Thanks Chuck -- I thought I had saved a copy but could not find it.
73 -- Larry -- W1DYJ
-----Original Message-----
From: Chuck Gooden
Sent: Wednesday, February 07, 2018 13:41
To: towertalk@contesting.com ; jim@audiosystemsgroup.com
Subject: Re: [TowerTalk] Is "The Truth about Trees and Antenna Gain" the
whole truth?
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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