Hello Bob 85:
It is posssible that Avanti did use 6061 T6. I know for a fact it is .058 inches wall thickness. Which allows the next smallest diameter in 1/8 inch increments to be telescope into each together.
Now here is a little unknow fact in the antenna business. Telescoping Alunimun Tubing is what antenna manufactures use mostly to make there antenans with. BUT!...........
The Aluminum Tubing Manufactures (The Mills) do not care about us antenna manufactures or aluminum tubing "Telescoping into each other" in order to make a antenna. So when aluminum tubing is ordered in mass production quantities or tons of aluminum tubing the customer must specify that the tubing will telescope or will be returned. So that is why the wall thickness's of .050, 049, .041 ect so that the tubing will telescope into each other, with out a big problem. Meaning the tubing is slightly weaker.
The Extruded 6061 T6 Aluminum Tubing has a wall thickness tolarance of +or- .008 inches.
The Cold Drawn 6063 T8 Aluminum Tubing has a wall thickness tolarance of +or- .004 inches.
So now do the math in wall thickness tolarance with .004 and .008 inches, you will see what 6063 T8 was used the most.
The Aluminum Mills will not gauntee that 6061 T6 will telescope into each other in 1/8 inch increments, at .058 inches wall thickness, as the wall thickness tolarance is to hard to keep. Also you will see Extruded Aluminum Tubing not being round but in a slight oval shape. There is also a spec for that.
That is why antenna manufactures do not always use 6061 T6.
The Avanti Sigma 4 can be internally reinforced to allow a stronger vertical assembly, if it is internally reinforced. But as the vertical elemment is internally reinforced the stress points move putting more stress on other places on the vertical elemment. And with the Physco wind you have in the UK you will need to rebuild or reinforce a bunch of the bottom tubing sizes on the Avanti Sigma 4 Antenna.
This is why my Interceptor 10K is internally reinforced, and can take the 100 MPH winds of thge Mojave Desert.
If you have a forgine made copy of the Sigma 4, I suggest you toss it in the scrape can.
Let me know what is the problem is with your existing Sigma 4 antenna.
Jay in the Mojave
http://www.a1antennas.com