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Re: LF: Re: lf andnoise and offshore.

To: [email protected]
Subject: Re: LF: Re: lf andnoise and offshore.
From: "Frank Gentges" <[email protected]>
Date: Wed, 14 Jan 2004 20:11:51 -0500
In-reply-to: <014d01c3dac6$cd3aabe0$f89a8418@Peter>
References: <014d01c3dac6$cd3aabe0$f89a8418@Peter>
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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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