W8JI i agree with you %100 . there are several machinist that make some of the more popular big external coil CB antennas and some of their claims are on the wild side . i guess it just comes with the territory or 11 meter band . one even says NASA tested his antenna , LOL . and no , i don't think i know a lot about antennas . i know enough to have built a few that have worked pretty well . i think i know more than some .... and a heck of a lot less than some others . i don't think i'd want to know everything there is to know about antennas , i really enjoy learning about them . this is just a fun hobby for me , my worth as a human being isn't defined by how accurate my antenna is or how much i know about the details of my hobby . it doesn't bother me when i'm wrong and get corrected , it's just a part of the learning process .
if anyone is interested , here's a thread that really got me interested in building this particular antenna .
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,
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 "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!"
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'sHIGH 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.