"Gary" <grwes...@...> wrote:
>Maybe there is something that the path simulators are missing.
It could be Gary and I think it's fair to say that propagation
simulators are just not capable of capturing all the variables of the HF
channel. They do, for the most part, capture the fundamental
characteristics of the real thing and that has been proven to be useful
when comparing the performance of different HF modems.
As far as NVIS, the simulator creates a multi-path condition that tries
to emulate both ground waves and sky waves arriving at the receiver; a
condition that can destroy throughput with certain modes regardless of
how strong the signals are.
NVIS field tests would have to meet this criteria in order to prove or
disprove whether an NVIS path simulation was useful in determining which
modes work best for this kind of propagation. The trick is recognizing
the presence of both ground waves and sky waves.
I doubt if there's enough ground wave signal strength to cause any real
problems on the longer NVIS paths; it's the relatively short paths that
give the most trouble and of course, that depends on terrain, antennas etc.
Thanks for the input Gary. I'd be interested in any NVIS recordings you
have. Feel free to send them and don't worry about file size (within
reason of course : ).
73 Tony -K2MO
.
--- In digitalradio@yahoogroups.com
<mailto:digitalradio%40yahoogroups.com>, "Gary" <grwes...@...> wrote:
>
> Your question is one that I have also. In our recent NVIS testing
with fldigi/flarq we found BPSK250 provided better throughput than
other modes we tested, and most notably MFSK32 which we thought would
be our safe, robust mode.
>
> This was with a variety of band conditions including strong signal,
weak signal, selective fading, lightning QRN, grungy power line noise,
and all the normal stuff we experience here in the Midwest. Path
distances varied from 40 miles to 150 miles which were the distances
we were interested in.
>
> There were a couple ideas we kicked around as possible reasons why
BPSK250 worked so much better than we expected. One was that when the
signal took a hit from something like a lightning burst, BPSK250
recovered and resynchronized very fast. The second was even more
speculative in that maybe the higher phase modulation rate (250 times
per second) was faster than Doppler path modulation allowing the BPSK
decoder to ride through.
>
> Anyway, we expected BPSK250 to be useless on NVIS but every time we
have tried it, it has worked. (and better than most modes.) Maybe
there is something that the path simulators are missing.
>
> Dunno... Just throwing some ideas out.
>
> Gary - N0GW
