Re: [digitalradio] VHF / UHF Digital Beyond line-of-sight [5 Attachments]
On 4/20/2010 3:32 AM, KH6TY wrote: Hi Tony, 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 - print is perfect. It sounds like there are two different propagation modes in play Skip. The steadier signals that tend to coincide with the Hepburn maps would appear to be coming from real tropospheric ducting (which says a lot for those maps) while the other mode may be tropospheric scatter. For what it's worth, the path simulator can emulate the rapid fade characteristics you mentioned by introducing low-frequency Doppler spread. This seems to coincide with the 2 to 3 fades per-second you mentioned (see profiles jpg). The fade frequency tends to become more rapid as the Doppler spread frequency is increased. It's difficult to say what's really going on, but the digital modes themselves may tell us something. We know for a fact that narrow-band PSK modes cannot tolerate Doppler spread while MFSK modes have little or no trouble coping. This seems to be the situation with your tests on 432 and suggests that the throughput failures are Doppler induced. I think you can determine if Doppler spread is present, but it's not going to show up in the waterfall with most digital modes; it needs to be fairly intense for that to happen. I've found that the best approach is to measure the spread of a carrier signal using Spectran or SBSpectrum. The frequency-spread carrier will appear broad compared to a normal signal; the software magnifies the effect -- see SBspectrum images 1 and 2. As you can see in the waterfall images (1 and 2) it's difficult to tell the difference between mild Doppler spreading at 0.25Hz and more intense Doppler spread at 5Hz, yet the difference is night and day in terms of throughput with narrow modes. Of course you can use the tuning indicator with PSK31, but it's not as precise. A few more questions: Are there times when the fading frequency increases beyond 2 or 3 Hz? Are the choppy signals generally weaker than those that coincide with the Hepburn maps? What are the distances between your QTH and the stations you work on VHF/UHF? Have the narrow modes like PSK31 worked at all on what seems to be tropo-scatter mode? Looking forward to hearing more about the VHF/UHF digital tests Skip. Thanks, Tony -K2MO
Re: [digitalradio] VHF / UHF Digital Beyond line-of-sight
Tony, thanks for images. We will try with some single carriers next week and see what happens. The conditions we are currently testing under are with no ducting at all on the Hepburn map, so I assume it is all tropospheric scatter. This morning, SSB phone was very badly chopped up, but signals varied from S1 to S4, so we had another opportunity to test digital modes. We tried DominiEx 11, DominoEx 11 with FEC, Thor 11, and Contestia 16-500. In each case, Contestia produced about 90% copy (there were a few words with errors), whereas DominoEX 11, DomimoEx 11 with FEC , and Thor 11 had over 50% errors. We could have tried the wider Thor and DominoEx modes, but then the minimum S/N would not be good enough. It was not quite good enough with Contestia for 100% print anyway. Next week we will compare Olivia to Contestia to see if we can confirm Jaak's simulation findings. No more tests possible this week. 73 - Skip KH6TY Tony wrote: [Attachment(s) #TopText from Tony included below] On 4/20/2010 3:32 AM, KH6TY wrote: Hi Tony, 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 - print is perfect. It sounds like there are two different propagation modes in play Skip. The steadier signals that tend to coincide with the Hepburn maps would appear to be coming from real tropospheric ducting (which says a lot for those maps) while the other mode may be tropospheric scatter. For what it's worth, the path simulator can emulate the rapid fade characteristics you mentioned by introducing low-frequency Doppler spread. This seems to coincide with the 2 to 3 fades per-second you mentioned (see profiles jpg). The fade frequency tends to become more rapid as the Doppler spread frequency is increased. It's difficult to say what's really going on, but the digital modes themselves may tell us something. We know for a fact that narrow-band PSK modes cannot tolerate Doppler spread while MFSK modes have little or no trouble coping. This seems to be the situation with your tests on 432 and suggests that the throughput failures are Doppler induced. I think you can determine if Doppler spread is present, but it's not going to show up in the waterfall with most digital modes; it needs to be fairly intense for that to happen. I've found that the best approach is to measure the spread of a carrier signal using Spectran or SBSpectrum. The frequency-spread carrier will appear broad compared to a normal signal; the software magnifies the effect -- see SBspectrum images 1 and 2. As you can see in the waterfall images (1 and 2) it's difficult to tell the difference between mild Doppler spreading at 0.25Hz and more intense Doppler spread at 5Hz, yet the difference is night and day in terms of throughput with narrow modes. Of course you can use the tuning indicator with PSK31, but it's not as precise. A few more questions: Are there times when the fading frequency increases beyond 2 or 3 Hz? Are the choppy signals generally weaker than those that coincide with the Hepburn maps? What are the distances between your QTH and the stations you work on VHF/UHF? Have the narrow modes like PSK31 worked at all on what seems to be tropo-scatter mode? Looking forward to hearing more about the VHF/UHF digital tests Skip. Thanks, Tony -K2MO
