Modified Vector 4000

[Marconi]

Bob,

what i have read from more multiple sources when people ask about coax dipoles using the transmissionline fuction to model coax to include coax losses it treats it as a 2 wire transmissionline with no outside surface of the braid to carry current & radiate unless you add a 3rd wire,

Bod, I vaguely recall reading some words in the Eznec manual that may have been along similar lines of what you're suggesting was discussed in your multiple sources.

So, did your sources show modeling examples along with the discussion about this coaxial dipole?

I have the model of Diptl.ez, and that is the FL templet that I used above.

Check the Antenna Notes for that model you referenced. Click the button in the Control Center near the top under the word Open. Those are the remarks by the creator of that Diptl.ez model.

I also added a do-it-yourself 1/2 wave vertical project called the Cobra in the CB book. This antenna basically looks similar to the GM, considering it's using coax for the radiator too.

I would like to see what you read in this case, and for sure if they used any modeling images to support their words.

Can you give me a good link?

 

[Marconi]

Eddie i can't see the wires even blown up many times,
i think what you are saying is the wire you add before you overlay the tl model is effectively the outside of the coax,

That is what I'm claiming Bob, but I could change my mind if I find out something different.

Sometimes I tell myself, I just know enough to be dangerous.

 

[bob85]

LOL sometimes you make me laugh Eddie,

i don't have links, its what i read on the www maybe something on qrz some time back from more than a couple of people,
like i said above i never saw anybody post a model of a coax dipole just people talking about models of coax that radiate such as a coax dipole,

just because i can't see the wire does not mean they are not there,

it could be that the wire you say you are overlaying the coax on is the wire they are talking about,
then you add the coax which is two more wires that i can't see because the tl function only deals with the coax ends but handles it like a two wire line that won't radiate,

if that's the case you don't need to change your ideas at all, you are adding the 3rd wire,

you have current & radiation so what im seeing that looks like a single wire on my screen can't be the tl model of the coax otherwise there would be no current & radiation.

 

[Marconi]

you have current & radiation so what im seeing that looks like a single wire on my screen can't be the tl model of the coax otherwise there would be no current & radiation.

You see the "T" in the green box on each wire in the chocking coil? I assume you can see this, and for others it is the symbolic indicator that means the wire has been converted to being coaxial. Sometimes we can see the 2 wires and sometimes we can't just like you have described.

I made the feed line from the bottom of the coil to 3" above Earth offset...so we could easily see these 2 wires for the coax. You might agree that there are only 2 wires and the coax shield outer surface presents yet another 3rd surface that allows the flow of currents. I agree too, but to be clear, there are only 2 wires, and IMO Eznec does not require an additional 3rd wires to make a FL radiate.

When I look into the end of regular coax...I generally see only two wires, the center conductor and the shield. The fact that currents can flow on the outside to the shield too, is granted, This is what Common Mode Currents are all about, and in this case the CMCs are our very beneficial.

The wire for the feed line to the radio, the wires in the coil, and the wire for the bottom section of this radiator design are all coaxial. They have a "T" inside a square box. It is near the middle of each wire that is noted to have wire dimensions in the Eznec TL tool spreadsheet.

Bob, I don't think you are confused about this issue, and if you want to say there are 3 wires then so be it. I have to think about what others might be thinking when they consider this back and forth over little or nothing between us.

You've now seen, possibly for the first time ever, the coil at the base of the GM, made as wire and then also made coaxial.

Bob, below is the model with just wires for the coil before I used the TL tool. Can you agree that the possible current distribution on this coil made with simple wires, caused the current magnitude at the base of the coil to be very low to start and steadily increase in magnitude toward the top of the coil?

I plan to do the same thing with the model where all these same wires are made coaxial and then check the current magnitude distribution of the coil again, and compare both.

