An oscillator, a small power stage, some modulation, and a tiny loop antenna make RF for experiments on at 187.5 kHz on the United States' FCC Part 15 Lowfer band (1600-1750 meters). This is a low power signal source I put together one evening to provide 187 KHz RF signals for an anticipated receiver investigation. Under Federal Communications Commission rules inside the United States, one is allowed to operate a transmitter without a license under certain conditions. Here, I have copied the relevant section of the most recent version of the Code of Federal Regulations that I could find..
(a) The total input power to the final radio frequency stage(exclusive of filament or heater power) shall not exceed one watt.
(b) The total length of the transmission line, antenna, and ground lead (if used) shall not exceed 15 meters.
(c) All emissions below 160 kHz or above 190 kHz shall be attenuated at least 20 dB below the level of the unmodulated carrier. Determination of compliance with the 20 dB attenuation specification may be based on measurements at the intentional radiator's antenna output terminal unless the intentional radiator uses a permanently attached antenna, in which case compliance shall be demonstrated by measuring the radiated emissions.
If you are thinking seriously about building a Part 15 transmitter, I suggest you read through Part 15 to make sure you are fully compliant. The FCC provides for very heavy penalties for those who step out of line.
Here is where I found this copy of Part 15:
I have known about the provision that allows 1 watt license free operation on this band for a long time, but didn't really take it seriously. The maximum antenna length is only 15 meters. This is electrically very short compared to the 1600 meter wavelength and is therefore be a very poor radiator.
However poor the efficency of the antennat some people using sophisticated coding and dedecoding techniques have managed to...