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Rich has presented good information about CW and propagation signal strengths and here’s some more communication theory which could perhaps stimulate some thoughts.
There are two equations which can apply to the situation of getting information through a system. One states that the noise power at a receiver is proportional to the square root of the bandwidth of the signal (and also the temperature and input resistance). The smaller the bandwidth, the less the noise. The bandwidth required depends on how much information is sent per second. Video and stereo music are examples which require a high bandwidth and the receiver will need a wide bandwidth thus will receive more noise. To overcome this high transmit powers are used so the signal to noise ratio is acceptable.
Voice is lower bandwidth so less noise is heard and lower powers will be acceptable. CW requires a very low bandwidth since the information rate is low and the S/N ratio can be acceptable if the receiver bandwidth is very narrow. In short, if you leave the receiver door open enough to receive a wide bandwidth it will also be open to receive more noise.
To take advantage of CW the receiver needs to have a very narrow bandwidth.
Another equation is Shannon’s equation which is more theoretical than practical but essentially states that the maximum rate of digital data transmission depends on the bandwidth and the signal to noise ratio. Web search it if you are interested.
An accurate estimate of the possible range would need to consider the noise figures of the receiver, the bandwidth, the signal strength, and the modulation method. Some QRP DXers use very slow frequency shift keying at bit rates of 0.2 per second for example. The Voyager space craft used a bit rate of 10 bps to send back pictures in order to overcome the signal loss over the distances in space and the compromise was good quality at the expense of transmission speed.
Carl speculated about combined modulation so here’s another story. During my campus CC days I hung out with broadcast techs, one of whom was a ham and worked at WLW. He said a couple of them experimented with shifting the 700 kHz carrier a few Hz. to send morse code. With the right receiver it could be copied.
Most digital systems in use today, such as your internet connection, use QAM coding which is a combination of amplitude and phase modulation. C-QUAM for AM stereo is such a system.
Part 15 rules do not restrict the modulation method per se but the problem is to get the receiver to match the modulation mode for best range.
Neil
