Avanti Sigma4: An alternative view point

[BOOTY MONSTER]

been searching the www and found some folks having problems with their sigma 4's bleeding on other electronics . i know this can be caused by improper installation of the antenna , cheap/poorly shielded/consrtucted electronics , overdriven amps and overmodulated radios as well as simple proximity .

is this antenna more inclined to bleed compared to other antennas ? is there a prefered way to install them ? minium height ? insulation from mast and ground ? i know any antenna can be made to bleed , just wanna know if theres anything special that can be done in the installation to help prevent that , dont wanna put it together and up more than once .

 

[bob85]

shockwave,
any news on your progress with the larger hoop on the 11mtr vector?,

i am looking for 6061-t6 drawn tube in the uk, looks like its going to cost 180-200 dollars shipped for a section of 1-58"x.058 plus four 3/8x .065" upper radial sections,
if i am going to end up shorter than 90" on the radials with a larger hoop i dont need the stronger upper radial sections, i can use radial parts from the 4 or 5 broken vectors i have in the garrage at least as a temporary test setup,
using four 60" avanti lower radial sections allows me to experiment with a longer than 90" radial using the stock 30"hoop or no hoop

i am undecided if i should go with a 2" bottom section with the 1-5/8" second section shortening the sigma4 sections to create a very strong antenna with near zero flex in the gamma area or go with double skinned 1-5/8" x .058" for the bottom section,
i guess i will have to make both bottom sections and test them to see if altering the diameter ratios hurts performance,

a while back i made a 6mtr vector 4000 out of a snapped 11mtr vector, the 6mtr version should be strong enough, just need a suitable gamma,
since you do the 6mtr dominator im surprised you dont offer the 6mtr vector too,

keep us posted on any news

 

[Marconi]

been searching the www and found some folks having problems with their sigma 4's bleeding on other electronics . i know this can be caused by improper installation of the antenna , cheap/poorly shielded/constructed electronics , overdriven amps and overmodulated radios as well as simple proximity .

is this antenna more inclined to bleed compared to other antennas ? is there a prefered way to install them ? minium height ? insulation from mast and ground ? i know any antenna can be made to bleed , just wanna know if theres anything special that can be done in the installation to help prevent that , dont wanna put it together and up more than once .

BM, a good question. IMO much of what we hear about TVI type problems are structural issues. I don't discount such issues as you describe however. There are a lot of connection points in the Sigma 4, so I would make sure they are all tightened well with SS connectors where possible, I don't like screws connecting tappered tubing so I use ss hose clamps over slotted tube ends instead. There are some good joint dressings available today and that is also a good idea to use during construction.

If you have ever had a loose connection, maybe you also noticed some interference as well. I recently installed an AstroPlane that had a stud installed in the end of my top element. This stud is compression fitted and it was loose, but not to the degree that it made the segment non-conductive from top to bottom and it would not allow me to fully tighten the element tightly into the hub. If the element had not been fully conductive, then the antenna would not have work well at all. The antenna did work, but it produced terrible TVI in my own home to TV, phone, radio, and computer. I secured the joint and all of those problems stopped completely.

 

[BOOTY MONSTER]

thanks marconi , ive got some noalox , ill use that on it .
id still really really like to build a homebrew antenna though . i throughly enjoy the suprise in folks voice when they ask about my antenna and i tell them im using a home made wire 1/4wgp . its given me some confidence to try something bigger . but it seems the sigma4 , vector 4000 and LW-150 do provide noticable gain over other 5/8wgp's . i even see folks posting saying they had just as good tx and rx with them as their beam . but that seems a bit much to me and i have to wonder if their construction or installation of their beam wasnt up to snuff . of course proximity to things can come into play there too .

hey marconi , mind if i give you a call over the weekend ?

 

[Shockwave]

Booty Monster: Unfortunately I have to admit there is truth in the Sigma IV design causing more RFI problems then a typical 5/8 wave. If you just replace a 5/8 wave with a Vector and do nothing else, you are almost sure to have more RFI then before. HOWEVER, with just 5 to 8 wraps of coax at the feedpoint, the RFI drops to the levels of the 5/8 wave with more signal. Don't let anyone tell you different, the Sigma / Vector style antenna is the highest gain single element antenna ever made. It comes close to the gain of smaller beams like the V quad and Y quad but it does not outperform a properly working beam.

