[TowerTalk] trees and verticals

[David Gilbert]

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
> VE3ZI
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>