]

The models say 318" which is shorter than a stock sigma or vector but the current on the radiator looks like its longer than 3/4wave, what's going on there Eddie ?

When I added the gamma match I had to connect the dog bone at a point a few inches up the radiator. I have no excuse, but I used 28" inches while the specs call for 33" inches.

Just an old man in a young man's world.

The extra length probably explains the high angle lobe dominating the low angle lobe Eddie,

Nope, I don't think so. I did a new model because I found some errors. I also changed the model when DB cautioned us for using a mast in his new video, so I removed the mount and the mast both. That caused me to re-tune a little, so I just made a new model with today's date.

Basically, all I did to get the pattern below...is to place the model over the Ground Description noted as "Very Poor: cities, industrial" instead of my regular use of "Average: pastoral, heavy clay." Again, I was trying to get a little closer to the results that DB was showing in his video.

something else i noticed while zooming to look at your feed-point, the segment lengths in the radiator are different to the radials,
is that just how the graphics work or do you have significantly different segment lengths ?

i thought segment lengths in models with parallel wires should be equal in length and as close to opposite each other / same height as possible,

Another good observation, Bob.

I fixed that problem as close as I could to get the segments closer and I saved a few segments from the model in the process. Making sure the segments are good is not as easy using Eznec as Steve describes using 4NEC2. Yes, the graphics do show a little skewing when the segments are off a bit.

The difference in the pattern was, IMO, primarily due to my using the "Average" ground characteristics rather than a more lossy value for ground. The model below is set to a different Ground Description and it shows, among other things, a different pattern and gain.

you can't get it perfect unless the radials are parallel to the radiator,

am i wrong or looking at the model graphics wrong?

Bob, I think you have a good mind's eye on modeling. This is partly why I tend to write so many words...trying to explain a bit more and do it as simple as I can for the folks interested in modeling details and maybe what does what.

1. I also posted several antenna views to show the dimensions and segment length for the primary wires in the model. I also show the Eznec Control Center image for the new model over real Earth.

The exhibits and models support the results for this model. IMO the difference noted was primarily due to a simple change in the ground conditions setting I used in this model.

2. I did nothing more to this new model. The model needs additional tuning, but I let it alone so I could show how Eznec can correct a model that has an AGT that does not = 1.

I also added the Free Space model for this model that shows it has AGT = 1.112 which is out of range and the model needs correction.
The likely accurate gain is = 4.74 - 0.46 = 4.28 dbi net gain at 8* degrees.

IMO. we can see here, that if we run the AG test and the results are off a bit...then we can use the AGT correction factor and adjust to a good gain results for the model.