G4CNN wrote:
Regarding the use of a high loading coil, Mike (XDV) comments:
> It is also worth saying this is effective when the
part beyond the coil is quite
> small (as in your straight vertical diagrams). For an
antenna with substantial
> top loading, the gains and losses (caused by the much
large inductance
> needed) can cancel each other out.
Perhaps I have misunderstood your point Mike, but this sounds wrong to me.
The less top loading, the greater the inductance required for resonance.
With little top loading above the high coil, a very large inductance is
required to have any effect, since the capacitive reactance will be
extremely high. A small coil will have little to no impact on this
reactance.
It would appear to me that the reduction of high voltages in those parts of
the antenna that are near ground, trees, buildings, etc. is probably where
the most gain is made. Moving the current node up the antenna obviously
makes a difference, but the reduction in losses because of the lower
voltages is probably much more significant. The high voltages are still
there of course, but they are now confined to the top load, which hopefully
is well in the clear.
Taking your own philosophy of experience being better that theory, the
experience of myself and others is that a short antenna will benefit from an
elevated coil much more than a long one. The argument about close-in
absorption applied to both types of antenna.
The point is that with, say, an inverted-L, the purpose of the horizontal section
is to load the vertical section such that the =effective= height is maximised. A
good size top section - perhaps 50m long - will already make the effective
height very close to the physical height. Therefore the gain to be made is
small. In this case the coil loss will probably be more than the gain available,
unless a very low loss coil is used, and this can be too heavy or bulky to
elevate.
With a much shorter top section - mine is some 18m - the gains to be made
from an elevated coil are much more, and greater than the coil loss.
I agree that there is a point where there is a trade-off between coil loss and
the inductance required, but I think is much less significant than the trade-off
between effective height and coil loss.
In any case, it works and that is the main thing.
I have a 7mH elevated coil but it is quite lossy. My plan is to use my similar
size 3mH Litz wire coil and load it with ferrite. This should be a better
compromise between loss and height.
73
Mike, G3XDV (IO91VT)
http://www.lf.thersgb.net
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