Dear Jim, LF Group,
I did some experiments a while back that compared the efficiency of a
top-loaded vertical (inverted L about 10m high and 40m long) in an
environment surrounded by trees, compared to the same antenna in the middle
of an open field. On 500kHz the antenna surrounded by trees was only about
1/6th as efficient as the field antenna, so trees certainly have a major
effect. Part of the effect is due to increased loss resistance - the trees
being a lossy dielectric in the capacitor formed by the antenna wire and the
ground plane. Also, the radiation resistance is reduced (i.e. less field
strength for the same antenna current), which I guess was due to induced RF
current flowing in the tree in the opposite direction to the current in the
vertical section, partly cancelling radiation from the antenna. It was
possible to measure a significant fraction of the total antenna current
actually flowing in the tree trunks. On 136k, the difference in efficiency
was even greater.
So I expect the tree has a significant effect, especially since the wire is
actually in contact. For my antenna, the wire was kept at least a few m away
from any tree, but it was surrounded by several trees. You will probably
find moving the antenna wire away from the tree reduces the loss resistance
significantly, and it should also lead to an increase in effective
height/radiation resistance.
Cheers, Jim Moritz
73 de M0BMU
----- Original Message -----
From: "James Cowburn" <[email protected]>
To: <[email protected]>
Sent: Sunday, November 08, 2009 3:19 PM
Subject: LF: Losses due to ant in tree?
Hi All,
The vertical 5m section of my inverted L runs up into the branches of a
silver birch tree. The wire goes over a large bough at 5m or so and then
comes out of the branches for some 22m down to a 4m high pole attached to
the garage.
Has anyone a guesstimation of the losses in signal caused by running the
wire through the tree?
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