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Avanti Sigma4: An alternative view point

Discussion in 'CB Antennas' started by bob85, Jan 3, 2009.

  1. bob85

    bob85 Supporting Member

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    do a search on the sigma4 and you will find plenty of people claiming that the sigma is a simple jpole and as such does not radiate from its lower 1/4wave,
    most of that has come from one source my old buddy MC who claimed it was a jpole and spoke to people who once worked for avanti and also claimed it was basically a jpole, mc claims his sigma2 outperformed his sigma4,

    in the absence of anything new from MC and the plethora of "its just a jpole" posts around the forums i feel its time for an alternative viewpoint,

    the claim is that because you have currents flowing in the lower portion of the radiator in opposite phase to currents flowing in the radials/sleeve or whatever you choose to call the basket surrounding the lower 1/4wave of radiator you have cancellation and no radiation, they assert that the lower 1/4wave is merely a means to feed that portion of the radiator above the basket, they also claim that the antenna is inferior to a 5/8 groundplane,

    the jpole idea has been posted on several forums, many people have taken that as gospel without doing tests for themself and posted it elswhere on the www,
    anybody interested will see i have always claimed that the avanti and its clones can outperform any single element groundplane, this is based on my own and other peoples experiments not something i read on any forum or book,
    i have believed the design was the best performer with the possible exception of the ham international big mac a 5/8 over 1/4wave colinear ever since the sigma4 first came out and i replaced my sigma2 with the sigma4,
    i have stated over and over that theres more to it than just following the owners manual and obtaining a low vswr, again this is based on my own experiments, i claimed this from day one before i ever spoke to any antenna guys about the sigma design,
    i started reading and talking about the sigma design because i have built jpoles for 27mhz and i do not agree with the nonesense i see posted about the sigma4, i am far from done with it or as bored as some of you guys probably are with the whole sigma4 topic,

    theres more to endfed antennas than you will find in any antenna owners manual for any endfed antenna i ever owned and its not cb hocus- pocus

    now there has to be a reason why i see what i do and a few people claim they dont,
    i have claimed that if you dont see better performance than a groundplane you are not tuning it correctly or you never did a proper test because your mind is set its just a jpole and thats that,
    i stated that the adjustments i make while tuning that style of antenna yield much more difference in distant signal strength than you see when tuning any conventional antenna and postulated that i must somehow be manipulating the angle of radiation as i adjust the relative element lengths and gamma tap/settings,
    i also stated that the 4 radial basket outperforms the 3 radial avanti setup and that the extended near 7/8 physical length version outperforms the 3/4wave for me and my friends that have made the same adjustments,

    i still have not got to the bottom of it yet, or maybe i have,

    you decide,

    i can prove the claim about opposite currents = cancellation of radiation is untrue in some cases,
    this has been known for a long time and published in the arrl antenna books for years, its called the skeleton sleeve fed monopole, which just happens to be a 3/4wave monopole with a 1/4wave sleeve that has opposing currents in the lower 1/4wave,
    not only does the lower 1/4wave radiate it does so in phase with the radiation from the upper 1/2wave and adds constructively giving an increase in gain and improved pattern than would be obtained with a simple 3/4wave monopole,

    heres an article on how to build one for 2mtrs, http://k6mhe.com/files/ssfm.pdf

    and a quote from the arrl antenna book 18th edition page 186-190, which gives a pretty detailed explanation of how the antenna works,



    "At length ratios approaching 3:1, the antenna mode and transmission line mode impedance become
    nearly equal again, and the central monopole again carries a significant portion of the antenna current. The
    radiation from the top 1/2 l combines constructively with the radiation from the 1/4-l sleeve elements to
    produce gains of up to 3 dB more than just a quarter-wave vertical element alone.
    Length ratios in excess of 3.2:1 produce higher level sidelobes and less gain on the horizon, except for
    narrow spots near the even ratios of 4:1, 6:1, 8:1, etc. These are where the central monopole is an even
    multiple of a halfwave, and the antenna mode impedance is too high to allow much antenna mode current"

    sleeve to monopole spacing effects impedance as does element diameter ratio, the closer spacing of the sleeve in the sigma design lowers antenna impedance thus we cannot direct feed the central monopole as in the skeleton sleeve antenna because the sleeve spacing is too small,
    i dont see a problem with that because we use a gamma to feed the antenna tapped up the monopole where we find a suitable feed point impedance,
    while some people talk about lossy feed systems ( mainly resistive ) others claim the gamma has the lowest operating q and is the most efficient matching method when it can be used and is correctly proportioned,
    Omega and Gama Matching

