The quick-n-dirty, or simplest answer is, as far as possible. The 'catch' to that is that to keep things practical, you'll have to settle for a less than 'best' distance. It's a mechanical problem, as in just how far apart can you get them without the top antenna waving around like a maniac in the slightest breeze?
In this type of situation, more is better, till something breaks. The typical answer is something like 9 feet, a 1/4 wave. Best distance? No, but it's at least within the realm of possibility. [Absolute most bestest distance is 1/2 mile. Yeah, right.]
Any two (or more) antennas near each other will affect each other to some extent. Keeping that 'affect' to a minimum, or to a point that's acceptable is what usually happens. And THAT just depends on what's 'acceptable' to you... and practical.
Don't you just hate answers like that?
- 'Doc
