Hi all, Finbar and I have been playing making some measurements on some
different simple aerials, as you may gather from the data sent to Jim on
ground loss v frequency.
Finbar has just errected a simple 'lowfer' type aerial. It consists of a 20
foot (6m) vertical bamboo with a horizontal cross on the top made of 2 x 8
(2.6m) foot canes, to support the capacity hat. The idea is not new and
Finbar says he saw it in Pat Hawker's Tech Topics some time ago as an idea
for local BC station aerials in VE. The thing is that instead of the usual
'spokes-and-rim' wheel style capacity hat, this design winds a spiral out
from the centre. Finbar used 169 feet (56m) of wire ti make the spiral. The
'drop' wire to the measuring RF bridge was 30 feet (9m) long.
I have managed to model the terminal impedance v frequency for the other
aerials we have tried with a simple L-C-R series circuit, and the
measurements have tied up well with the calculations. The above design does
not.
The first surprising point (to me anyway) was that the aerial's natural
resonance was at 1750kHz !! Note that is without any coils added. When one
measure a simple end fed wire with the same natural resonant frequency , the
resistance drops and the capacity starts to rise as the frequency starts to
approach the resonance. (above about 500-600kHz in this case) This is due to
the inductive reactance cancelling out some of the capacative reactance as
Jim explained, leaving a lower overall reactance and thus a higher measured
capacity. The capacity of the the above design does not rise as rapidly as a
simple series LCR circuit would predict, and the inductive component is
about twice that of a inv 'L' at the same natural resonance. (56uH compared
with 31uH for a inv 'L' wire)
My first thought is that we have a limiting case of 'inductive top loading'
here. In which case the spiral 'top hat' may be a useful construction for
those with limited real estate. No only does it substantially load the
aerial, but presumably it has the same advanageous effects found by Mike
G3XDV and Rik ON7YD of reducing 'environmental' losses. The downside was
that the total capacity of the aerial was still only 149 pF which would mean
a substantial coil to resonate it at 136kHz. (but maybe no worse than a
vertical with a 'spoke-and-ring' hat.) What does occur to me is that this
may be a very good way of making a reasonable resonant Top-Band aerial in a
limited space. (it will also need anti-corona treatment on the end of the
spiral)
We have probably been 'reinventing wheels' .....but it is great fun.....and
we would be interested in any comments on the format, and what should the
equivalent circuit look like? Would it be usefull to add further (3 or 4)
radial capacity loading wires (say 5m) to the outer turn, pulled out away
from the top and sloping slightly to ground in 'guys'??
Cheers de Alan G3NYK and Finbar EI0CF
[email protected]
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