Hi Tony,
The aircraft reflections are usually recognizable. You can hear the
pulsations in the background noise change in rate as the airplane flies
around and sometimes even see the frequency shift on the waterfall, but
the big problem is that there is an almost constant, often fast, shift
in frequency on UHF that causes the phone signal voice pitch to change
in frequency so much and so fast that it is "chopped up". That seems to
be most prevalent when trying to cross several levels of propagation
enhancement and signal are stronger due to the enhancement, but also
when no enhancement is evident at all on the Hepburn maps
http://www.dxinfocentre.com/tropo.html, and signals are weaker. If both
stations are within the same level of enhancement, there is usually no
problem, even if both signals are weak. Since Contestia and Olivia keep
printing after the transmission has ceased, I suspect that the
interleaving and redundancy is carrying enough data over the peaks and
valleys that we hear to produce perfect print, but I am no theorist on
this, for sure!
In the case of PSK125R, you can see the carriers severely shifted in
frequency on the waterfall and becoming jagged lines instead of straight
ones. Decoding is perhaps 10% compared to Olivia and Contestia which
will generally be 95% to 100% under the same conditions, depending upon
the S/N. A CW note at the same time sounds like a "buzz saw" instead of
a note. Even the very narrow Contestia or Olivia modes often fail, so
we have found that 16-500 is the best compromise between sensitivity,
speed, and bandwidth on UHF. Olivia or Contestia 8-250 theoretically
should be 1 dB more sensitive, but usually have more errors than 16-500
when signals are near the noise threshold. The wider PSK250R and PSK500R
modes, even though wider, are not usable because they require a greater S/N.
We have been looking for a mode that will provide 24/7 coverage of our
state (over a 120 mile radius) with reasonable antenna gains (14 dBi)
and power (100 watts). HF does not do it because propagation changes
over the course of 24 hours, 6M propagation is always very spotty, and
often propagation, when there is no enhancement on 2M, is worse than on
70cm, so we are fortunate to find that Olivia and Contestia work well
enough on 70cm to do the job.
73 - Skip KH6TY
Tony wrote:
FWIW, PSK125R does not survive the Doppler disturbances on UHF.
Olivia or Contestia does. Therefore, the mode we have found works
best under the severe conditions of multipath, Doppler shift, Doppler
spread, and very weak signals is Contestia 16-500 at 30 wpm. the
minimum S/N is -12 dB, which is essential for weak signal UHF and VHF
digital operation as every dB of S/N we can get is important for weak
signal work.
73 - Skip KH6TY
Skip,
Thanks for the info. The path simulator results seem to agree with
your observations on the high bands. I bet you have a fair share of
problems with aircraft Doppler? I've noticed multiple reflections from
multiple aircraft while monitoring VHF beacons. Spectrum analysis
reveals how great those Doppler shifts can be; the mixing of 2 or 3
multipath signals can play havoc with throughput. .
While it's not quite the same, we didn't get a chance to test on HF
today. Andy tried his best to accomodate (always there for us Andy),
but conditions weren't good between us on 80 meters. Tomorrows another
day.
Thanks again.
Tony -K2MO
