Numbers stations are real and are very simple. Back in the day all major spy agencies would operate them. Simple but effective. You can send coded messages via

Numbers stations are real and are very simple. Back in the day all major spy agencies would operate them. Simple but effective. You can send coded messages via radio waves, to receive them you only need a radio. You can send them from your country so enemy state(s) cannot stop you from doing it short of invasion/sabotage. If spies are using none reused one time codebook its impossible to decode message without the codebook. Because of its nature you cannot really catch anyone receiving signal. The only downside is that its one way communication and if somehow spy gets caught than their codebook can be used for decoding of the messages (but each spy would have unique book). So even that could have small fallout.