LF Antenna _in_ a tree

[Andre' Kesteloot]

Rik, ON7YD wrote re DF3LP's elevated feed antenna:

Hi Peter & LF-gang,

I just had a look on Peter's homepage (very nice !) and especially to
the
pictures of his 'birch-three' LF-antenna.

http://www.qsl.net/df3lp

First of all I have to confirm that the antenna works fine as Peter
has a
very good signal with me, compared to the power he runs.

The antenna can be briefly described as a T-antenna, 17m high in the
middle
with 2 x 30m topload ending at 13/11m hight. The loading coil is at
12m hight.

Having a closer look at the antenna I noticed that is special is some
ways :

1. The vertical section just runs within a tree.
2. The feeding-point is at 12m (just below the loading coil), the
antenna
is fed by coax with the inner conductor of the coax connected to the
loading coil and the shield of the coax connected to a metal pipe that
goes
the 12m down toward the earth and is grounded there.

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.

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).

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.
The value of this current depends on 'grounding conditions' and wether
a
'balun' is used between TX and antenna. But in any case this downgoing
current will only create a magnetic field (that suffers much less from
losses in the earth, threes etc... compared to the electric field).

Anyway, a very interesting antenna.

73, Rik


Rik Strobbe  ON7YD


Mike Dennison G3XDV wrote 
> 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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