-------- Original Message --------
Subject: Re: WOLF TX freq question
Date: Tue, 25 Dec 2001 06:35:16 +0100
From: Stewart Nelson <[email protected]>
To: Dexter McIntyre W4DEX <[email protected]>
References: <[email protected]>
<[email protected]> <003f01c18613$7ade1580$0400000a@parissn2>
<[email protected]>
Hi Dexter,
I am quite curious about the S/N being better at 7.45 kHz. I would
think that the Tx antenna efficiency would be proportional to at least
the cube of frequency, and noise should be lower near 9 kHz as well.
Perhaps you have some local QRM which is higher near 9 kHz. If so,
you should observe changes in receiver output when tuning. If not,
there might be a Tx antenna coupling problem at higher frequencies.
Do you have a way to measure antenna current?
IMO, there are two aspects of VLF which are very interesting. One is
that the path stability should permit systems which "work at any SNR",
by employing some sort of frequency and time synchronization. The
easiest implementation would use GPS receivers with 1 pps outputs, or
very accurate oscillators (Rb or better), but LORAN signals could also
be used. We would need to do some real-time software. On the Tx
side, we would feed the 1 pps, or audio from an LF receiver tuned to
LORAN, into the sound card; the software would output the modulated
signal directly. For Rx, the amplified and filtered antenna signal
would feed one sound card channel; the reference would feed the other.
The software would simply sum multiple copies of the repeated message
until decoding was successful. I believe that such a system, if
allowed 10 hours, would be 20 dB more sensitive than the present WOLF,
which has a maximum useful integration time of about 10 minutes, and
loses 3 dB for its reference signal.
At VLF, the earth and ionosphere form a "waveguide" which makes the
signal at 10000 miles only slightly weaker than at 1000 miles. It
also varies little from day to night or with the seasons. I don't
know if it is possible for a reasonable amateur Tx antenna to achieve
such distances, but measurements at shorter distances should show if
it might be feasible. This type of propagation falls off quite
rapidly below 10 kHz, which is one reason for my concern above. Do
you have any figures or charts for this type of propagation?
73,
Stewart
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