The radio transmitter is responsible for generating electromagnetic waves paw that can be transmitted and then received by the receiver. The example of the figure is a transmitter circuit developed with a triode tube (an example is not necessarily functional) just trying to illustrate the note. This tube is the predecessor transistor. You can see diagrams of transmitters here
The person responsible for generating electromagnetic waves is called high frequency oscillator, it emits waves into space at a frequency determined by a coil and a variable capacitor, which will select the frequency at which it will pass. We are talking about a simple transmitter, transmitters commercial use quartz crystals to generate a frequency stable and unique.
To do this simply say that the transmitter converts the signals audible in electromagnetic signals.