Below is the matching details for the coil with just wires, and no TL Tool was used yet. I noted earlier that this model was making the match inductive with +600 ohms of inductance at the feed point, and it was also making a lot of RF from the coil. You commented on those effects if you remember. Click on the PDF file below.

 

[Marconi]

it could be that the wire you say you are overlaying the coax on is the wire they are talking about,

Bob, I knew those two images would make for more confusion. That happens sometimes when I have currents turned on for the model and then try and zoom in real close to get details, I get two images because in fact the wires are in one file and the overlays of the feed line tools results are in another file. I know nothing more, I did not intended that to happen, but I forgot to turn the currents off right before I printed the coil.

If you could overlay those two images you would see one color covers the other pretty much, but not always totally. Then if I check real close with a magnifying glass...I can see both wire on my monitor. I doubt the scan and PDF file handling would never let this idea be seen easily.

I was surprised when you told me you were seeing the segments dots, not lining up correctly, on the antenna view for the NV4K/S4.

You don't miss anything do you Bob?

 

[Marconi]

Here is the model with the coil wire working with coax as intended for the GM choking the CMC at the bottom of the radiator compared to the coil that is not coaxial. The inductance dropped like a rock, down to about +25<> ohms at the feed point, far from the 600> ohms I started with on adding the coil to the base of the radiator.

The match is not good yet because I haven't decided how to setup the stub and move the capacitor to the bottom of the upper radiator wire instead of the top of the bottom radiator wire.

It is not right yet, but it is getting real close.

Note; the magnitude of the current (amps) represented by the red lines for currents is; the further away the red line for the current is from the wire that generates the current...the more current (amps) there are in the wire.

It is not hard to see a difference between these two coils in the PDF file below...even if you don't understand all that is going on here. This coil is supposed to curtail the RF (Common Mode Currents) from radiating below the radiator onto the feed line and mast below.

This is my opinion, until I'm blessed to learn different. I now see the GM model working close to what I might expect from such a design.

Below is the model over real Earth.

 

[bob85]

Eddie
yes its the first time i have seen an actual model of what people talk about when modeling coax dipoles or any coax that carries common mode on the braid & radiates,

i think what's happening is i only see the wire you add between the coax ends,
you see both wires at your side when technically there are 3 wires because the tl function treats coax as a 2 wire none radiating line,

i see the current taper on the choke as you describe,

if that's not whats going on here i don't see how you have current & radiation from the coax,

if its right i learned something about what i can expect to see on my screen in peoples models of coax that radiates

i expect the gain-master to have a bit more gain than a 1/2wave dipole since its 5/8wave with currents in phase & current maximas spaced a little wider apart than a 1/2wave dipole.

 

[Marconi]

i expect the gain-master to have a bit more gain than a 1/2wave dipole since its 5/8wave with currents in phase & current maximas spaced a little wider apart than a 1/2wave dipole.

Bob, I don't have a good match like I use to see on my real GM. You may be seeing a very good match on your GM too.

This GM model in FS does not show a good AG result. I don't have a tuning stub included in the mix, and the capacitor seems to be in the wrong place too.

 

[Marconi]

i think what's happening is i only see the wire you add between the coax ends,
you see both wires at your side when technically there are 3 wires because the tl function treats coax as a 2 wire none radiating line,

Bob your words here seem familiar again. Is this idea based on something you read in the Eznec manual, or is it the way you see TL in your minds-eye?

 

[bob85]

The cap is in the middle coupling the coax center conductor to the top wire Eddie,
the stub is part way down the coax, its placement & value are important,

when you get it right you should see a double dip in the vswr curve as seen in other antennas using coupling capacitor to help with broadbanding such as the the x-pole,

unfortunately for us the highest vswr point between the 2 dips is around our ch19,
that could be why the gain-master won't handle anything close to what sirio claim continuously for us,

the idea about 3 wires where i only see 1 is from what you say above and my interpretation of the eznec manual.

 

[Dr_DX]

Dr_Dx,

if it was easy people would not be tuning for resonance through coax...hope this helps.

