I am an both a Ham radio operator and an electrical engineer. I own one of the antenna tuners pictured in the referenced pdf and my boat was equipped with a backstay antenna when I got it. I have talked hundreds of miles with "long wire" antennas.
It would be helpful to know what is in that tube but if it just an insulator and you are just talking about the electrical connection to the active part of the backstay then here goes. The referenced pdf article has a great line in it. It says to connect the antenna center coax to the swage connection and not around the wire. I think this is key advice. The swage is large and a connection can be made without covering any important part of the swage. If it were me, I would wrap the wire around the swage and use a thin hose clamp. I would not try and seal it from the weather as my opinion is that you can only seal water in, you cannot seal it out. I would use 12 gauge house wire for the connection and not bring the coax above deck. Any antenna has to have two elements. One is the backstay. The other is probably the ocean through a keel bolt. The idea of keeping the antenna high to avoid getting in the field seems silly to me unless this is a dual system with both elements on the backstay. If you are using the ocean as the "ground plane" you are going to be in the field. I have transmitted with 100 watts and never worried about this. Had I had 1000 watts as many hams do, perhaps I would worry. The reason I would use coper is that it is close to SS electrically and many center conductors in cable are steel. Even if they are coper, they are small gague wires so a 12 gauge will last longer.
Just my 2 cents. I have not done this as I didn't want all the bonding my boat had and pulled it all out. I replaced the backstay to remove the two insulators. I still wonder what is in that big tube.
Allen
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