[TowerTalk] trees and verticals

[David Gilbert]

#1   Antennas inside of an attic are often really poor performers 
precisely because there is enough stuff up there to absorb a lot of the 
RF, although not all of it is wood.  I don't think shingles are all that 
transparent to RF, for example, especially when wet.  Attic antennas do 
enable contacts, of course ... after all, even if only 5% of your 100 
watts is escaping the attic you're no worse off than if you were running 
QRP to a decent antenna.  The problem is that most people using an attic 
antenna don't have anything better to compare it against.  if they did, 
not many of them would be using the attic antenna.  Besides, wood inside 
an attic is almost always bone dry ... people used to make insulators 
out of that stuff.

#2  Just because steel has lower loss than a wet tree doesn't mean that 
it would be smart to built an antenna inside of a steel structure when 
it would be better all the way around to put it in the clear.  Why would 
you ever do that?   It's like saying I'm going to cover my TV screen 
with waxed paper because waxed paper lets more let through than tar paper.

#3  Tuning and SWR of an antenna has NOTHING to do with the inherent 
loss of the antenna itself.  Nearby structures can affect the tuning 
without adding any loss of their own to the system.

#4  Actually, at least a few people have already answered your 
questions.  Conductive but mostly lossy structures absorb RF ... 
conductive but mostly non-lossy structures deflect or block RF.  Neither 
is desirable, but as N6FD tried to tell you quite a while ago deflected 
RF is better than dissipated RF because at least it's going somewhere 
that might potentially be more useful than heat.  Nobody is "having it 
both ways" ... you just don't grasp the difference.

It's your turn to think about it.

Dave   AB7E



On 12/27/2011 4:03 PM, Eddy Swynar wrote:
> On 2011-12-27, at 4:17 PM, N6FD wrote:
>
>> A lot of people (myself included) have been very effective with attic antennas in wood structures.
>> But steel does often have lower loss than a wet tree.
>>
> Hi Eric,
>
> I'm going to play "devil's advocate" here,&  try to put my "spin" onto what you've just stated above...!    :>)
>
> QUESTION #1: you've stated earlier that trees are very lossy affairs,&  because of that will dissipate RF fields radiated into them. If that's so, then why would your "...attic antennas in wood structures" be so effective...? Why wouldn't that environment have a lossy influence, too...?
>
> QUESTION #2: Why not build antennas in steel structures, if said steel has "...lower loss than a wet tree"...?
>
> Again, I hope you might see where I'm going with this. You can't have it work one way one time,&  not the other, another time...at least not the way that I grasp this.
>
> Another question: why is it that in some antenna articles (QST, CQ, etc.) the author muses aloud as to the possibility that some nearby metal structure may have had an adverse effect upon the final SWR / tuning of his antenna...? It's never, "The nearby gooseberry grove may have affected my antenna," but rather, "The nearby aluminum-sided house / water tower / steel chicken coop / etc...." that had a play in things...
>
> And I'll ask my question anew, because nobody has even come close to answering it, specifically, your gut reaction: in what environment would you rather have your new 160-meter 1/4-wave vertical in, (A) the middle of a forest of 120' tall trees, or, (B) the centre of a parking lot surrounded by ground-mounted 120' tall metal lamp posts placed about every 20', or so...?
>
> Think about it! Hi Hi
>
> ~73~ de Eddy VE3CUI - VE3XZ
>
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