Charlie Gallo wrote:
> On 5/11/2010 jimlux wrote:
>
> ...snip....
>
>> I didn't get the impression from the original poster that he had a roof
>> mounted antenna.
>
>
> ...snip...
>
> Nope, not roof mounted - back edge of the lot, near the corner of the garage
> is where it would have to go - the ROOF is where the Triband beam is going.
> The issue is I really don't have much on 40 or 80 - some, but not a lot
>
> As for cracking - that's what they call expansion joints, and why I said "run
> jumpers under them"
>
> BTW - I got Jim's reply, but still have not seen Jeff's original message
>
If they run mesh through the slab continuously, and you're just talking
the saw cuts/thin slots, it might crack all the way through, but the
wire in the mesh bridges the crack. If you're talking the 1" or so gap
filled with a squishy filler of some sort, I don't know. On the one
hand, it seems that running a continuous wire would be a good idea, on
the other hand, since all you really care about is making the concrete
look like a "good" conductor, it probably doesn't matter much. You've
got a lot of surface area on either side of the joint, so the
capacitance is large, and if the joint is wet, then it's like a resistor
in the gap, so it's hardly an insulator.
The idea isn't to duplicate a solid metal plate, but to be "better than
soil"...
Ballpark calculations:
slab 6" thick with a 1/2" gap that's 10 feet long. Call it 15cmx3
meters by 1cm gap..
for nothing in the gap
C = A * epsilon0/d
A is 3*.15 or about .45 square meters. d is 0.01, epsilon0 is 8.85 pf/m
45*8.85 pf = 400pF (and easily 4 or 5 times that if there's wet
anything in the gap, because water has a really high epsilon)
I suspect that if there's water around, the resistance of the filler is
much less than the RF impedance of the capacitor.
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