Given the location of that river, it is quite possible that someone
dumped something in there that changed the conductivity :-)
Erich N6FD
Dan Zimmerman N3OX wrote:
> "Could a fresh water pond have sufficient minerals dissolved in it to
> be helpful? Even if not salt in the water, I think it could be useful
> in the same way."
>
> I can tell you that water that's been corroding some steel and brass for a
> few months doesn't make a very good coaxial transmission line dielectric at
> VHF/UHF... but it's not exactly a dead short either.
>
> I filled up a three foot long slotted line with such water today and
> measured transmission through it (in fact, with the intention of measuring
> the RF conductivity of this water) and got about 17dB loss in 88cm of line
> at 315MHz.
>
> So I think i'm calculating this right:
>
> line attenuation in dB/m = 8.686 * pi * sqrt(permittivity) * loss tangent
> / free space wavelength
>
> I know I was in the ballpark of 19dB/m, and have dielectric constant = 80,
> so I get an effective loss tangent of about 0.07...
>
> I find elsewhere that loss tangent = 2*conductivity / (permittivity *
> frequency)
>
> Backing out the conductivity from that gives about 0.007 S/m... a tad higher
> than EZNEC "Average Ground"
>
> Seawater is 4S/m
>
> Errors may abound given the hour.
>
> For a totally different perspective on fresh water, the stream near my house
> over the last week seems to have fluctuated from 0.08S/m to 0.18 S/m.
>
> http://waterdata.usgs.gov/md/nwis/uv?cb_00095=on&format=gif_default&period=7&site_no=01649500
>
> Seems like maybe my tank's water is a lot fresher than the stream , but
> neither is quite seawater. I don't know how USGS measures conductance or
> at what frequency.
>
> 73
> Dan
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