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1) The difference in r-f ground loss is due the practical length likely for buried radials for Part 15 systems on 175 kHz relative to 1600 kHz or so.
If both systems used buried radials with the same free-space wavelength at their frequency, then for the same number of radials buried in the same earth, their r-f ground connection loss would be the same.
The r-f currents flowing in the earth within 1/2 wavelength from the monopole need to be collected by the ground system and returned to the “ground” terminal of the antenna system
2) The lower limit of the audio output of the receiver could extend down to 20 Hz or less, if the transmitter and program supported that (as long as the r-f bandwidth of the signal was least 40 Hz),
If the r-f bandwidth of the antenna system is 4 kHz, the upper frequency limit for the receiver output audio will be 2 kHz.
For DSB AM signals the upper frequency limit of the audio at the receiver output always is 1/2 the r-f bandwidth.