Today's Radios have AM, FM, Shortwave, Ham, VHF, UHF, and/ or other radio frequency bands. But; they began with the theory of electronics of the early Crystal Radio, that eventually were redesigned and developed; until they were replaced by regular AM radios.
Antenna Towers transmit waves from Long Wave transmitters. The aerial long antenna Long Antenna connects to a lead in wire that on a Crystal Radio goes to a Coil, that has about 150 turns on it. The Variable Capacitor, or a 2 foil aluminum foil that is taped on cardboard; goes to the coil that has about 150 turns on it.When the foil plates are close together then the first then the first coil plate then the first foil plate flows back to the coil and to the bottom plate, always at the same rate speed. The Radio Stations CPS (cycles per second) are different so there are different frequencies; for different radio stations.The Antenna travels to Ground. The variable capacitor off other frequencies; so, one radio station remains tuned in. The crystal radio mineral is sensitive to Radio Waves in the long wave band.The thin Cats Whisker Wire, the mineral makes sound electricity vibrations. That is how the Simple Crystal Radio works.In the long wave are the Medium Wave radio stations.
The flow of electricity through a coils effect; and the effect with a capacitor, is similar to radio waves. Using an Aerial and a Ground added, to this circuit; so, you can tune in radio wave electricity. Then; if you add an Amplifier you can hear the radio waves; but, you can even use a Crystal Diode to hear the same sound through a crystal radio earplug or headset, without adding an amplifier.
Opening the variable capacitors foils; opening the plates the capacitance increases. When closing the foils the capacitance decreases, and the resonance frequency decreases; for the lower radio wave frequencies. With an amplifier you can hear the radio stations louder, that are tuned in with the variable capacitor. The transmitters send a Carrier Wave that contains many waves in Cycles, that is known as Amplitude Modulation radio waves, that is known as the desired signal amplitude modulation radio waves, that is the Audio Signal. The Crystal Radio Diode, or mineral with a cats whisker; is removing the bottom half of the radio waves, and makes current flow one way.
The Crystal Radio Set uses basic electronic parts, designed in a circuit to use radio waves, and separate the radio wave signal frequencies. With a Germanium Crystal Radio Detector, the Cats Hair Whisker wire must connect to a strong radio signal area of the Germanium Mineral, that is called, the 'sweet spot'; because the signals are the strongest, there.
The crystal radio is still in the long wave KHz band. Medium waves are in MHz. A second coil with about 50 turns are added to the circuit. In the long wave 2 or more radio stations can sometimes be heard and can not be separated, because the aerial stays the same; but, the ground changes from the contour environment, creating some capacitance between them, that is limiting the resonance circuit from working properly. A Coupling Capacitor aerial circuit, which is a second variable capacitor; that varies the amount of the radio signal. So; the antenna wire goes to the coupling capacitor, to the smaller coil; and to the ground on the crystal radio circuit, and because it is similar to the larger coil in the circuit it makes a Transformer, so that they`are quite isolated,but there is a slight loss of Gain. Then it connects to where the antenna terminal was on the original simple crystal radio. The long wave is now a medium wave.
[antenna Mica Capacitor #1].
The Simple Crystal radio; sometimes needs an additional Variable Tuning Capacitor, that is used to tune the strength of the radio wave signals. So; The Antenna's radio waves that's tuned with the additional Variable Capacitor, can be adjusted to the grounded Ground Connection, and then the radio wave frequencies can be separated individually, that are close together.
In an amplified crystal radio; changing from a long wave crystal radio to a medium wave crystal radio, the radio stations become more faint, as the stations were separated more. To Amplify the circuit that is now a Complex Circuit, a Valve is used that is a diode; and how the diode works, it has a Filament and a Anode, that is inside a glass tube vacuum. Connecting a battery the the filament on both ends of the wire, that is positioned on the bottom of the tube; creates a flow of Electrons (-) that have a negative charge and stay near the filament. Then putting a battery from the anode (+) to the filament (-), the electrons are attracted to the anode, and flow up to it through the vacuum tube in space of the diode valve.The Electrons flow around the circuit from the filament to the anode; but the diode only allows the current to flow one way. Then invented was another diode valve that had a Grid in the middle; when the electrons pass through the grid some of the electrons are caught by the grid, so the grid created a positive charge. This diode with a the grid is a Triode; or an Electron Tube. It rectifies the radio modulation signal, and that is how it amplifies the signal. Because the radio signal is going into the grid the positive side of the wave goes in, and the negative side is suppressed; since in a diode current can only flow one way. Since, the anode is positive and the grid is positive, it accelerates and increases the flow of electrons; so the current to the anode is increased. The current keeps increasing and decreasing, by the grids positive charge in the grid; and that is amplification. The triodes are made for higher and higher voltages; so the amplification can be louder and louder. Transistors are now used that needs less voltage, than the electron tube. The sound can be be heard through headphones
[headphone Mica Capacitor #2], better; or with a powerful enough amplifier it can be heard through a speaker
By using electric voltage for electric components in the circuit, and with an Amplifier, or with more stage amplifiers; a more regular type AM Radio can be built, To receive regular AM radio frequency radio station Broadcasts. The newer regular radios still use a Arial but there is no grounded connection that is needed, and sometimes they have FM tuners, or multi band frequency widths, that also use the more complex electronic amplifier circuit; circuitry.
Mica Capacitor #1 - Makes waves longer and more separatable.
(connects at beginning of the antennas lead in wire)
Mica Capacitor #2 - Uses less of the sound electricity radio waves whole wave; so, the volume that is heard sounds louder.
(Connect between the terminals to the headphone)