Thank you for the answers guys.
It makes sense to ignore the concrete if I am talking about HF frequencies and
the grid spacing of the re-bars or, better yet, the contraction by temperature
mesh is less than lambda/12, way more less than that for sure. 1E5 is quite
more than the 4E3 of the sea water, nice.
Cutting the concrete is out of question, I think the HOA... I mean...
Facilities Management at the campus probably would not take that so well. :o)
73
David
HK1A
EC5KXA
AE5XQ
> Date: Thu, 4 Apr 2013 06:41:44 -0700
> From: jimlux@earthlink.net
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] Conductivity of a reinforced concrete rooftop
>
> On 4/4/13 4:25 AM, David Robbins wrote:
> > better than dirt, worse than solid metal.
> >
> >
> > Apr 4, 2013 12:27:03 AM, hk1kxa@hotmail.com wrote:
> >
> > Any hint on the values of conductivity and relative dielectric constant of
> > a reinforced concrete rooftop?
> >
> > David
> > HK1A
> > EC5KXA
> > AE5XQ
> >
>
>
> At what frequency? I have a lot of data on concrete and other building
> materials.
>
> At 1 GHz, a typical number is epsilon =6, sigma = 20-40 mS/m
> Brick is around 4-5, 17.5 mS/m
>
> That is for the concrete, without rebar.
>
> If you add rebar, it becomes a very good conductor at frequencies where
> the wavelength is > spacing between the bars. A decent approximation
> would be to take the conductivity of iron and the conductivity of
> concrete, and combine them with the relative cross sectional area.
>
> Take a slab that is 4" thick with 1/2" bars on 6" centers. For every
> foot, the total cross sectional area is 48 square inches. There's two
> bars in that foot with cross sectional area 0.39 square inches (or about
> 1%).
>
> So you would approximate conductivity as 0.99*20E-3 + 0.01*1E7
>
> I would just ignore the concrete.. the conductivity is 1E5 S/m..
>
>
> (this is why a Ufer ground works so well..it doesn't much metal in the
> concrete to make it a "really good" conductor compared to soil)
>
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