Avanti Sigma4: An alternative view point

[CDX-007]

Interesting Read Bob, Thanks for sharing



My .2....

I Lived in a very rural area, had a nice tall 3 story House, easy roof to work on, and an about perfect test base. On the roof I had a pair of 10 foot Tripods separated by 45 feet of roof, 20 Feet of Well pipe leaving the base of the antenna at 63 Feet. At the time was no such thing as Antrons, Sirio, larrys, Macos, Etc...

I did have the Sigma IV, Starduster, Penetrator 500, and a Sigma II. Surrounding Terrain was Flat except a large Hill (Ski Hill) to my SW at about 8 miles. While I wasn't perfect in my test I was pretty thorough.

Radios used were:
Base: Johnson 500/Collins R390, Tram D201 D&A Phantom.
Mobile: Hygain V, 102 Whip (roof mounted Aprox. center of car roof), and an Old School Palomar 4 transistor mobile.


The tests were run over about a week. Consisted of talking to the mobile without interruption to a local (1940 foot elevation)Mountain, about 60 miles as the crow flies. For base to base comparison I used 3 contacts. One at about 20 miles Moonraker 4, and a Penetrator. One at about 55 miles, Starduster. And one about 110 miles with a PDL II.


Back in the day gas was cheap, time was plentiful, and I was a pretty full of energy.


Up until about 15 years ago I still had the Paper with all the appropriate signal strengths, and distances. Unfortunately not anymore.

I do recall the antennas were all within a small range on my R390, The Tram was a bit more difficult to read but results were the same.

stationary mobile to base, and base to base the Sigma IV was the winner. though for the first 20 miles the difference was minimal. Signal was measured about every 5 miles at a quick stop. it wasn't until 50+ miles the antenna began to really shine.

I was able to talk to the mobile the full 60 miles QRP except in the case of the Starduster, was a few times I lost signal. This was rolling and could be attributed to conditions.

Base to Base was pretty much a repeat. The Sigma IV beat all the others with a noticeable signal increase both TX and RX. at 110 miles the Sigma IV made enough difference to allow a reliable conversation with less then 25 watts possible.


Also years later I did some tests, which consisted of a Penetrator and a Starduster copy, VS a Larry... Same results.

Interestingly I also had a very cool fairly close copy (Spec wise) of the Moonraker 4... I don't remember the maker (Cushcraft maybe?) but it was pre-assembled, you just bolted the boom together, telescoped the elements, locked the hinged elements in place and you were done... Very impressive antenna, or so I thought.

Carefully measured and tuned the antenna. Put it on my 10 foot tripod in place of the Larry.... Boy what a huge disappointment, the larry did better both TX, and RX.

Now in this case, it shouldn't have been, but it was... antenna was mounted about 36 feet center boom. The Larry was obviously a bit higher due to its profile.

Only thing I can figure is possibly because I was in a slight hole. Lived right on a lake, and even to the South where it was level the Larry still kicked its butt.


Moving forward to the mid nineties I compared the Sirio Sigma IV with the Antron.. What a huge difference. No matter the distance. I will say the V58 was also a big difference, was not as strong as the Sirio though.


Over the years I've owned the original Avanti, the Larry LW150(think that was it) A gold anodized copy CTE?, and the Sirio. All were decent antennas, the gold version was very annoying to get tuned up, but once I managed it was great. None of them matched the build quality of the Original. I will add Attention to detail in assembly was important on all of them.

Since my Start in radio in the 70's I've owned Just about every antenna made My all time favorite is hands down the Sigma IV. Copies 2nd, then Sigma II, and Penetrator. Would like to try the some of the modern antennas but I just cannot Justify the cost.

Excellent test. Wonderful testing equipment!

I had a Larrys LW-150 and I received different results, more along the lines of boring standard antenna theory, as the Hy-Gain .64 Penetrator outperformed the LW-150 at ~45miles by 1.5 S-units both RX & TX, due, I believe, to the lower take-off angle of radiation.

