[TowerTalk] Conductivity of cement
Larry E Sheriff
sher9000 at bellsouth.net
Sat Sep 6 20:43:14 EDT 2008
Hi, as putting ground rods and tying to the rebar; has any of you seen what
happens to concrete when high heat hits it???
I live out in the country and alot of us have a well with concrete slabs
around it. Lighting has hit a few. The concrete explodes into many peices.
Which would just leave the guy wires to manage the tower.
I will NEVER ground anything in my base!!!
Just my 2 cents worth.
73, Larry - W4LES
----- Original Message -----
From: "jimlux" <jimlux at earthlink.net>
To: "K4SAV" <RadioIR at charter.net>
Cc: <towertalk at contesting.com>
Sent: Saturday, September 06, 2008 6:21 PM
Subject: Re: [TowerTalk] Conductivity of cement
> 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
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