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Author Topic: home-brew sigma 4 11 meter antenna  (Read 71388 times)
PA1ZP
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Posts: 236




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« Reply #15 on: June 23, 2011, 12:08:24 PM »

Hi Booty monster

I read your building with great interest

I used a sigma 4 for many years and I liked the antenna a lot.
I do think it has gain over a GP or a half wave because we tried different antennas at the same time in QSO's

And I must say even antenne moddeling in MMANA or 4NEC2 shows the difference between these antennas.

Now I have build the BIG BOOMER on 10 mtrs .
It is a portable colineair 2 x 5/8 wave for 10 mtrs.
And I know a beam is better in performance, just was going somewhere where no one has gone before.

And it has more gain then just 1 GP and the take of angle is seriously a lot lower then a GP.

If you like the experiment just build it and have fun.
That is what our hobby is meant to be .
Have fun and experiment.

73 Jos
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W8JI
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« Reply #16 on: June 27, 2011, 04:15:32 AM »


the DX contacts too . IMO the 1.5ish db gain a 5/8 has over a dipole is trivial ,



Why do you think a 5/8th wave has 1.5 dB gain over a 1/2 wave?? That gain is electrically impossible, unless the 1/2 wave has a design problem.

Quote
but where the lobes on a antenna put the signal on the horizon makes a great deal of difference . this antenna is essentially a scaled up to an 11 meter version of the Dominator antenna .

http://lpfmantenna.com/dominator.html

When the mast and feedline is included in the model, the "Dominator" falls apart. The exception to this is if we add a groundplane at the junction of the cone and the mast and coax.

Many antennas get modeled without proper representation of feedlines.

Quote
my experience tells me there definitely differences between 1/4 wgp's and 5/8 wgp's . where the antenna puts the dominate lobes will dictate how effective it is . as close to parallel to the horizon works best for me and thats my purpose in making this antenna . BTW i do use a air core coax choke on my antennas at the feed point when i put them up to prevent CMC's . 5 loops on a 4 1/4 inch former .

An air coil choke can just as easily make common mode worse as better. It is a reactance with fairly high Q, and as such the choking ability depends on the common mode impedance of the system at the point where it is inserted. You have a 50/50 chance of the choke making things better or worse.

Quote
i also think the imax is a poor antenna design that is very well known for bleeding and splattering across multiple channels .


So you are saying an antenna can cause "bleeding and splattering"? Exactly what is the non-linear mechanism in the antenna that does that? I wonder where statements like that come from, because unless the antenna has non-linear components in it...it cannot possibly cause intermodulation.

Quote
i experimented with shorter than 1/4 wavelength ground elements on my 5/8 adventures and im a firm believer that any elevated antenna should have 1/4 wave length ground elements ..... longer than 1/4wl may be better or worse , but i didn't go longer than 1/4 wl on mine . no ground elements is a definite no-no IMO , but we all know what opinions can be like Smiley Smiley

That makes sense. The radials serve to decouple common mode from the mast and coax.

The way a 5/8th element makes gain is not the magical mystical thing most Hams and nearly all CB'ers think. The 5/8th produces gain only because the current maximum is .375 wavelength above a reflective surface. To be a reflective surface, it has to be virtually solid and extend out for several wavelengths. This then forms an "image antenna" that has a current maxima 3/8th waves below the reflective plane. The result is two in-phase current maximas spaced 3/4 wave apart, a pseudo-collinear antenna.

A 1/2 wave vertical over the same infinite groundplane would have 1/2 wave spacing between current maximas, and thus has about 1 dB less collinear gain. This is not new science, and was well know many years before antenna modeling made everyone an expert designer.

Maximum possible gain from the spacing of current maximums is show in this graph:

http://www.w8ji.com/images/Antenna/Collinear.jpg

With a perfect vertical half wave over perfect lossless infinite groundplane the current maximums are separated 1/2 wave between the imaginary image antenna and real physical antenna. That is just barely under 2 dB collinear gain.

The 5/8th wave would separate current maximums by 3/4 wave distance, and gain would be just exactly 3 dB *if the 1/8th wave section with out-of-phase currents did not exist**.

The result is, in a perfect world, just barely over 1 dB difference. Since the 5/8th wave carries out-of-phase currents in the lower 1/8th wave area, gain difference will always be less. The only exception is if we build a bad 1/2 wave reference.

Maximum possible gain is shown here:
http://www.w8ji.com/stacking_broadside_collinear.htm

Now the problem that prevents the 5/8th from reaching the maximum theoretical gain of about 1 dB over a 1/2 wave is the small out-of-phase section that is 1/8th wave long. That section "fights" the low angle radiation, and enhances higher angle radiation.

Where the nonsense comes in is where people think without that infinite groundplane, the 5/8th has gain. Without that infinite perfect flat groundplane sticking out several wavelengths, there is no collinear gain. A properly designed half wave has more low angle field strength.

