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

[Larry Banks]

Hi Roger,

I asked this same question in 2009 -- of the QST's "The Doctor Is In," but 
more oriented toward VHF.  Here is the answer I received from Gene, W3ZZ 
(SK).

73 -- Larry -- W1DYJ

--------------------------------------------------------
Hi Joel and Larry

Everyone appears to have an opinion on this subject but definitive 
scientific works are more difficult to find - at least on the Internet. From 
a practical observational standpoint, I have found that low band verticals 
particularly 80 and 160 do not seem to be bothered by deciduous hardwood 
trees. However I used these during contests that occurred mainly in the 
colder months though I am reasonably sure that the sap has NOT drained by 
the end of October and I never noticed a difference between late October 
[leaves have turned but half of them are still ON the trees] and February 
[leaves gone and sap drained if it really does drain in MD].

My station has all its yagi antennas mounted on a 24 ft mast beginning on 
top of an 83 ft tower on a 1/4 acre lot. The good news is that the tower 
sits at the edge of a group of hardwood trees so it is nearly invisible in 
spite of its size. The bad news is that the trees, once 70 ft tall are now 
approaching 90-100 ft range. I don't think the trees bother my HF tribander 
at 83 ft. Or my 7 el 6 meter beam at 87 ft. But the 2 meter beam at 95 ft 
may be impacted. I think my 2 m signal on moonrise/moonset EME is at least 3 
dB below what it should be. I also think that at 432 and above -especially 
above- trees are bad news [see below].

The best reference I can give is section 2.3 [on p. 18] in a 1978 paper by 
A.G.Longley at the U.S. Dept. of Commerce.

http://72.14.205.104/search?q=cache:IEqG7929jj4J:www.its.bldrdoc.gov/pub/ot/ot-78-144/complete_report.pdf+radio+wave+attenuation+trees+HF&hl=en&gl=us&ct=clnk&cd=22

That paper and others that I have found agree that avoid trees entirely is 
the best course. Anything at 100 MHz and more is unacceptably attenuated by 
trees with non-deciduous pine trees being somewhat worse than deciduous 
hardwoods. At HF the effect may be quite a bit less noticeable. Verticals at 
HF may be more affected but again the difference is only a very few dB more. 
Pine trees at HF are worse for the reasons you have already noted.

Therefore I would say if you are moving ... you need to take the dense tree 
cover seriously. On 2 meters you will be impacted and above 2 meters unless 
you are clear of the trees you may be severely impacted. I suspect on 160 - 
40 meters you won't have much trouble but a tribander buried in the trees is 
also likely to see some attenuation - maybe more than you'd be comfortable 
with. The same with 6 meters. For less dense trees I think the tribander and 
6 meters would be o.k.

Good luck.

73 Gene W3ZZ
World Above 50 MHz
FM19jd MD
50 => 10 GHz
Grid Pirates Contest Group K8GP
Member, CQWW Contest Advisory Group
-----------------------------------------------------------------------------------------------

-----Original Message----- 
From: Roger Parsons via TowerTalk
Sent: Wednesday, February 07, 2018 10:57
To: Tower and HF Antenna Construction Topics.
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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