Tony, thanks for images. We will try with some single carriers next week
and see what happens.
The conditions we are currently testing under are with no ducting at all
on the Hepburn map, so I assume it is all tropospheric scatter.
This morning, SSB phone was very badly chopped up, but signals varied
from S1 to S4, so we had another opportunity to test digital modes. We
tried DominiEx 11, DominoEx 11 with FEC, Thor 11, and Contestia 16-500.
In each case, Contestia produced about 90% copy (there were a few words
with errors), whereas DominoEX 11, DomimoEx 11 with FEC , and Thor 11
had over 50% errors.
We could have tried the wider Thor and DominoEx modes, but then the
minimum S/N would not be good enough. It was not quite good enough with
Contestia for 100% print anyway.
Next week we will compare Olivia to Contestia to see if we can confirm
Jaak's simulation findings. No more tests possible this week.
73 - Skip KH6TY
Tony wrote:
[Attachment(s) <#TopText> from Tony included below]
On 4/20/2010 3:32 AM, KH6TY wrote:
Hi Tony, When both stations are within the same ducting level, the
only audible Doppler effect is usually reflections from airplanes,
and sounds much like your recording. When there is no propagation
enhancement showing on the Hepburn maps, there is usually a fast,
constant, "chopping up" of the SSB phone signal, and when we switch
to a relatively wide digital mode - print is perfect.
It sounds like there are two different propagation modes in play Skip.
The steadier signals that tend to coincide with the Hepburn maps would
appear to be coming from real tropospheric ducting (which says a lot
for those maps) while the other mode may be tropospheric scatter.
For what it's worth, the path simulator can emulate the rapid fade
characteristics you mentioned by introducing low-frequency Doppler
spread. This seems to coincide with the 2 to 3 fades per-second you
mentioned (see profiles jpg). The fade frequency tends to become more
rapid as the Doppler spread frequency is increased.
It's difficult to say what's really going on, but the digital modes
themselves may tell us something. We know for a fact that narrow-band
PSK modes cannot tolerate Doppler spread while MFSK modes have little
or no trouble coping. This seems to be the situation with your tests
on 432 and suggests that the throughput failures are Doppler induced.
I think you can determine if Doppler spread is present, but it's not
going to show up in the waterfall with most digital modes; it needs to
be fairly intense for that to happen. I've found that the best
approach is to measure the spread of a carrier signal using Spectran
or SBSpectrum. The frequency-spread carrier will appear broad compared
to a normal signal; the software "magnifies" the effect -- see
SBspectrum images 1 and 2.
As you can see in the waterfall images (1 and 2) it's difficult to
tell the difference between mild Doppler spreading at 0.25Hz and more
intense Doppler spread at 5Hz, yet the difference is night and day in
terms of throughput with narrow modes. Of course you can use the
tuning indicator with PSK31, but it's not as precise.
A few more questions:
Are there times when the fading frequency increases beyond 2 or 3 Hz?
Are the "choppy" signals generally weaker than those that coincide
with the Hepburn maps? What are the distances between your QTH and the
stations you work on VHF/UHF? Have the narrow modes like PSK31 worked
at all on what seems to be tropo-scatter mode?
Looking forward to hearing more about the VHF/UHF digital tests Skip.
Thanks,
Tony -K2MO