Dear Jim, LFers,
James Moritz wrote:
> 10m high top section [...]
> centre of the antenna span raised to 14m [...]
> increases the overall efficiency by a factor of 4 [...]
A very interesting experiment Jim!
I find it amazing that such a small increase in average height
can make such a dramatic improvement in efficiency! Were
there any trees near the center of the topload wire?
> This could be interpreted as the effective height of the antenna being
> reduced by the screening effect of the surrounding trees, buildings etc.
Yes, but I suspect that the improvement also may have something to do with
"Laurie's footprint theory" even if the horizontal length is not changed.
My rough guess is that most of the electric field between the topload and
the ground, and hence the displacement current, is confined in a cone (well,
roughly) of a certain angle (neglecting trees and other vertical objects in
the vicinity...). When the topload is raised, and if everything scales
nicely, the area of the base of the cone increases with the square of
topload height. A larger "pickup area" on the ground means more
"grass straws in parallel" :-)
73
Johan SM6LKM
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