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@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|