antenna specialist . Sigma IV mod AV-174

[yogi bear]

help with sigma 4 type project

Hi Guys, i know this is a bit of an old thread so perhaps i had better start a new one or look for a newer one but here go's.
I am rekindling my passion for cb, it's been many years but i did once own an original sigma 4, i had also owned a fake one amongst many other antenna.
I have a few questions if anyone can help.
I am building a copy with parts i have lying around but there are a few things i am not sure about, i am going to build a gamma match but i don't know the exact dimensions of the parts, does anybody know the length of the tube and rod? Must they both be aluminum? Even if the gamma match must be aluminum could the basket be made out of soldered copper tube?
I was going to make a version from wire and suspend it between two 40M pine trees but then found that i had a large glass fiber wind surfer mast and an extendable fishing pole that fits exactly in the end, i can achieve the length required and was planning on using aluminum or copper wire, to be hung on the outside (rather than inside) of the fiberglass tubes (to avoid some shielding).
So basically the questions are:
1? Gamma match dimensions, and is alu necessary? (also is there a good source for a teflon sleeve or equivalent)?
2? Can copper be used as a basket or must it be made from alu?
3? Can the main 27' 7" radiator be copper wire? or must this also be alu?
4? Does anybody know of any interesting mods. other than the obviouse waterproofing and filing the corners of the gamma match etc?

So with the glass fiber poles i should be able to get the antennae to the top of the largest 50M pine tree just behind the shack although i am probably barking up the wrong tree since i live at 1200M almost at the bottom of a narrow valley on a fairly steep hillside surrounded by 3000-4000M peaks
James

 

[BOOTY MONSTER]

if youre thinking about home-brewing or modifying a sigma id highly suggest reading this thread ........

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

the dimensions for the stock gamma are here .......


http://www.worldwidedx.com/cb-antennas/31799-avanti-sigma4-alternative-view-point-26.html

the stock 7/8 vector gamma use

tube
.512" x
.049" x
14-3/8" ( not including the 1-1/4" fold )

rod
.236" x
34"

maximum capacitance ) is 50pf

heres another option for a gamma

http://www.worldwidedx.com/cb-antennas/31799-avanti-sigma4-alternative-view-point-27.html

Making a gamma with a 36 inch 3/8 rod and 10 inch 1/2 tube should have more then enough capacitance and length to match this antenna. The tap point from rod to radiator is typically around 27 inches up from the connector bracket. If you correct the angle of the radials at the base, your radial support disk is an insulator, and your gamma is close to these specs, I'm at a loss as to why you can't match it. You can change element lengths on this antenna by feet, making the antenna completely ineffective but yet the gamma can still be tuned to provide a perfect match in nearly every case.

 

[Marconi]

marconi im not sure about exact heights, more than likely i would have put the hoop of the astro level with the bottom of the sigma,

not punch through but raise the point of maximum radiation and have a lower angle than the groundplane mounted at the same low height, i can set the antenna tips the same,

dont know about smaller footprint been the cause but the sigma does work very well when mounted low compared to other antennas,

what about a sigma style with a cap hat? do you think that may give any advantage over a stock sigma?

Bob maybe you remember, at some point after this thread was originally posted, I installed my Sigma4 and I used the top section from my Wolf .64 that has a small top hat instead of the shorter S4 top element. I did it in order to make the antenna appear longer, because the antenna was tuning higher in frequency than I thought it should. I checked all the other radiator elements exposed lengths and they were all correct as per specs.

I can't say for sure, but in that case I didn't see any differences while using that very small top hat on top of my S4. If this is correct, then maybe it confirms my opinion that such a small sized top hat only serves, more or less, as a static or lightening suppressor/arrestor and does not act like a truly effective capacitance hat device.

With that said, hopefully you remember that I ultimately found the problem to be the wrong positioning of the gamma between the radials, and when fixed the original shorter top element worked fine...to set resonance in the middle of the CB band without re-tuning the gamma match.