Peter,
AMRAD conducted several winter trips to the Outer Banks of North
Carolina to listen to LF signals in the days before transatlantic
amateur signals were common. The Outer Banks are a long narrow strip of
land several miles offshore from the North Carolina mainland. We
observed LF broadcast signals and compared to what we were seeing at
sites more inland were stronger. We attributed this signal difference
to sea gain and would put its value at 10 or more dB in the few cases we
observed although we did not make careful measurements.
The ITU has put together a model for radio propagation and includes a
factor for sea gain. It only works out to be worth a couple of dB in
the test cases we ran. I think the ITU model does not recognize the
amount of sea gain that is available from moving to the sea coast for
the really best conditions at least below 200 kHz.. I am convinced it
is there and is significant.
Also the noise was low but we could find quiet sites inland and we just
could not hear the Transatlantic signals nearly as well inland.
<caution, speculation on> I speculate that the LF waves are combining.
and adding in phase due to the long wavelength, at the sea water
interface and traveling in as surface waves stronger than the low angle
sky waves that created them.. Similar to glints or mirages we see
optically. <speculation off> We can hear other surface waves at 770
kHz on the Outer Banks from New York City duirng the day and night. We
cannot hear them much past a mile inland as the land kills the surface
wave propagation. While we did not conduct a similar experimant on LF I
suspect they would also fade down to the sky wave strength if we went
inland.
Frank K0BRA
Peter van Daalen wrote:
Sorry,
I made a most ennoying typo :
I wrote :
snip.....
" Does sea again ( if any at all ? ) and low man made and evironment noise
add
substantially to the LF performance ? "
snip.....
While I meant to write :
" Does seagain ( if any at all ? ) and etc... add substantially to LF
performance ? "
I do know something on sea gain on 144 Mhz ( for /MM EME purpose on moonrise
and moonset ) but I am unaware of the seagain effects on LF.
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