Hi Tony, Thank you for the information. This is the issue with field tests, there are always several variables.
I will proceed with some other field tests, trying to eliminate some of the variables. In my case I have an inverted V on 40 and 80M at only 9 meters peak over the ground for the server. So if I use a low dipole for the client next time (2 or 4 meters high) instead of the vertical I assume I should be able to safely eliminate ground waves over that distance (95 miles). Am I correct in my understanding that there is still multipath and therefore selective fading in pure NVIS (no ground wave) conditions? Thanks again, Regards, John --- In [email protected], Tony <d...@...> wrote: > > John, > > The first thing that comes to mind is whether there were any ground wave > signals mixing with sky waves during your field tests? It's been shown > that NVIS throughput can fail when the sky wave echoes interact with > ground waves. The sky waves take more time to arrive at the receiver so > you can imagine what the difference in timing does to copy when the two > signals interact. This is what the NVIS simulations were based on; two > channels, one with no delay (simulated ground wave) and the other with a > 7 ms delay (simulated NVIS sky wave). > > January's path tests showed that PSK-R appeared to be less robust than > BPSK under NVIS simulation while the white noise tests clearly showed > PSK-R the winner in terms of sensitivity. Your field tests seem to > reveal the same results in terms of which modes have the edge in > sensitivity, but not necessarily the edge in terms of dealing with > multi-path timing delays. I could be wrong though and there may have > been strong evidence of ground wave interaction? It can be difficult to > tell; some paths are more obvious than others. Hellschreiber is the only > mode I know of that can visually indicate this sort of thing, but that's > not an option with PSKMail. > > Hope to hear from you soon John. > > Tony -K2MO > > > > > > n 4/1/2010 9:45 AM, vk2eta wrote: > > > > To Tony (K2MO) in particular, but not exclusively: > > > > Following your simulation results on these modes in January I have > > done a few tests in the field and I have to say that I don't > > understand the results. > > > > Please note that I am not trying to make a point, but to understand > > why the theory does not seem to match the practical side. > > > > My tests simply revolve around examining the bahaviour of the Pskmail > > server adapting speed to the conditions. > > > > We have in the latest version a table of modes that the server can use > > by shifting up and down, one mode at a time. It does so by relying on > > the s/n report gathered from Fldigi and the number of repeats due to > > damaged ARQ frames. > > > > The list is arranged in an empirical order of speed vs robustness and > > is the following for regions 2 and 3: > > > > THOR8 MFSK16 THOR22 MFSK32 PSK250R PSK500R PSK500 > > > > The MFSK/IFSK family of modes are normally the modes of choice for NVIS. > > > > This week I did some tests at 95 miles in a strait line from my server > > on 40 and 80M between about 1PM to 2PM local time so obviously in NVIS > > conditions. > > > > What I noticed every time I would connect in MFSK16, the server would > > progressively shift the TX mode up into the PSKR modes, up to PSK500R, > > but never to PSK500. > > > > I also noticed that there would be no fallback from PSK250R to MFSK32 > > after a shift up from MFSK32. > > > > So my interpretion is the following: > > > > If the PSKR modes had a weakness in NVIS conditions, I would see the > > server moving continuously between MFSK32 and PSK250R: good reception > > in MFSK32, speed up to PSK250R, poor reception, return to MFSK32, etc... > > > > Also since it did not go up pass PSK500R to PSK500 it indicates that > > in these particular cases the PSK500R modes was starting to show signs > > of limitations and the server calculated that there was not enough s/n > > margin to shift the speed up. > > > > Selective fading is very visible especially on the PSK500R mode of course. > > > > So my question is: in the simulation you performed, are there > > parameters that maybe would need to be looked at to explain why these > > modes seem to behave well in these conditions or are there other > > variables to consider? > > > > Also trying to get a more formal comparison, how would you design some > > practical tests that minimize the effects of variation in propagation > > in the field? > > > > On this point I was thinking of sending a set text in different modes > > and repeating the test several times, interleaving the modes so that > > in average it would be unlikely to be just propagation. Mode1, Mode2, > > Mode3, Mode4 then again Mode1, Mode2, Mode3 etc... repeated say 5 > > times. Then taking the average result for comparison. > > > > Best regards, > > > > John (VK2ETA) > > > > --- In [email protected] > > <mailto:digitalradio%40yahoogroups.com>, "vk2eta" <vk2eta@> wrote: > > > > > > Hi Tony, > > > > > > Thank you for the simulation results. I will report any field > > results for PSKR modes in NVIS conditions. > > > > > > Regards, > > > > > > John > > > > > > > > > > > > > __________ Information from ESET NOD32 Antivirus, version of virus > > signature database 4993 (20100401) __________ > > > > The message was checked by ESET NOD32 Antivirus. > > > > http://www.eset.com >