Re: [digitalradio] VHF / UHF Digital Beyond line-of-sight [1 Attachment]
On 4/21/2010 3:25 PM, KH6TY wrote: This morning, SSB phone was very badly chopped up, but signals varied from S1 to S4, so we had another opportunity to test digital modes. We tried DominiEx 11, DominoEx 11 with FEC, Thor 11, and Contestia 16-500. In each case, Contestia produced about 90% copy (there were a few words with errors), whereas DominoEX 11, DomimoEx 11 with FEC , and Thor 11 had over 50% errors. Skip, Your results seem to agree with the Doppler tests I ran with the path simulator. I found that there's an obvious difference in how much Doppler spread each mode can handle and Olivia tends to be the most tolerant. Frequency spreading does cause the rapid fade effect we spoke about and in this test, the fades are faster than the 2 to 3Hz you mentioned. There's a sample of the Doppler spread audio attached to this mssage. The first half is a normal MT63 signal without distortion; the second half shows the effect of frequency spreading (7 Hertz). Tony -K2MO Path Simulation: Frequency Spread 7 Hz SNR -3db THOR11 tiq Rck brown fox juc ekver the la0 nr e;5yd G to lsGa tmps over the lazy dog taAHk brown fox jumpoOireCoer DominoEX11 riefox zukpl over theeizydqtT theepuick brocrfak Iuksl ower te layy dty the quidT ßtwn xox jpsovtr the lazj hoz DominoEX11 FEC e quick bÄwn fox jumps over the laonithe q¸?yeXºe ecteips oveords oo¯he quixoôroc ávs over the lazy d o Aquick bmt ª?ox jumps over the lazy dog Olivia 8-500 the quick brown fox jumps over the lazy dog the quick brown fox jumps over the lazy dog the quick brown fox jumps over the lazy dog. Olivia 16-500 the quick brown fox jumps over the lazy dog the quick brown fox jumps over the lazy dog the quick brown fox jumps over the lazy dog. PSK31 (no print) PSK63 the /ui btown fox 6smps om r laoty dogt he qutc^(TM)I own fo jumps over tme la_ogl the |-ipk yrown fox j om on er hlazb dog Tony wrote: On 4/20/2010 3:32 AM, KH6TY wrote: Hi Tony, 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 - print is perfect. It sounds like there are two different propagation modes in play Skip. The steadier signals that tend to coincide with the Hepburn maps would appear to be coming from real tropospheric ducting (which says a lot for those maps) while the other mode may be tropospheric scatter. For what it's worth, the path simulator can emulate the rapid fade characteristics you mentioned by introducing low-frequency Doppler spread. This seems to coincide with the 2 to 3 fades per-second you mentioned (see profiles jpg). The fade frequency tends to become more rapid as the Doppler spread frequency is increased. It's difficult to say what's really going on, but the digital modes themselves may tell us something. We know for a fact that narrow-band PSK modes cannot tolerate Doppler spread while MFSK modes have little or no trouble coping. This seems to be the situation with your tests on 432 and suggests that the throughput failures are Doppler induced. I think you can determine if Doppler spread is present, but it's not going to show up in the waterfall with most digital modes; it needs to be fairly intense for that to happen. I've found that the best approach is to measure the spread of a carrier signal using Spectran or SBSpectrum. The frequency-spread carrier will appear broad compared to a normal signal; the software magnifies the effect -- see SBspectrum images 1 and 2. As you can see in the waterfall images (1 and 2) it's difficult to tell the difference between mild Doppler spreading at 0.25Hz and more intense Doppler spread at 5Hz, yet the difference is night and day in terms of throughput with narrow modes. Of course you can use the tuning indicator with PSK31, but it's not as precise. A few more questions: Are there times when the fading frequency increases beyond 2 or 3 Hz? Are the choppy signals generally weaker than those that coincide with the Hepburn maps? What are the distances between your QTH and the stations you work on VHF/UHF? Have the narrow modes like PSK31 worked at all on what seems to be tropo-scatter mode? Looking forward to hearing more about the VHF/UHF digital tests Skip. Thanks, Tony -K2MO __ Information from ESET NOD32 Antivirus, version of virus signature database 5048 (20100421) __ The message was checked by ESET NOD32 Antivirus. http://www.eset.com
Re: [digitalradio] VHF / UHF Digital Beyond line-of-sight
