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Current Into Radio Waves
Name: Rafael J. A.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: 2/26/2003
Question:
Sir, I am an accounting teacher. When I operate a
radio transmitter, it is plugged into an electrical outlet, and the
transmitter is drawing current. I presume that current is
flowing up and down the antenna at a certain frequency, but how is that
current "used up" ? Where is the "load"? I can see electricity being
turned into light and heat in a light bulb, or into kinetic energy in a
spinning electric motor, but nothing is happening in an antenna. I understand
that every time the current reverses, an electromagnetic wave is radiated
outward (is that right?), but there is a fair amount of current going into
the transmitter, and nothing coming out but these electromagnetic waves.
Replies:
You are absolutely correct in that whenever an electric charge is
accelerated it radiates an electromagnetic wave. What you may
not appreciate is that these waves do indeed carry energy (and
momentum). For example, a 50,000 watt "clear channel" station
such as WOR in New York does indeed radiate 50,000 watts of
power. Of course, as always, a radio transmitter is not 100%
efficient in transforming the input power into radiated
electromagnetic waves; much is used up in producing heat energy,
non-useful radiation, etc.
You may be interested in knowing about "populating space" as
proposed by the late Gerry O'Neil. He conceived of 2 mile long
space stations with perhaps a million inhabitants. One of the
advantages to earth of this plan is that electric generators
powered by sunlight could produce megawatts of electric power
which would then be transferred to earth not along electrical
wires, but by electromagnetic waves. That part of his plan, at
least, was quite feasible. Antennas on earth, perhaps a mile
wide would pick up the power, which would then be transformed to
a form suitable for transmitting over the usual network of
transmission lines.
You can personally feel the power transmitted by electromagnetic
waves simply by going to the beach on a hot sunny summer day and
feeling the heat of the sun, which is, of course, transmitted by
electromagnetic waves.
The "load" is caused by the forces of the electric and magnetic
field produced by the accelerated charged particle producing a
reaction force back on the charged particle. Lenz's Law
guarantees that the reaction force will be in a direction
opposing the motion of the charge producing the wave.
Best, Dick Plano, Professor of Physics emeritus, Rutgers
University
Rafael,
To the eye, NOTHING is leaving the antenna. But I assure you that an
appreciable amount of power that originates from the 120 V outlet current,
or whatever source one may be using is ultimately being radiated from the
antenna hooked into the transmitter.
Electromagnetic waves (EM waves) do seem sort of magical in a way. But just
because you cannot see them does not mean that they are not there. A
light bulb
will radiate power at a wavelength ~ 200 nM - 800 nM. Your eyes can
actually detect EM emissions at this wavelength. However, for example, in a
cellular phone communication band (2140 MHz). That wavelength is ~ 0.14
meters (5.5 inches). Your eyes are no designed to receive EM emissions at
that frequency. But I assure you that the emissions are there. Also, that
cell phone antenna is just as worthy of the name "load" as the tungsten
filament in a light bulb and to the copper wire windings in that motor that
you spoke of.
The frequency of the incoming wall current; 60 Hz for home; 0 Hz for DC
(batteries) HAS ABSOLUTELY NOTHING TO DO WITH THE FINAL EM EMISSION LEAVING
THE ANTENNA. That final frequency is determined by more complex tuning
systems and mixers within the transmitter / receiver.
APPARENTLY, nothing is happening in an antenna. However, if you were to
stand back with a device called a Spectrum Analyzer, you would be able to
see that, indeed, the antenna is emitting an RF (radio frequency) signal. I
really do not want to go into any more detail. However, I will if you ask
more detailed questions related to antenna design / radiation patterns,
etc...
I hope this has helped.
Best regards,
Darin Wagner
Rafael,
These electromagnetic waves are the same as the light from a light bulb,
exactly the same. The only difference is the frequency.
Your eye has a huge number of "rod" and "cone" cells that are actually tiny
antennae. They pick up the light wave radiation similar to how a radio
antenna works. There is energy in the waves.
Dr. Ken Mellendorf
Physics Instructor
Illinois Central College
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