I intend to perform a series of longwave experiments with ground contacts or limited antennas.  Here is the first, varying frequency only at a range of 25 yards with an input signal of one volt.  Earth contacts were what was readily available.

20 KHz received signal 0.1 mV, noise .02 mV

137 KHz received signal 2mV, noise .05 mV

472 KHz received signal 3.5 mV, noise .008 mV.

As you can see, 472 KHz was clearly superior.  That is where I shall concentrate further experiments.