    notice how w8ji claims "[FONT=Times New Roman, Times]Note that bandwidth increased while efficiency increased. This happens in many cases. Popular folklore tells us narrow antennas are efficient, but that is true only in a few specific cases. In most cases, bandwidth by itself tells us nothing about system efficiency!"
    [/FONT]which fits my claim that i see best performance when i have the widest bandwidth, that bandwidth is wider than my groundplanes,

    avanti claim that the sigma4 produces 1db gain over a 5/8wave groundplane in its current configuration with more gain available if the angle between vertical monopole and radials is increased to around 30 degrees, they also state that the central monopole can be extended in increments,
    avanti acknowledge that in regular antennas the 5/8 wave provides the most gain and that a 3/4wave provides less usefull gain, but they go on to make all the other claims of the sigmas superiority,
    remember that avanti also made what many consider to be the best 5/8wave groundplane at that time,

    consider that on the box it stated "highest gain base antenna available", "whole antenna radiates"
    those claims dont fit a jpole but they could fit a skeleton sleeve monopole or a modified form thereof, especially one with a modified sleeve tilted towards the horizon,

    heres the avanti patent, is it all advertising hype as has been claimed or is it true?
    United States Patent: 4282531


    i am not alone in my claims, others think the same or similar to me and each of them i believe have more oil in their lamp than an average person,

    jack ( freecell ) claims the lower portion radiates and that the design is the highest gain vertical cb antenna ever made,
    he did not dispell my modified skeleton sleeve idea, he said maybe,
    we do agree it is the best performer out there,

    eddy ( wolf ) claims the sigma 4 is a form of phased array, he has built them for other frequencies and claims they out perform any groundplane including his .64,

    eddy ( marconi ) after reading the evidence and owning a sigma4 a wolf .64 and a i10k amongst his other antennas thinks theres more to it than meets the eye too,

    lb cebik ( rip ) told me there was more going on with that design than is immediately apparent to most people,
    i fully explained what i was doing and what the results were,
    he said that in the real world of earth masts and feedlines as opposed to freespace models, the radial angle makes little practical difference to the angle of radiation, that is untill we fold the radials up towards the radiator,
    then a none aparent colinear array is possiblle that will outperform any conventional groundplane,
    he said my test results were very much possible but was reluctant to get involved with the pages of meaningless arguments that such misunderstood antennas can generate,

    then we have the people making antennas for fm broadcast, they claim the design will outperform any groundplane, they say it will put more signal on the horizon or your money back, they also make 5/8 waves,
    fm broadcast is one place where bullshit wont fly,
    their game is reaching their audience, thats a lot of people demanding a refund when they change to a sigma design and listeners are bombarding the studio with calls about crappy reception,
    FMBroadcastAntenna.com | FM Broadcast Antenna's

    HIGH GAIN COLINEAR ALUMINUM ¾ Wave EXTERMINATOR®

    this is my current position on the avanti sigma4 style antennas,
    untill somebody shows me an alternative i am of the opinion that herbert r blaese the named inventor on the avanti patent took the basic idea of the skeleton sleeeve monopole and modified it into a sturdy and easy to manufacture antenna of compact radial dimensions and superior performance, i also believe that the claims clearly outlined in the patent are true and can be demonstrated in field tests.
     
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    Last edited: Jan 4, 2009
    2 people like this.
  2. DIRTYWHITEBOY

    DIRTYWHITEBOY Active Member

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    Great post Bob,got sleepy reading all that:)
     
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  3. mackmobile43

    mackmobile43 Jock Supporter

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    I have recorded this as a book on tape for anyone who would like to purchase a copy:D

    GOOD reading bob.
     
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  4. rfoverlord

    rfoverlord guardian of freedom

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    couldn't get the patent info to come up, but excellent post Bob. something worth actually reading, instead of skimming.
    works good now thanks Bob!
     
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    Last edited: Jan 23, 2009
  5. bob85

    bob85 Supporting Member

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    i edited the link, see how that works, click on images for the full patent;)
     
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  6. Master Chief

    Master Chief Guest

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    I read up to "you decide", then stopped because I have to go. Let me just say that everything I've read from Bob so far is 100% correct! He has always maintained his claims.

    I do want to be clear on one item. My stock Avanti 5/8 DID outperform my stock Sigma IV. This was based on personal experience way back in the 80s......before I even knew what a j-pole was.

    I can't wait to read "the rest of the story"! I have always loved this antenna!
     
