KV9U wrote: > Good points, Paul, > > One thing that I found with longer distance FM signals on HF, even > though 10 meters can be close to the MUF when it is open, is that there > is a lot of frequency inversion or other anomalies from the ionosphere > that make it rather annoying and unsatisfactory. This is not true with > narrow bandwidth modes such as SSB. If you were just using FM for local > communications it is much more satisfactory and we have found it can > compete well with 2 meter repeaters in some cases. Instead of dropping > out, the signal just gets weaker, but often still readable.
Given the comment in the original posting about the older receiver supporting NBFM and therefore it must "have once been permitted", I was reading the main question as the one of legality. I haven't tried AFSK over FM on 10 meters, but given what I've heard on 10M FM during the last sunspot maximum, I would definitely agree with you that the propagation characteristics -- which are obnoxious enough on FM voice -- would probably destroy a packet signal. During the sunspot minimum, though, it might provide convenient local links, although a different modulation scheme would give better results even for that. > This is almost like the attempt to use a mode such as digital voice on > the HF bands. It needs a very good S/N ratio to stay locked in. > > Even digital SSTV/FAX modes which fit into a regular narrow voice > bandwidth will display almost continuous damage to at least some of the > tones at any one time when you observe them on the waterfall. I've watched lots of MT63 on the lower end of the HF spectrum, and observe the diagonal lines from the fading so some extent *most* of the time. With 1200-baud AFSK/FM -- and NO FEC -- you'd get an overall raw data rate of 1200 bits per second *IF IT DECODES*. MT63 (in 2 kHz rather than 15 kHz bandwidth) would do 20 symbols of 7 bits, or 140 bits per second raw throughput. 1200 bps / 15 kHz = 80 bps per kHz of spectrum, compared to 140 bps / 2 kHz = 70 bps per kHz of spectrum: not significantly different for strong signals. But the difference on weak or fading signals would be 70 vs. nothing. So, while packet-over-FM on 10m is certainly legal, I'll certainly agree with you that it's still not necessarily the best idea. 73, - ps
