Avanti Sigma4: An alternative view point

[bob85]

you could be right shockwave, its not how it looks to me but im not arguing
over a few inches of mast with the guys at sirio who use cst microwave studio, their new vector shows that they too believe that changing relative element length ratios alters radiation angle, broadcast engineers knew that in the 1940's,
there seems to be good reason why sirio make the best performing cb antennas

 

[AudioShockwav]

We do see yellow and sky blue in the same phase and shape as the currents above, just with less magnitude.

With the dark out of phase currents (red and dark blue) trapped inside the cone.

Is that what you are saying Shock?

73
Jeff

 

[Shockwave]

With the dark out of phase currents (red and dark blue) trapped inside the cone.

Is that what you are saying Shock?

73
Jeff

The "out of phase" radiation is from the lower 1/4 wave of the main radiator. This is shown in dark blue on the left and red on the right. This deconstructive radiation is indeed "trapped" inside the cone. Even better is that the cone replaces this radiation with it's own beneficial radiation.

 

[CDX-007]

My inspired guess, (if anyone really gives a rip... ) is that the differential currents in the lower ¼ should cancel, leaving the upper active part both well elevated above what one would get from a standard bottom-fed 5/8, plus, (and due to the cancellation of the lower ¼ wave currents) should be allowed to fully radiate as an unimpeded ½ - 5/8 wave, thus producing roughly the same gain as seen from the SGM, which is also an unimpeded full 5/8 wave unlike the active 3/8 left to do all the performing in a standard bottom-fed 5/8.

OK, now just TRY to disagree, ...I DARE YA!

 

[Shockwave]

For quite some time there was debate as to if the lower portion of the antenna radiated constructively with the upper portion. Some claimed the lower 1/4 wave did not radiate while Avanti claimed it did and CST proves it does. When you look at the colors of the radiation currents you can see the currents from the cone are radiating constructively with the upper 1/2 wave. The cone does not simply cancel unwanted radiation, it replaces it with desired radiation and forms an antenna that is active across it's entire length when you include the cone.

 

[jazzsinger]

Perhaps this model of the Vector in CST Microwave Studio will help explain the radiation currents from the antenna. They are shown in color, phase, and magnitude. The currents on the radials are lower individually but remember the current is divided into four radials. This model is the work of Mr. Lorenzo, one of the engineers at Sirio.

It is much easier to understand why Cebik referred to the antenna as a non-apparent collinear. One can see how the unconstructive radiation is confined within the cone while the cone emits it's own beneficial radiation. This in not the conventional collinear however, it does produce the same effects based on the same theory.

Assuming the radials radiate in a 360 degree plane like the radiator, then it stands to reason the inward radial radiation cancels the bottom 1/4 wave of the radiators radiation, and the outward radial radiation adds to the top 1/2 or 5/8 wave radiator radiation at some point in the far field,

i'm assuming this is the reason sirio have returned to the 3/4 wave radiator to keep all outward radiation in phase,as this wouldn't be the case if the top radiator portion above the basket was 5/8 wave rather than 1/2 wave.

i'm also assuming that the angle of those radials is critical for maximum gain in the far field as the outward radiation from the radials will be angled towards the earth (due to the radials being angled earthwards and EM waves generally travelling in straight lines) and be reflected by the earth and at some point in the far field both the radiator radiation and radial radiation will recombine, prefferably i imagine exactly in phase to maximise far field gain.

which if i'm not mistaken is what gives this antenna an advantage at extreme line of sight distance.

its definately a j pole,pmsl.

 

[CDX-007]

For quite some time there was debate as to if the lower portion of the antenna radiated constructively with the upper portion. Some claimed the lower 1/4 wave did not radiate while Avanti claimed it did and CST proves it does. When you look at the colors of the radiation currents you can see the currents from the cone are radiating constructively with the upper 1/2 wave. The cone does not simply cancel unwanted radiation, it replaces it with desired radiation and forms an antenna that is active across it's entire length when you include the cone.

Well, I do wonder how it could simply remove the unwanted out-of-phase current? Cancelling it seems probable, but wouldn't they cancel one another in doing so? It seems the bad current would cancel the good current. You can't simply expect energy to vanish, it has to go somewhere so I'd expect it would go into the same space as the good current thus causing cancellation of both.