Same radios, Yaesu FT-107M for SSB, and a Johnson Messenger 223 for AM, (with it's commercial piece of test equipment-like meter & circuit) same coax, mast, etc.

What surprised me was the receive being lower than that of the Penetrator. I expected at least better RX due to the LW-150 being about 5 feet taller, but it seemed to follow the radiation angle of the TX, and dropped considerably in comparison to the Penetrator, especially at greater distances. The greater the distance the more the Penetrator excelled.

I absolutely loved the construction of the LW-150 but had to let it go, reluctantly, due to it being a poorer performer in comparison to the Hy-Gain Penetrator.
I was under no pressure to sell it, and adored the forged aluminum cylinders which joined each section to the next, plus the weight being that of TWO Penetrators / I-10Ks attesting to the exceptionally high-quality construction, etc., but what good would an inferior performer of an antenna do for me?

Last I heard it's now living in Boise, Idaho along with my old, beloved, White Yaesu FT-107M, a radio I truly miss.

I have also talked with others who have replaced their 3/4 wave whatever antenna with a .64 (Penetrator / Sigma5/8 / Wilson Apha / I-10k, etc.) and have seen improvement, especially over greater distances.

One local radio op switched from the MacoV-5/8 to the 4000 then back to the Maco because he said they performed the same but he didn't like the light-duty construction of the 4000. WOW, and he thought that tissue-paper-weight Maco was better built?? Eeeek! No wonder the 4000s are losing their 'heads'! (top elements)

I'll wager just the radials from the Penetrator outweigh the entire MacoV-5/8

The swr was less than 1.2:1 on each antenna I tested, and built/assembled 'by-the-book'.

The Penetrator has one issue for some, the ground shunt part of the match has often been assembled incorrectly and attached to the wrong side, thus causing some slight degradation in performance. It's easy to do if you have Slisdexia like eM, or don't pay attention to the diagram & pictures, which might account for your apparent sub-level performance.

Still, excellent test, I like to hear about those, especially when so much care is taken to keep it fair!

73

 

[Toll_Free]

Bob,

Here ya go.

I sent this to Master Chief via email earlier, then remembered my password.

Been following the thread Bob85 started on WWDX.

I, too, for three years, have argued that it's a modified slot antenna, and have NEVER seen the patent. I didn't even know the patent existed!

HOWEVER, I, HAVE MODELED IT!

It has max gain at 34.3 degrees, 8.23 dBi (referenced over REAL ground, and antenna GROUND mounted. You can skew the results ANY way you want by just changing lengths. You also HAVE to Balun feed this type of antenna, coil of coax works fine, 6 to 7 1/2 turns for 15 mtrs on up. JUST ENSURE ITS SOLENOID WOUND, NOT BUNCHED UP!!!) . At 27 mhz. 14.4+494 ohms. 14 ohms resistive, 494 ohms of inductive reactance... Hence the necessity for the cap (gamma match).

At 12-14 degrees, the Avanti Sigma antenna gives 2.7dBi of gain as well.

Calling it a J-Pole is complete, and udder bullshit. A J-Pole is called a J-Pole because of the configuration of the matching network. This is antenna 101, and can be verified by looking up a J Pole and a Slot and comparing them in either the ARRL Antenna Handbook, or almost ANY of Orr's manuals. The "designer" that called it a J-Pole should have been ignored at that point, simply because of the fact that the J portion is the matching network. Had the Avanti BEEN a JPole style antenna, no gamma match. The quarter wave stub will match almost ANYTHING to ANYTHING!!! Hence the reason we use them at V/UHF for matching our linears. You can match from 1 ohm to a megohm with a quarter wave stub (I mean, a "J Pole"). Incidentally, for more 'backing up' of my statements, search zeppelin antenna, not the end fed crap we hams use today, but the original zeps... They used the "J" style match, but didn't call it a J Pole because it didn't stand on it's end, resembling a pole... They trailed out.

