Dialogue between G4CNN and ZL2CA:
> If this is so then elevating the loading coil, either by putting it high
up the vertical element or simply by raising it as high above the ground as
possible must contribute to an improved signal. Presumably as usual this will
appear as a reduction in the resistance at resonance. This I think will be my
next experiment. Alternatively a longer coil should raise the high voltage part
of the coil and also reduce losses. Comments?
Lower system resistance allows more current to circulate, for a given
applied power. So doing things that lowers system resistance is always "a
winner". The height above ground for the loading coil is mainly a matter of
impacting on CURRENT DISTRIBUTION and that is reflected in a moderately higher
value of radiation resistance. The radiation resistance is very difficult to
test, as it is of the order of milliohms versus tens of ohms for other losses.
Mechanical support of a low loss LF loading coil is challenge, as is
weatherproofing (for high voltage).
I thought we had put this one to bed.
Practical experiments by several of us, including myself and ON7YD, have
shown that in some cases raising the loading coil produces a significant
improvement.
This is normally when local objects - houses, trees - are causing absorption,
and when the length of the top section is small compared to the overall length.
I can go into why this works, but this has been done before. It is sufficient to
say that under those conditions the case has been shown to be true.
What I have not yet seen is an experiment with raising the loading coil of an
antenna way out in the open. My gut feeling is that this will still show an
improvement.
It is not worth bothering with if the top section is large. My antenna has 14m
vertical and three 18m parallel top wires spaced 0.5m apart.
As for mechanical problem, I have not experienced any, though I have had to
be ingenious. Certainly, you can't put a huge coil high up, but the gain to be
had in some cases far outweighs any reduction in coil efficiency. In the case
of a 'T' antenna supported at the ends, the coil must be made very lightweight.
This is not a real problem. In the case of an inverted-L there should be no
problem at all as the coil can be mounted on top of the mast. As for
weatherproofing, I find black plastic tape quite adequate and long-lasting.
My elevated coil uses a coke bottle as the former and is suspended
horizontally between the mast and the antenna by some polyprop 'string'
running through the inside of the former.
In short, if you have a small antenna with close-by trees, it is well worth going
to the trouble of designing an elevated coil. If not, it is probably not worth it.
Mike, G3XDV (IO91VT)
http://www.lf.thersgb.net
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