In these several years of discussion and learning about AM antenna systems and the limiting factor of poor ground conductivity, I have stood in respect of the scholared radio engineers we're so lucky to enjoy at this website, while accepted their pronouncement that the laws of physics are set and the best discoveries in antenna performance have been known for almost a century.
Yet a part of the brain has reserved belief that there is something else to be found; a breakthrough idea; a eureka invention. Tonight the lightbulb lit.
I call it "ground biasing," and the idea is an upside down variant of what is known as "tape bias" from the days of magnetic recording tape. What is tape bias?
Tape bias is a steady sine-wave signal fed to the record head at a frequency above the audible range, to avoid being heard as a whistle when the tape plays back. A frequency of 100kHz would be typical, and the purpose of this constant signal is to excite the magnetic particles on the tape as it rolls by causing them all to align in the same direction, dramatically reducing distortion experienced with no bias. Tuning the bias to a proper level reminds us of the class-E tuning of the AMT5000, because tape bias is set so that audio level going onto the tape peaks, but minimum distortion occurs not at peak, but a little bit past peak.
To achieve ground biasing we foresee injecting a steady sine wave tone at a frequency below the audio spectrum. Why so? For one thing, we don't want to hear this as a "hum" on the transmitter's modulated signal, but the lucky second reason is that earth resonance resides down low, around 8Hz, a fact exploited by the Navy with ELF (Extra Low Frequency) buried antennas for sending radio signals to submarines submerged in the ocean. The ELF signal excites the magnetic corridors of the earth and travels thousands of miles in earth and water.
By "exciting" a set of AM ground radials at around 8Hz, we anticipate a marked improvement in antenna/ground conductivity. This should increase antenna efficiency by "N," to employ John WXCD's unknown number figure.
Even if it doesn't work, think of it, we can sell thousands of 8Hz Bias Generators and move to New Zealand where 1 Watt FM awaits.
All joking aside, my invention of ground bias will be my creed whether it works or not.