Avanti Sigma4: An alternative view point

[chale]

if you have room to put one together and raise it then you have room to build it . you can also build one much stronger than what you can buy . here's two examples of making one .

http://www.worldwidedx.com/cb-antennas/104894-homers-qv4k-2.html

http://www.worldwidedx.com/home-brew-mods/102961-booty-4-a.html

if you haven't read this thread about modifying/tweaking them for peak performance id highly recommended it

http://www.worldwidedx.com/cb-antennas/36412-modified-vector-4000-a.html

i have read all of those and am quite familiar with Homer's work and yours...and NO i really don't have a ton of room to work with, band saw, metal working tools etc currently in storage i have my yard to erect it and tune it so.. again i have the room to erect it and tune it but do not currently have the basement space for fabrication

this isn't my first rodeo with antennas...i would just like the original question answered

 

[BOOTY MONSTER]

i understand what you mean . i only have a drill/bits and a skill saw with metal cutting blade along with some pliers , vise grips and socket/wrench set along with a pocket square .
it's a challenge :headbang but i enjoy it

 

[HomerBB]

I think my brain is leaking out of my ears after trying to absorb all of this

I definitely want to play with one of these antennas, so here is my question, all things considered..which one should i buy and play with? i guess which one will give me the largest room for adjustment? i've thought about "rolling my own" so-to speak which i sure have the ABILITY to do, but currently don't have the ROOM/SPACE to do so what i'm thinking is buying either the LW or V4K and then modifying it

I have never purchased either, so I can only speculate as to which is the one to buy.
based on what you'd like to do, I'd say get the one with the longest tubing on it so you have the maximum to play with.
I'd also say be sure it has all four radials on it. Perhaps a 7/8 wave one would be the place to start. More flexibility in the amount of materials.

 

[chale]

I have never purchased either, so I can only I'd also say be sure it has all four radials on it. Perhaps a 7/8 wave one would be the place to start. More flexibility in the amount of materials.

Ok the V4000 has 4 radials but there are no overall lengths given in the manual other than the approximate lentgh of the main...all other measurements are given off of a set point such as a joint, etc

OAL for the V4K is 8480mm or 27.82'
OAL for the LW150 is at 31.17' but has 3 radials

i'm looking for a little SPECIFIC guidance here not general references...one would think with all the knowledge here someone could answer a simple question

 

[BOOTY MONSTER]

here's the simplest answer/suggestion i can give . if you're NOT in a area that gets hurricanes , tornadoes or heavy snow/ice id get the vector 4000 since it already has 4 basket elements and they are longer @106 inches . and id get rid of the loop/ball on the top and add tubing to get up to 29-30 feet on the length on the vertical so you can find out what length works best for you .

if you live in a area with rougher weather id suggest home brewing a stronger version or getting an original sigma 4 and extending both the vertical element and basket elements . you could also try adding a 4th basket element but you'll have to redo the bottom bracket for the proper spacing of the four .

having some patience is a good thing on any mods done to it , take the time to do it right .

 

[Marconi]

Bob posted the actual measurements for the New Top One below. You can check the manual for the Antenna Specialists Sigma IV on CbTricks website and get the actual dimensions and measurements for the original model.

IMO, the older models for the LW150 and the Vector 4000 didn't have very good instructions.

 

[chale]

thanks guys, exactly the info i was looking for, i'm on the east coast so we CAN get crazy weather but it's not a "regular" occurrence...but neither is 20 tornadoes in April like this year...i was leaning towards the V4K based on the info provided here, but to be honest there are SO many threads and SO much info at this point on these 3 antennas it is overload trying to catch up with it

i THINK there might be a local with a Sigma 4 NIB (i've heard rumors) and if that doesn't work i have a good idea on what to do off the bat with the 4000...THANKS

 

[BOOTY MONSTER]

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"

hey bob , that is from your first post starting this thread .
this line in particular caught my attention .

"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"

any thoughts on what might be expected from this style antenna with a full wavelength center driver ?

 

[Marconi]

BM, here are my thoughts on your question to Bob.

