[Lowfer] MLS QRSS120

[Garry Hess]

Mark, operator of lowfer MLS, inquired about QRSS120 operation at 
http://lwca.org/mb/msg/3707.htm. Since I can't include small images 
there I'll make some comments here.

Speeds even far lower than QRSS120 have been successfully used at VLF. 
See, for example, the "dream" band work of Stefan, DK7FC at 
http://www.qsl.net/dd7pc/dk7fc_vlf_tx.htm. However, for lowfers I think 
there is a limit to sensitivity gain by increasing the transmission 
length and QRSS60 is about it (of course if you have a callsign like EH 
you can send dots at QRSS180 to be equivalent to dashes at QRSS60). 
That's because signals near sensitivity tend to appear as a jumble of 
shorter transmissions separated by dropouts on the waterfall rather than 
a continuous line, however weak and faded that might be. See the 
attachment of MLS daytime reception here just taken. MLS is sending 
QRSS60 but ARGO at QRSS30 is used for that capture.

Hifer experiments with VK2XV show that better detection can be achieved 
if the frequency is ramped instead of maintained at a fixed value. See, 
for example, http://vk2xv-hifer.blogspot.com/2012_05_01_archive.html. 
That's because the dropout jumble is then not important - all that's 
needed is a clear up-slope or down-slope. I'm planning on implementing 
such "slant" coding for the SIW lowfer next winter on days it operates 
at 185.185 kHz. Slant code has a time efficiency advantage like DFCW but 
double the spectrum width unless the starting points for dots and dashes 
are offset.
-- 
73, Garry, K3SIW, EN52ta, Elgin, IL
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