Personally, I wouldn't expect too much from a 10 - 20 feet distance. That's far - field indeed.
Near - field extends six inches from the tube in all directions, but we prefer the plasma to be actually touching our body. It seems more effective that way.
After six inches it becomes far - field, which isn't as effective but still works. John White recommends you remain within 5 feet of the tube for decent efficacy but says it continues to work even further away albeit that the strength decreases.
I have used Plasma in limited far-field, but only spasmodically and not for any serious issues.
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