a transistor + a couple of caps + resistor + a wire = a transmitter;
an active crystal oscillator controlled by your mcu pin + a wire = a transmitter;
a mcu pin flipping fast and furious + a wire = a transmitter;
...
a transistor + a couple of caps + resistor + a wire = a transmitter;
an active crystal oscillator controlled by your mcu pin + a wire = a transmitter;
a mcu pin flipping fast and furious + a wire = a transmitter;
...