SIGMA IV Details

[HomerBB]

. . .
i don't see the sigma design meeting any of these requirements for a none radiating transmission line, what do you guys think????

if cebik was correct in saying a form of co-linear was possible i don't know of another mechanism that could cause radiation from the sleeve to be in phase with the upper 1/2wave,
. . .
im open minded to new ideas, if anybody has an alternative view on the issue.

I tend to see it just this way so far, too.

Marconi raises the issue of Shockwave's statement to the effect that there is a high voltage at the ring at the top of the cone so that CMC would effectively be stopped from manifesting on the outside of the basket in the same way that a coaxial choke works to prevent CMC. I see his point, but I also am aware of the bottom of the cone being connected directly to the vertical and consider the possibility of the in-phase current on the cone coming from there. Any insight on this?

 

[bob85]

i don't understand the phasing stub deal donald, i know cebik used a 1/4wave stub on the co-linear j-pole

http://www.cebik.com/content/a10/vhf/jp4.html

i don't know where we can go from here with vector, its all one sided,
in almost 9 years of debate with the j-pole camp they have never posted anything but

"its a 1/2wave"

their thinking is parallel wires cancel radiation,
even when dealing with j-poles that is only true under certain close spaced conditions where there is only a small imbalance at the top of the sleeve & little cm current on the leg,

nec can show there is radiation from the short leg on a wider spaced j that does modify the pattern in the direction of the short leg even though its not shown in the current log or current view, what do the 1/2wave j-polers say about that? absolutely nothing,

i think they never posted alternative ideas because they can't find what does not exist outside of citizen band mythology,
im a very patient guy, i waited 9 years already,

its not that im closed minded, i don't have a horse in this race,
its the fact they never posted any alternative & what they have said can be shown to be wrong from multiple sources.

 

[bob85]

homer,
if you can find any evidense of this voltage node at the top of the sleeve in any way stopping cmc on the sleeve i would like to see it,
the way i see it the opposite is true,

the radials would only be in phase with the lower 1/4 of monopole if you folded them down like a gamma fed diple or sirio top-one.

 

[Marconi]

I'm not feeling good, so this might be more confusing than I intended. I know the video did not turn out like I wanted, but maybe if you print out my currents report from the "Sigma 4 Manual w mast 36'" model this won't be so much of a problem. It may raise more questions, but maybe it will help understand better what I see. When I get to feeling better I will do another video and try and get my act together better.

The issue is that sometimes I can't see what is being recorded, when I try to get the model to show up on the full screen, so sometimes the image I'm referring to is not in view. I should have moved the models into the center of the screen so you could see better, but I could not see what was on my monitor at the same time.

This is a 3/4 wave ground plane with 4 x 107" radials and I am trying to demonstrate where I see voltage and current nodes.

Marconi and Eznec, a demo - YouTube

Sorry guys, this video was terrible and did not make the point I intended.

 

[Shockwave]

I was not under the impression that changing the position of the radials had any affect on current phase. Phase has more to do with the length of the conductor and location of the source. I have never seen a version of the Sigma that could produce anything other then out of phase currents between the cone and vertical portion inside it. That is true for my versions of the antenna too. I'm somewhat confused by Marconi saying mine are in phase at the base because they are not.

The reason for this phase inversion has less to do with the radials or the fact they are sweeping upwards on this antenna and more to do with the length of the 3/4 wave radiator itself. Remember when you extend the vertical radiator past 1/2 wave it causes current inversion at its base. That's why the Sigma design has inverted currents between its 1/4 wave cone and the base of the 3/4 wave radiator.

The only way I can see bringing the base of the vertical into the same phase as the cone would be to shorten the length of that radiator from 3/4 wave to 1/2 wave. You can model that in EZNEC. It would not be an improvement on the design because you lost 1/4 wave of constructive radiation but all elements would be in phase.

I do agree there is a voltage node at the top of the loop just like you would find in any other end fed 1/4 wave element. It also lines up with the voltage node on the vertical radiator but in the complete opposite phase. Creating a very large voltage difference between the two. I do not believe there is anything that resembles choking action on the loop that could prevent CMC from radiating off the cone.

Perhaps this idea came from the use of a coaxial sleeve around the coax at the feed point to reduce CMC on the coax? Similar principles may be at work here but I ask you to consider how the sleeve works to stop the coax from radiating? By giving the current an alternative low impedance, resonant element to radiate the energy off of. The sleeve still radiates CMC to prevent the coax from doing so. The length of radiator above the sleeve will determine if the sleeve's radiation will be in a constructive phase or not.

Choking uses non-resonant, high impedance to prevent radiation where sleeves provide a low impedance path that is resonant for the current to radiate from. Each have their application but work in opposite ways.

PS: I hope you are feeling better soon Marconi.

 

[HomerBB]

homer,
if you can find any evidense of this voltage node at the top of the sleeve in any way stopping cmc on the sleeve i would like to see it,
the way i see it the opposite is true,

the radials would only be in phase with the lower 1/4 of monopole if you folded them down like a gamma fed diple or sirio top-one.

I am not able to do so. I am simply raising the question by extracting it from a larger post because I wanted to hear/read a reply to the point.
I referred to a statement by Marconi concerning what he had said had been said by Shockwave.

Shockwave, thanks for your reply in reference to this point. I was unable to adequately respond to the point, but wanted someone to help sort out what may have been actually occurring. The differences in the impedance, and the resonant vs non-resonant characteristics of the sleeve and the choke restore a sense of what is going on here.

When the opportunity affords it, I think I would like to try another one of these antennas home-brewed. The problem for me is that in the multiple threads of discussion so many variables of dimensions emerged that I can not easily determine which one of them is the one to try to emulate. I thought i was done with building this type. My last one failed to give me the kind of excitement I got with the first. It seemed to lack the vibrance, and in fact, my EFHW has done an better over all job for me. The first was made with the 4 radials cone, and at 3/4ƛ, while the second was a 3 radial cone also at 3/4ƛ. Another thing I experienced was the inability to match the antenna at virtually any reasonable length change with the gamma. A good SWR demanded I stay within a very close few inches of length on the vertical. The final issue with the second one was that I used the MFJ-259b to set match, something I did not own when I made the first, and for the life of me never could get the antenna to show desirable X=/0= values when I found a good SWR match, a sought after condition in any resonant well match antenna system.

 

[bob85]

i can't find anything either homer but im willing to look at other ideas if anybody finds something,

i don't know if the longer radials work better yet, i can only tell you a longer monopole than a sigma4 and shorter than an old style vector works best here.

 

[HomerBB]

i can't find anything either homer but im willing to look at other ideas if anybody finds something,

i don't know if the longer radials work better yet, i can only tell you a longer monopole than a sigma4 and shorter than an old style vector works best here.

Thanks, Bob.
I'll just have to figure it out when the time comes.