"Longwave" has no universally agreed defintion, but good news, amateur radio already has usage of 135.7 kHz to 137.8 kHz.
Building equipment that works on frequencies this low, and avoiding natural interference, can be extremely difficult.
Building equipment that works on frequencies this low, and avoiding natural interference, can be extremely difficult.
Actually generating the signal you could do with a moderately expensive USB soundcard, directly synthesizing it with simple VST-like plugin. Mad to think about, really.