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LF: 73kHz Today

To: [email protected]
Subject: LF: 73kHz Today
From: "James Moritz" <[email protected]>
Date: Tue, 1 Feb 2000 15:26:44 +0000
Organization: University of Hertfordshire
Reply-to: [email protected]
Sender: <[email protected]>
Dear LF Group,
Very pleased to work G3XTZ(589) again, and G3YXM (579) and G3LDO (569) for the first time on 73kHz this morning, in the gaps between transmissions from the Rugby TX. this brings my total to 6 stations worked on the band. I also heard G2AJV (559) sending a beacon signal, from about 0830 until 1040 when I left for work.
The noise in the sidebands of the Rugby signal is certainly - well,
prominent. Looking at the signal with Spectrogram, it seems to be
a pretty ordinary FSK type signal with 100Hz shift - has anybody
any ideas as to why it radiates so much noise? When it was off,
the band sounded like 136kHz on a quiet day, with very little QRN,
and the noise floor seemed to be mostly LORAN.
I also sent some QRSS calls between about 1000 and 1030, but
didn't see any others.
Later.... went home during lunch break to have another look. Rugby
was still off & on, and G2AJV was still 559 when it was off. Noticed
a mains buzz on the background - at first, I thought it must be a
dimmer or some such, but eventually it became apparent that
Rugby TX was transmitting an unmodulated carrier, and the 50Hz
was sidebands on the Rugby signal. Managed to look at the
spectrum using Spectrogram in bar graph mode - the sidebands
were between 45dB and 55dB down on the carrier, extending over
at least a few kHz bandwidth at intervals of 50Hz. Even multiples of
50Hz were stronger than odd multiples by roughly 10dB.
I would guess that the sidebands are due to power supply ripple,
and that when FSK modulation is on, all the sidebands get
modulated too. Since the data is probably not syncronised with the
mains, the end result is the amazing racket we are used to.
I haven't been able to make a direct comparison between noise
levels with Rugby on and off yet, but would guess it must bring the
noise level up by at least 20dB at my location. Roll on the first of
next month!
I tried giving G2AJV a call when he paused for a few minutes, but
no reply. I would guess a QSO should be possible, though.
Cheers, Jim Moritz
73 de M0BMU




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