Avanti Sigma4: An alternative view point

[Shockwave]

CDX, The CST model is of the new Vector 4000 with the shorter radiator and longer radials. I believe these measurements are posted elsewhere in this tread. I do not have access to the active version of this model. The still image was kindly provided by Sirio and I do not want to trouble them by asking too many questions. I request everyone to do the same because the possibility has occurred to me that this information may not have been for public consumption. The active version would provide very little extra information. You would basically just see all of the currents shift to a perfect mirror image over a 180 degree phase shift.

Booty Monster, The CST computer models are completely new to me. This model was graciously provided by the engineers at Sirio who are apparently using some very sophisticated modeling software. From what I gather without having any instructions, these colors represent the intensity and phase of radiation from this antenna. If you look at the chart to the right it appears to show radiation in some unit per meter with the color green indicating 0 radiation. Red indicates maximum radiation and dark blue is also maximum radiation in the opposite phase.

When you see all red emitting from one side and all blue emitting from the other, this indicates all radiation from the antenna is constructive. Pay particular attention to the colors inside the cone section. You can see the radiation from the lower 1/4 wave on the central main radiator is deconstructive by the colors not matching the top half. Notice none of that radiation makes it outside the shielded cone. Also notice that the cone is a radiator producing currents that propagate away from the base which reinforce the top section. At this phase angle shown, it's red on left and blue on right

To sum things up, intensity is described in terms of the shade of colors emitted. Phase is considered to be constructive when the colors that actually emit from the outside of the antenna are one color on the left side of the antenna and the opposite color on the right side as shown on the graph. The antenna accomplishes this at the base by the shielding action of the cone that blocks the radiation inside while the cone radiates a constructive current on it's outside surface.

This model also explains why longer ground planes in the 3/4 to 7/8 wave range like the Jo Gunn Son of a Gunn fail miserably. Consider what happens to the radiation currents when you pull those radials down like a typical ground plane. You've just unleashed the full force of the deconstructive radiation in the lower 1/4 wave by removing the shield around it! Whatever beneficial radiation the radials could offer, is now taking place in the horizontal plane at a loss of 20 db. Just because it matches up nice doesn't mean it works good.

 

[Shockwave]

CDX, I misunderstood your question about the length of my Vector used to compare the GM. The length of my Vector was a little longer then the new Vector main radiator length. Around 28 feet. The difference was with my cone section. My cone was larger in diameter and shorter in length. If you read my initial reports, I thought this improved gain over the old stock Vector I had up for over a year with improved bandwidth into 10 meters.

Additional testing to many more stations I normally worked showed almost no difference in gain but it did have wider bandwidth. I've found testing at 50 miles can be hard even when changing the antenna out within 20 minuets. It almost requires an average signal check taken over time to the same station to spot the difference between propagation and antenna gain.

I think you can tell from my later posts about the larger loop on the Vector that I was somewhat disappointed as I got to use it more and recommended others not copy my modification. I feel shortening the radial length boasted 10 meter performance at the slight expense of 11 meter gain. That did take time to notice because at 50 miles an S-5 signal can easily drift an entire S-unit within seconds due to propagation!

 

[BOOTY MONSTER]

Booty Monster, The CST computer models are completely new to me. This model was graciously provided by the engineers at Sirio who are apparently using some very sophisticated modeling software. From what I gather without having any instructions, these colors represent the intensity and phase of radiation from this antenna. If you look at the chart to the right it appears to show radiation in some unit per meter with the color green indicating 0 radiation. Red indicates maximum radiation and dark blue is also maximum radiation in the opposite phase.

When you see all red emitting from one side and all blue emitting from the other, this indicates all radiation from the antenna is constructive. Pay particular attention to the colors inside the cone section. You can see the radiation from the lower 1/4 wave on the central main radiator is deconstructive by the colors not matching the top half. Notice none of that radiation makes it outside the shielded cone. Also notice that the cone is a radiator producing currents that propagate away from the base which reinforce the top section. At this phase angle shown, it's red on left and blue on right

To sum things up, intensity is described in terms of the shade of colors emitted. Phase is considered to be constructive when the colors that actually emit from the outside of the antenna are one color on the left side of the antenna and the opposite color on the right side as shown on the graph. The antenna accomplishes this at the base by the shielding action of the cone that blocks the radiation inside while the cone radiates a constructive current on it's outside surface.