You can also google Jagi or any other variant of the "J" matching method.

The slot, BTW, is common at microwave freqs... . UHF as well.

CB Ground Plane antenna. 4 Radials, all 1 meter long (each side, so a total of a bit over 6 feet... Didn't really matter if I made them longer in any type of near field (model-able) crap.

Antenna height 3M over real ground, default ground setup (typical of > 50 percent of earth, according to my modelling software)


Max Gain, 2.53 dBi at 41.8 degrees. 64+163, so 64 ohms with 164 ohms of inductance.
At 13 and 14 degrees, you have 1.8dBi of gain as well...

So, that's what I get, using a NEC2 style engine. Feel free to post the results, I forgot my password and associated email addy on that site.....

You CAN really screw up the Sigma's pattern by NOT balun feeding it. I can take it to a negative gain antenna by playing with feedline lengths, etc.... Source the voltage / current AT the feedpoint, and the proof is in the model.

I haven't even tried to increase (optimize) the gain.... My software allows for optimization on anything, f/b, gain, swr, impedance, whatever you want.

I'm using one of my Sigma's as my radiator for 160-10 now... Have it fed at the base, mounted at the 2nd story balcony. Using a marine tuna at the base, and a single ground rod (constantly soaked, NICE loamy soil, can't really ask for better), and I do OK. 1.2 swr ANYWHERE, butthe RF voltage is a bit high on 160!!!

Anyway, as I said, feel free to post the results. I DID model the antennas, and the sigma, properly installed will blow nearly ANYTHING else out of the water. Modelled as a sigma (I DID use a triangular "ring" for the top in my model. I can't do circles, each point becomes a new wire, but I did try using all different shapes, triangle, square, octagon, etc... And it didn't change the models nearly 1 bit... All within a dB. The problem with the sigma is this (and this is ANY antenna): The installer and installation!

Penetrator and nearly ANY style of groundplane that uses a decent matching system, the feedling currents will be decoupled by the ground radials. At least, in theory, this is how it works. HOWEVER, you WILL get some decoupling from them regardless, making the installation of a GP style antenna almost idiot proof.

The Sigma, you actually have to know what your doing, and install it as a system. A nice feedline choke, W2DU style balun, or nearly any other method of decoupling (qw sleeve, etc) will make a Sigma sit up and dance.

I compared my Sigma IV to a Radio Shack ground plane for a year up on the mountain in ******. All tuned MFJ, blah blah blah. The Sigma, for LOS comms, was the HANDS DOWN winner. The Radio Shack ground plane would eat it for lunch, however, when it came down to getting over a particular mountain, and after modelling them both, I can see why..... The Sigma had ALL it's RF below a particular point..... The RS ground plane was a LOT shorter, and had some interesting patterns.

--Toll_Free

edited for spelling error

 

[bob85]

thanks toll-free, i knew you were a sigma fan but i did not know you were keeping an eye on me lol,

not sure what type of slot antenna you relate the sigma style to, i dont know much about them at all but i am keen to learn,
i see it as a modified form of open sleeve antenna,
i agree it will beat a groundplane,
they will talk to and hear stations at distance that a 5/8wave mounted at the same height just cannot hear,

i agree the person installing the sigma style antenna will make or break the performance depending on what they believe is the correct way to install and tune those antennas,
they can be made better at distance by playing with the relative sleeve and radiator lengths then retuning,
i said it felt like i was actually manipulating the radiation angle to pick up 1.5 s-units over the stock antenna on the ft990 meter at 60+ miles,
sirio have now lengthened the radials by 16" and claim a lower radiation angle, is that the truth or is it marketing hype ( they do have an rf anechoic chamber to test models ) seems like somebody is listening,
i never went over about 91-92" but i now have the tubes to go longer if its needed,

thanks for the model it is very interesting,
how about modelling the sigma with 16" added to the radials, lets see if what sirio are claiming is backed up with your software,
i tried modelling the sigma but soon realised i needed to learn more about nec before i could do what i wanted so im back to field tests to see if i can improve on what i started with,

welcome to the "ITS NOT A J-POLE" club, your badge and teeshirt are in the mail

 