The real problem with all of this is people build crummy unpredictable 1/2 waves, and then compare the crummy unpredictable half-waves that do not work nearly as well as possible to 5/8th wave antennas. It becomes a matter of pure dumb luck which works better, because not only are the antenna unpredictable but the test methods and physical installations are just as poorly understood.

With so many random things going on, it all becomes a matter of emotions selecting what the cult wants to follow as fact.

73 Tom



 

« Last Edit: June 27, 2011, 04:54:36 AM by W8JI » Logged
BOOTYMONSTER
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Posts: 73




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« Reply #17 on: June 27, 2011, 11:07:18 AM »

for my particular situation its a moot point , i can't guy here . i can do about 28-29 ft max feed-point height here for a permanent install , or maybe 19-20 ft for something i can raise and lower to play with . a starduster would limit my tip height to 29 - 38 ft . a 5/8 about 44 1/2 - 51 1/2 ft tip heights ..... add six inches for the all mighty .64wl . a .82 will be at 49 1/2 - 58 1/2 ft tip height . for my modest feedpoint height limitations the longer antenna allows me to get the antennas tip over 20- ft higher than i could with a 1/4wgp .


now ......... keeping with what WB6BYU had to say . if TOA isn't really an advantage and tip height is all that matters .......
since there are numerous ways to make almost anything tune to make a meter reasonably happy .... why not just go to a 36 ft or 54 or even a 72 ft vertical ?
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W8JI
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« Reply #18 on: June 27, 2011, 01:02:40 PM »

for my particular situation its a moot point , i can't guy here . i can do about 28-29 ft max feed-point height here for a permanent install , or maybe 19-20 ft for something i can raise and lower to play with . a starduster would limit my tip height to 29 - 38 ft . a 5/8 about 44 1/2 - 51 1/2 ft tip heights ..... add six inches for the all mighty .64wl . a .82 will be at 49 1/2 - 58 1/2 ft tip height . for my modest feedpoint height limitations the longer antenna allows me to get the antennas tip over 20- ft higher than i could with a 1/4wgp .


now ......... keeping with what WB6BYU had to say . if TOA isn't really an advantage and tip height is all that matters .......
since there are numerous ways to make almost anything tune to make a meter reasonably happy .... why not just go to a 36 ft or 54 or even a 72 ft vertical ?

I don't know why anyone would focus on tip height. Any antenna has a series of elevation nulls.

There are two things that determine the position of nulls.

One is where the various current peaks in the antenna occur, and this includes accidental or unintentional currents on the mast and the outside of the coax that everyone pretends don't exist, and the phase relationship of those current peaks.

The second effect is a multiplier by ground reflections that varies in angle of re-enforcement with height of the current maximums.

Together those things form the pattern, and where nulls appear.  Radiation then occurs only where there are no nulls being forced, the result being a multiplication of the two effects.

Another factor, feedline and mast coupling, is virtually ignored. It is virtually impossible to fully decouple a feedline without having radials at the element base.

The bottom line is this. Aside from doing something wrong to reduce gain, there is no major difference between a 1/4 groundplane with sloped radials, a 5/8th wave vertical, or a 1/2 wave vertical when all are at the same mean height for current maximums. It is where the current maximum is that matters, not the tip. 

73 Tom
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K9FON
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Posts: 1012




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« Reply #19 on: June 28, 2011, 12:00:22 PM »

Antennas DO NOT cause intermod or "bleedover" if you will. Hacked radios cause harmonics. I never can understand why CB ops always say this antenna bleeds and that antenna doesnt. Just one mans opinion...
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PA1ZP
Member

Posts: 236




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« Reply #20 on: June 29, 2011, 09:31:20 AM »

Hi All

As ever Tom is right.

Instead of horizontally stacking you can do it vertically.
Ofcourse it is very difficult to do so on 80 mtrs or on 40 mtrs.

It might we will try 2 colineairs on 10 mtr in end firing and broadside firing.
But that is only because we are bit loony and love big vertical antenna's
That is why we are building a 4-SQUARE for 20 mtrs with half wave verticals.
We can always upgrade the antennas and 4-square  to 40 mtr GP's. 

At the same feedpoint hight a 1/2 or a 5/8 wave will show some gain because of its higher current point and it will radiate at little lower take of angle if the hight is correct.

As Tom explains.
If your vertical is at wrong hight influence of ground will force it to create a higher radiation angle.
If your vertical is to long without proper fasing between the sections it will also create a high radiation angle.
I know as my 1/4 wave fer 80 is a 1 1/4 wave fer 18 MHz and it doesn't work good on 18 MHz at all though SWR is fine.
My 1/4 GP fer 20 works much better on 18 MHz though its SWR is not good at all

And if you have a Sigma-4 vertical it is very easy to rebuild as a 1/4 wave GP for 40 mtrs.
That is what I did several yrs ago
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N0SYA
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Posts: 369




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« Reply #21 on: June 30, 2011, 09:57:28 AM »

Getcha more bird watts if you only use multiples of 9 feet for your coax, kinda like engines make more pawr if you let the pistons smack the heads a lil. Then you can play on da bowl!