Also, if the lower ¼ wave radiates good current in conjunction with the upper section, I'd expect you'd see gain over the SGM since it would effectively be a collinear monopole, but your tests confirm the performance of the two as equivalent.

 

[jazzsinger]

eddie, i would say the current situation m8ji shows is more than possible,
if i wanted to portray the same info and make it easier to visualise with ezbob id do it like this, im not saying its the correct way but it is easier to visualise the concept.

i think these type of 2 dimensional current loop diagrams are what confuse many people, in reality that current loop is uniformly the same around all 360 degrees of the radiator at any given point. think of the current loop more like a typical dipole's radiation pattern in that it is all around the radiator and the picture gets a bit clearer.

 

[Shockwave]

Well, I do wonder how it could simply remove the unwanted out-of-phase current? Cancelling it seems probable, but wouldn't they cancel one another in doing so? It seems the bad current would cancel the good current. You can't simply expect energy to vanish, it has to go somewhere so I'd expect it would go into the same space as the good current thus causing cancellation of both.

Also, if the lower ¼ wave radiates good current in conjunction with the upper section, I'd expect you'd see gain over the SGM since it would effectively be a collinear monopole, but your tests confirm the performance of the two as equivalent.

It doesn't remove or cancel the unwanted radiation. That out of phase radiation is very much intact inside the cone. The cone simply confines or shields the radiation within the cone. It is not a canceling effect as though the cone was a parallel transmission line. This effect is not dependant on the currents within the radials because it is a passive shielding action not active cancellation. The radial currents control what the radiation on the outside of the cone will be and they just happen to be constructive to far field gain.

With respect to the Gain-Master versus the Vector / Sigma, If the Vector was not performing like a collinear it would radiate as a unity gain half wave end fed. Examination of that CST model clearly shows it behaves nothing like a half wave. It's longer length takes complete control of the undesired radiation with the upward flaring cone that radiates constructively with the rest of the antenna. Testing that puts the GM on the same playing field simply confirms the advanced design of the GM for lower power applications.

 

[Marconi]

a no radial 5/8wave endfed on a mast with total height 2 wavelengths is what i intended to illustrate eddie, i guess my drawings sometimes get the message across lol

Bob, I know the best we can do to depict AC currents flowing on a wire is to indicate them at some particular point...frozen in time, so-to-speak. But, what determines the direction those arrows point when EzBob plots such an image?

i have no idea about what the lower 1/8wave of a 5/8wave does in the far field with regards canceling radiation from the 1/8wave of radiator above it above it, i saw it posted recently, its not something i have looked at,

I think I saw that post also, and thought it was interesting. If the phase idea works with this 1/8 wavelength idea like current flow is reported to work on a 1/2 wave dipole, then the antenna system would necessarily have to waste another 1/8 wave above it on the good phase portion of the element...in order to show cancellation due to opposing forces. IMO, that is why the dipole is depicted to have the currents on both sides of the source flowing in the same direction (in-phase) and hopefully both legs with the same magnitude. And if there is no in-phase flow, then the RF is canceled just like it is inside the coax. If the magnitudes are not equal then we can experience common mode currents of the feed line shield as well.

For a long time I've had this thought - even if the RF from the 1/8 wave segment at the base of a 5/8 wave radiator was canceled somewhat, that its major contribution to the antenna's effectiveness was to raise the 1/2 wave current portion higher up on a longer and higher radiator. I assumed this would show an improvement in maximum radiation...and this is why the 5/8 wave is said to show more gain at a low angle. Notice that I said "...at a low angle," not a lower angle. I was never convinced however, that this small increase in elevation was quite enough to really lower the maximum angle of radiation in a primary lobe. In the modeling I do I most often see changes affecting the higher angled lobes rather than the lowest lobe. I believe it likely works that way in real life antennas too, except to some degree how antenna elevation changes have their affects the lower lobe may be affected a little. In the 18' to 50' range we often see in CB, I doubt this low angle change would be much however.