[Toll_Free]

thanks toll-free, i knew you were a sigma fan but i did not know you were keeping an eye on me lol,

not sure what type of slot antenna you relate the sigma style to, i dont know much about them at all but i am keen to learn,
i see it as a modified form of open sleeve antenna,
i agree it will beat a groundplane,
they will talk to and hear stations at distance that a 5/8wave mounted at the same height just cannot hear,

i agree the person installing the sigma style antenna will make or break the performance depending on what they believe is the correct way to install and tune those antennas,
they can be made better at distance by playing with the relative sleeve and radiator lengths then retuning,
i said it felt like i was actually manipulating the radiation angle to pick up 1.5 s-units over the stock antenna on the ft990 meter at 60+ miles,
sirio have now lengthened the radials by 16" and claim a lower radiation angle, is that the truth or is it marketing hype ( they do have an rf anechoic chamber to test models ) seems like somebody is listening,
i never went over about 91-92" but i now have the tubes to go longer if its needed,

thanks for the model it is very interesting,
how about modelling the sigma with 16" added to the radials, lets see if what sirio are claiming is backed up with your software,
i tried modelling the sigma but soon realised i needed to learn more about nec before i could do what i wanted so im back to field tests to see if i can improve on what i started with,

welcome to the "ITS NOT A J-POLE" club, your badge and teeshirt are in the mail

I've said it wasn't a jpole for years now... With antenna modeling,it's starting to sink in.

I should model a JPole as well.

I'll have to fudge with the program to get the sigma with different sleve lengths... I tried it, and it made it look like a friggin dipole! I reloaded the antenna, tried it again, and got the same results... I know it's me, and I can't get the source right... As I said, play with the load and source, and you can REALLY accomplish amazing things with that antenna.

The matching network sucks. A gamma can lose up to 6 dB.

--Toll_Free, off to dindin

 

[Toll_Free]

Another thing I noticed after reading the patent info that Bob posted the link to, that got me thinking...

About 2 years ago (last time I frequented this board), I left after having a heated discussion with someone about antennas, and the fact that these "idiots" making 5/8 antennas today where really making .64 wave antennas, etc. because NOBODY was taking electrical length into account. I was basically handed my hat by the "supposed" experts (who couldn't actually be BOTHERED to read anything by the ARRL, Orr, or anything else) that told me there was no such thing as "electrical length" of antennas.

Well, to hand you your asses back, on the proverbial platter, take a look at the radial terminology in the patent. Then take a friggin look at the REASON for the impedance loop on the Sigma IV style.

It broadbands the antenna, as well as shortens the ELECTRICAL LENGTH of the 'radials'.

There, smart-asses. Have it your way, even if it's wrong.

Antennas generally require a 4 to 6 percent shortening, depending on a number of factors, but that's the generally accepted rule for aluminum tubing, NOT covered. Heat shrink your antenna, and the dimensions all CHANGE!

Thought I'd throw that back into the game.... Seems some supposed, recognized "experts" have just been proven wrong.

--Toll_Free

 

[CDX-007]

Is this Sigma4 in question about 27' or more like 31' feet tall?

The LW-150 I played with for a while was only about 3/4 wave long. I wasn't impressed against the Penetrator but perhaps there's more going on with the longer version...?

And Toll_Free you're right, of course, about "electrical length", otherwise how could coax have a velocity factor which needs to be taken into account if you are, say, making a 1/4 wave matching transformer from it?