Bob85,

I never said anything about tuning for resonance through coax.

I said you can use your analyzer to tune for the freq the coax was cut to a 1\2 wave multiple of (I would use the analyzer's SWR function).

 

[bob85]

Yes you can use the analyzer to tune a length of coax for 1/2wave electrical,
you said it won't be reactive which I also agree with on the frequency you tuned the coax,

The problem is people get an analyzer & stop tuning for minimum vswr & start tuning for resonance through coax because its the latest buzzword,
some even tell people that tune for minimum vswr with a good vswr meter or analyzer they are doing it wrong,

That's not true, they should look at what they are doing with their analyzer in more detail.

 

[Marconi]

Bob, do you have the Free Version of Eznec on your computer?

In playing around with Eznec Demo in the past, can you do anything with the Eznec program?

I think I figured out, a little bit, of what we are dealing with regarding the Eznec file you talked about Diptl.ex.

This file is found in Eznec, among the demonstration models that Roy included with his software. IMO, this model is not what it appears to be.

W8JI also uses a similar model on his Website to demonstrate the same issues about Transmission Lines. See link below.

https://www.w8ji.com/common_mode_current.htm#Dipoles_and_Common_Mode

If this link opens the file at the top of this article, then look for the item: "Jump down to the dipoles," in the list of topics.

My only Point here is: you may be right saying Eznec TL line tool does not show any currents when used.

Also, when we look at these dipole models I see some strange things going on with the model Roy posted, and the model that W8JI used shows absolutely no modeling details. and he uses Eznec +Pro which is very descriptive if one does not turn the information details OFF.

Even my cheap Eznec 5 does a better job of describing some data for the models...unless I deliberately turn the features OFF.

 

[bob85]

Eddie,
i don't have any nec versions & never played beyond briefly trying to make a sigma model years ago
when i realised it would never do what i wanted with the because of the segment limit i lost interest,

What about the two dipole models seems odd?
w8ji is saying that a horizontal dipole with 1/8wave coax feed-line often does not need a balun & the current log for the coax braid seems to show that's true how he has it setup in the model

not sure what he's saying about the same dipole with 1/4wave coax as theres no current log.

 

[Marconi]

Eddie,
i don't have any nec versions & never played beyond briefly trying to make a sigma model years ago
when i realised it would never do what i wanted with the because of the segment limit i lost interest,

I understand the limit is not useful in modeling Bob. I asked if you had the Eznec Demo version, so maybe you could follow me in checking out the model DipFL.ez.

It is a very interesting model in what and how it does what it does. BTW this file was made to work with the Eznec Demo version and has only 20 segments...the limit. So you could could check this model out and see the issues I'm talking about.

What about the two dipole models seems odd?

The DipFL.ez file that Roy Lewallen includes with his Eznec program has the Feed Point in a strange location...at the end of the Feed Line. There are other things strange about this model too, but they are not discussed in the Antenna Notes, and I can only speculate as to possibly why. This might be hard to describe if you can't see the model in detail.

W8JI made a model very similar to the DipFL.ez model and it produces similar results. He also talks about the currents on the FL, and shows us the current magnitude on each segment in his model. That also makes me wonder, but I can't check his model out, because he give no details...even the frequency is omitted.

So, we see no currents on both models, and again I don't understand that yet.

You may be right about however, this Eznec FL Tool is not able to model a FL using the FL model + another wire...and showing us Common Mode Currents as a result.

Bob, did anybody in the group you head talking about the FL feature in Eznec...ever discuss the idea that the FL Tool could not model a FL + 1 added wire to the model and show CMC if they were present in such a model?

Let me know if you want to look at this model, DipFL.ez that Roy included in his software, for us to use and demonstrate this Eznec Feed Line feature.

Here is the link to W8JI article on this issue. Go to "Jump down this page to the Dipole models."

https://www.w8ji.com/common_mode_current.htm