If the backstay "is" the
antenna, then you would use GTO-15 wire from the tuner to the backstay. The GTO-15 wire is insulated and looks like coax, but it is not. It is a
single wire within the
insulation. The connection would be made where the GTO-15 and the backstay meet.
From what you are describing, the
antenna is not the backstay, but a wire that is attached to the backstay with standoff insulators. If you want, you can use insulated wire for protection.
The connection from the tuner to the antenna should not be one piece of wire, but two separate wires. This will allow for any
repair work to be done if needed. You can use whatever is convenient for that connection as long as you keep it
water tight. Meaning, put protection on the connection.
When measuring the antenna length, the wire from the tuner is part of the total length. i.e. the antenna is 30ft and the wire from the tuner is 10ft. Total antenna length is 40ft.
There will be some more coming on to add to this.