At 12:15 22/02/01 -0000, G3XDV wrote:
Others have tried elevated coils with some success, notably Rik,
ON7YD, who has a similar problem to me - small top section and
local obstructions. He was originally very critical of my claims, but
has become a convert.
I can confirm the results that Mike got. Last year I added a 2mH loading
coil at the top of my inverted L (at that time 11m high with a topload
consisting of 3 parallel wires spaced 75cm and 25m long). The base loading
coil xwas reduced from 3.5mH to 2mH.
Based on improved current distribution the benefit should have been few
tenths of a dB (confirmed by simulation) but in practice it seemed to be
about 4dB. Measurements made by PA0SE and DK8KW (over distances of abt. 150
and 400km) confirmed this.
My 'guess' to explain this unexpected improvement is that the coupling
between the antenna and the many surrounding trees was mainly via the
vertical section (as trees tend to be mainly vertical objects), so by
adding the elevated coil I could reduce the voltage on the vertical section
by abt. 50%, reducing also the unwanted current flow from the antenna to
the trees.
After I reported the good results with the elevated coil to the reflector I
got some mail from other hams who tried the same and it looks as if only
those whose antennas are surrounded by trees or other obstacles seem to
benefit from an elevated voil.
Stimulated by Mikes results (once more) I will try to get almost 100% of
the loading elevated, leaving a small coil at ground level for tuning
purposes.
73, Rik ON7YD
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