Measure current of 51 kHz signal and ideas for increasing range of AM signal
What radiates the most is the looped short circuit at the end of the wire.
The limiting factors on the current are;

how you measure it? peak or average
FET loss resistance
wire loss resistance and length of extension cord.
100 Ohm resistor

I can't say why it was designed this way or guess the unstated assumptions.  It doesn't seem to resonate at AM frequency in a small loop with stray capacitance coupling. AM antennae are normally dozens of large diameter loops. But this is intended to resonate with some plastic dielectric,  moist soil and not dry earth.  Moisture will add more capacitance than plastic by volume 80 / 4 ratio.
But I would suggest a slightly larger loop are of exposed shorted end wires as much as possible.  Then orientation will affect Radio signal strength, with max facing the loop area.
What counts is not the current drive but the Q of the end loop voltage and E-field created by a large loop 1 area ~ 1m = 1uH * 100 nF ( large capacitance ) or which would resonate at 520 kHz  but any sudden increase in dielectric approaching this product will increase level.  I was estimating a 20ft extension cable 300 Ohms.
So start with small changes to see if it improves.