This model also explains why longer ground planes in the 3/4 to 7/8 wave range like the Jo Gunn Son of a Gunn fail miserably. Consider what happens to the radiation currents when you pull those radials down like a typical ground plane. You've just unleashed the full force of the deconstructive radiation in the lower 1/4 wave by removing the shield around it! Whatever beneficial radiation the radials could offer, is now taking place in the horizontal plane at a loss of 20 db. Just because it matches up nice doesn't mean it works good.

cool , i was kinda in the ball park then . and the different colors makes perfect sense because ever cycle has a up and down swing .

freecell made a pretty interesting post about the sigma 4 . LOL , you can almost imagine an antenna growing from a 1/4wgp into a sigma 4 .

******************************
i have posted this some time ago in The Members Portal, i've decided to
release it here first in the Antenna Group. let me know what you think........

Where the story begins........

when the ground wave field strength of vertical antennas is examined, it is found that if a 1/4
wave antenna has 100 mV. field strength at 1 mile, a 1/2 wave vertical will have about 125 mV.,
a 5/8 wave will have about 140 mV., but a 3/4 wave will have only about 80 mV..

the conditions under which these tests were conducted assume a fixed amount of transmitter power
with all antennas tested matched to the feedline so that Z=50 or R=50, X=0 at the target test
frequency and that all antennas are tested at the same exact feedpoint height.

as the antenna design with regard to wavelength^ is extended from 5/8 wave to 3/4 something not
unexpected occurs. additional lobes appear in the upper elevation plane reducing the amount of
field strength in the major lobe at the lower takeoff angle and the measured ground wave signal
strength is reduced.

(^ electrical, not physical)

and so, conventional antenna theory is confirmed and vindicated not only by the test results
above but given further credibility and affirmation by the many college level engineering texts
that have been written on the subject.

whether or not this was the starting place for the engineers at Avanti when they contemplated
the design of the controversial Avanti Sigma IV is not known but it makes sense that this would
be as good a point as any. herein was the challenge presented to not only defy conventional
theory and design but also in the process to come up with a design that eliminated the formation
of these additional and wasteful high angle lobes while increasing the ground wave gain and at
the same time reducing the sensitivity of the design to objects in the near field which tend to
wreak havoc with feedpoint matching and distort the radiation pattern.

first let's re-establish a few facts so that it's easier to follow along with what lies ahead.

beginning with a basic 1/4 wave ground plane with radials at a 90 degree right angle to the
active radiating element, typical input impedance is roughly 36 ohms at a predetermined height
above earth ground. as the radial elements are lowered towards earth ground the input impedance
rises. conversely, as the radial elements are raised the input impedance is lowered.

as the radials are raised towards the radiating element something else interesting occurs. the
radiating element becomes less sensitive to the influence of surrounding objects and terrain
in the near field, allowing the antenna to be mounted in locations and at heights above ground
that would be extremely deleterious not only to the feedpoint match but also to the radiated
pattern emanating from the radiating element.

as the radials are swept upwards towards the radiating element capacitance between the radials
and the radiating element increases, effectively raising the resonant frequency of the system
or causing the electrical length of both the radials and the radiating element to be shortened.

so, the closer the radials come to the radiating element the more the coupling capacitance
increases, the higher the resonant frequency climbs and the lower the feedpoint impedance drops,
approaching single digits and 0 ohms.

also remember that the gamma match is used to match 50 ohm feedline to loads representing less
than 50 ohms. let's assume at this point that we have raised the radials to the point that the
angle represented at the apex (bottom of the antenna) is such that it approximates a value of
15 degrees. at this point we have gross mismatch, (single digit impedance) a 3/4 wave vertical
element that started out resonant in the middle of the cb band (or whatever) and is now resonant
much further up the band (1/4 radials included) but we have eliminated the sensitivity of the
vertical element to surrounding objects and terrain which provides a distinct operational
advantage, mentioned previously. now we have to match the feedpoint to the feedline and provide
some compensating influence to counter the rise in the resonant frequency of the radiating
element.