Yes, the chopping of the SSB voice is very similar to what the second half of the MT63 signal sounds like. Next week, we'll try MT63 and just compare the sounds to SSB phone over the air. Thanks for the simulation! If you get a chance, please run Contestia against Olivia and see if you get any difference in print. 73 - Skip KH6TY Tony wrote: [Attachment(s) #TopText from Tony included below] On 4/21/2010 3:25 PM, KH6TY wrote: This morning, SSB phone was very badly chopped up, but signals varied from S1 to S4, so we had another opportunity to test digital modes. We tried DominiEx 11, DominoEx 11 with FEC, Thor 11, and Contestia 16-500. In each case, Contestia produced about 90% copy (there were a few words with errors), whereas DominoEX 11, DomimoEx 11 with FEC , and Thor 11 had over 50% errors. Skip, Your results seem to agree with the Doppler tests I ran with the path simulator. I found that there's an obvious difference in how much Doppler spread each mode can handle and Olivia tends to be the most tolerant. Frequency spreading does cause the rapid fade effect we spoke about and in this test, the fades are faster than the 2 to 3Hz you mentioned. There's a sample of the Doppler spread audio attached to this mssage. The first half is a normal MT63 signal without distortion; the second half shows the effect of frequency spreading (7 Hertz). Tony -K2MO Path Simulation: Frequency Spread 7 Hz SNR -3db THOR11 tiq Rck brown fox juc ekver the la0 nr e;5yd G to lsGa tmps over the lazy dog taAHk brown fox jumpoOireCoer DominoEX11 riefox zukpl over theeizydqtT theepuick brocrfak Iuksl ower te layy dty the quidT ßtwn xox jpsovtr the lazj hoz DominoEX11 FEC e quick bÄwn fox jumps over the laonithe q¸?yeXºe ecteips oveords oo¯he quixoôroc ávs over the lazy d o Aquick bmt ª?ox jumps over the lazy dog Olivia 8-500 the quick brown fox jumps over the lazy dog the quick brown fox jumps over the lazy dog the quick brown fox jumps over the lazy dog. Olivia 16-500 the quick brown fox jumps over the lazy dog the quick brown fox jumps over the lazy dog the quick brown fox jumps over the lazy dog. PSK31 (no print) PSK63 the /ui btown fox 6smps om r laoty dogt he qutc^(TM)I own fo jumps over tme la_ogl the |-ipk yrown fox j om on er hlazb dog Tony wrote: On 4/20/2010 3:32 AM, KH6TY wrote: Hi Tony, 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 - print is perfect. It sounds like there are two different propagation modes in play Skip. The steadier signals that tend to coincide with the Hepburn maps would appear to be coming from real tropospheric ducting (which says a lot for those maps) while the other mode may be tropospheric scatter. For what it's worth, the path simulator can emulate the rapid fade characteristics you mentioned by introducing low-frequency Doppler spread. This seems to coincide with the 2 to 3 fades per-second you mentioned (see profiles jpg). The fade frequency tends to become more rapid as the Doppler spread frequency is increased. It's difficult to say what's really going on, but the digital modes themselves may tell us something. We know for a fact that narrow-band PSK modes cannot tolerate Doppler spread while MFSK modes have little or no trouble coping. This seems to be the situation with your tests on 432 and suggests that the throughput failures are Doppler induced. I think you can determine if Doppler spread is present, but it's not going to show up in the waterfall with most digital modes; it needs to be fairly intense for that to happen. I've found that the best approach is to measure the spread of a carrier signal using Spectran or SBSpectrum. The frequency-spread carrier will appear broad compared to a normal signal; the software magnifies the effect -- see SBspectrum images 1 and 2. As you can see in the waterfall images (1 and 2) it's difficult to tell the difference between mild Doppler spreading at 0.25Hz and more intense Doppler spread at 5Hz, yet the difference is night and day in terms of throughput with narrow modes. Of course you can use the tuning indicator with PSK31, but it's not as precise. A few more questions: Are there times when the fading frequency increases beyond 2 or 3 Hz? Are the choppy signals generally weaker than those that coincide with the Hepburn maps? What are the distances between your QTH and the stations you work on VHF/UHF? Have the narrow modes like PSK31 worked at all on what seems to be tropo-scatter mode? Looking forward to hearing more about the VHF/UHF digital tests Skip. Thanks, Tony -K2MO __ Information from ESET NOD32
Re: [digitalradio] VHF / UHF Digital Beyond line-of-sight
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. 73 - Skip KH6TY
Re: [digitalradio] VHF / UHF Digital Beyond line-of-sight
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
[digitalradio] VHF / UHF Digital Beyond line-of-sight
Skip, Thanks for the UHF screenshots. There has to be motion or change taking place within the refractive medium to cause Doppler and it would seem this holds true for the type of atmospheric enhancement found on VHF/UHF. I can see why it's necessary to use the most robust modes if both Doppler spread and Doppler shift are present much of the time; I'm sure aircraft scatter is no help either. Speaking of aircraft reflections; I recorded a short video of the effect today using a local 2 meter beacon as the signal source (see link below). The beacon is only 10 miles from my home and the AC Doppler can be quite strong; often competing with the S9+ signal. You can hear (and see) the two signals beat together as the AC Doppler sweeps across the spectrum. There's quite a bit of air traffic out this way so it would be easy to test how much of an effect this has on digital. I'll have to pursued a local buddy of mine to participate. Long distance VHF/UHF will have to wait until I can find a place for my Yagi's. Aircraft Doppler recording -- http://www11.zippyshare.com/v/10329668/file.html 7 Zip extraction utility -- http://downloads.sourceforge.net/sevenzip/7z465.exe Tony -K2MO
