[Lowfer] TOP-HAT; how long is not useful anymore ??

[Alan Melia]

Hi Andy, I will be playing the old record again :-))
It doesnt matter what anyone tells you (!!),  your site may be different
from theirs. The only way is build a little RF bridge and measure the wire
(untuned) it will look like a capacitor and resistor in series.
http://www.alan.melia.btinternet.co.uk/aelossbr.htm  The resistance is
virtually all loss resistance.

Basically there are two parts to the answer. The top-load wire with incease
the radiation resistance (or effective height) up to a length about equat to
the height, after that you will get no further significant increase in Rrad.
However (and it seems that "noralot of people know this" ) If you double the
capacity of the aerial by adding further top-load in any direction, provided
it is over "clear" ground, and not over not lossy trees or bushes, you will
halve the ground loss UNLESS you are over very conductive ground. The
top-wire can be straight, zig-zag, folded, whatever but you should try not
to keep it horizontal. Sloping top-wires shield the radiating vertical
section a little and reduce the Rrad. If you get it wrong and are coupleing
too much into the trees the bridge measurement will show and increase in
ground loss rather than a decrease.

My ball-park idea to minimise environment losses is to avoid any metallic
section of the antenna approaching trees or building any closer than a
distance equal to the antenna height. Not always practicable but the bridge
will tell you what is happening without you wasting a lot of time in the
engineering side of it, to find it make little difference.

I hope that helps a bit.....much of this was due to the experiments of the
late Laurie Mayhead G3AQC, and also Finbar EI0CF. Laurie and Finbar
increased their top-load to get something like 1200 to 1500pF. I have also
done the experiment by increasing my top-load to 850pf (from 400pF) and
watched my ground loss (sandy heath-land soil) drop from over 100ohms to
leass than 60 ohms.  See the plot towards the end of this article.
http://www.alan.melia.btinternet.co.uk/spiraltop.htm

Best Wishes
Alan G3NYK