Have we any Carrier Current gurus in the house that might explain my observations?
I've finally gotten my Radio Systems TR-20 with matching CP-15 coupler going. Transmitter needed work - turned out to be a shorted zener diode on the 12v supply as well as both oscillator IC's (nand gate and flip-flop) bad. Plus an intermittent crystal that, after warm-up, jumped to a higher (and non-standard) frequency. Anyhow, I ordered a few more crystals, and was testing out a couple of them to compare performance.
I tried the lowest (600) and highest (800) frequencies I had crystals for. The result was that at the lower frequency of 600KHz, I had almost double the usable range when compared to 800KHz. OTOH, while 800KHz is almost hum-free, 600KHz is significantly more hummy.
I imagine the lower frequency might couple more efficiently into the power line, and/or do a better job of jumping the transformers, resulting in a bit more range, but what would account for the lower frequency also being more hummy?
I also observed that, into a dummy load, the transmitter was putting out a bit less power on 600 than on 800, though it might be that my power meter (part of a ham antenna tuner) might just not be accurate at that low of a frequency. And it's possibly also a red herring.