[Elecraft] [QRP-L] Antennas and trees

[Martin Gillen]

Hi, Jim.

I have run an experiment to determine the effect of
trees... not sure the type of tree but they were very
green leafy trees, full foliage in Summer, and it
had not rained for a week or so so there was no water
around, except for tree sap :).

I erected an 18.5ft vertical antenna (teflon coated
wire)
in one tree, close in near the main trunk, so it
had to be pulled through leaves and branches on the
way 
up, right in the middle of the tree.

I erected another 18.5ft vertical antenna in a
neighbouring pine tree but this time from the end of a
long branch about 25 feet up, so the antenna was
mostly
outside the main body of the tree, at least 6 feet
from
any other branches, leaves, etc.

I used an 18.5ft wire as a counterpoise which was
run at 4ft off the ground, the counterpoise was in
the same position relative to the antenna when the
measurements were made.

I then contacted several stations in the US and
asked them for comparative signal reports.  The
signal reports were taken by me holding the rig in
transmit for 30 seconds while the station read the
minimum and maximum S meter reading then I averaged
it (((max-min)/2)+min), to account for fast QSB.

I repeated each 30 second measurement 3 times for
each antenna for each station contacted, switching
between the antennas as quickly as possible (maybe
1 minute), to account for longer term fluctuations
in propagation.

Station 1 antenna 1 30 seconds.
1 minute switchover.
Station 1 antenna 2 30 seconds.
1 minute switchover.
Station 1 antenna 1 30 seconds.
1 minute switchover.
Station 1 antenna 2 30 seconds.
1 minute switchover.
Station 1 antenna 1 30 seconds.
1 minute switchover.
Station 1 antenna 2 30 seconds.

Station 2 antenna 1 30 seconds.
1 minute switchover.
Station 2 antenna 2 30 seconds.
1 minute switchover.
Station 2 antenna 1 30 seconds.
1 minute switchover.
Station 2 antenna 2 30 seconds.
1 minute switchover.
Station 2 antenna 1 30 seconds.
1 minute switchover.
Station 2 antenna 2 30 seconds.

I discovered that the antenna which was mostly out
of the pine tree always got between a half and two S
points higher than the antenna whih was mostly inside
the really leafy tree.

I retuned the antenna system with my KX1 autotuner
after each switchover.  I did not check the effect
of the tree on the impedance characteristics but I
did conclude that the leafs and branches had an
effect on the effective radiated power.  I theorise
that this effect is due to absorbtion of the RF
energy by the nearby leafs and branches.

So I now have a fiberglass panfish fishing pole
on order... I'll repeat this experiment once it
arrives to compare a nearby tree with an antenna
out in the middle of a field with only a fiberglass
pole as support.

Martin.
VA3SIE/QRP/P.




		
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