John,
The "raspy" sound is similar to that associated with aurora, but this
far south, aurora is very rare, and the "raspy" tone is there almost all
the time, every day, if there is no propagation enhancement. So I don't
think it is caused by aurora, but if you picture how aurora looks
visually, with curtains of light moving about, it makes one wonder if
the tropospheric scattering is also unstable in a similar way. The
general consensus is that VHF/UHF communication over the curvature of
the earth (i.e. past line of sight ) is mostly by either tropospheric
scattering or by ducting. What makes the medium unstable in the manner
observed does not seem to be well understood. Check the Hepburn
prediction page for an excellent discussion of tropospheric scattering:
http://www.dxinfocentre.com/tropo.html scroll down to the bottom, past
the maps, and see the links in yellow - really fascinating reading!
73 - Skip KH6TY
Jon Maguire wrote:
Skip,
Just a thought, but "raspy" signals on VHF/UHF are usually associated
with aurora. Can you correlate that?
73... Jon W1MNK
PS Great discussion!!
KH6TY wrote:
Hi Jose,
We will be starting with tests of ROS 1 baud tomorrow but I will not
have any results until next week, after we have been able to make
tests over several days and under many different conditions. The
tests with ROS 16 baud have been finished and our results are as I
have already reported.
Perhaps if the spreading were much wider, say as much a 10 kHz or 20
kHz, the result "might" be better, but then nobody on UHF SSB has an
IF filter wider than 2.5 kHz anyway. It would probably take at least
a SDR on both ends, I think, but so far those are still rare, even
though they make excellent IF's for VHF and UHF transverters. So,
wider spreading is just not practical.
Whatever it is that is causing a "raspy" CW note, and "raspy"
sounding ROS tones, must be destroying the data modulation on the
carriers, but I do not know enough about the modulation technique or
the autocorrelation function that ROS uses to understand why that is
causing ROS to fail. Perhaps it is because EVERY tone in the bandpass
is so badly distorted that autocorrelation is not possible and
decoding fails (i.e. is the Doppler shift perhpas moving the carriers
outside some very narrow DSP filter?). As best I can remember from my
college days (50 years ago!), autocorrelation will only work if
reoccurring signals are identified among random noise, but if the
tones are distorted so they appear too much like the noise,
correlation may not be possible. I am sure experienced communications
theorists can make a better guess than I can! The Olivia tones are
also "raspy" sounding, but Olivia survives and ROS does not. When the
tones sound pure, ROS does OK, but that does not happen very often at
fringe area reception on UHF, and mostly only when there is
propagation enhancement.
73 - Skip KH6TY
I promised to post the results of our attempts to use ROS on UHF on
this reflector, and this is what we have found. So, it looks like
Olivia is currently still the best digital mode to use on UHF, VHF,
or HF for normal (not EME) digital QSO's.
Skip, please do tell us. I am particularly quite curious about the
results of your tests.
73,
Jose, CO2JA
