On Tue, Apr 20, 2010 at 03:32:18AM -0400, KH6TY wrote: > Hi Tony, > > Thanks for making the recording of aircraft reflections. Yes, we also > see and hear aircraft reflections mixed with atmospheric disturbances > all the time. The aircraft reflections sound similar to what you hear on > the beacon, and you can identify those because they vary in frequency > and intensity as the airplane approaches or recedes, just like you hear. > > However, what we experience on UHF over longer paths is a constant > "chopping" up of the SSB phone signal, or narrow digital signals, and > that seems to correlate with the Hepburn propagation maps, especially > when the path crosses two or more levels of ducting, when signals can be > strong, but SSB is still not very understandable. 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, like > Olivia or Contestia, which continues to print for a couple of seconds > after transmission has ceased (due to the interleaving and FEC, I guess) > print is perfect. The frequency of the audible chop is generally around > two to three times per second, which is less than the latency of the > digital mode. Those modes which display very little or no latency seem > to be the ones that fail to print. > > Over the next few weeks, we are now going to compare Contestia > variations with different bandwidths and latency to see how print > compares to the observed period of "chop" on SSB phone.
I find the above very interesting indeed. Jim, WB5UDE, and I have just begin a series of experiments on VHF (2m) digital mode communications. We live about 20 miles apart as the crow flies, I in Norman, OK, and Jim in west OKC, OK. We've both been using Yaesu FT-897D with 2m J-poles. Jim has been using FLDigi, and I have been using HRD with DM780 and MixW. Since I live down in the South Canadian River valley, there isn't much chance I'll get LOS contacts with anyone outside my immediate area, until I get antennas up on a 400' tower. I wish. So far we have found that the FH-style modes (OLIVIA, Contestia, THOR, etc.) don't work at all well for us. We tried some changes to bandwidth and bitrate for OLIVIA and Contestia, than tried very slow THOR-4 just to see if _any_ FH mode would work; no go, which was very most surprising. I'll see the signal level go well up past the squelch marker, but can't get a good decode. In contrast, the PSK-style modes (BPSK-31 and -63, so far) work very well, with solid copy once we get the decode passband overlaid on the received signal. This has been difficult for some reason: we have found we have to do it manually. The audible received signal isn't at all choppy, and I haven't heard fading, choppiness, or aircraft reflections, though there may be some, as Jim lives 2 miles S of Wiley Post airport and I live 1 mile S of Max Westheimer Airport in Norman, with the approach/takeoff paths for Will Rogers Airport directly between us. Jim and I are planning to have lunch together today, and I expect to work out a formal test program with him, setting out the mode/BW/rate list. Readers in the Metro OKC/Norman area are invited to join us. -- Mike Andrews, W5EGO mi...@mikea.ath.cx Tired old sysadmin
