One thought that came to mind living in a forest myself, and in California
always under the threat of fires; and that is the potential for arcing from
the antenna to a tree (depending on distance) potentially starting a fire.
I've seen it happen.
73s, Paul, W6IBU
-----Original Message-----
From: W1JCW John
Sent: Wednesday, February 7, 2018 8:31 AM
To: towertalk
Subject: [TowerTalk] Is "The Truth about Trees and Antenna Gain" the whole
truth?
Roger -
Good thoughts.
I was wondering the same about this article as well and expected a different
scope of explanation, such as I've run many dipoles through and over trees
and wanted
to read thoughts of how efficient these may be along with pros and cons.
There were charts and graphs but didn't recognize related information of
what I was interested in.
I have noticed depending of foliage density winter verses summer
interactions on certain frequencies but this is the fun part of the hobby
discovering these things.
73-
W1JCW
John
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Roger
Parsons via TowerTalk
Sent: Wednesday, February 7, 2018 9:58 AM
To: Tower and HF Antenna Construction Topics. <towertalk@contesting.com>
Subject: [TowerTalk] Is "The Truth about Trees and Antenna Gain" the whole
truth?
I had been expecting a discussion here on this recent QST article, but there
has been very little. So I thought I would jump in. Answering my own
question, I do not feel that the article does present the whole truth.
It seems to me that there are two self evident cases where an object placed
close to an antenna does not cause loss:
(1) Where the object is perfectly conducting, it may change the radiation
pattern, but as it has no resistance there can be no losses;
(2) Where the object is perfectly insulating, it may affect the
characteristics of the antenna (by changing the dielectric) but as it can
pass no current, there can be no losses.
In all other cases a loss may occur, and I have no reason to doubt the
general methodology described in the article.
However. The NEC based analysis is based on an antenna and a broadly
resonant tree in free space. A tree in free space is considerably less
likely than an antenna being there! (Actually, as there is currently an
expensive motor car in orbit perhaps I am wrong...) The analytical
simulation considers an infinitely long tree next to an antenna, again in
free space.
Perhaps a right circular cylinder is an accurate representation of some
particular tree, but it doesn't seem to fit the generalised case. Trees are
ground mounted and have a ground system which probably has higher
conductivity than their trunks and foliage - and which actively seeks out
water. They also have top loading of almost infinite variety. The cedar tree
that I can see from my window has very complex and spread out branches and
foliage, whereas a palm tree (which I can unfortunately not see) appears to
be quite close to a monopole with a some top loading.
Because a tree is lossy it will have a very broad resonance, but it seems to
be stretching credibiity to suggest that a 5m high tree would significantly
influence a 1.8MHz vertical. Or that a 50m high tree would have significant
coupling to a 28MHz vertical. In each case the tree is likely to be very far
from resonance.
I could go on, but my feeling is that although the conclusions reached in
the article are reasonable for the model adopted, they are likely to greatly
overstate losses in the real world.
73 Roger
VE3ZI
ps Perhaps there has been discussion on another reflector?
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