Question... doesn't the size of the of the mentioned piece (or pieces) of
metal play a significant role in its re-radiating? IIRC there is a
resonance issue at work here too. Grounding of that metal?? Only if it
affects the "tuning" of the metal structure.
Also, there is the matter of wave guide theory at work. If the holes in the
screen, or the spacing of the stand of metal supports in the building are
too small the signal will act as though there was a shield in place.
"Waveguide below cut-off" comes to mind. The same phenomenon which allows a
1//4" thick window screen to allow the ventilation of a chamber hot with rf
energy, without any of it leaking out.
Stan
----- Original Message -----
From: "David Gilbert" <xdavid@cis-broadband.com>
To: <towertalk@contesting.com>
Sent: Tuesday, December 27, 2011 4:30 PM
Subject: Re: [TowerTalk] trees and verticals
>
> 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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