Think about the metal as being part of an antenna system. A yagi has
the reflector often shorted to the boom and driven element. The metal
reflects or redirects the signal with minor losses. The signal may not
go where you want it to, but it does go somewhere. At RF a short is not
necessarily a short. If you short the end of your coax, the energy
doesn't disappear, but is reflected back up the coax to either be
dissipated in the radio or the coax.
In the case of a building with your signal inside, think microwave
oven. The signal bounces around the interior, being dissipated in the
meat (you, your furnishings, etc.) inside. Not much of the signal gets
out. A steel frame building generally has openings much less than a
wavelength long at HF, and is therefore a Faraday Cage, keeping the
signals inside. A lot of people (myself included) have been very
effective with attic antennas in wood structures.
The reason that we don't build antennas with steel is that it is lossy
compared to aluminum and copper. Were it cost effective, silver would
be the best antenna material. But steel does often have lower loss than
a wet tree.
On 12/27/2011 12:35 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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