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Theoretically (and practically to some extent) if you have a low frequency square wave then you have higher frequency components though at profoundly lower power levels than the fundamental frequency. A filter might help push down the stuff you don't want - leaving the part that you do want. This seems clumsy but it might serve your purpose.
I know in amateur/shortwave radio they used to have a crystal "calibrator) that delivered RF at various specific intervals all over the RF spectrum. I recall seeing some solid-state circuits that I'd guess were square wave generators rich in RF. One would adjust the marker against a known standard like WWV.
Is this an FM transmitter? In high power output stage of a transmitter a MOSFET in class c is practically making a square wave, often the gate driven by logic IC or MOSFET driver. The LC 'Pi filter' which matches the antenna turns it into a sine wave.