Here is a better link to the document:
https://www.its.bldrdoc.gov/publications/2674.aspx
On 2/7/2018 10:16 AM, Larry Banks wrote:
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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