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the radio
The changing electric field results in a changing magnetic field,
and vice versa, and this causes the emergence of electromagnetic
waves that transmit sound or information from one place to
another, and this is a simple explanation of what was discovered
by the two scientists Faraday and Maxwell, discoverers of the
electromagnetic theory. On this basis, scientists and researchers
conducted experiments to transmit sound from one place to
another through magnetic waves, which led to the discovery of
the radio "radio" in 1889 by the scientist Henry Hertz, where he
worked on creating a device that creates electromagnetic waves
by generating an electric discharge. He made a frequency
detector, a device that could receive electromagnetic waves and
convert them into electricity. The scientist Nikola Tesla
developed the idea of an oscilloscope in 1896 until he made the
first radio in the world to receive electromagnetic waves, which
carry sound as information that is transferred in the air into
electrical signals, which we can hear through simple circuits and
speakers that convert the electric vibrational energy into
audible
sounds.
Crystal radio or crystal radio
(In English: Crystal radio) is a very simple device to receive
radio waves and was widely used in the early days of the radio. It
is distinguished by that it does not require an electrical source
and depends on the energy in the radio waves that is received by
the antenna.
The name was derived from the most important part of this
radio, which is the crystal detector. This was originally a piece of
metal crystal or today it is called a diode.
A crystal radio is the simplest type of radio receiver and can be
manufactured in some inexpensive parts, such as an antenna
wire acting as antenna, a copper electrode coil for tuning, a
capacitor, a current crystal or diode detector, and an earphone.
The crystal radio receives the AM modulated signals and can be
designed to receive any radio frequency beam, but the majority
of the crystal receives the amplitude modified frequency beam.
From a channel
Technical ideas and projects
the design
The crystal radio circuit
The crystal radio can be considered as a radio receiver reduced
to the radio core. The crystal radio is composed of at least the
following elements:
An antenna that converts the received radio waves into an
electric current
A tuning circuit capable of choosing the desired frequency for
the radio station from among all the frequencies received by the
antenna, and rejecting the rest of the frequencies. This circuit
consists of an electrical coil and a capacitor connected in parallel.
Therefore, it forms a resonant circuit at the desired station
frequency and thus the tuning is done. Either capacitor or coil
can be tunable (i.e. change its physical value) allowing the circuit
to be tuned to different frequencies.
A semiconductor crystal that acts as a detector which extracts
the modulated audio signal from the carrier frequency signal. It
works with a rectifier and allows the current to pass in one
direction.
An earpiece that converts the audio signal into an audible wave.
The low power produced by a crystal radio is not usually
sufficient to power loudspeakers, so headphones are used.
A good ground wire because the antenna used in a crystal radio
is a monopole antenna that generates the voltage on its output
relative to the ground.
The synthesis circuit
The tuning circuit consisting of a capacitor and a coil in parallel
acts as a resonant circuit. This circuit has a high impedance at
the desired frequency of the radio signal and a low impedance at
the rest of the frequencies, so the signal at the unwanted
frequencies passes directly to the ground while the signal of the
desired frequency passes to the detector and from there to the
speaker audibly. The receiving station frequency is the resonant
frequency "f" in the tuning circuit which is determined by the
capacitance value C and the inductance L according to the
relationship:
