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
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