Google "waveguide beyond cutoff" and it will tell you why a signal will not
get out of a steel building unless the opening is large enough.
Many signal generators were built with variable attenuators based on this
principle. They simply moved the probe inside a small diameter tube further
away or closer to the open end where a coupling loop was placed. Without the
small tube the coupling would be much greater to the loop.
This operates as a "waveguide beyond cutoff". (the tube diameter and opening
are much smaller than a 1/4 wave length needed to support propagation)
73
Gary K4FMX
> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of Eddy Swynar
> Sent: Wednesday, December 28, 2011 8:27 AM
> To: David Gilbert
> Cc: towertalk@contesting.com
> Subject: Re: [TowerTalk] trees and verticals
>
>
> On 2011-12-27, at 8:29 PM, David Gilbert wrote:
>
> #2 Just because steel has lower loss than a wet tree doesn't mean that
> it would be smart to built an antenna inside of a steel structure when
> it would be better all the way around to put it in the clear. Why would
> you ever do that?
>
> ---I guess you'd do it because, using the logic of "wood" vs. "steel"
> that's been utilized here so far that says there are absorptive losses
> in wood / trees, whereas RF would simply reflect off of metal---correct?
> So why not do it...? The signal would simply "...reflect its way" out of
> the steel enclosure with minimal loss, if I understand things so
> far...in a wood enclosure, it would be absorbed & lost.
>
> #4 Actually, at least a few people have already answered your
> questions. Conductive but mostly lossy structures absorb RF ...
> conductive but mostly non-lossy structures deflect or block RF.
>
> ---If the structure is conductive and non-lossy, would it not then
> simply and efficiently re-direct all that RF directly to ground,
> creating an even GREATER loss in our far-distant signal? Think of your
> OT VFO enclosure: shielded, bypassed, and GROUNDED. Taking the logic of
> this a step further, if a well-grounded low resistance cover over a VFO
> was ineffective because it'd merely "deflect" RF, then why not simply
> place a small wooden apple basket over top of it...? Surely the
> lossiness of the wood would "absorb" any RF fields emanating from the
> VFO itself, right...?! And a grounded cover would thus be rendered a
> moot point!
>
> Nobody is "having it both ways" ... you just don't grasp the
> difference.
>
> ---No Dave, I guess I do not grasp the difference...and I'm glad of it!
> If I were to accept such an "ism" as the gospel truth, I would never
> have gotten my station on the air 40 years ago...and I would have razed
> each & every tree in the immediate field of the tower that I have here
> now, too. Trees were the gifts of nature that allowed me to get on the
> air years before I could even dream of ever getting a free-standing
> steel tower...and I think that any prevailing "...tribal knowledge" that
> alleges they are bad to the hobby is anathema to sustained growth in our
> ranks, too...think "stealth", think inexpensive aerial supports.
>
> It's your turn to think about it....
>
> ---There. I've thought about it! And I remain unconvinced... : >)
>
> ~73~ de Eddy VE3CUI - VE3XZ
>
>
>
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