[TowerTalk] Conductivity of cement

[jimlux]

K4SAV wrote:
> 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...

Jim, W6RMK