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

Wed Feb 7 13:41:00 EST 2018

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?
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>