Bob: Almost done with the modified Vector. Should be complete by Monday if I don't get any emergency orders in before then. I built mine from the parts of two Vectors and doubled up on the lower sections and top loop supports as you were considering. I have made them for 6 meters too. The production cost is higher on the Sigma style antenna then the 5/8 wave. There is also very little competition with 6 meter antennas. The 5/8 wave we sell already takes care of any 6 meter competitors so we didn't need to market the 3/4 wave for 6......yet.

 

[bob85]

booty,
i never noticed any rfi problems with the sigma style even without a choke at the feedpoint but i do have a significant run of coax laying on the ground,
i dont doubt what shockwave claims but i cannot confirm it with current measurements at the moment,
im sure you have read what i claim for the sigma hybrid vs 5/8wave before and i stand by what i claim 100%,

shockwave,
glad to hear you are making progress, i understand the 5/8wave is less work and expense than converting the sigma style using a quality gamma and connector,
you will not be getting any competition from any individual this side of the pond as materials are far more expensive over here, it would need somebody with the bulk buying power of sirio to make such an antenna viable,
its catch22 because the guys that can obtain decent tube at relatively inexpensive usa prices dont believe in the sigma style antennas capabilities, nobody gets the opportunity to buy a sturdy built best performer.

 

[Shockwave]

Funny you should mention the long run of coax on the ground Bob. My first Sigma went up in a 90 foot oak tree. Only took me a year to finally talk someone into climbing the tree. The tree was about 50 feet away from the window that had my radio gear in it. As soon as the antenna went up I ran the coax right into the window on an angle down from the tree. Antenna worked great but had RFI issues. I soon took a garden edger and cut a slice in the grass from the tree to the house and ran the coax under ground and into the basement window. I'm sure this made a difference because I no longer messed up the phone or audio equipment in my house after the wire was capacitively coupled to ground.

With the antenna at this height and 50 feet of wire in ground contact the only RFI began about 1000 feet from my house. Just what you would expect if the only significant radiation was coming off the antenna itself. This was in 1980 and I didn't know about the coax balun then. I bet with one on the Sigma I could have gotten away without having to bring a length of coax in contact with the earth. The stock Vector I have now was installed on my roof and had no balun originally. It replaced a Sirio S-827. First thing I noticed was RFI in the computer speakers that was not present with the S-827. Adding the coax balun removed the RFI. I only used 5 turns about 6 inches in diameter. Could have done better if I had more coax to spare.

 

[Shockwave]

Almost forgot, I have been working with Sirio to get them to make improvements. It seems hard to convince them that it would be economical to take most of my advice. I still have not heard from them if they can do the majority what I've asked but some interesting developments seem to be taking place. They have a new version. I've been informed that my next shipment of FM models is going to receive an upgraded base collar with the connector mounted on it. This part has been notorious for corroding away in location near water with a high salt content like Bermuda. I've asked Sirio to extend each lower whip section to the full shipping box length without reducing the diameter of the top 6 inches of each tube by compressing them. This would result in a whip with a much stronger top section and larger diameter. Also asked them to slice each section so a hose clamp can be used in addition to the screw.

If people want to see this happen, email Sirio. Perhaps if more people did this, the company would see there is a demand for a version of their Vector that is more mechanically sound. If Jay can get over $300 for a 5/8 wave then Sirio could learn a lesson in quality from this and produce a 3/4 wave that that could handle Mother Nature. I understand that Sirio has to balance production cost against profit however, I think they need to hear from more people willing to pay $300 for the best because what we are asking for is going to cost. They aren't going to make it unless they see a market. If we write Sirio, it may happen. Sirio is a great family run company with a polite and professional sales staff. Wouldn't it be nice if they took the design to the max and expanded that loop like the Avanti patent suggests? About 129 inches in circumference should make a change in gain.

