Hello Gang,
Thanks for interest and discussion!
Mike Dennison wrote:
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.
I agree and hope that it radiates. But below the loading
coil it is "cold" against ground, so there is only current
flowing i.e. a magnetic near field and the dielectric
properties of objects in the vicinity will have only minor
influence. The same amount of current flows just above the
coil but at high potential of AC voltage and you have to
take the dielectrics into account. But there are only some
branches.
The distances of the coax feed running down the Al-tube to
ground doesn't have any influence.
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.
Yes, coils for LF are always too small but I have the luck
that this tree has grown like a staircase. So it's easy to
climb and to heave those 10 kg upstairs. It is attached
there by a sewed belt system made of polypropylene and bound
by a second PP-rope for safety.
Since I had some flash over to the fresh grown leaves I will
replace the two vertical wires running parallel up to the
top loading wires by heavy ignition cable and cut some
smaller branches. Furthermore a main problem is to hold the
distances between these wires and the supporting metallic
mast section to be constant (30cm here) since at this point
small variations of capacitance to ground will have a great
influence to the feed point impedance. You can use this
effect for tuning the system.
Interestingly (but probably coincidentally), not only does DF3LP get
out well but he hears well, too.
TNX, but the LORAN-C rattling noise (S9 now) is a serious
problem and a hotel at neighbourhood is equipped fully with
low energy bulbs.
If you don't like to cut those fine trees *use* them...
54°16'N / 10°04'E, JO54ag
73 es gl de Peter, DF3LP
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