All true. I'm not even sure at this point, but I think the original
message that began this thread discussed a vertical wire running
alongside and in close proximity to a tree trunk, which would of course
put the potentially lossy medium within a high intensity portion of the
EM field and therefore have a greater impact.
On 12/28/2011 11:59 AM, Jim Lux wrote:
> 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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