73

 

[bob85]

007, im sure toll free modelled a 27ft sigma as thats what he was using a while back and thats what he referes to in his post,

according to the arrl the central monopole can be extended to about 3.2 x the sleeve length, after that higher angle lobes start rearing their ugly head, ( explains why i lost signal performance as i went too long with the central monopole )

i cant explain why you had the 5/8wave in front, thats not how it pans out here,
i still have not seen a pic of the fabled larry lw150 antenna,

cebik had something interesting to say about ground conductivity that he felt needed to be further investigated, it may or may not help explain why some folk just dont see what they expect,

5/8waves at moderate heights over anything but very poor soil dont provide their highest gain at low angles like they do in the free space plots,

in the real world over average to very good soils the 5/8waves major lobe is up at 35 degrees with a smaller lower lobe down towards the horizon giving only a fraction of a db improvement in gain at desireable angles over a 1/4wave when mounted at the same base height,
that more than likely explains why my low mounted i10k works superbly for short hop skip but not as good as the sigma style over distance locally or in longer hop skip,

another thing cebik notes is that a 90 degree radial 5/8wave provides its highest gain and lowest angle of radiation when mounted over very poor conductivity soil,
over very poor soil the 5/8waves lower lobe becomes the major lobe,
over better conductivity soils the 5/8 waves secondary upper lobe is dominant with less energy going into the more desirable lower angles,

it seems not everybody will get the same results depending on your soil conductivity all else been equal,
im sure the same antenna will not work best at all locations

 

[Marconi]

007, im sure toll free modelled a 27ft sigma as thats what he was using a while back and thats what he referes to in his post,

according to the arrl the central monopole can be extended to about 3.2 x the sleeve length, after that higher angle lobes start rearing their ugly head, ( explains why i lost signal performance as i went too long with the central monopole )

i cant explain why you had the 5/8wave in front, thats not how it pans out here,
i still have not seen a pic of the fabled larry lw150 antenna,

cebik had something interesting to say about ground conductivity that he felt needed to be further investigated, it may or may not help explain why some folk just dont see what they expect,

5/8waves at moderate heights over anything but very poor soil dont provide their highest gain at low angles like they do in the free space plots,

in the real world over average to very good soils the 5/8waves major lobe is up at 35 degrees with a smaller lower lobe down towards the horizon giving only a fraction of a db improvement in gain at desireable angles over a 1/4wave when mounted at the same base height,
that more than likely explains why my low mounted i10k works superbly for short hop skip but not as good as the sigma style over distance locally or in longer hop skip,

another thing cebik notes is that a 90 degree radial 5/8wave provides its highest gain and lowest angle of radiation when mounted over very poor conductivity soil,
over very poor soil the 5/8waves lower lobe becomes the major lobe,
over better conductivity soils the 5/8 waves secondary upper lobe is dominant with less energy going into the more desirable lower angles,

it seems not everybody will get the same results depending on your soil conductivity all else been equal,
im sure the same antenna will not work best at all locations

Yes Bob85, what you describe here is what I see with my 1/4 radiator Marconi when I compare it to my I-10K while both are over very good soil conductivity in my area. According to ARRL Hand Book, this area shows 30 millisiemens/meter conductivity and that is very high. The I-10K still typically shows a better local RX signal on my Signal Reports, but not with every station---plus the differneces are also very marginal at best and not nearly as varied as some have previously reported. The only thing that I can say to help explain about my result is the soil conditions---just as you described.

It would be interesting if everyone reporting their antenna comparison results would also included the condition of the soil in their area, or describe generally speaking what the soil around the antenna looks like. I have attached a picture of the ARRL Map of the US for your reference. You can open the image into another window and enlarge and see every detail.

