Let us consider two conductors, two conducting electrodes, one close to the other.
There is atmospheric air between the two electrodes.
In standard conditions, atmospheric air does not contain a significant number of mobile charges and therefore does not carry current. Thus, it is considered to be a very good insulator.
A perfect insulator does not exist because even insulators contain small numbers of mobile charges. If we apply an electric field between the two electrodes, then (the few) electrons that exist in the gap (from cosmic rays or background radiation) will be accelerated.
As they collide with air molecules, they create additional ions and newly-freed electrons which are also accelerated. We therefore have exponentially-increasing electrons and ions and this density of carriers will at some point suddenly allow the conductance of electricity from one electrode to the other.
“The rapid transition from a non-conducting to a conductive state (due to ionization) produces a brief emission of light and a sharp crack or snapping sound. A spark is created when the applied electric field exceeds the dielectric breakdown strength of the intervening medium. For air, the breakdown strength is about 30 kV/cm at sea level.”
This phenomenon is similar to lightning. Wikipedia: "Lightning is a sudden electrostatic discharge during an electrical storm between electrically charged regions of a cloud(called intra-cloud lightning or IC), between that cloud and another cloud (CC lightning), or between a cloud and the ground (CG lightning)."
We saw in the section "LC circuit" how a dipole antenna works.
In the picture above from https://en.wikipedia.org/wiki/Heinrich_Hertz we see Hertz' dipole antenna which constitutes the first radio transmitter: "a dipole resonator consisting of a pair of one meter copper wires ending in 30 cm zinc spheres". A spark gap exists in the center.
Hertz used the experimental setting below. He connected the antenna above with an inductor and coupled this with an LC circuit that we show in the section "LC circuit". Starting from a charged capacitor, the LC circuit will generate an alternative current with a specific frequency that will depend on the L and the C of the circuit. The current will increase, decrease, become zero, change direction and increase etc and this according to a specific frequency. We will be able to follow the periodicity of the phenomenon by seeing the sparks in the spark gap of the antenna. How we will know that the signal is transmitted to a distance?
We will place a circular wire with a spark gap at a distance. Upon transmission of the signal we will see sparks in this part as well.
As mentioned in wikipedia: "The frequency of the waves was roughly 100 MHz, about that used in modern television transmitters."
This is how Hertz created and detected radio waves.