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If some one said;
well you're radio ham what exactly is radio? You may answer by
saying that it is electro-magnetic radiation similar to light and
hope that, that was a sufficient enough answer. Because this is a
complex and curly question and if you investigate it you may be
surprised at what is understood as being the true nature of radio
and the other manifestations of electro-magnetic radiation including
light.
Albert
Einstein was once asked to explain how radio worked, Einstein said:
"You see, wire telegraph is a kind of a very, very long cat.
You pull his tail in
New York
and his head is meowing in
Los Angeles
. Do you understand this? And radio operates exactly the same way:
you send signals here, they receive them there. The only difference
is that there is no cat."
The
nature of radio
Yes; radio is
regarded as being electromagnetic radiation just as light, x-rays
etc are, but at a different wavelength or frequency. The wavelength
refers to the spatial period of the electromagnetic wave, the
distance over which the wave's shape repeats or one complete cycle. It
is usually determined by the distance between consecutive
corresponding points of the same phase, such as crests, troughs, or
zero crossings. The frequency refers to how many complete cycles
occur per second with the unit of measure of one cycle per second
being 1 Hertz.
The
complete range of all possible frequencies or wavelengths is
referred to as the electromagnetic
spectrum and includes frequencies well below the lowest operational
ELF (Extra Low Frequency) to Gamma radiation at the extremely shot
wavelength end.
Electromagnetic
radiation is defined into a number of broad groups according to the
frequency of its wave; these include radio waves, microwaves,
infrared radiation, visible light, ultraviolet radiation and X-rays
etc. Within these broad groups there are many subgroups such as VHF
(Very High Frequency), UHF (Ultra High Frequency) etc.
The colour of light
for example is known to be a result of its wavelength or frequency
e.g. 450 nanometres or 660 Terahertz being blue.
The Electromagnetic radiation takes the form of self-propagating wave in space consisting of two
components an electric field and a magnetic field. According
to my copy of the ARRL's handbook 2003, electromagnetic
radiation or the electromagnetic wave is the interaction of an
electric and a magnetic field. An oscillating electric charge in a
piece of wire for example, creates an electric field and a
corresponding magnetic field that in turn creates an electric field
which creates another magnetic field, and so on.
The
two fields sustain themselves as a composite electromagnetic wave,
which propagates itself into space. The electric and magnetic
components are orientated at right angles to the direction of
travel. The polarization of a radio wave is usually designated the
same as its electric field. Unlike sound waves or ocean waves,
electromagnetic wave needs no propagating medium, such as air or
water. This property enables electromagnetic wave to travel through
the vacuum of space.
Earlier theories of electromagnetic
radiation did however suggested that an unseen undetectable medium
known as the Aether was needed to support the electromagnetic wave,
this idea is now largely out of favor.
All this implies
that electromagnetic radiation is in fact a wave, however electromagnetic
radiation also has in away that is near to impossible to visualise
particularly for radio; a second identity, that of a particle or
Photon or packet of energy.
While the Photon is normally associated with light it is a feature
of all electromagnetic wave radiation irrespective of the
wavelength.
Prior to the
twentieth century science largely fell into to two distinct camps
when it came to describing light for example, those that felt a
particle model best described all or most of the nature of light and
those that felt that a wave model best described light radiation.
We had two
distinctly different models for electromagnetic radiation at least
as far as light was concerned; wave or particle, the particular
model is applied subject to the application or observation. As
science moved into the twentieth century this became more than
convenient as many experiments demonstrated that both models can be
shown to be valid.
In
1909 a scientist named Jeffrey Taylor determined to settle the
controversy by conducted a series of experiments to determine the
true nature of electromagnetic radiation ultimately concluded
in the end that in fact both model were valid and electromagnetic
radiation had a dual identity.
The apparently
conflicting concept that later became known as particle/wave duality
is within the realms quantum physics and well beyond anything that I
can comfortably rap my head around. According to most physicists
however the duality concepts is accepted as being fact for the
entire electromagnetic radiation spectrum.
It is thought that
the photons as an elementary
particle having no electric charge or resting
mass are field particles that are the carriers of the
electromagnetic field. If you can imagine it, this is how it is
thought that the two concepts fit together.
As
a particle the photon
is seen as a discrete bundle (or quantum)
of electromagnetic energy. Photons are always in motion and have a
constant speed of light to all observers. The photon known as a
force carrier is categorised as a Gauge Boson and is exchanges when
ever Protons and Electrons interact, it has as stated earlier no
mass or charge and seems to carry just enough energy to excite a
single molecule in your eye's photo receptor cell. Despite its
ethereal existence the photon is were it's at; and is fundamentally
involved in everything that you have or will ever do or experience.
Australian
physicist/writer Paul Davies
said 'The idea that something can be both a wave and a
particle defies imagination, but the existence of this wave-particle
"duality" is not in doubt. ... It is impossible to
visualise a wave-particle, so don't try'.
It is however
important to realize that while the particle/wave duality theory is
generally accepted, there are other views including that a wave only
model may be able to explain the entire observable phenomenon
electromagnetic radiation.
What ever the model
of what radio and the other electromagnetic radiations are, will
also be a model of how in fact energy and matter interact and is
therefore a fundamental corner stone of our understanding of physics
and a model of the very realty we all take for granted.
Creation
of the radio wave
The electrical
energy generated at a particular frequency in a radio transmitter
and sent up the transmission line and loaded into the antenna is
something that most radio amateurs pretty much understand and are
comfortable discussing and experimenting with. But, how many know
how this energy transmutes from electrical energy to the more
nebulous electro-magnetic radiation that is after all radio.
