Rik, ON7YD wrote re DF3LP's elevated feed antenna:
So the lower 12m of the vertical part is just a connection to the ground.
This might explain why the antenna is working so well and is not suffering
from too much loss in the three it is mounted in.
Until the lower part reaches the ground it will still radiate. It does
not matter (except for engineering and matching reasons) where an
antenna is fed - it still works the same.
In some way it also confirms the 'theory' that the goal of a good
LF-antenna is to bring as much 'power' as possible as high as possible.
According to this theory the ony function of the vertical part of the
antenna is to 'transport' the power to the top (and any radiation by the
vertical part could be considered as waste).
Hmmm. I think you need as much current as possible in as much
of the vertical section as possible. The trouble with the conventional
extremely short vertical is that the current drops off rapidly with the
maximum at the bottom of the vertical section. In a conventional
very short Marconi antenna this drop off will be approximately
linear. If you add an elevated loading coil (often difficult for practical
reasons) the current distribution is distorted to give more current
higher up the mast.
An alternative way to look at it is to make a difference between the
creation of the electric and the magnetic field :
The electric field is created by voltage where the magnetic field is
created by current.
So the top-section of the antenna (all above the loading coil) has a high
voltage and there is a current flowing through it, so this section will
create an electric and a magnetic field.
But the 12m vertical section going from the loading coil down to the ground
is at zero voltage but it is possible that some current goes through it.
Yes. Another advantage of the top inductive loading is that the
horizontal wires (the capacitance) are immediately shifted in phase
so that there is less cancelling out of the vertical radiation when the
top wires are drooping. Without the coil, the top of the vertical is
very nearly equal in phase and amplitude to the start of the
drooping element, hence cancellation of some of the vertical
component.
I have tried several inverted-L configurations at several locations
and have always found that a top inductance gives an improvement.
This applies much more for very short antennas - at home my top
section is less than 20m long. Ideally the whole antenna should be
self-resonant, or slightly LF (ie no bottom loading coil, or even a
capacitor) but I have not achieved this as lots of top L is needed.
The snag is that it is difficult to engineer an efficient and light
weight coil which will sit on top of a mast. I use a 3 litre cola bottle
(made rigid by the method suggested by Rik - put in freezer with lid
off, take out and screw lid on, then the pressure builds up inside as
the air expands). I have streamlined the bottle by taping the funnel-
shaped top from another bottle to the flat bottom of the first - so it
has a top at each end! It is then wrapped with black tape as it
looks silly when you have a cola bottle on a pole! It slips over the
end of the fibreglass top section of the mast.
Interestingly (but probably coincidentally), not only does DF3LP get
out well but he hears well, too.
Mike Dennison, G3XDV
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