http://www.youtube.com/watch?v=RGPLz2nX0GM
http://www.youtube.com/watch?v=21iItd5opGs&feature=related
http://www.youtube.com/watch?v=c-I0GTLTCNw
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If you have a clumsy child, you make them wear a helmet. If you have death prone children, you keep a few clones of them in your lab.
NO6L
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Posts: 179




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« Reply #22 on: July 29, 2011, 04:33:37 AM »

Technically, I could never understand the CBers fastenation with the old Sygma 4 "3/4 wave" antenna. They're nothing more than a J-Pole. And, regardless what advertising hype or rumor says, they don't have any more or less gain than any other 1/2 wave antenna. No, the bottom 1/4 wave does not contribute anything, except to match the 50Ω feedline to an end fed 1/2 wave vertical, also known as an End Fed Zepp.

Visually, now that's another story; They're quirky looking, big and defy explanation by the layman. The perfect CB antenna.

Their one saving grace is really no advantage, the current node is  1/4 higher than a less quirky looking and physically smaller 1/2 wave vertical. Why not just add 1/4 wave of structure under the 1/2 wave and call it a day? It's cheaper and more structurally sound.
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NO6L
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Posts: 179




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« Reply #23 on: July 29, 2011, 05:11:10 AM »

Antennas DO NOT cause intermod or "bleedover" if you will. Hacked radios cause harmonics. I never can understand why CB ops always say this antenna bleeds and that antenna doesnt. Just one mans opinion...

I can explain it, knowledge and common sense. Most CBers are not aware of the problems caused by improper feedline decoupling, corrosion induced "mixing diodes", etc. What happens is our intrepid CB type finally gets around to replacing an older antenna with a fancy-shmancy "Bushwacker Special II" and his problems are history, when all he really needed to do was clean up the the mess he had on the end of the mast in the first place.

Also, J-Poles, Sygma 4s and end fed Zepps, which are all the same thing have one thing in common besides theortical gain, epic level feedline radiation if they are not properly decoupled, which a Sygma 4, and latter day clones, never are.

By the way, I know you were only being modest, that was fact, not opinion.
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NO6L
Member

Posts: 179




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« Reply #24 on: July 29, 2011, 05:30:42 AM »

LOL , i didn't build it to make you happy .
got any articles about antennas you have built to share ?

I do, and yours stinks. You can remedy it by:

1. Instead of just hitting the keys close to the ones you intended, actually have pride in what you write.

2. Use a word processor to assist instead of direct entery into the browser.

3. Use an outline so you can get your of discussion in the right order ahead of time.

4. Actually have in depth knowledge in the subject being presented.

5. Choose the proper audience to present your article to.

Sir, you have not met even one of these basic rules for writing a quality paper/artical. Therefore, you are not qualified to pass judgement on others regarding the same.
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BOOTYMONSTER
Member

Posts: 73




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« Reply #25 on: July 31, 2011, 09:49:48 AM »

many thanks to W8JI and the others that have made on topic and helpful comments .

to the the few that just cant get over me being just a CB'er or thinking i (or anyone else) need or want their approval of a forum post .... get a life , learn not to be so self righteous and generally feel free to go have sex with yourself ......... and learn how to make a post that's on topic !!!!
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AD6KA
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Posts: 2237




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« Reply #26 on: August 01, 2011, 03:43:34 PM »

Ya know, whatever criticisms people have about this antenna's performance,
claimed performance, theory behind it, or whatever, I  really have to give kudos to a guy
who actually got off his butt and built it himself. I bet he learned a lot in the process.
Ken  AD6KA
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W8JI
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Posts: 9296


WWW

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« Reply #27 on: August 02, 2011, 02:03:29 PM »

Ya know, whatever criticisms people have about this antenna's performance,
claimed performance, theory behind it, or whatever, I  really have to give kudos to a guy
who actually got off his butt and built it himself. I bet he learned a lot in the process.
Ken  AD6KA

On the other hand, sometimes people learn wrong things when building antennas. There are entire careers, manufacturing operations, doctorate thesis, and patents based on totally incorrect nonsense theories and antennas that work nothing at all like claimed.

I'd say the first several dozen antennas I built, I didn't learn a thing about how they really work. To see an extreme of not understanding electromagnetics and how antennas work, look up the CFA and EH antennas.

73 Tom

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BOOTYMONSTER
Member

Posts: 73




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« Reply #28 on: August 23, 2011, 10:42:50 AM »

thanks AD6KA Smiley
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 .

http://www.worldwidedx.com/cb-antennas/31799-avanti-sigma4-alternative-view-point.html

Quote from: bob85;126263
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 "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'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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BOOTYMONSTER
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Posts: 73




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« Reply #29 on: November 14, 2011, 02:20:58 PM »

its up in the air . no coax on it , im beat , my back is aching , ill get it tuned and on the air tomorrow .
i did make a few changes to the antenna , ill post pics of those tomorrow also .



http://i867.photobucket.com/albums/ab234/BOOTY-MONSTER/IMG_0158-1.jpg
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