Lets you do precisely what you are describing running a separate ground wire to an existing point on the equipment grounding system originating from the same panel.
Ground wire in attic.
As provided in 250 130 c the equipment grounding conductor shall be permitted to be run separately from the circuit conductors.
Air movement in your attic should not pose a problem.
The copper wire at the base of the antenna mast attaches to the copper vent tube that runs to the roof through the attic in a 55 year old house.
You have to inspect it along its full length until you reach a working ground wire.
This is usually a moveable piece of drywall or plywood that covers a rectangular hole in.
Bare copper antenna wire stapled directly on wood rafters causes loss of rf signal energy.
Grounding of mounting hardware is not required indoors.
Use wire nuts to connect.
With time bare copper oxidizes the wood where the wire is tightly stapled to absorbs some of this oxidization and forms a loose yet lossy bond with the wire.
The attic shortwave antenna wire this is the most importantaspect to improve.
You can use the outside of the electrical conduit provided the conduit has a continuous reliable electrical connection to earth ground.
How to run wire through an attic step 1.
Measure the distance between the two sites you want to run wire between and make sure you will have enough wire.
Find your attic access.
Check to ensure that the vent tube is firmly grounded before connecting the wire.