 

[W5LZ]

I agree, soil conductivity can play a big part in how an antenna performs. The biggest 'problem' with that is that most people have no idea how, or the ability to measure it. And while the ARRL's reference is very 'generally' true, boy is it NOT exact at all! It can change from day to day, and almost by the 'foot'. So, without actually measuring it over a fairly large area and then averaging the results, it only amounts to a guess.
Once you assemble all the stuff you need to do that measuring it isn't a biggy. But getting there can really be fun !
Considering the 'state' my yard is usually in, I can pretty well figure on good soil conductivity. The only thing my yard and rice-paddies don't have in common is rice. It's got all the rest of that sh... ah, stuff.
- 'Doc

 

[bob85]

doc, me and mm200 metal detect so we are well aware of soil composition and moisture content effecting radio waves and how it can change from field to field or over much smaller areas,

if cebik is correct about upper hf monopoles it implys that to make your 5/8wave 90 degree groundplane mounted at moderate heights perform at its best having a dominant lower lobe you need to be in an area with very poor soil conductivity,
better conductivity soil causes the higher angle lobe to dominate,

that may partly explain why MC sees the i10k pull ahead of others when hes testing antennas in the high desert,
it could also explain why eddie with his very good conductivity soil does not see the gains hed expect when testing his marconi against other antennas,
eddie has done a lot of testing recently with some surprising results that mostly fit what guys like cebik and w8ji claim.

 

[CDX-007]

Never more a Sigma4, 4 me 4 evermore.

How high above ground are the 1/4 & 5/8 waves in your modeling?

Is the ground effect anywhere near as serious a factor when the antenna radials are raised to 1.5-2 wavelengths above good soil conductivity ground?

The limited reading I've done with regard to ground effect has always been focused toward low band HF and considered the ground to be only 1/4 - 1/2 wavelength beneath the antenna.

I have my Penetrator atop a 4 section telescoping mast(36') atop a 20' rooftop, so the high current node is nearly 2 wavelengths from the top of my ground, and well above 2 wavelengths above the water table which is 7'-10' below the surface.

I read an extremely long strand on eham yesterday about vertical performance, ground effects, differing antenna radiation angles, etc. and found it to be more confusing than enlightening. Even Tom W8JI mentioned that he has found modeling isn't always correct, and he designs commercial antenna systems. Check out his rather impressive bio: http://www.eham.net/callbook/w8ji

"The proof is in the pudding" is often heard and in this case it means when real world tests are conducted the truth comes to light even when theoretically it shouldn't be as experienced.

Modeling is fun and can help bring ideas to life without so many errors one might otherwise experience if the simple 'trial & error' method of designing was instead employed, but it will never beat real-world comparisons, and I have done them on my roof and with the same coax, radio, mast, etc., with everything from a 1/4 wave R/S GP thru an Astroplane, Starduster, Sigma 5/8, MaCo V-5/8, A99, Imax, LW-150, Penetrator, CLR-II, Taylor GLR4, Wilson Alpha V-5/8, Shakespeare Big Stick and assorted others, (mostly short radial 1/2 wave designs) and only five stood out as superior performers well above the rest, and they were:

(in Alphabetical order)

1) Avanti Sigma 5/8
2) Hy-gain Penetrator
3) Radio Shack .64
4) Taylor Grandslammer GLR4
5) Wilson Alpha V-5/8

- and they are all ~23', four radial, .64 / 5/8 antennas.

That was here, 56' at the radials, 84' of 213 coax.
Not two different masts, two different runs of coax and two different locations on the property, no, the same mast, coax, placement, and height at the mounting bracket.

I have yet to try the I-10k but I fully expect it would mirror the other ~23' GPs in performance.

I was almost ready to blow some cash on another Sigma4 due to all of the convincing sounding hoopla surrounding it's mysteriously magical design, but I've already been there, done that, so why go back to the same results expecting them to be better the 2nd time 'round?

I'm satisfied that it is a decent, sub-.64 performer at this location.
Perhaps elsewhere, at a different location / elevation above ground / coax length / surrounding terrain, etc., it might just beat all others, but I have done my testing and the real world has favored the .64 design in my application.
And your 3.2x sleeve length numbers make sense as I remember the LW-150 being a little under 27' total length. Closer to 26' 7" - if I remember correctly. It's been well over 10 years since I sold it.

...and I don't see how anyone can experience a 1/4 wave GP as nearly as effective an antenna. Here it wasn't even in the same ballpark. Geez, even the A99 beat it.

