A radio wave is strong enough to use an earphone (no batteries required).
The audio signal is extracted from the radio wave using a mineral crystal.
The crystal is touched with a fine wire ("cat's whisker") until a useful sensitive spot is found.
Tuning is done by using a shorter or longer amount of the wire coil.
An outdoor antenna (50 - 100 feet) and a connection to earth ground is needed.
This radio could easily be home made.
Geoff Roberts in the UK has built some beautiful modern crystal radios.

Became available once the vacuum tube was invented.
The newly invented vacuum tube could amplify the radio signal enough that a speaker could be used, so the whole family could listen to music and shows.
Since tubes were expensive, the radio was designed to use a single tube.
"Regenerative" means that the weak radio signial is amplified, then a portion is sent through the tube over and over, re-amplifying each time to make the signal strong enough for a speaker.

A new design once tubes were less costly.
The first and second tubes amplify the radio signal. The third tube extracts the audio signal from the radio signal. The fourth and fifth amplify the audio signal for the speaker.
This design enabled better sensitivity to get weak stations and greater selectivity to separate close signals.
Here is the user manual for the 1924 Stomberg-Carlson Model 1-A. And, a 1922 Montgomery Ward radio catalog.
It also solved a problem (accidental oscillation) with regenerative radios.
But, the first three circuits each must be tuned to the radio station individually with separate tuning dials (see photo). This is not always easy to do.

Turning the station tuning dial also tunes a internal radio frequency signal.
The internal signal is varied so that, when it is combined with the tuned station signal, it always produces a new signal with a specific frequency, called the intermediate frequency.
The benefit of having a standard IF frequency (typically 440 kilocycles) is that the circuits that amplify it can be more sensitive and more selective, yet are also less expensive. And, only one tuning dial (not three) is needed.

Click here for a summary of major innovations in the 1920s

Click here to listen to a mid 1920s radio advertisement for Atwater Kent radios.

THE ASHEVILLE RADIO MUSEUM