Follow up with Bob on the Sigma 4/New Vector 4000

[Marconi]

that's what i am trying to understand eddie,
the way it sounds to me is he says it won't radiate because transmission-lines in the numerical model always have equal and opposite currents and do not recognise that coax is a 3 conductor affair unless you add the extra wire to simulate the outside of the coaxial braid ,

I think you’re right Bob, but you are ignoring the broader context in the text.

IMO this is what Roy is saying. Isn't that the way the theory of coax is supposed to work, based on the conditions he stated in his quote below.

That is, the currents in the model’s two conductors are always equal and opposite currents, so the line doesn’t radiate or have current induced by coupling.

Roy explains this in the first paragraph of that section on “Using Transmission Lines.” See the manual below. Here Roy explains how using Eznec works to solve the 3rd conductor issue, and he explains why. IMO Roy is not ignoring the facts about a third conductor. He goes on to tell us how Eznec handles this when the load is not purely resistive for some reason.

I don’t understand all the ramifications of Transmission Line theory like you do Bob, but these words describing NEC rules and how Eznec handles a TL…seems pretty clear to me.

the cone is a transmission-line and part of the antenna structure, it should act similar to a tapered coaxial cable,
the radials can be replaced by a tapered solid cone according to avanti,

IMO, if we tapered a coaxial cable like this cone, we would have to insure that the ratio of the differences in the radius for the center conductor to the radius for the shield was maintained the full ¼ wavelength of the cone. IMO, if we don't maintain the correct coaxial ratio, then we sure can't call the cone coaxial.

A 4” coaxial pig tail will radiate antenna currents just because the correct coaxial ratio as been skewed. This is why we do our best to keep pig tails short, to help minimize radiation.

You can also extend the wire lengths of a pig tail out to near a 1/4 wavelength each, and you’ll have a working dipole radiating antenna mode currents into the far field.

For a while back in the good old days I believed Freecell…when he too used to try and convince me that the cone on the S4 worked just like a coaxial feed line. He said the cone feeds the bottom end of the S4 ½ wave radiator, and thus made a smooth transition to a match for the end of the ½ wave element above. I didn’t see it and I didn’t believe it, because in order to feed the end of a ½ wave element requires a very high impedance connection. Coax presents a need for a very low impedance.

the arrl article tells us transmission-line currents flow and are responsible for radiation in the lower 1/4wave in phase with the upper 1/2wave,

Are you referring to the ARRL Open Sleeve article? This idea is new to me.

I was thinking that transmission-line mode currents follow the rules of feed lines…and do not radiate into the far field, because of the equal, opposite, and cancellation rule that applies to coax construction?

Am I wrong?

Bob, to be clear here, I also think this theory or rule regarding coax also includes that the load is perfectly matched, and maybe this is why Roy talks about the subject this way. Apparently this issue is a NEC thing.

Consider this

The coaxial cone serves several important functions. In some ways is does mimic a transmission line with it's ability to confine or shield radiation from the base of the center vertical radiator. Because it's tapered, it transforms the impedance of the antenna so that it can be efficiently matched to 50 ohm coax. The cone is also tuned to act as an effective counterpoise and has both transmission mode and antenna mode currents flowing through it. Transmission mode currents are confined within the cone and antenna mode currents are allowed to radiate constructively on the outside of the cone.

the arrl does not tell us that there is no radiation due to cancellation even though the conductors are parallel to the monopole, what am i missing?

Again, are you getting this conclusion from the same article above?

if i remember correctly your models show very little imbalance in currents at the top of the radials,

is that because it sees the cone as a none radiating coaxial cable with equal and opposite currents?

Bob, my S4/NV4K models do show very little imbalance in currents between the monopole and the sum of the radial currents. These currents are also out of phase and cancellation occurs.

I think it was Freecell that first referred to the cone area as a coaxial feed line. Later I think Donald was suggesting the same thing. I’m not sure if you ever mention this however.

W8JI wrote that a cone or sleeve basically helps decouple antenna CMC’s, similar to what radials do with any other vertical radiator, but he says nothing about the cone being coaxial. He shows Eznec model results with the cone attached to a J-Pole.

Bob, there are more of your comments below, but for now I’m going to stop here. I’ll try and get to the rest later, OK?

 

[bob85]

Roy explains this in the first paragraph of that section on “Using Transmission Lines.” See the manual below. Here Roy explains how using Eznec works to solve the 3rd conductor issue, and he explains why. IMO Roy is not ignoring the facts about a third conductor. He goes on to tell us how Eznec handles this when the load is not purely resistive for some reason.

roy seems to be talking about feedlines using the tl function eddie,
im asking about the radial sleeve that forms a taperd transmission-line and is part of the antenna structure,


For a while back in the good old days I believed Freecell…when he too used to try and convince me that the cone on the S4 worked just like a coaxial feed line. He said the cone feeds the bottom end of the S4 ½ wave radiator, and thus made a smooth transition to a match for the end of the ½ wave element above. I didn’t see it and I didn’t believe it, because in order to feed the end of a ½ wave element requires a very high impedance connection. Coax presents a need for a very low impedance.

