[TowerTalk] Is "The Truth about Trees and Antenna Gain" the whole truth?

[Paul Booth]

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 at 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 at 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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