CLAIM OF ZL TO W4 AMATEUR LF DX
4 December 2001
On Saturday 1 December ZL6QH sited near Wellington, New Zealand transmitted
a low speed frequency shift keyed test signal in the 136 kHz band. This was
the first time that DX testing in he 136 kHz band has been tried by ZL6QH,
since having the 135.7 - 137.8 kHz band recently added to the club station
Low Frequency Permit. Various DX listeners had prior knowledge of the ZL6QH
test transmission, which ran for all hours of local darkness.
Dexter W4DEX managed to receive and positively identify signals from ZL6QH.
The path length from ZL6QH to W4DEX in North Carolina has been calculated as
13,633 km. W4DEX used receiving equipment with the audio fed to a computer
sound card for filtering and display of the signal. This is now a popular
technique used for "below the noise" detection of amateur LF signals, as
ZL6QH was using 120 second dot length (each transmitted bit was 2 minutes)
and there can be nil detected by ear, yet coding of the wanted signal can be
seen after DSP filtering and displaying on the screen. The screen display
can be captured and saved as a file, as a record of what was received, and
for passing to others by electronic means.
Best reception was reported as being between about 0904 and 0950 UTC, which
is approximately one to two hours after ZL6QH sunset at 0800 UTC and about
two to three hours before W4DEX sunrise at 1212 UTC.
The radio equipment used by W4DEX was a square loop antenna, 2.3 metres per
side, balanced pre-amplifier feeding an HP3586B selective level meter.
The ZL6QH transmitted signal used dual frequency keying, with 0.4 Hz
frequency shift. The uniquely coded transmission consisted of repetitive
sending of QQQQQ. sent as 137.7900, 137.7900, 137.7896, 137.7900 and then a
gap, with all elements being of 120 seconds (2 minutes) duration. This
means sending a single Q takes ten minutes. The timing was such that the
start of a fresh Q was on the hour, so coding by absolute timing was another
factor that could be used for analysis of weak signal detection.
The ZL6QH LF signal was generated using a modified TS-850SAT, in SSB mode,
fed with audio from a lap top PC, using soundcard software developed by
Steve VK2ZTO. TS-850 frequency control was by means of a high performance
TCXO master oscillator. The low level LF signal from the TS-850 transverter
port was fed to an external LF power amplifier, and the output power was
around 500 watts. The radiated power is estimated to be 4 watts. The
antenna was a long wire at the Quartz Hill club station, which is run by the
Wellington Amateur Radio Club. The ZL6QH operator was Bob ZL2CA.
The next test from ZL6QH in the 136 kHz band will be on 15 December.
Bob Vernall ZL2CA
Organiser of the ZL LF DX tests
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