How do radio antennas convert electrical energy into photon energy?

[MichaelsTech]

How do transmitters convert electrical signals into photon energy?

 

[Nugatory]

Moving charged particles back and forth produces electromagnetic waves, just as you can produce waves in water by moving an object back and forth in the water.

A radio transmitter works by applying a changing voltage to the antenna, causing electrical charges in the antenna to move back and forth. These in turn create electromagnetic waves which travel outwards, carrying energy with them.

 

[MichaelsTech]

How can a voltage be applied if it is not a complete circuit?

 

[Nugatory]

How can a voltage be applied if it is not a complete circuit?

You need a complete circuit to get current to flow when your circuit is in equilibrium but you don't need a complete circuit to apply a voltage or to get current to flow for a very short time.

Google for "LC oscillator" for a simple example.