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
|