(crossposted to NVIS and Digital Radio) The effect of the ionosphere on digital signals was discussed on a webpage that I coincidentally read this morning while looking at soundcard modes.
Murray ZL1BPU has created 'DominoEx', with the details at http://www.qsl.net/zl1bpu/DOMINO/Index.htm NVIS is specifically mentioned, with the mode tolerant of multipath signals. Murray writes on the page that the mode was intended for use with NVIS, so it might be worth considering. It was announced to the world December 2005, so there might be some people who have used it. I'd be interested to hear of people's operating experiences with digital modes and NVIS. Cheers, David. -- David Ingram (VK4TDI, ZL3TDI) Brisbane, Queensland, Australia http://www.ingramtech.com/ MGRS: 56J MQ 991583 Grid Square: QG62lm expeditionradio wrote: > As I understand it... > The path loss for the ground bounce at HF is likely to attenuate the > delayed single-bounce signal, and especially the multi-bounce signal. > This attenuation from the ground bounce signals usually results in a > significantly exalted no-bounce skywave direct signal for NVIS. > > The question then becomes: How much immunity is there with PSK > decoding to the time shifted interfering signal's amplitude? > > Digital decoding of PSK may not be much of a problem, because there is > somewhat of a threshold effect with decoders, but still, phase shift > symbol rate becomes the important factor here. What we need is a > relative figure of merit for comparing PSK at various symbol rates and > various other signal methods in these sort of channel conditions. The > flavor of noise in the channel can often be a factor... especially > impulse noise. > > The RF reflectivity of the earth at the bounce point has a significant > effect on the bounce's attenuation value. So, the attenuated bounce > may be more of a problem on sea water (-3dB to -12dB?), than when the > bounce is on land (-6dB to -30dB?). I'm guessing at these attenuation > values, for the purpose of discussion, rather than looking up the real > data :) > > Also, when operating freq is far below FoF2, or in propagation > conditions that have QSB, with a rapidly varying sharp D layer > absorbtion cutoff frequency knee, the multi-bounce signal may become > larger than the primary NVIS signal when the lower angle incident > skywave signal is attenuated. But, most of the time, this isn't seen > much though, since normally the D layer absorbtion is a rather smooth > curve vs frequency and doesn't have a sharp cutoff at such high angles. > > The bounced multipath interference signals for NVIS can be much > stronger at MF frequencies than at HF, due to sometimes sharper > ionospheric high angle critical incidence at MF and the increased > reflectivity of the ground at MF in some areas. This is a common > problem for AM broadcast DXing where phase cancellations often distort > the signal at night. > > This was a significant factor in LORAN, especially when flying over > the ocean, and if you have ever watched the old LORAN scopes in > aircraft, it can sometimes be rather tricky to discern which is the > real signal to use at certain times of the day... fortunately, most > experienced navigators were pretty good at it! > > Well, enough rambling for now... I gotta get back to work. > > 73 Bonnie BA7/KQ6XA