The Starduster was the most surprising antenna I tested, beating the Astroplane, A99, Big Stick and, of course, the 1/4 wave, and it kept up with most of the other, taller, 1/2 wave GPs! - I believe it to be a center-fed 1/2 wave dipole, not a 1/4 wave GP as some have said, including me from time to time, mistakenly.

73

 

[dt1561]

Most interesting reading,I have 2 Sigma 4's up and running here.Just put up #2 fresh outa the box,I have had #1 antenna since it first hit the market many moons ago,I have the gamma;s in use and I get a decent match,If I am reading correctly The antenna would perform better if I used a 1/4 wave stub? Would love to hear more,Better yet a picture of the setup would be handy also.....Jay on de Sandpile~~~~~

 

[CDX-007]

Follow-up

You bastages now have me wondering if there isn't something to this longer version that I should try... thing is, every time I go against my intuition I end up disappointed wondering why didn't I listen to my intuition...

We'll see, but I'm never going back to that dang 3/4 wave, POO-POO!

(Intelligent way to close, don't you think?)

 

[Shockwave]

The Sigma IV explained in simple terms.

Checking the statistics of my FMBroadcastAntenna.com | HIGH GAIN FM BROADCAST ANTENNA web site allowed me to see traffic resulting from links in this forum. That prompted me to explore the link and I've enjoyed reading the posts here. In a way I felt obligated to share what I've learned in the past three decades working with this "Sigma IV" style antenna. This is not a plug for my FM scaled version of the antenna since CBers have no use for that product. However, the same principles of how the antenna works and what improves it's performance still apply on 11 meters.

First, the antenna is definitely not a J-Pole. This name originates from Sirio who called it a coaxial J-Pole. There is no 1/4 wave element driven with RF to inductively couple to the main radiator. Therefore the name does not fit. There is also much confusion as to if the bottom 1/4 wave portion of the antenna radiates, or not. Using a simple field strength meter will confirm almost nothing is being radiated in the area of the cone. The more important question should be "how does the cone section improve performance"? The cone makes this antenna superior in two ways that only this design has been able to accomplish.

The cone "forces" RF up the main radiator with very little lost in the first 1/4 wave. The longer wavelength main radiator causes much more RF voltage to build across it's length then could ever be possible by simply end feeding a half wave. Higher RF voltage on a resonant radiator creates a stronger electromagnetic field. The "cone effect" as I call it also provides much needed focusing of the radiation pattern in the .82 wave design. Without it, the antenna would have the disastrous calculated results of a 45 degree radiation angle! Thankfully the real world results are closer to 12 degrees above the horizon. Much more useful for land based communications even with low antenna height.

Back in the days before getting into FM broadcast, I found this radiation angle to give a profound advantage in ability to work DX on HF. This doesn't happen too often at VHF but I can tell you for lack of better terms, it sure kicked butt over any other single element antenna. I now refer to the new generations of the Sigma IV style antenna as a .82 wave coaxial antenna due to the coaxial cone structure at the base. I've tried every vertical antenna possible with on the tower tests and no single element vertical puts more signal on the horizon then this antenna. The only possible exception is the Big Mac. This antenna is not a single element and they are so scarce I could not find one to test.

It is important to understand that the Sigma IV gets it's gain by lowering the angle of radiation on the horizon. This is not like the effect of a beam that concentrates most signal in one direction. The result with the Sigma IV is that the improvement in signal is typically not noticed locally. The advantage only becomes obvious in testing over 20 or 30 miles. It is the shape and electrical length of the coaxial base that is entirely responsible for lowering the angle of radiation. The Sirio is not designed for maximum gain in this area. It has been slightly compromised to reduce the mechanical size of the loop.

