Principles of radio
One of the more fascinating applications of
electricity is in the generation of invisible ripples of
energy called radio waves. The limited scope of this
lesson on alternating current does not permit full
exploration of the concept, some of the basic principles
will be covered.
With Oersted's accidental discovery of
electromagnetism, it was realized that electricity and
magnetism were related to each other. When an electric
current was passed through a conductor, a magnetic field was
generated perpendicular to the axis of flow. Likewise, if a
conductor was exposed to a change in magnetic flux
perpendicular to the conductor, a voltage was produced along
the length of that conductor. So far, scientists knew that
electricity and magnetism always seemed to affect each other
at right angles. However, a major discovery lay hidden just
beneath this seemingly simple concept of related
perpendicularity, and its unveiling was one of the pivotal
moments in modern science.
This breakthrough in physics is hard to
overstate. The man responsible for this conceptual
revolution was the Scottish physicist James Clerk Maxwell
(1831-1879), who "unified" the study of electricity and
magnetism in four relatively tidy equations. In essence,
what he discovered was that electric and magnetic fields
were intrinsically related to one another, with or without
the presence of a conductive path for electrons to flow.
Stated more formally, Maxwell's discovery was this:
A changing electric field produces a
perpendicular magnetic field, and
A changing magnetic field produces a
perpendicular electric field.
All of this can take place in open space,
the alternating electric and magnetic fields supporting each
other as they travel through space at the speed of light.
This dynamic structure of electric and magnetic fields
propagating through space is better known as an
electromagnetic wave.
There are many kinds of natural radiative
energy composed of electromagnetic waves. Even light is
electromagnetic in nature. So are X-rays and "gamma" ray
radiation. The only difference between these kinds of
electromagnetic radiation is the frequency of their
oscillation (alternation of the electric and magnetic fields
back and forth in polarity). By using a source of AC voltage
and a special device called an antenna, we can create
electromagnetic waves (of a much lower frequency than that
of light) with ease.
An antenna is nothing more than a device
built to produce a dispersing electric or magnetic field.
Two fundamental types of antennae are the dipole and
the loop:
While the dipole looks like nothing more
than an open circuit, and the loop a short circuit, these
pieces of wire are effective radiators of electromagnetic
fields when connected to AC sources of the proper frequency.
The two open wires of the dipole act as a sort of capacitor
(two conductors separated by a dielectric), with the
electric field open to dispersal instead of being
concentrated between two closely-spaced plates. The closed
wire path of the loop antenna acts like an inductor with a
large air core, again providing ample opportunity for the
field to disperse away from the antenna instead of being
concentrated and contained as in a normal inductor.
As the powered dipole radiates its changing
electric field into space, a changing magnetic field is
produced at right angles, thus sustaining the electric field
further into space, and so on as the wave propagates at the
speed of light. As the powered loop antenna radiates its
changing magnetic field into space, a changing electric
field is produced at right angles, with the same end-result
of a continuous electromagnetic wave sent away from the
antenna. Either antenna achieves the same basic task: the
controlled production of an electromagnetic field.
When attached to a source of high-frequency
AC power, an antenna acts as a transmitting device,
converting AC voltage and current into electromagnetic wave
energy. Antennas also have the ability to intercept
electromagnetic waves and convert their energy into AC
voltage and current. In this mode, an antenna acts as a
receiving device:
While there is much more that may be
said about antenna technology, this brief introduction is
enough to give you the general idea of what's going on (and
perhaps enough information to provoke a few experiments).
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REVIEW:
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James Maxwell discovered that changing
electric fields produce perpendicular magnetic fields, and
visa-versa, even in empty space.
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A twin set of electric and magnetic
fields, oscillating at right angles to each other and
traveling at the speed of light, constitutes an
electromagnetic wave.
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An antenna is a device made of
wire, designed to radiate a changing electric field or
changing magnetic field when powered by a high-frequency
AC source, or intercept an electromagnetic field and
convert it to an AC voltage or current.
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The dipole antenna consists of two
pieces of wire (not touching), primarily generating an
electric field when energized, and secondarily producing a
magnetic field in space.
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The loop antenna consists of a loop
of wire, primarily generating a magnetic field when
energized, and secondarily producing an electric field in
space.
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