On 12/28/11 10:21 AM, David Gilbert wrote:
>
> An element that is totally non-conductive introduces no losses because,
> as you say, no currents are introduced.
But could change the pattern, because it changes the field distribution.
(e.g. polyrod antennas and dielectric lenses in microwaves)
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.
>
The saving grace, for trees, I think, is that a forest is a "low
density" heterogeneous dielectric. if the trees themselves are lossy,
they occupy a very small fraction of the total space (<1%?.. think 1-2
foot diameter trees spaced 20 ft apart).
So it's like operating your antenna and propagating in a medium that is
0.05 mS/m conductivity and epsilon of 1.0X
Barring some frequency selective effects (which I think is worth looking
into) you should be able to compute what the propagation loss is.
> 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
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