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The schematic shown at http://www.mfjenterprises.com/man/pdf/MFJ-956.pdf shows the best circuit to load a part 15 antenna, namely a loading coil with a big series cap for fine tuning. The high end of the band will most likely tune at switch position 4 (330 uH)
The problem is that this is a small antenna tuner intended for receiving applications, so the inductors are physically small. Small, relatively high value inductors like the 330uH are usually wound on ferite cores with very fine wire. The DC resistance will be high (because of the fine wire) and inter-winding capacitance will be high. This adds up to big LOSSES for transmitting. A relatively huge coil (3.5″ diameter, #16 wire) will perform dramatically better. I have experimented with both.
In the part 15 AM world nothing comes cheap or easy. We have to deal with crippled power and crippled antenna length. To get the best range, everything must be done to minimize losses.