Imagine a hoop, made out of flat copper strip, about 3" wide, tens of feet long. There are tens of Amps of RF current (3-15MHz) flowing along the strip. Do you use 3" or 6" (one side or both sides of the strip) when figuring the cross-section of the surface that the RF flows along?
Suppose you wrap the strip around a plastic pipe formed into a loop (like a hula-hoop) as a mechanical support, such that the strip wraps three-fourths of the way around the pipe, leaving a gap between edges of the strip. Has anything changed? Where does the current flow?
Now, suppose you wrap the strip around a smaller diameter plastic loop, such that the copper strip actually closes over itself, and you solder the seam? You have effectively made a tube over a plastic mandrel. Does the RF current still flow on both the outside of the tube and inside of the tube, or only on the outside?
Suppose you wrap the strip around a plastic pipe formed into a loop (like a hula-hoop) as a mechanical support, such that the strip wraps three-fourths of the way around the pipe, leaving a gap between edges of the strip. Has anything changed? Where does the current flow?
Now, suppose you wrap the strip around a smaller diameter plastic loop, such that the copper strip actually closes over itself, and you solder the seam? You have effectively made a tube over a plastic mandrel. Does the RF current still flow on both the outside of the tube and inside of the tube, or only on the outside?