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? If cement is really
conductive, it seems like if you totally enclose an RF source inside it,
that there wouldn't be much RF transmitted thru it. It it's not
conductive a ufer ground wouldn't work. Is there some unusually
differences here between wave transmission and conducted emissions? It
would have to be something really unusual because wave transmission is
usually blocked by conductors.
Jerry, K4SAV
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This data is for cured Portland cement, moisture included. Here is a
brief summary of the data.
The report is from Rome Air Development Center RADC-TR-67-446, Feb. 1968
by C.M. Brennan et al titled EM shielding of building materials. It is
referenced in a more recent Sept. 1984 report by J.C. Wyss titled
Building Penetration Project that is available through NTIS, number
PB85-126001. The loss through Portland cement concrete, aggregate to
cement ratio = 3:1, that was saturated with moisture is reported from
the former in the latter, with a 10% "assigned uncertainty", to be:
Frequency Attenuation
Hz ______ dB/cm
1.0 E4 ___ 2.98 E-7
1.0 E5___ 4.41 E-7
1.0 E6 ___1.50 E-6
1.0 E7 ___2.58 E-6
1.0 E8 ___4.84 E-5
1.0 E9___ 7.6 E-4
1.0 E10 __7.6 E-3*
* denotes extrapolation.
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