heres a post i made on cbjunk when MC jumped ship explaining how i came down this road of thinking.
Re: avanti sigma4 an alternative view
by bob85 » Wed Feb 18, 2009 4:53 am
thanks for the comments guys, the sigma4 is not an antenna you can look up in a book like other commonly used antennas, consequently as cebik said it is very much misunderstood,
you may wonder how i came to my conclusions,
well it all started when i first posted about the sigma on wwrf, discussion/arguments ensued,
MC put forward a compelling argument backed by what an avanti engineer told him, it all sounded very plausible apart from one problem,
i had built jpoles for 27mhz and the sigma did not perform like a jpole at all for me, i had a gut feeling that there must be another explanation some addition from the basket that overcame the traditional problems of pattern breakup with radiators longer than .64wave but the only facts i had was the fact i knew what i and others had observed,
comming to terms with that been an illusion was not on the agenda just yet, not until i had done some research,
since then me and mc have taken almost every opportunity to have a poke at each other whenever the sigma4 gets a mention 
,
we had something that a few people were intersted in but we did not agree on how it worked or how well it worked in practice,
i knew what i had been seeing for years but i wanted to know why some folk did not, i was pretty sure avanti had not invented something that nobody else had ever seen before, it must work on known principles,
i looked at the other things lou and herb had designed for clues and found none, lou was a kind of inventor, his field of invention was not restricted to antennas,
after the usual trawling the net / antenna books and finding nothing that seemed applicable other than an idea that if it did outperform other antennas it could not be any single element antenna i could find so in my mind the whole antenna must radiate just like it said on the box,
as unusual as it may seem for cb antennas i actually believed what it claimed and what the patent claimed but had no real idea how that could be, what they claimed fit what i observed but why?,
the patent seemed long winded for a tall story and it was herbs name on the patent not ampowers,
the sigma was not the only antenna avanti made that had people baffled, the astroplane does the same thing but more on that one later,
avanti thought out of the box, not following convention,
back then only one person agreed with me that i knew of, he claimed the lower portion radiated but getting anything else from jack ( freecell ) is hard work or should i say he likes to make you work for your information,
he did say one thing, that i should not give up and throw years of experience in the bin,
after corresponding with cebik and him not falling out of his chair laughing but on the contrary said it was possible and you can make a none apparent colinear array i wanted to know more,
i could not find anything about none apparent colinears,
what did he mean "its not apparent to most people" and usually generates lots of arguments,
hell yeah, he was right about the arguments that bit was easy to understand, what about the rest,
i started searching different ideas like "radials radiate in phase with vertical radiator" etc and eventually came across the open sleeve antenna or skeleton sleeve fed monopole,
holycow batman, this antenna has all the ingredients im looking for,
it can be 3/4wave or slightly longer,
it has radials folded up beside the radiator like cebik said,
the radials radiate in phase with the upper portion of the radiator in a colinear effect improving gain like cebic said,
it could operate in two modes ( antenna mode and transmissionline mode ) i figured if id never heared of two mode antennas then it was probably not apparent to most other people like cebik said,
some referances to the skeleton sleeve antenna call it a form of jpole like mc's avanti engineer said,
a form of jpole with a twist as mc said it may be ( after some poking ) lol,
i punted the idea around a few people with no negative comebacks and one positve then pondered on it while still looking for alternatives, so far i have found none that seem to fit as closely as a modified form of open sleeve antenna,
it ticks all the boxes imho,
cebik told me that the sigma would not be easy to model and get accurate results, i tried and got kind of lost in eznec4,
sadly by the time i had gained enough confidence in the idea to contact cebik and ask him "am i right in thinking the sigma style antennas operate on the principles of open sleeve antennas" hed gone silent key 
,
thanks to all who have been involved in the sigma debate over the last 5 years or so 
both on the forums and in pm/email,
i suggest you do your own tests and decide for yourself the validity of my conclusion, let us hope we have it figured out within the next 5 years 
