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

Larry Banks larryb.w1dyj at verizon.net
Wed Feb 7 13:55:53 EST 2018


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 at contesting.com ; jim at 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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