[TowerTalk] Conductivity of cement
jimlux at earthlink.net
Sat Sep 6 18:21:16 EDT 2008
> It just occurred to me that I have two pieces of data that seem
> contradictory. Most people know that you can build a Ufer ground for a
> tower using concrete and rebar. The concrete is said by most people to
> be more conductive than the surrounding soil. Several years ago before
> GFIs, if you were ever unfortunate enough to touch a lamp that had a
> short, while standing barefoot on a concrete floor, you can attest to
> the conductivity of concrete.
> On the other hand..... There have been studies to look at the
> attenuation of RF signals by cement. This is important because it
> affects signal transmission thru building materials. The results of
> these studies show that cement is virtually transparent to RF. A
> summary of that data is listed below.
> So how do you reconcile these differences?
Different "applications".. what's conductive from a grounding standpoint
isn't particularly conductive from a "shielding" standpoint. Look at
skin depth for a sort of cue. Skin depth in soil (or concrete) at HF is
many meters. If you think about shielding, and you've got a signal on
one side of something 1 skin depth thick, on the other side, you've got
the same field, just reduced by 1/e... not even 5 dB down.
In the safety grounding (or lightning grounding) situations, you're
really looking at DC conductivity (or, maybe LF/MF conductivity at
1MHz). It's not really a wave propagation thing, more of a bulk
resistivity thing. The wavelength is enormous compared to the
dimensions of any physical thing you're looking at.
Upshot.. concrete is a good conductor as a bulk electrode, terrible as
an RF shield. Good question though, because "good conductor" kind of has
a variety of meanings, depending on context. (and we won't even get
into bulk resitivity of semiconductors)
Very useful data you found, too...
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