As noted in Avanti's original patent maximum gain on the horizon occurs with a cone angle between 25 and 30 degrees with respect to the main radiator. Since the coaxial base is electrically equal to a 1/4 wavelength, we must shorten the stock four support rods and increase the diameter of the loop to reach the 25 degree angle. Yes, four support rods work better then the original three that Avanti used. Adding more then 4 will not help much. Sirio was on track when they increased the wavelength from 3/4 wave but they past the maximum gain by going to 7/8 wave.

The Sigma IV design provides more gain when the main radiator is inductively reactive. This is why it requires the capacitance of the gamma match to cancel out the inductive reactance. Some here have suggested the gamma match has inefficiencies that can cause a loss of 6 db. Totally 100% false! If you give up the gain added by going beyond 3/4 wave you can directly shunt feed the 3/4 wave without the gamma. The radiator can even be isolated from ground and directly fed. The result in either case is less gain. Therefore any loss introduced by the gamma is more then compensated for. On FM broadcast, on the tower testing has proven that .82 wave gives the most gain. Just slightly shorter then the 7/8 wave.

With respect to modeling this antenna with NEC, this has proven to be a complete waste of time. I've had at least a half dozen "RF engineers" over the years tell me that NEC shows the primary lobe at 45 degrees above the horizon. This couldn't be anymore inaccurate! If that were the case my "money back guarantee" would have put me out of business years ago. The FM version has replaced countless 5/8 wave Comets and in every case provided a larger coverage area. Didn't matter if the antenna was 30 feet AGL or 200 feet.

The biggest mechanical failure with the Vector 4000 results from screwing together the telescopic whip. Use a saw to slit the top of each section so that a hose clamp can tightly lock the sections together. Then install the bottom screw only in each section so they can't slide down. This will stop the top sections from breaking off. The RF components on the stock antenna leave a lot to be desired if you run power. The SO-239 is specifically designed with little space between the center conductor and ground ring so that it will arc under power. The stock gamma is no better.

Remove the original SO-239 and drill out the mounting hole so that it will accept a single hole Teflon insulated UHF connector. Solder an eye connector to the center pin so that you can bolt a Maco 5 KW match to the connector. Some have made comments about making the antenna handle 25 KW. You bet it can be done if you can weld a DIN connector to the original bracket and use a vacuum variable cap. I know of one person that is dumping the full output of a 4CX15,000A into one as I described.

When using this design on HF the Sigma definitely benefits from the use of a coaxial balun in the feed line. This can be as simple as a few loops of coax just below the feedpoint. When testing the antenna against the Interceptor 25K we clearly noticed an improvement in gain by no less then 1 db. When the loop is expanded to the 25 degree angle this goes up to almost 2 db over the 5/8 wave. The drawback was that an increase of unwanted RF interference was noted due to radiation taking place on the coax line. While I can't confirm that adding the balun increased signal any, it certainly reduced RF interference to the levels of the Interceptor.

As a side note, I've been working with Stefania at Sirio to improve the Vector 4000 for CB and 10 meter use. I've provided her with both my mechanical and electrical improvements and she has passed this along to her research and development team to see if they can economically implement my suggestions. Unfortunately the anechoic room at Sirio has been designed to model antennas operating at UHF or higher. According to Stefania it was installed at their facility to design 800 Mhz. and Wi-Fi antennas.

I hope this information has been helpful to those searching for the best omni directional antenna. While the Sigma IV design is often misunderstood or not even heard of, I can assure you beyond any doubt you cannot purchase a more effective ground plane. You can spend more money on an Interceptor but you can't buy more signal unless you install a beam. To the few that have claimed the Vector does not outperform the 5/8 wave, your tests absolutely contained other variables that effected your results in an adverse way.

 

[loosecannon]

excellent post shockwave!

welcome to the forum.
i hope you can find time to join us here more often.

what about isolating the sigma IV from its support pole?

when you were noticing increased RFI because of coax radiation, and you added the RF choke to the feedpoint; did you isolate the antenna from its support structure?
if not, do you think that the CMC's that you "choked" off of the feedline appeared on the metal mast?

LC