 

[bob85]

shockwave,
i have well over 100ft of my coax laying on the ground, i would bury it but i need some mobility when im experimenting, i guess it must absorb common mode currents on the outside of the coax as i have never had rfi in the shack with the antennas i have tested , if there was a problem my neighburs would soon let me know,

it would be nice if sirio did build a sturdy vector, it would have to be MUCH stronger than the old version to stand 100+mph winds,
as you must be aware sirio use some common parts and tubes across their antenna range, that results in compromises but without it we would likely not have the range of antennas they produce,
the problem imho is not the 6063-t832 tube as that is what almost all decent antenna manufacturers use nowadays,
imho their problem with the vector for 11mtrs is wall thickness and taper schedule,
an extended base tube from the 827 without the swaged end and .058 wall tube in the rest of the antenna would be a much better starting point,

i have not yet tested or even seen the upgraded vector to comment, nobody local will buy one as they say once bitten twice shy, they favour the much stronger 827, my local ham shop will not stock them which leaves me with a near 120mile round trip to see with my own eyes what sirio did with the new version.

 

[Marconi]

thanks marconi , ive got some noalox , ill use that on it .
id still really really like to build a homebrew antenna though . i throughly enjoy the suprise in folks voice when they ask about my antenna and i tell them im using a home made wire 1/4wgp . its given me some confidence to try something bigger . but it seems the sigma4 , vector 4000 and LW-150 do provide noticable gain over other 5/8wgp's . i even see folks posting saying they had just as good tx and rx with them as their beam . but that seems a bit much to me and i have to wonder if their construction or installation of their beam wasnt up to snuff . of course proximity to things can come into play there too .

hey marconi , mind if i give you a call over the weekend ?

Booty, I just got back from a trip out of town. Call when you can.

Hey guys I'm enjoying this thread very much.

 

[CDX-007]

At 26'7" your antenna was approximately .77 wave.The extra length could be down to improper setting of the gamma match and having to offset it with the radiator for good swr.

How are you measuring?

I was tuning for our local frequency, Ch.30, 27.305mHz

So:

11803 / 27.305 = 432.25"

432.25 / 2 = 216.125 = 1/2 wave

plus

234 / 27.305 = 102.825" 1/4 wave

216.125" + 102.825" = 318.95 or 319"

319" / 12 = 26.58' or 26' 7" or an electrical 3/4 wave for Ch.30

A mere .75 for the center of the band would be longer and a .77 for 27.185mHz would be even longer still at 329" or 27' 5"

 

[bob85]

as you sweep the radials up towards the central monopole the resonant frequency of both monopole and radials moves up while transmissionline mode impedance comes down,
you end up with element lengths somewhat longer than freespace calculations for a given resonant frequency,
26'7" is over a foot shorter than the sigma4 and a few feet short of a correctly tuned 4 radial vector 4000.

 

[CDX-007]

as you sweep the radials up towards the central monopole the resonant frequency of both monopole and radials moves up while transmissionline mode impedance comes down,
you end up with element lengths somewhat longer than freespace calculations for a given resonant frequency,
26'7" is over a foot shorter than the sigma4 and a few feet short of a correctly tuned 4 radial vector 4000.

OK, well that makes it a lot less than .77 then. It seems to calc right on .75, just like a J-pole length, and performance in comparison to the .64.

Well it's been about 2 decades but I do recall I set it by the paperwork which had a few different lengths for certering it at varying frequencies, and I arrived at that length by simple calculation and averaging between the lengths given for 27.0 and 27.5.

It worked too well to be off. Saw a difference at ~30-35 miles and further, more difference the further the station with whom I compared them.

I imagine this newer 7/8 design may be better, and I'd be game to try it, I just can't get the numbers from anyone. I've asked and no one yet has offered them. I'm sure it's no problem to duplicate whatever seems to be working.

73

 

[bob85]

shockwave posted his final dimentions here which may get you in the ballpark if you use his other dimentions and tube diameters,
http://www.worldwidedx.com/cb-antennas/36412-modified-vector-4000-a.html

he used almost the same monopole length measured from bottom of radial hub as i did but shockwave experimented with a wider radial angle which according to the avanti patent can yield a further increase in gain over the 5/8wave groundplane in the 3/4wave version.

 

[CDX-007]

Thanx Bob!!

I may take on this project during the coming Winter months and if so, I'll have it tuned, ready and installed in the early Spring, if not sooner, weather permitting.