This is where I ordered the PTFE material for the Super Penetrator 500 upgrade ......
http://catalog.fluoropolymerproduct...fe/extruded-tube-teflon-and-ptfe-2?&forward=1
I do not remember the size I ordered ...... I ordered two 6' long pieces (12' total), to ensure I had enough PTFE material to experiment with.
Turns out I did 2 SPT 500 antenna bases without wasting any material.
You need to measure the O.D. of the vertical radiator , the I.D. of the bracket, and go from there.
I had to very slightly enlarge the hole in the bracket to accept the PTFE, and I also had to machine the I.D. of the PTFE tube so as to slide over the vertical radiator. I used an adjustable inside reamer and removed a little inside material at a time until I reached the size to permit the tube to slide over the radiator .......
With my top insulator upgrade, I improved the performance of the antenna in wet weather, no more high VSWR's when wet,
And the power handling capability of the antenna increased. I will run as much as 2KW PEP of power without any more arcing at the top insulator. I'm sure it can handle much more than that now.
Hope this helps. I am retired, living out on my ranch. I am not really interest in producing these for resale, but will gladly share what I learn.