Metal re-radiates whatever it RF absorbs, assuming no ohmic losses.
Think about how a yagi reflector or director works. In the case of
random metal you will get random re-radiation, but the RF goes somewhere
just as N6FD stated. In the case of fully enclosed metal you will get
Faraday shielding because the currents exist only on the outside of the
structure and the re-radiated signal goes elsewhere than inside the
structure.
Grounded metal doesn't necessarily dissipate RF anywhere, although it
might do so if the current distribution is such that a significant
portion of it passes through lossy earth. Think about how a grounded
vertical antenna works. We're not talking DC here.
Fundamentally, when RF hits a conducting element (wire, piece of tubing,
tree, side of a building, etc) it induces currents in that element ...
currents that are no different than if you had been able to somehow
connect your transmit coax to it. Those currents generate an
electromagnetic field around that element that is in fact re-radiated
RF. If the element is lossy (wood, dirt, wet mattress, etc) the induced
currents are dissipated as heat instead of being re-radiated. If the
element is non-conductive the RF passes through it without loss
(although there might be some dielectric diffraction depending upon the
material and the frequency involved). If the element is highly
conductive without appreciable loss the energy is re-radiated in a
pattern determined by the physical structure of the element and it's
proximity to other conductive elements in the vicinity.
Lastly, to your comment about energy just reflecting from conductor to
conductor until it reached our antenna, each time a re-radiation occurs
the energy heads in all directions. Only a small portion of it
re-induces currents in the next element closer to you. If indeed the RF
originated INSIDE a structure fully comprised of perfectly conductive
metal, the RF would bounce around until it was all absorbed by a receive
antenna placed inside (that just happens to be the principle behind
integrating spheres used for light intensity measurements), but you
don't get that situation if the RF comes from OUTSIDE the structure.
Dave AB7E
On 12/27/2011 1:35 PM, Eddy Swynar wrote:
> On 2011-12-27, at 11:55 AM, N6FD wrote:
>
>> Trees are absorbtive, metal is reflective. At least with the metal,
>> your signal goes somewhere.
> Hi Eric,
>
> Now THAT'S what I find to be so contradictory& confusing in all this...
>
> How can metal possibly be reflective, vis-a-vis a tree, in a similar
> situation...? The metal is grounded, and conducts FAR better than wood---if
> your signal is "absorbed" by the tree and consequently dissipated by its
> ohmic losses, would not, in turn, your signal be routed directly to ground by
> a metal post, rather than being "reflected" as you suggest, by virtue of the
> fact that the resistance of metal is miniscule, compared to wood...?
>
> And taking this a step further, why is it NEVER desirable to have an indoor
> receiving antenna, housed in a building with a steel structure...? Using the
> logic of "...wood BAD, steel GOOD," we should be able to receive nothing on
> our portable sets when we're in the woods, right...? The signal would be
> absorbed by all those lossy trees around us---but by comparison, reception
> should be great when we're inside a building made of steel beams...the
> signal(s) we're copying would just "reflect" from beam to beam until it
> reached our antenna.
>
> I'm not trying to poke fun what you're saying, Eric, but rather, attempting
> to understand how---to ME, at any rate!---the laws of radio (if you will)
> apply in one instance, but not in another...
>
> Does any of this make sense to anyone else, besides just me...?!
>
> ~73~ de Eddy VE3CUI - VE3XZ
>
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