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