as we can see from the above the gamma match is the logical choice for impedance matching and we
can re-resonate both the vertical element and the radials by increasing the physical lengths
until resonance is re-established at the intended operating frequency. we can lengthen the
radials and simultaneously improve the frequency vs. bandwidth attributes AND provide additional
physical support for the radials by simply connecting an aluminum loop that will intersect the
radial ends while extending the physical lengths of each from 89.5" to 107".

with that done we can now direct out attention to re-resonating the vertical element for the
intended operating frequency by providing additional tubing length (inductance) beyond what is
dictated by the standard 3/4 wavelength formula. with that accomplished all that is left is to
implement the gamma match and then adjust these last two variables to match the feedline while
maintaining resonance at the target frequency. this can be the tricky part as the gamma strap,
referred to by SigmaIV enthusiasts as the "Dogbone" also comes into play with these other two
adjustments.

after all of the hard work what you end up with is a full-size modified 3/4 wave vertical that
easily produces twice as much signal as a conventional 3/4 wave @ 80 mV..

as to all of the questions posed by the pundits from the J-Pole camp i have only this to say.
you have no clue what you're talking about. even the most uneducated person can look at these
two designs based on nothing but geometry and determine that they can't be the same antenna
design based on the dissimiliar symmetry alone. what seemingly escapes most of you is the way
that the feedline smoothly transitions into the SigmaIV while closely maintaining the same
basic concentricity (inner conductor to outer conductor diameters) as that of the feedline with
no exceptions as to the size and type of feedline used. this efficiency factor coupled with the
full-size 3/4 wave radiator and revolutionary upswept radial design is why the SigmaIV was ahead
of its time when it was first introduced to the market and lends itself to the design of even
longer antennas (in terms of wavelengths) to produce even higher gain figures without the
restrictions presented by the development of multiple lobes as was the case in the conventional
designs that we were previously limited to.


Coming Next: The Avanti AstroPlane DeMystified!
292 Radio Shop / FireCommunications Network.......The Export Radio Specialists!

 

[CDX-007]

Originally Posted by Shockwave
CDX...
...Please look at the detailed picture of the radiation currents below shown in CST, courtesy of Sirio. We are not likely to ever see any more conclusive proof that the Sigma / Vector radiates from the cone in a phase that is constructive with the upper radiator. This antenna can be effectively configured to perform as anything ranging from a 1/2 wave over a 1/4 wave to a 5/8 wave over a 1/4 wave.

CDX, I misunderstood your question about the length of my Vector used to compare the GM. The length of my Vector was a little longer then the new Vector main radiator length. Around 28 feet...

Odd, if the currents are aligned as that single frame Vector CST image shows, then why shrink it back down to a seemingly lesser-gain 1/2λ over a 1/4λ when the longer 5/8λ over a 1/4λ version should show gain and a lower TOA?

Which brings to mind another set of questions:

- With only a remaining 'active' 3/8λ above the canceled 1/4λ of the Bottom-end-fed 5/8λ design, such a the I-10K, why then does a 5/8λ show a lower TOA and 1.2dB gain over a fully 'active' 1/2λ such as the Ringo with it's reasonably efficient (non-A99) Maco-type matching network, and even after I tried adding a set of 1/4λ counterpoise radials to the Ringo? (Even when tuned to near zero reactance, I cannot get anywhere near the performance from the Ringo which I get from the Penetrator, as in MINUS 7dB vs the P500 on TWO iCOM meters @ 10 miles.)

- Therefore, wouldn't it hold true to see the same improved performance resulting from comparing a 7/8λ Vector (5/8λ over 1/4λ) to a 3/4λ (1/2λ over 1/4λ) Vector?

 

[Marconi]

Odd, if the currents are aligned as that single frame Vector CST image shows, then why shrink it back down to a seemingly lesser-gain 1/2λ over a 1/4λ when the longer 5/8λ over a 1/4λ version should show gain and a lower TOA?

007, I think it is possible that Sirio shortended the Vector and lengthened the radials trying to see if they could improve the durability and not loose much, if any, advantages compared to their 3/4 or longer wave. Maybe a couple of feet less at the tip would stop a lot of complaints with customers. IMO, they did suggest this, but I could be wrong however.

