VLF,
Last saturday, Michael Oexner has successfully and undoubtly
received the message i transmitted on the 600m spaced earth
electrode antenna on 8970 Hz. His RX antenna is a PA0RDT
design. SpecLab was used to receive the signals.
The applied TX power was about 250 W and the mode was DFCW-600. The
antenna current was measured to be 530 mA. The QRN was reasonably low
although it could have been better to transmit 2 hours before.
There are 2 screenshots in one picture: The upper (in UTC) shows the
result with a SpecLab internal band filter (BW= 6 kHz, centered arround
9 kHz). A clipping level of 6 dB was applied + the noise blanker set to
0.002/9/0.05. The lower (in UTC+2h) shows the same signal without using
the internal SpecLab filter and without clipper, so just the blanker.
You can see the difference! This means, as impressive demonstrated by
Paul Nicholson on http://abelian.org/vlf/ss100801/, that bandfiltering
is very important on VLF. Mostly it means to exclude the mains hum and
the strong MSK transmitters. A too narrow filter is not optimal since
the blanker has to "see" the QRN to blank it then. Thus i modified my
VLF grabber circuit and have thrown out the 9 kHz band filter.
After increasing the earth antennas efficiency soon, the signal should
be up to 10 dB stonger :-)
73, Stefan/DK7FC
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