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