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Re: [TowerTalk] trees and verticals

Subject: Re: [TowerTalk] trees and verticals
From: David Gilbert <>
Date: Wed, 28 Dec 2011 11:21:11 -0700
List-post: <">>
An element that is totally non-conductive introduces no losses because, 
as you say, no currents are introduced.  An element that is perfectly 
conductive introduces no losses, but it does re-radiate and cause 
pattern distortion (desirable in the case of a yagi, usually undesriable 
in the case of a tower or steel light pole).  But trees are wet wood, 
which is partially conductive ... i.e., receives induced current from 
incident RF --->AND<--- dissipates it as heat.

Once more with feeling ... RF impinging on a lossy material that is 
conductive enough to receive induced currents will be dissipated.

Why is that so difficult to understand?  In terms of loss, the 
approximate circuit analogy would be a short versus an open versus a 
resistor.  The highly conducting structure is the "short", air or dry 
wood would be the "open", and a wet tree would be the "resistor".

Dave   AB7E

On 12/28/2011 8:50 AM, Roger Parsons wrote:
> Eddy and others
> I am a little unconvinced by some of the arguments that have been presented.
> Consider a Yagi in free space with nice copper elements. It would work quite 
> well if one could find a way to connect the transmitter.
> Now consider replacing one of those copper elements with one made out of 
> wood. Because wood is a poor conductor, very little current would be induced 
> into it from the driven element, and to all intents and purposes the Yagi 
> would become one having one less element. The pattern would (probably) be 
> distorted because of the missing element, but the efficiency of the array 
> would be almost unchanged.
> By extension, keeping all the original elements in place, and introducing an 
> additional wooden element into the array (even if resonant), would have very 
> little effect because the current induced into that wooden element would be 
> negligible. This is a close parallel to having a tree near an antenna.
> The question of whether it is good to have an antenna in the middle of a 
> forest is completely separate. If we put our nice Yagi into a big wooden box, 
> it will not work so well because there is now a lossy medium through which 
> the wave must pass. The thicker the walls of the box and the more conductive 
> its material the greater its effect will be.
> 73 Roger
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