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  7. bob85

    bob85 Supporting Member

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    story lmao.
    mc, i dont doubt you at all about your sigma 2 vs 4, i dont think you would make something like that up and run with it for 4 years:D

    i half expected jack jumping in and giving me a public thrashing, maybe he's prepairing his case:eek:
     
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  8. MisterFatty

    MisterFatty Poppin Fresh!

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    Interesting Read Bob, Thanks for sharing (y)



    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.
     
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  9. Master Chief

    Master Chief Guest

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    Lets get testing! Your opinions are solid and definitely have merit! Your own personal experience is the icing on the cake!

    Its too bad L.B. didn't want to look into this further as he would have had the last word on this subject.

    I was looking at buying the LW-150 from H&Y and building up one of my original IV's. Publish your tubing diameters and lengths so we can duplicate the antennas.

    I'd still like to see a true model of this antenna. As I get better at it I plan on trying to model it. WIRES from K6STI would be the right program to use, but it is unavailable (although I've heard rumors that a professional version can be bought for $$$).

    If this antenna is a true colinear, I would think that a multi-element colinear (like the Hustler G7 series or the Super Station Master) would out perform the FM versions. Also, a multi-bay folded diple array has more gain than the 6.3 claimed by the seller.

    In the end, the proof is in the pudding! Build them, test them, and report the conclusions.

    You will have to share your tuning techniques on your Big Mac. I know you have touched on it, but I would like to hear them in detail. Send a PM if you want to keep that more or less confidential.

    Nice job Bob!
     
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  10. bob85

    bob85 Supporting Member

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    mc im looking locally for a place that can turn down a piece of pole for the bottom 1/4wave, the next sigma style i build has to be as physically strong as the i10k or stronger,
    after running the 10k for 4 years without the slightest worry or bit of trouble im done with weak antennas, i aint going back to fixing antennas everytime we get strong winds,
    the next one will be strong i hope i dont spoil the performance,

    you wont be at all happy with the vector 4000 construction quality, they do work great untill somebody in the next town farts and the top 3 or 4 sections fall off, get one all the same, some usa folk claim siro is better made than maco in both beams and verticals, id like your opinion on that as sirio are shit quality imho, i cant believe maco could be even worse.


    a fatter high wattage gamma would be nice so long as the spacing does not upset the mix,
    sigma10k or 20k yes please, wheres my vacume variable;)
     
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  11. BobinFla

    BobinFla Mud-Duck

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    I love all the info here, its very educational. Now I hope someone here can help me with some measurements. I am trying to resurrect 1 of 2 old Sigma 4 antennas I had out back. 1 was an original with the 3/8" aluminum tube for the ring while the other had flat aluminum bar. The tube ring was destroyed in a hurricane that hit herein Florida some 10 years or so back, I just stored the remaining antenna parts after that. I came across the Flat bar ringed Sigma a few years back (guy was taking his stuff down and gonna toss the antenna). I telescoped the poles into each other and left the ring alone, got it home and never did anything with it after that. Now I want it up and talking. I lengthened it yesterday but it loks so much longer than I remember my original Sigma looking. I believe my son has gotten ahold of my meaqsuring tape so I cannot measure it until I get another tomorrow.

    My question for you guys is this, what are the specs on the Sigma 4, primarily the length? Would the specs for the tube ringed antenna be the same for the one with the flat bar?

    I will be using this on the CB freqs (primarily channels 14 & 19).

    Mucho Gracias for the help up front.
     
    #11
    Last edited: Jan 10, 2009
  12. Peddler

    Peddler SWR (sandwich with rum) Expert

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    #12
  13. this is a great thread ! ive really been geeking out on antennas for a while and theres so much good info in here and from folks that consistantly make sense , IMO . but im an antenna building/theory duck egg , LOL .
    ive got a few questions on the skeleton sleeve antenna Bob85 linked to , but ill start a new thread to ask it because im really enjoying the way this one is going and i dont want to side track it . im actually more interested in the coaxal stub fed monopole antenna in fig. 1 of the pdf. . but i did want to thank yall for the informative and inspiring comments .
    get ready for more PMs Bob ! ;)
     
    #13
    Last edited by a moderator: Jan 10, 2009
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  14. BobinFla

    BobinFla Mud-Duck

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    #14
  15. Master Chief

    Master Chief Guest

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    Careful, you are giving away secrets now! I know a number of people who replaced their gammas with vacuum variables. A gamma is just a capacitor after-all.

    I have a 20K (its just an I-10K with a 7/16 DIN). I also modified the main element tubing sizes and diameters, but I'm not telling how!:love:
     
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