There was some packet activity on 29 MHz in the 90's . While I could do a LOT of forwarding at 1200 baud on 28.18 MHz at 1200 baud using a SSB radio, I was NEVER lucky to get a connection at 1200 baud FM AFSK, even when I heard some of them. SNR was too bad.
The numbers tell that such a link is entirely disadventageous using a "subcarrier" on FM against raw 1200 baud FSK over a SSB radio. Jose, CO2JA --- Paul L Schmidt, K9PS wrote: > 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 >
