"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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