Very good information, Skip, If the signals are in the marginal range, how do you do the coordinating between the stations?
How do you calculate the error rate (such as the 6% mentioned)? If I understand this correctly, the test was between a 5/8 vertical to quad for vertical polarization vs. quad to quad for horizontal, wouldn't it be about right to see 6 dB difference considering that you are increasing the path budget with the inclusion of the quad? Several ways to do this is with quad to quad vertical and quad to quad horizontal polarization, or some other gain antenna that can switch properly between polarizations. I wonder if you would see such a difference? 73, Rick, KV9U kh6ty wrote: > Hi Rick, > >> Have you found that DominoEX is the best overall digital mode for FM? I >> know that PSK modes can have doppler errors from aircraft, but otherwise >> seem pretty good for weak signal. >> > > Yes, definitely! DominoEx is a frequency shift keying mode, not a phase > shift mode, but doppler problems are still sometimes a problem, but not > nearly as much as on PSK31 or PSK63, so that is one reason why we now use > DominoEx. Once the reflected signal arrives 180 degrees out of phase with > the direct signal, it cancels out the direct signal for a while and there is > no mode that is going to print under that condition. The wider, multitone > modes have less problem because the data is redundant and spread over the > width of the signal, but even they are no completely immune. However, on our > twice-weekly net, since we switched to DominoEx, the number of multipath > problems is considerably down, even on SSB. Initial tests suggest that > MFSK16 might even be better on FM, since it is the most sensitive mode we > currently have with almost enough speed for messaging. It is completely > unusable on VHF SSB, though, because many transceivers in the field are not > frequency-stable enough to stay tuned. On FM, the carrier frequency sweeps > over the entire passband, so only the audio frequency stability is > important. DominoEx is especially valuable for drifting signals on SSB, > because it can tolerate mistuning of 50% of the signal width. The IC-746Pro > and the FT-857D, if without an optional TCXO, just drift too much to be > usable on SSB, but are OK on FM, even though the S/N of FM is worse than on > SSB. Note that any multipath cancellations simply cause repeated blocks when > using ARQ, so they only slow down the transfer while the reflected signal is > moving across the direct signal. > > Last night on our net, we had positive confirmation of the better > performance of SSB over FM. The error rate between two stations was running > at 6% on SSB, but when we all switched to FM, there was zero copy. The fact > that there were any errors at all on SSB indicated that the stations were > fringe area to each other, so it was a good demonstration of the advantage > of SSB over FM. One station was beaming toward me and the other was 45 > degrees away from the beampath of that station. It was the same as if a > station with a high gain yagi were pointed away from me and even if I > pointed directly at him, he was not radiating enough energy in my direction > for me to copy him. We have to make more tests, but I think the secret of > the OptimizedQuad is that the pattern is bulbous instead of being > pencil-shaped - more like an omnidirectional pattern, but with gain over a > wide beamwidth. Stacking OptimizedQuads vertically would increase the gain > by 2.5 dB and still retain the wide beamwidth. It sure is interesting stuff! > > >> Your point is well taken that many of the hams who participate in public >> service activities, may tend to be the younger ones who are Technician >> class and can mostly operate on 6 meters and up with their vertical >> antennas and FM only rigs. The number of hams with the >> multimode/multiband rigs is increasing, at least in our area. It is not >> easy to get them to try SSB, much less SSB digital though. >> > > I have found that the main problem is lack of VOX with the FM transceivers, > which cost under $200 for a single band one, so you need to spend another > $100 for a SignaLinkUSB interface in order to use macros to do the PTT > switching. The FM Transceiver Interface solves that problem for only $10. I > have built 10 of them which I will be giving out to the first few people who > want to join the net but have only FM transceivers, but they also need to > have an OptimizedQuad, or small yagi, horizontally polarized. > > >> The claim about the ground gain for horizontal antennas may be true but >> I have not seen this definitely tested. Have you done some comparisons >> with low 2 meter antennas, such a mobile to low base antenna with V and >> H and found H consistently better? I don't hold too much stock in >> software modeling and only would go with empirical data for that kind of >> test. >> > > I have done only one test so far, as it is difficult to arrange, since both > stations have to switch polarization, but that first test did show a huge > advantage using horizontal polarization. Range on FM between a 5/8 > wavelength whip mounted on a Prius and my quad turned for vertical > polarization was only 25-30 miles, depending on whether or not the mobile > was clear of trees, but 70 miles was a piece of cake between the > OptimizedQuad and my own quad turned for horizontal polarization. We could > have gone even farther if we had time. Next opportunity, we hope to be able > to keep going. I am now more than convinced that the difference is real. > There was once a reference, which I cannot find, that found that a quad near > a ground surface retains a low takeoff angle, but the takeoff angle of a > yagi of the same gain increases to as much at 40 degrees off the horizon, > which means a quad may be the best choice for portable operation anyway. > > >> We will probably bite the bullet eventually and put a rotor back up on >> the low tower and maybe go with a Gulf Alpha 11 element V and H antenna >> for some reasonable gain. Then we could do the test. The ham that was >> going to help us lost his QTH and will not be able to relocate his VHF >> antenna farm. Of course they are quite high so maybe there would not >> have been as much difference in such a case. One of the best known VHF >> ops in my Section says that after running many tests he has never found >> either polarization is any different. But he has high antennas so maybe >> that accounts for it. >> > > Yes, high antennas are probably the reason. At seven wavelengths from real > ground, the disadvantage to using vertical polarization over horizontal > drops from 6 dB at two wavelengths to only one dB at seven wavelengths, but > portable stations or mobiles generally are not going to be able to get have > antennas much higher than 2 wavelenghts. The jury is also out whether > horizontal polarization is an advantage over several hundred miles. I will > not be able to test this until the coastal tropo scatter season comes back > in the spring. > > If your yagi has more gain that you need, you can just rotate it 45 degrees > and cover both polarizations, but with a 3 dB gain loss on both. > > >> We hope at least soon do some digital mode comparisons on 2 meters, >> whether SSB or FM. >> >> 73, >> >> Rick, KV9U >> > > That would be great! We need as much information as we can get, especially > since lower South Carolina is quite flat, with no hills until you get to the > upper part of the state. We do know this for sure - using a sensitive > digital mode with either SSB or FM greatly extends the range over using > phone, simply because the digital mode can copy under the noise level and > phone cannot. The average modulation of a phone signal is only 30%, or maybe > 50% with compression, but the passband needs to be over 2 KHz. With a narrow > digital mode, the DSP filters in the software (and at IF if available) can > be used to narrow the noise window by at least four times, improving the S/N > by 6 dB or more and still use 100% average modulation for another 3 dB or > more improvement in S/N. You simply cannot do this with phone and remain > intelligible, and you cannot use redundancy with phone as you can with > digital modes. > > > 73, Skip KH6TY > NBEMS Development Team > >