Which brings to mind another set of questions:

- With only a remaining 'active' 3/8λ above the canceled 1/4λ of the Bottom-end-fed 5/8λ design, such a the I-10K, why then does a 5/8λ show a lower TOA and 1.2dB gain over a fully 'active' 1/2λ such as the Ringo with it's reasonably efficient (non-A99) Maco-type matching network, and even after I tried adding a set of 1/4λ counterpoise radials to the Ringo? (Even when tuned to near zero reactance, I cannot get anywhere near the performance from the Ringo which I get from the Penetrator, as in MINUS 7dB vs the P500 on TWO iCOM meters @ 10 miles.)

- Therefore, wouldn't it hold true to see the same improved performance resulting from comparing a 7/8λ Vector (5/8λ over 1/4λ) to a 3/4λ (1/2λ over 1/4λ) Vector?

According to my modeling of the Vector, I think it shows the shorter Vector shows a bit more gain at even a little shorter than the 3/4, but that is modeling not real world. I'm not sure what I did about the length on the radials, but I think I left them like the regular Sigma4 and they might should be made longer like the New Vector.

Bob experience shows the longer Vector is best near 7.8 wave. I'm not sure what he did about the radial length.

 

[CDX-007]

Marconi, I could make a 50' antenna last through 100mph winds if I wanted to, I don't see why they couldn't simply increase the base diameter tubing a size or two and use perhaps a few more sections in order to achieve durability if that's the issue. Sure it might cost more, but I doubt it would be anywhere near $339 USD but what a performer it should be, right?

Anyway, whatever radial length they decide is fine, I just don't see what that has to do with the overall radiator length, and performance is only increased by extending to a 7/8λ since a 5/8λ is a better performer than a 1/2λ.

...or is it?

- Your AP/TO seems to be kicking ass, it would sure be interesting to see a comparison with the I-10K if both were topping out at the same 55' mast location & coax.

...or would the bottom 1/4λ-fed 5/8 design of the Vector eliminate the lower 1/8λ of reverse current so that it would be a more effective 5/8λ top than is an I-10K type 5/8λ?

Perhaps your AP/TO would take the cake if it were compared to an I-10K at an identical location & height...? :blush:

 

[Shockwave]

CDX, the model shows the new short Vector. I'm thinking they shortened it so that all of the currents would be productive as shown on the model. You're questions regarding 3/4 wave versus 7/8 wave are good and it's somewhat unclear as to what works best. Testing in the field that both Bob85 and myself have done indicated that a main radiator length above 3/4 and less then 7/8 wave provided the most gain in the far field. Modeling does show that when you go past 3/4 electrical wavelength that the tip of the radiator begins to radiate in the wrong phase.

Which brings us back to the question of how is it possible for a radiator to begin showing signs of deconstructive radiation in the opposite phase and still appear to show an increase in far field gain? As you point out the Vector design is not the only antenna to show this characteristic. The 5/8 wave ground plane has apparent issues with the phasing in the lower 1/8 wave. I believe it's more complex then to assume if the lower 1/8 wave is in the wrong phase it will equally cancel out an additional 1/8 wave of the 5/8 wave. There is not an equal cancellation as though they were two parallel conductors.

Most of us agree that the end fed 5/8 wave beats the end fed 1/2 wave any day. This pretty much undermines the theory that due to cancellation, only the top 3/8 wave is functioning as an active radiator on the 5/8 wave. When it comes to the Vector there are other variables to consider beyond the radiator length. As I suspected earlier, maximum far field gain is dependant on the top of the radials and loop lining up with the dip in current on the main radiator. This is the point where the phase inverts and requires shielding of the radiator below it from the cone.

I recognize I did not answer the question of how it's possible to extend the 1/2 wave to 5/8 wave with the bottom 1/8 wave of radiation in the reverse phase. That's going to take someone who knows more then me to explain fully. I do agree it is the case with more then one type of antenna.