Bob, I'm surprised you never looked at what is going on at the base of a 5/8 wave antenna. Surely you've had some thoughts about how the 5/8 wave element works.

eddie, i don't see how you made models that showed the currents to change phase, how they are presented when you turn things off and on in nec may not fit what you expect or how you expect them to be presented, i did not even use the same method of presenting my currents in all my drawings but in my minds eye i was portraying the same thing, the cst plot is just another much more detailed way to display the same data,

the cst microwave studio plot posted thanks to shockwave and mr lorenzo at sirio is the best i have seen by far, no plot has ever given me so much pleasure

Bob, I don't really know either why the currents are as they appear and you're right about my lack of expectations. I don't make the models to show anything in particular. I'm sure whatever I do has some affects, but my activities in the area of currents are not deliberate or planned. I just try to get the physical dimensions right and keep the Eznec errors from popping up. I figure if the dimensions are close enough, then let Eznec worry about the scientific calculations.

I did fix the model to agree with your argument about mast currents the other day, but I can't honestly say what I did to get those results. When I do a model, I don't plan with currents in mind. I just don't understand enough about currents, direction, or phase to plan that stuff. I'm sure to be very accurate in modeling it is desirable to get currents right...since the whole NEC engine uses pre-determined scientific factors regarding currents to do its calculations.

changing the relative lengths adding hoops etc, will shift phase relationships to some degree, thats plain physics,
one theory (from a forum member) is that extra gain comes from the increased separation of current maximas which extending the sigma does,

how i think that could be effecting the farfield is speculation from me (remember cebik did not fall off his chair laughing, he said it was perfectly possible) i first observed what the sigma could do, argued with everyman and his j-pole, explained it to mr cebik, got the answer i had hoped for and went looking for his illusive "none apparent collinear array" that could be formed by folding radials up toward the radiator and caused pages of meaningless arguments from people who did not understand how they worked, (his words not mine), the only antenna i could find that fit how claimed the sigma worked was the open sleeve and its derivatives,
look at the plot and read the open sleeve article in the arrl, all will become clear, ezbob also shows the contraflowing transmissionline currents in the gamma section, they are there too,

Yes Bob, you've done an excellent job of getting your experiences out and finding a logical source for the collinear idea and I personally believe that is what makes the Sigma 4 design work like it does.

yes the plot shockwave kindly posted shows that the antenna mode currents are where ezbob predicted and the radiation from the sleeve is in phase with the upper 1/2wave very much like a modified OPEN SLEEVE MONOPOLE, there is no more arguing about what the sigma4 is for me, its taken 6 years im a patient guy, if anybody wants to argue with j-pole/endfed 1/2wave nonesense it won't be with me:headbang, im sticking with the folk that understand antennas,

Well Bob, that leaves me with some very big steps to try and follow. I have to say, I don't think I'll ever be able to satisfy such goals, even if I promise to never mention the J-pole. Personally, I think the J-pole is a mute issue when considering the Sigma4 design.

i don't recall exactly when i realized that i could tune a sigma better than my buddies, probably around the time they started asking me to come help tune theirs, remeber your tuning settings are no good for our cb band, i saw with my own eyes that many settings would provide a good vswr but some worked better than others, i don't see anything odd about modifying anything, i mod everything not just radio related stuff, whats odd to me is thinking you can't make anything better,

Bob, you mention your CB band is different from the US. I think it is higher in frequency, right? Why then, back in the day, did you ever consider to making the antenna longer? I would have thought that would be putting me further away from the desired frequency range. Just curious about that.

There is absolutely nothing wrong with modifying stuff. I admire folks that experiment in life. Among other things, I cut my teeth on modifying a couple of 55' Chev's in my younger adult days. One for street modified, with a 283" small block. I competed successfully and won often in NHRA "C" & "D" Modified Stock classes in the 50's and 60's. I was never satisfied. I could change nothing on the exterior of the engine, drive train, body, or suspension, but I worked midnights under a shade tree on the guts of the business, to run that thing in the high 12's, over 100 mph @ 1/4 mile tracks around Texas. That is nothing with today's technology, but it was hot back then, or at least I thought so as l left a lot of better cars in the rearview mirror. I've always done things that you might call "out of the box" even if I don't see the subject that way. I hate buzz words.

