Very interesting experiment, Jim.
Did you try checking the coil output with it positioned alnogside one
of the trees, as opposed to actually wrapped around it?(i.e. wrapped
round an imaginary tree trunk right next to the real one)
I used Rogowski coils quite a lot, back when I was routinely
measuring longitudinal noise currents in multi-core cables and cable
sheaths etc-very useful devices. We made ours up using some special
coaxial delay line cable we got in from the USA. It was rather
expensive stuff back then. They (Rogowski coils) are readily
available commercially these days, so it's easier to go out and just
buy one. Good to hear you did the homebrew thing successfully though.
It certainly would have been cheaper than buying one!
73
Dave, ZL3FJ
----- Original Message -----
From: "James Moritz" <[email protected]>
To: <[email protected]>
Sent: Tuesday, June 12, 2007 9:43 PM
Subject: LF: Tree current
Dear LF Group,
It has often been said that trees surrounding an LF vertical antenna
can be
responsible for a substantial proportion of the antenna loss, and
at my
QTH there are several trees that have gradually been getting bigger
over the
years, so I decided to try to measure how much RF current was
actually
flowing in some trees.
To measure the tree current, I used a Rogowski coil - this is like a
toroidal RF current transformer, but without the magnetic core;
instead of
measuring the secondary current, the open-circuit voltage across the
secondary is measured, and is proportional to the total current
flowing
through the area enclosed by the coil, the number of turns, the
area of
each turn, and the frequency. My Rogowski coil used 1.2m of rubber
hose as a
former, with about 500 turns of insulated wire, and had a scale
factor of
around 100mV out = 1A in. I measured the voltage with a SPM-3
selective
voltmeter. The advantage of this type of current sensor is that the
loop of
flexible hose former can be made large enough to fit round a tree,
and can
be opened out, wrapped round the tree to be measured, and closed up
again. A
practical difficulty is that the output is quite small compared to a
normal
current transformer, and an aluminium foil electrostatic shield had
to be
added in order to reduce the effect of pick-up of the intense
E-field
directly under the antenna. This did not entirely eliminate the
problem, but
reduced it to a reasonable level.
I measured the current near ground level in the trunks of 8 trees -
these
are scattered around within about 10m horizontally of the antenna,
with
heights of about 5 - 10m (antenna height is around 9 - 11m). With an
antenna
current around 4A, the currents ranged from 50mA to 190mA, with the
total
for the 8 trees being 930mA. Generally, trees closer to the antenna
and with
bigger areas of foliage had higher currents, as you would expect.
The 8 measured are only a sample of the small trees and large bushes
in and
around my garden, also at about 20 - 40m distance there are larger
trees all
around my QTH as well. So it seems likely that a large fraction of
the
electric flux of the antenna is intercepted by a tree at some point,
and a
substantial proportion of the total antenna current is flowing to
ground
through trees. Since wood is a poor conductor, it is not surprising
that
trees close to the antenna increase the loss resistance. But also,
current
flowing vertically in the trees will contribute to the overall
radiation of
the antenna. However, since a current flowing "up" the antenna will
be
flowing "down" the tree, and the spacing of tree and antenna is a
very small
fraction of a wavelength, the effect of the tree current will be to
partly
cancel the overall radiation, and so reduce the radiation
resistance. If
the current flowing in the trees is a large fraction of the total
antenna
current, and the height of the trees is comparable with the antenna
height,
one could expect a substantial reduction in radiation resistance and
effective height of the antenna, as the field strength measurements
I made
last week would seem to indicate.
Cheers, Jim Moritz
73 de M0BMU
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