 

[CDX-007]

CDX, the model shows the new short Vector. I'm thinking they shortened it so that all of the currents would be productive as shown on the model. You're questions regarding 3/4 wave versus 7/8 wave are good and it's somewhat unclear as to what works best. Testing in the field that both Bob85 and myself have done indicated that a main radiator length above 3/4 and less then 7/8 wave provided the most gain in the far field. Modeling does show that when you go past 3/4 electrical wavelength that the tip of the radiator begins to radiate in the wrong phase.

So how would one move that reverse phase to the center and away from the tip?

Which brings us back to the question of how is it possible for a radiator to begin showing signs of deconstructive radiation in the opposite phase and still appear to show an increase in far field gain? As you point out the Vector design is not the only antenna to show this characteristic. The 5/8 wave ground plane has apparent issues with the phasing in the lower 1/8 wave. I believe it's more complex then to assume if the lower 1/8 wave is in the wrong phase it will equally cancel out an additional 1/8 wave of the 5/8 wave. There is not an equal cancellation as though they were two parallel conductors.

Now here I disagree, like a full wave antenna won't radiate because the reverse current cancels the forward current even though they are one above another, and not parallel lines.

Most of us agree that the end fed 5/8 wave beats the end fed 1/2 wave any day. This pretty much undermines the theory that due to cancellation, only the top 3/8 wave is functioning as an active radiator on the 5/8 wave.

No, I'm not certain, but I bet it's more about the elevation of the top 3/8λ and attraction to the counterpoise field which causes a lowering of the TOA, thus showing more signal on the horizon / gain. My best inspired guess, anyway...:laugh:

When it comes to the Vector there are other variables to consider beyond the radiator length. As I suspected earlier, maximum far field gain is dependant on the top of the radials and loop lining up with the dip in current on the main radiator. This is the point where the phase inverts and requires shielding of the radiator below it from the cone.

I wonder if there's some extending of the reverse current in the cone area which may cover the extra bottom 1/8λ, if it's out of phase on the 7/8λ, and which is what is allowing you & Bob to see gain over the 3/4λ?

I recognize I did not answer the question of how it's possible to extend the 1/2 wave to 5/8 wave with the bottom 1/8 wave of radiation in the reverse phase. That's going to take someone who knows more then me to explain fully. I do agree it is the case with more then one type of antenna.

I still believe it has only to do with the TOA lowered due at least partly to the counterpoise system, along with the canceled bottom 1/4λ acting as elevation for the top 3/8λ...

 

[bob85]

007,
radial number/length/diameter /proximity to monopole directly effect monople resonant frequency,

shockwave,
did you have nec set to show currents in their correct phase and magnitude when it indicated out of phase radiation near the tip?

from what i have seen the upper 1/2wave always assumes a sinusoidal current distribution with maximum voltage at the tip and imax 1/4wave down from the tip,

i do think that extending too far will cause high angle lobes to apear as seen in open sleeve antennas with monopole to sleeve ratios in excess of about 3.2:1 using equal diameter elements,
imho this could be why we both see a drop in performance as we extend towards the vectors original length and beyond,

taking all the vectors variables into account id say its impossible to guess the electrical length from physical measurements of length,
id also guess that it won't be much past .8wave before we see high angle lobes starting to form when using 1/4wave radials,
other length radials may change the situation,

the whole antenna needs more experimentation ,
we understand the sigma in principle but i don't think we understand the effects of relative element length adjustments with the gamma fed version,

the effects of the hoop and the question of isolation and choking also need looking at in more detail,
then we can start looking at what the astroplane is

 

[Marconi]

007, tell me how you get the "λ" to show up in your post? I know it's from a table of symbols, but I forgot how to call up the table.

 

[Marconi]

007, radial number/length/diameter /proximity to monopole directly effect monople resonant frequency,

shockwave, did you have nec set to show currents in their correct phase and magnitude when it indicated out of phase radiation near the tip?

