Why does a spark produce radio waves?

Air normally does not conduct electricity. However, if the voltage is high enough (for a given amount of air) the electrical energy causes the air atoms and molecules — composed of electrons, neutrons and protons — to let go of their electrons. This is called ionization. Because the electrons are negatively charged, they are immediately attracted to the positive electrode and this movement produces an electrical current. The electrons bump into other atoms along the way, causing further ionization and therefore greater current. The ions are constantly moving about, and this movement of charge produces changing electric and magnetic fields that create the radio wave energy. Most of the energy is produced as heat and light but a little goes to form the radio wave energy we need for transmitting. The spark is not a very efficient way of generating radio wave energy, but it is the simplest!

A conductor in its natural state — negative electrons (blue)
and positive ions (red) evenly distributed along its length.

The same conductor in a state of imbalance where all the
negative particles (blue) have been forced to one end.

How can we make a spark transmitter? 

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