We also see now that Sirio has elected to go longer on the radials, but shorter on the radiator while they claim to have retained all their previous gain and angle advantages compared to their previous Vector 4000. This one is a sleeper too. I've not heard a sole proudly reporting on his New Vector 4000, even though several guys on this forum have one. Bob, you told me that it's still not built well enough for the UK climate. You mentioned that Multimode said something, but I can't recall what. I don't think he had a report however. That is strange that no body in Europe or the US has tried it and reported what the New Vector 4000 design can or can't do? I don't think it was a new design that Sirio discovered, I think they were probably forced, due to antenna failures, to make it shorter somehow and to try and mitigate the bad reputation it has.

good luck in your tests with the gainmaster, we are still awaiting delivery, i would like to see your test against your stock sigma at your location, when i tested on two poles in the field id swap them around and try to average the results out, you know my feelings on veracity two antenna tests but so long as you enjoy what you do its all good to me,

i do have that in mind even though it is the hardests one to put up and tune. My new mount is close to the house and I can still get up that high safely. I designed it that way for that reason.

the gm had and probably still has its detractors, the "its just a wire dipole in a tube" brigade, these guys learned radio from the same book as "its just a j-pole" camp, i won't knock it before i try it, shockwaves test gives me reason to believe there could be something to it,
there is more than one way to beat a 5/8wave groundplane and this could possibly be another method, those who think "inside the box" are forever stuck "inside the box",

i hope the gainmaster does what is claimed, that opens up new homebrew avenues

take it easy eddie.

I'm impressed by the Gain Master, but there are some exceptions to my stellar reports so far. None exceed the SGM, but I do see a few signals that match it and one today where a guy reported the GM showed him less signal. Bob, it is like summer before last, I'm seeing conditions in terrible shape and I have a lot of static. As our weather becomes cooler and less humid the noise seems to really affect signals and hearing, at least that is what I think. Our recent summer was very dry late in the year so I may be seeing some change that is affecting how the antennas here typically act. This may even be affecting the water table in my area. I'll know more about that when I get the I-10K up if I do that too. My new mount allows me to work those bigger antennas without having to build them and attach them to the mast and then raise all that up.

73's
Eddie

 

[Marconi]

For quite some time there was debate as to if the lower portion of the antenna radiated constructively with the upper portion. Some claimed the lower 1/4 wave did not radiate while Avanti claimed it did and CST proves it does. When you look at the colors of the radiation currents you can see the currents from the cone are radiating constructively with the upper 1/2 wave. The cone does not simply cancel unwanted radiation, it replaces it with desired radiation and forms an antenna that is active across it's entire length when you include the cone.

Shockwave, you're right about the in-phase issue a while back. I think when I got to modeling my version, there was not question in my mind that we concluded wrong back then, and the bottom is in phase with the top portion just like the Sirio model shows. I wrote Bob that he was right. We were all confused by how Eznec handles and smooths the depictions of currents. I don't know why that is or even if I'm right, but that is what I think.

I tend to agree with Shockwave. If we could get further back from the model that SW posted, then I think we would see a larger RF pattern that is complete from bottom to top with no nulls across the entire Sigma radiator including the base. For me this larger tear drop shaped lobe shows the pattern in the far field. That part is also like CDX007 suggests about the full radiator working on the Gain Master.

I also think if we weren't looking at a collinear affect in this antenna, we would see such a far field pattern. Also notice that the pattern show does not indicate much if any skyward RF. If this is Sirio's model of the New Vector 4000, then I don't see how it can be beat unless better tuning would put more current into the segments. SW, do you know if Sirio is showing us a tuned model? Did you get any more output of their model that would show they included mast, feed line, and/or matching of some sort? That would also be great for us to ponder. I think Bob may be right however, it looks like a free space model again. For Sirio's purposes, that may be all they consider relevant to their antenna designs. How bad could a good free space model turn out to be, pattern wise, just by placing it over the real Earth feature in the software?

I would like to see a 3D view of that model.

 

[Marconi]

i think these type of 2 dimensional current loop diagrams are what confuse many people, in reality that current loop is uniformly the same around all 360 degrees of the radiator at any given point. think of the current loop more like a typical dipole's radiation pattern in that it is all around the radiator and the picture gets a bit clearer.

A J-pole eh?:headbang What'd he say? George, you're picking on Bob again. Bob, don't loose your cool, he'll get over it soon.

George, I think you are right and I also think the depiction you describe can be done with a 3D view of the pattern.

SW, I would love to see the wires description from that Studio software if it works similar to Eznec. Does it use the NEC engine like Eznec?

 

[Marconi]

Well, I do wonder how it could simply remove the unwanted out-of-phase current? Cancelling it seems probable, but wouldn't they cancel one another in doing so? It seems the bad current would cancel the good current. You can't simply expect energy to vanish, it has to go somewhere so I'd expect it would go into the same space as the good current thus causing cancellation of both.