How long is a length ratio in excess of 3.2:1 for a radiator that is supposed to be close to resonant for the CB center frequency of 27.205 mhz? I estimate it to be about 28.936' feet, based on a 3/4 wavelength being about 27.1275' feet. BTW, the radiator length for my Eznec5 model for my Sigma4 ends up at 28.6' feet long, where the value for X = -J 0.1293 ohms. This will vary a bit due to diameter, but............

If this is so, what does that suggest about radiators at 27.205 mhz that are longer even up to a wavelength as you suggest?

I don't know how much less gain on the horizon will be realized if we exceed the 3.2:1 ratio, so maybe the resultant problem is insignificant and then I might wonder why raise the issue at all. When I read or hear folks make claims with such limiting conditions attached, I will respectfully ask, "...how much loss or what is the magnitude of this limiting affect?"

I wonder if the lost gain noted in the article is significant enough so as one can tell from operating our radio, or is it so small that it takes a Spectrum Analyzer on a certified Antenna Test Range to tell the difference?

If not, then how else do we explain the fact that both Bob and Shockwave noted improved gains, over longer distances? I think they both used lengths that exceeded this limiting ratio (3.2:1) by what seems to me to be considerable amount?

I recently got a 72" piece of 1/2" aluminum that is about twice as long as the tip for my Sigma4. I think now I can make my antenna at least 3' longer and see what happens.

On the models below, I added 36" length to the tip of my Eznec5 model for my Sigma4, and it increased the mis-match by about 50%. Even so, I think the gamma should take care of that easily. As far as gain is concerned, it dropped very little and the angle did not change at all. I did however, notice that the currents flowing on the mast increased noticeably, and that may be a problem to consider. I see little to no advantage in adding length to the Sigma4...based on this model, but I hope to test the idea in a real world test real soon.

I also noticed little to no disadvantage in making the antenna shorter either. I did not change the length of the radials however, like Sirio has done with their New Vector 4000. I did not analyze the changes in match here, so maybe that is why Sirio made the radials longer as they made the radiator shorter. Personally, I think the only reason Sirio did this was to try and overcome some of the problems with ice and wind on this very tall radiator while not sacrificing any performance.

How say you?

Notice how the current increased on the image with the longer top element.
View attachment Current differences when making S4 longer..pdf

 

[BOOTY MONSTER]

thanks for the comments OGP . maybe my reading comprehension skills are lacking ........... but this comment seems to suggest there may still be some more performance to be had from the antenna at lengths not tested yet (that i know of) . a 36 ft vertical would be a tall SOB for sure but definitely doable .

" .............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"

i need to get my crap up and start playing with it

 

[HomerBB]

i need to get my crap up and start playing with it

Uh huh

 

[Shockwave]

Save your time with the 36 foot full wave Sigma. Once you go past about .82 wavelength, you'll have to add a lot more length before you start seeing reasonable gain again. At 36 feet the top would be a 3/4 wave vertical. They have big 45 degree lobes at the expense of power aimed at the horizon.

 

[MrSuburban]

I did however, notice that the currents flowing on the mast increased noticeably, and that may be a problem to consider. I see little to no advantage in adding length to the Sigma4...based on this model, but I hope to test the idea in a real world test real soon.

why not just model the antenna instead of adding a mast and coax to the equation. Unless you are trying to model your particular setup.

 

[BOOTY MONSTER]

thank you very much shockwave . i guess given the length of the antennas you make if going to 1.5 or even twice the wavelength had a positive effects that you could easily do that and have a better antenna . that's a cool benefit of that frequency . im gonna play around between 27 and 29 1/2 -30 feet , 28 seems like a good starting point .

 

[Marconi]

Well guys here is Ole Grampa doing his thing with modeling again. I used my Sigma 4 model to compare it to the same model with the radiator extended out to 36' feet. I then generated a pattern and captured a trace of the pattern to overlay with the default model Sigma 4 model that I published earlier here on the forum.

Hopefully you will see that what happens is exactly what Shockwave tells us happens when you extend the Vector radiator out longer to a full wavelength.

YouTube - ‪Booty Monster's question re: a full wavelength Vector 4000‬‏