common mode currents/decoupling

[Bushmaster]

Bushmaster,
You're getting into that 'isolate/insulate' thingy here. Electrical tape will insulate the antenna from the mast, but it won't 'isolate' it. Best 'isolator' would be distance to reduce any capacitive/inductive reactance between antenna and mast. Convert that 'distance' to a longer insulator (non conductive mast?). How long? Whatever you can manage without breaking the bank and is still reasonable/practical electrically and mechanically, sort of. After that, you are 'stuck' with whatever it happens to be. While it ~can~ be, it doesn't necessarily have to be a 'big deal', and you won't know till you try it.
Half the fun of it is in the 'doing', not the results...
- 'Doc

I came up with a good non-conductive mast. I took a twelve foot 1 7/8 (OD) piece of schedule 80 PVC pipe, and slid a 1 1/2" diameter wooden closet rod dowel rod inside. This made it RIGID. Then I slid the whole kit and kaboodle inside the metal push up pole base. A perfect fit, and a perfect 20 foot mast, that isn't conductive (on the top half).

Viola!