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February 1st 04, 04:35 PM

Paul Keinanen wrote:

On Sun, 01 Feb 2004 01:33:02 -0000, (gudmundur)
wrote:

I was thinking of building
my small portable Loran-C receiver into a waterproof box, and towing an
insulated antenna wire of perhaps 10 feet length behind me.

You should include a low noise preamplifier in front of the receiver.
Due to the very high atmospheric noise levels on LF and MF bands, the
sensitivity for a typical receiver is very low (especially if a
ferrite rod antenna is used).

Below the surface, both the level of the wanted signal as well as the
atmospheric noise level will drop rapidly and sooner or later the
signal will be swamped by the receiver internal noise (high F figure).
A low noise preamplifier between the antenna and the receiver will
allow a weaker signal to be detected before being swamped by the
preamplifier internal noise when going deeper.

However, if the preamplifier is used close to the surface, it most
likely will overload the receiver, so a pressure switch should switch
out the preamplifier when you are (say less than 3-5 m from the
surface) on include the preamplifier into the receiver AGC loop.

Paul

In the case of a LORAN receiver, over load is not a serious matter. The
receiver only has to decide
if a signal is there or not. The LORAN is a pulsed signal and you only
have to detect timing of the pulses.
If the receiver limits strong signals and does not lose sensitivity you
should be OK

Good luck
Bill K7NOM