Use wire nuts to connect.
Ground wire in attic.
How to run wire through an attic step 1.
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.
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.
Insert the end of the 12 2 wire into the fans junction box.
Bare copper antenna wire stapled directly on wood rafters causes loss of rf signal energy.
Air movement in your attic should not pose a problem.
You have to inspect it along its full length until you reach a working ground wire.
Strip away about 6 inches of outer insulation and 1 2 inch of individual insulation from the end of each wire.
Measure the distance between the two sites you want to run wire between and make sure you will have enough wire.
Grounding of mounting hardware is not required indoors.
You can use the outside of the electrical conduit provided the conduit has a continuous reliable electrical connection to earth ground.
I found that a firm grounding of antenna mast gave a better reception as well.
The attic shortwave antenna wire this is the most importantaspect to improve.
This is usually a moveable piece of drywall or plywood that covers a rectangular hole in.
Find your attic access.