From: "Mike Dennison" <mike.dennison@rsgb.org.uk>
Organization: Radio Society of Great Britain
To: rsgb_lf_group@blacksheep.org
Date: Wed, 28 Apr 1999 14:02:25 +0100
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Subject: Re: LF: DF3LP homepages, update...
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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
Publications Manager

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