The answer to the top loading is that the top is essentially the same as if it was an L on the antenna - just with the connection point moved to the center. I would add maybe 5 ft on either side of the top to that equation and see what it looks like when you install. And, like other verticals. If it gives you a good SWR right off the bat, then your ground losses are too high. It ideally wants to be around 20 Ohms. I can't get mine down past 30 Ohms because of the really poor ground in Vermont. I started with 24 and kept adding 6 radials (1/4 wave) at a time until I saw no difference with the feed point impedance adding the next 6. That had me end at 48 radials per T on a 2 el phased array.
I was surprised by Frank's comments. I would get 50 - 75 feet away - but "hundreds of feet away" would seem too far to have a parasitic element have effect on a yagi. Especially a 10, 15, or 20M yagi. I guess it would be easily modelled to see. Ed N1UR _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
