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).
- REVIEW:
- James Maxwell discovered that changing
electric fields produce perpendicular magnetic fields, and visa-versa,
even in empty space.
- 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.
- 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.
- 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.
- 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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