Also, if the lower ¼ wave radiates good current in conjunction with the upper section, I'd expect you'd see gain over the SGM since it would effectively be a collinear monopole, but your tests confirm the performance of the two as equivalent.

Scott, I kind of think that is the way sleeve type devices work, but Bob is better at this understanding. Maybe it has to do with the idea that RF flows on the surface and then imagine how currents work inside of coax, so as not to cause radiation when the coax remains in tact.

I also think in some ways the Starduster design has some similar affects on protecting the feed line from radiating and ill-affecting the radiated pattern. We haven't discussed symmetry yet, but I think I could argue that it has something good to do with all this chatter we're making.

I think it goes without saying that how an antenna deals with the feed line has a lot to do with how effective it ends up being. In my little efforts at modeling I see very little currents flowing below the Vector/Sigma4 design. Relative to the 5/8 wave the magnitude is Nil.

 

[Marconi]

It doesn't remove or cancel the unwanted radiation. That out of phase radiation is very much intact inside the cone. The cone simply confines or shields the radiation within the cone. It is not a canceling effect as though the cone was a parallel transmission line. This effect is not dependant on the currents within the radials because it is a passive shielding action not active cancellation. The radial currents control what the radiation on the outside of the cone will be and they just happen to be constructive to far field gain.

With respect to the Gain-Master versus the Vector / Sigma, If the Vector was not performing like a collinear it would radiate as a unity gain half wave end fed. Examination of that CST model clearly shows it behaves nothing like a half wave. It's longer length takes complete control of the undesired radiation with the upward flaring cone that radiates constructively with the rest of the antenna. Testing that puts the GM on the same playing field simply confirms the advanced design of the GM for lower power applications.

SW, I plan to compare my Sigma4 vs. my GM. I don't think you do side by side testing, but did you have the antennas at the same height at the coax connections or were they staged some other way?

I haven't decided yet how I want to set mine up in that case, but I've found in the past, doing side by side, that the longer antenna always wins when the feed points are the same height, and the shorter radiator always wins when the antenna tips or the same height.

If I lower my GM so that the tip is no higher than the AstroPlane tip, then the GM is at a real disadvantage with the bottom being close to something on the ground or house. No way the GM can equal the AP. However, if I put the GM and the AP at the same feed point height, no way the AP can compete with the top of the GM towering almost 18' feet above the AP. The difference is not as much as one might think, but the GM is superior with both coax connections at the same height. I do think in this situation the GM will really shine at longer distances however. In fact right now I have the antennas about centered together in height with the tip of the GM about 8' feet above the AP. The GM shows better signals the further out the stations is which I would expect. I haven't been able to do much comparison work in this configuration, but every time conditions get real quiet, I test if I can hear someone on the air. I will switch the antennas and compare signals before going on to another antenna combination with the GM.

 

[CDX-007]

SW, I plan to compare my Sigma4 vs. my GM. I don't think you do side by side testing, but did you have the antennas at the same height at the coax connections or were they staged some other way?

I haven't decided yet how I want to set mine up in that case, but I've found in the past, doing side by side, that the longer antenna always wins when the feed points are the same height, and the shorter radiator always wins when the antenna tips or the same height.

If I lower my GM so that the tip is no higher than the AstroPlane tip, then the GM is at a real disadvantage with the bottom being close to something on the ground or house. No way the GM can equal the AP. However, if I put the GM and the AP at the same feed point height, no way the AP can compete with the top of the GM towering almost 18' feet above the AP. The difference is not as much as one might think, but the GM is superior with both coax connections at the same height. I do think in this situation the GM will really shine at longer distances however. In fact right now I have the antennas about centered together in height with the tip of the GM about 8' feet above the AP. The GM shows better signals the further out the stations is which I would expect. I haven't been able to do much comparison work in this configuration, but every time conditions get real quiet, I test if I can hear someone on the air. I will switch the antennas and compare signals before going on to another antenna combination with the GM.

Since it's the Christmas season I can get away with posting a, "Bah, HUMBUG!" to your side by side comparison.

I'd rather see you live with the Sigma4 AS YOUR ONLY ERECTED ANTENNA for a week or so, all the while notating signal strength from as many stations & directions as possible, then swap to the SGM on that same pole and retest over the next week, taking many readings, then average all the data and find the winner.