I believe it means "High Frequency".
Amplitude Modulation (AM) and Frequency Modulation (FM) AM tuning changes the amplification size of the waves being received, FM tuning controls the frequency of the waves being received. As...
AM on a radio stands for amplitude modulation.
Amplitude
"Amplitude Modulation" Note: If it's a clock radio, then that label could also denote "Ante Meridian".
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