Bob, is your question referring to the tip of the mast or the radiator? Shockwave if bob is referring to the mast, then I would also like confirmation for how this feature turns off and on with Exnec. At this point when I construct a model the first time I click the "Antenna View" on, the currents are not shown. When I click on details for the "Currents" then the currents shown are displayed in the "Antenna View" as well. But, I'm not sure if I'm seeing both magnitude and phase, or just the magnitude. Can you explain this for me?

from what i have seen the upper 1/2wave always assumes a sinusoidal current distribution with maximum voltage at the tip and imax 1/4wave down from the tip,

Bob, do you mean you see the max (not imax) current node 1/4 wave down from the tip and max voltage at both ends of the upper 1/2 wave?

i do think that extending too far will cause high angle lobes to appear as seen in open sleeve antennas with monopole to sleeve ratios in excess of about 3.2:1 using equal diameter elements,
imho this could be why we both see a drop in performance as we extend toward the vectors original length and beyond,

When modeling I think I saw these higher lobes at a bit less than 3.2:1, but I wasn't sure my calculations were exact either. I did see a tipping point in this are however. Give me your idea of the dimensions for 3.2:1 at 27.205 and I will model it again. Do you think it would have to be the same as the model in the ARRL book or will the Vector model work too?

taking all the vectors variables into account id say its impossible to guess the electrical length from physical measurements of length, id also guess that it won't be much past .8wave before we see high angle lobes starting to form when using 1/4wave radials, other length radials may change the situation,

I think I saved that model Bob, and I'll check and see how long I made the radiator. I have not modeled the Sigma4 yet with the long radials, my most accurate model has measurements for the Sigma4. Maybe soon I'll model the Vector with the dimensions you and MultiMode provided and check your length idea again with the New Vector dimensions. I'm swapping antennas right now. I just took my GM down and put my A99 up without radials. Next, I'll add 4 x 72" A99 GKP radials horizontal, and then add 3 x 102" slanted together with the horizontal. Then remove the horizontal. Lots of work to do, and then the Imax. Some are not going to like my results.

the whole antenna needs more experimentation ,
we understand the sigma in principle but i don't think we understand the effects of relative element length adjustments with the gamma fed version,
the effects of the hoop and the question of isolation and choking also need looking at in more detail, then we can start looking at what the astroplane is

Shockwave or Henry, have you modeled with a mast vs. insulating the mast? My models all show insulating does not improve gain, but I can't explain why. I believe that insulating should improve the current flowing in the radials if available and that should improve something. This confuses me.

 

[bob85]

eddie, im talking about the tip of the antenna,

by imax i mean current maxima 1/4wave from the tip,

you may have more luck with the arrl open sleeve design as it uses equal diameter conductors and a parallel sleeve which nec can handle, the downside is its not exactly a sigma,

id be guessing if i said any particular length due to the variables, i don't even know if the length that worked best here is longer than it should be.

 

[Marconi]

The only part on my Sigma4 model that shows to be out of phase with the rest of the radiator...is the first 4 x 7.15" inch segments at the bottom around the area where the gamma would be.

For some reason however, my radials show a little more that half of the radial, from the bottom up, are in phase and the rest, up to the hoop, are out of phase and I don't understand that.

Well, I took 106" for the radials and figured the radiator at 3.2 times 106": 106" x 3.2 = 339.2" x 12 = 28.2666' for the radiator being the longest radiator according to the rule. I think I actually noticed a tip-over point, where the pattern skewed badly, at a tad less than 28.26' in the model however. Real antennas are different and modeling can do everything.

I posted the other day that I see a similar tip-over point around .625 wl for the 5/8 wave where the current maxima changes from its low angle of about 8-9* degrees to a higher 40* degree angle with the .64 wl model. Like I told 007 however, in that case the gain in the 10* degree lobe was only a fraction behind the primary lobe, so it was still good, but diminished rapidly beyond .64.

If the model of the Sigma4 don't work in this regard, I wouldn't waste my time trying to model that antenna in ARRL Antenna Handbook.

None of my models have had any kind of verification as to accuracy.

 

[Marconi]

In my post above I typed the following in error: Real antennas are different and modeling can do everything.

It should read: Real antennas are different and modeling can't do everything.

 

[jazzsinger]

007, tell me how you get the "λ" to show up in your post? I know it's from a table of symbols, but I forgot how to call up the table.

On XP Eddie its click start,all programmes,accessories,click system tools then click character map. gives you all sorts of fonts/letters/characters to use including the lambda symbol.

i'd imagine its in a similar place on all microsoft operating systems.


Alternatively just copy and paste it.