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