the top end of the sleeve is fairly high impedance eddie, its 30" wide with about 1" center conductor, i agree its not a smoothe taper,


coax always has equal currents on the inside eddie, it will radiate if the load is unbalanced,
and if im understanding roy correctly that radiation won't show as radiation in eznec unless you model the coax as a two wire transmission-line and add a wire to simulate the outside of the braid,

he also says nec is no good for modeling 1/4wave stubs because in the model the stub will have equal currents and no radiation but in the real world the stub will radiate due to unbalanced currents.

what does eznec see the sleeve as?

the "open sleeve antenna" article was in the arrl antenna books for years eddie,
its not new to you, i quoted from the article in the first post of the alternative viewpoint thread, we have talked about it several times,

you need to read it to understand what i mean by transmission-line mode currents and antenna mode currents with regards to the antenna they describe,
it will answer your other questions,

you could model the antenna they describe, it only has two sleeve elements, like a ssfm with two legs removed, see if you get similar numbers.

 

[The DB]

coax always has equal currents on the inside eddie, it will radiate if the load is unbalanced,
and if im understanding roy correctly that radiation won't show as radiation in eznec unless you model the coax as a two wire transmission-line and add a wire to simulate the outside of the braid,

he also says nec is no good for modeling 1/4wave stubs because in the model the stub will have equal currents and no radiation but in the real world the stub will radiate due to unbalanced currents.

In the third .pdf that Eddie posted at the end of his post there is a section on modeling coax that includes non-radiating and radiating coax.

Modeling Coaxial Cable
A non-radiating coaxial cable is electrically identical to a non-radiating two-wire line, so an ordinary transmission line model can be used for modeling a nonradiating coaxial line. A radiating coaxial cable can be modeled quite well with a combination of transmission line model and a wire. The transmission line model represents the inside of the coax, and the wire represents the outside of the shield. The wire is the diameter of the shield, and connected where the shield of the actual cable is. lt should follow the same physical path as the real coaxial cable. One of the example files, DipTL.ez, shows how this is done. (See its Antenna Notes for more information.) If the coax cable doesn't radiate, the current on the additional wire will be zero, and the additional wire contributes nothing to the analysis so can be removed.
There's no way to use the transmission line model to accurately model a radiating two-wire transmission line (one with common mode current present). If it's necessary to do this, the line will have to be modeled as two parallel wires. See Closely Spaced Wires for precautions.

So if there are common mode currents on the outside of the cone in the Vector you need a second wire for each element of the cone to simulate said common mode currents.


The DB

 

[bob85]

that is what i want to know DB, am i wording my question wring?
been having a hard time for a few years getting a straight answer from people that are using eznec,

either eznec will or eznec will not show radiation from a transmission-line when it is part of the antenna structure,

does the way eznec handles transmission-lines such as 1/4wave stubs mean that it also sees the vector as a none radiation transmission-line or does the rule only apply when you are using the TL function.

 

[The DB]

that is what i want to know DB, am i wording my question wring?
been having a hard time for a few years getting a straight answer from people that are using eznec,

either eznec will or eznec will not show radiation from a transmission-line when it is part of the antenna structure,

does the way eznec handles transmission-lines such as 1/4wave stubs mean that it also sees the vector as a none radiation transmission-line or does the rule only apply when you are using the TL function.

I don't know about EZNEC specifically, but I can tell you that in NEC2, which is what EZNEC either uses or is based on, if I model two equal length parallel wires close together, with a source across the bottom ends I get gain and a pattern. It seems NEC2 radiates when you have two parallel wires with equal currents and opposite phase, and radiates fairly well. The pattern looks like two separate lengths of wire fed 180 degrees out of phase.


The DB

 

[Marconi]

I don't know about EZNEC specifically, but I can tell you that in NEC2, which is what EZNEC either uses or is based on, if I model two equal length parallel wires close together, with a source across the bottom ends I get gain and a pattern. It seems NEC2 radiates when you have two parallel wires with equal currents and opposite phase, and radiates fairly well. The pattern looks like two separate lengths of wire fed 180 degrees out of phase.

The DB

DB, your description of you model idea is very brief, but here is what I think you are talking about. I made my wires 1/4 wavelength and 4" apart for 27.385 mhz, but I'm only guessing at what you actually did with your model.

You said the other day that our models should produce very close to the same results because both Eznec and 4Nec2 use the NEC2 engine, but I don't get positive gain from my model here, like you claim you do with your 4Nec2 model noted above. I get negative gain and it is a large value. I also get an infinitely bad match.

 

[The DB]

Now that I go to the model and run an average gain test I get 0.1 dBi gain. Apparently something got by me with that model...


The DB

 

[Marconi]

Now that I go to the model and run an average gain test I get 0.1 dBi gain. Apparently something got by me with that model...


The DB

DB, don't try and fix something that is not broken. The part that got by you in modeling with 4Nec2 might be what the AG results tell you.

You must have done the source right for this model in order to see such a result. If you can get close to this value on a CF dipole, which is not really hard to do, you have managed to put the source in the middle of the wire(s).

Average Gain, the .01 dbi you report is a very good result. It is desirable to try and get the Average Gain result between 0.00 dbi and .20 dbi in these NEC models.

Can you post your results?

 

[Marconi]

Here is Eznec's Manual topic on Average Gain.