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Date: Thu, 15 Jan 2004 21:17:14 EST
To: rsgb_lf_group@blacksheep.org
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Subject: LF: Re: lf andnoise and offshore.
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>From either an engineering standpoint or a scientific standpoint, I'm not 
very comfortable with the term "sea gain" being applied at LF.  The sea is vastly 
less lossy than land--conductivity of 5000mS/m for the former, versus a range 
of anyone from one-half to 30mS/m for the latter.  There is no gain at work 
here; the received signal strength is never as much or more than the inverse 
distance law would suggest for the same signal over a perfectly conductive 
surface.

Note that this statement, as phrased, encompasses the varying 
skywave/surfacewave proportions one would encounter at different frequencies and over 
different path lengths.  Taking all those into account, observed field strengths on 
and near the ocean still do not exceed expected values by the conventional 
propagation models.  One could easily fall into superstition by casually thinking 
of it as actual gain.

The conductivity difference between seawater and land (ranging from 150-to-1 
up to thousands-to-1) is not trivial.  It makes a huge difference in surface 
wave propagation, obviously.  It reduces attenuation for each earth reflection 
of skywaves.  It even affects efficiency of the receive antenna in some cases.

To address Bryan's comments about longwave and mediumwave comparisons, the 
effects are actually quite similar between LF and MF.  Surface wave attenuation 
is greater for a given surface conductivity as one increases in frequency, of 
course, but in this regard 136kHz, 770kHz, or even higher, are all just points 
on a continuum.  Much the same is true of D-layer effects between LF and MF. 
The resulting differences are in such things as the distance where one first 
runs into fading zones and in the amount of day/night signal variations.  But 
again, these are points on a continuum.  One has to go down to mid-VLF before 
one encounters fundamental differences in how propagation works (increasingly 
by waveguide effect), and even that is more because of differences in the 
ionosphere's behavior than the earth's or the ocean's.

Skin depth of the ocean increases as frequency falls, but even at "classical" 
submarine frequencies in the 10-30kHz range, the sea is *far* from 
transparent to radio waves, experiencing 6 to 10dB of attenuation per meter of depth.  
That's pretty drastic.  You have to trail a very long wire, and you're still 
uncomfortably near the surface given the capabilities of spy satellites.

It is necessary to descend to 1kHz to achieve a mere 1dB/m, and even at the 
ULF signaling frequencies of Sanguine and Zevs, it runs one or two tenths of a 
decibel per meter--manageable at depth, albeit only at very low data rates.  A 
good graph of the attenuation values can be found in M. L Burrows' "ELF 
Communications Antennas" (1978, Peter Peregrinus Ltd on behalf of the IEE).

(OT:  In the other direction, the attenuation rises uniformly to roughly 
1000dB/m (!) at 10GHz, then jumps roughly another decade of loss, where it remains 
more or less constant until we reach the visible light spectrum, where the 
curve takes a sharp dip down to about the 1kHz level.  The ability of 
electromagnetic radiation to penetrate liquid water to reasonable depths at that 
frequency is exactly the reason why the chlorophyll molecule, and by extension, its 
later quasi-analog, the hemoglobin molecule, are at the heart of nearly all 
life on earth.  The evolutionary chemical consequences of which, are why we know 
that octave of spectrum as "visible light" in the first place.)

John D