I actually thought
that I did understand it, well to a degree anyway with out having
really thought about it. I think the term electro-magnetic leads me
to think electricity and magnetism. I felt that I had a good working
grasp of electrical and magnetic principles, so electro-magnetic
radiation shouldn't be to different should it!
Electric charge
fields and magnetic fields can be powerful things, but are
relatively undetectable at fairly modest distances unlike radio,
light or other electro-magnetic radiation that can be detected after
travelling across the entire universe. So electricity and magnetism
do not equal electro-magnetic radiation, it is something related but
different.
Perhaps as a simple
analogy the relationship between electricity, magnetism and
electro-magnetic radiation is a bit like in chemical terms oxygen
and hydrogen forming water after a suitable chemical reaction. The
properties of water bare little resemblance with its constituent
elements as electro-magnetic radiation differs in many ways from its
fundamental components electricity and magnetism.
It is interesting
that while publications like the RSGB and ARRL amateur radio
hand-books cove subjects like antenna principles
(Electrical and Magnetic) and radio propagation principles
(Electromagnetic radiation) they do not cover at all the point when
one becomes the other.
I begin to
appreciate a comment made by a fellow radio amateur and technician
that antenna theory was 15% science and 85% black magic!
The reason I
decided to write this article is as I said earlier I didn't even
realise that I had no idea
about creation of electro-magnetic radiation by my antenna. I
stumbled over it a couple of years ago in a copy of the United
States Navy Electricity and Electronics Training text book.
The training manual's explanation of how electro-magnetic radiation
is created by the antenna is covered in a non-mathematical
relatively easy to follow descriptive presentation. A secret had
been reviled.
I'm
not sure about most people, but when learn something new, something
in this case I didn't even know I didn't know, it's a beautiful
thing and worth sharing.
The
following is an exert from the
United States
Navy Electricity and Electronics Training Series Module 10
- Introduction
to Wave Propagation, Transmission Lines, and Antennas
ELECTROMAGNETIC
FIELDS
The
way energy is propagated into free space is a source of great
dispute among people concerned with it. Although many theories have
been proposed, the following theory adequately explains the
phenomena and has been widely accepted.
There
are two basic fields associated with every antenna; an INDUCTION FIELD and a RADIATION
FIELD. The field associated with the energy stored in the
antenna is the induction field. This field is said to provide no
part in the transmission of electromagnetic energy through free
space. However, without the presence of the induction field, there
would be no energy radiated.
INDUCTION
FIELD
Figure
2-1, a low-frequency generator connected to an antenna, will help
you understand how the induction field is produced. Let's follow the
generator through one cycle of operation.
Figure
2-1. - Induction field about an antenna.
Initially,
you can consider that the generator output is zero and that no
fields exist about the antenna, as shown in view A. Now assume that
the generator produces a slight potential and has the instantaneous
polarity shown in view B. Because of this slight potential, the
antenna capacitance acts as a short, allowing a large flow of
current (I) through the antenna in the direction shown. This current
flow, in turn, produces a large magnetic field about the antenna.
Since the flow of current at each end of the antenna is minimum, the
corresponding magnetic fields at each end of the antenna are also
minimum. As time passes, charges, which oppose antenna current and
produce an electrostatic field (E field), collect at each end of the
antenna. Eventually, the antenna capacitance becomes fully charged
and stops current flow through the antenna. Under this condition,
the electrostatic field is maximum, and the magnetic field (H field)
is fully collapsed, as shown in view C. As the generator potential
decreases back to zero, the potential of the antenna begins to
discharge. During the discharging process, the electrostatic field
collapses and the direction of current flow reverses, as shown in
view D. When the current again begins to flow, an associated
magnetic field is generated. Eventually, the electrostatic field
completely collapses, the generator potential reverses, and current
is maximum, as shown in view E. As charges collect at each end of
the antenna, an electrostatic field is produced and current flow
decreases. This causes the magnetic field to begin collapsing. The
collapsing magnetic field produces more current flow, a greater
accumulation of charge, and a greater electrostatic field. The
antenna gradually reaches the condition shown in view F, where
current is zero and the collected charges are maximum. As the
generator potential again decreases toward zero, the antenna begins
to discharge and the electrostatic field begins to collapse. When
the generator potential reaches zero, discharge current is maximum
and the associated magnetic field is maximum. A brief time later,
generator potential reverses, and the condition shown in view B
recurs.
NOTE:
The electric field (E field) and the electrostatic field (E field)
are the same. They will be used interchangeably throughout this
text.
The
graph shown in figure 2-2 shows the relationship between the
magnetic (H) field and the electric (E) field plotted against time.
Note that the two fields are 90 degrees out of phase with each
other. If you compare the graph in figure 2-2 with figure 2-1, you
will notice that the two fields around the antenna are displaced 90
degrees from each other in space. (The H field exists in a plane
perpendicular to the antenna. The E field exists in a plane parallel
with the antenna, as shown in figure 2-1.)
Figure
2-2. - Phase relationship of induction field components.
All
the energy supplied to the induction field is returned to the
antenna by the collapsing E and H fields. No energy from the
induction field is radiated from the antenna. Therefore, the
induction field is considered a local field and plays no part in the
transmission of electromagnetic energy. The induction field
represents only the stored energy in the antenna and is responsible
only for the resonant effects that the antenna reflects to the
generator.
RADIATION
FIELDS
The
E and H fields that are set up in the transfer of energy through
space are known collectively as the radiation field. This radiation
field is responsible for electromagnetic radiation from the antenna.
The
radiation field decreases as the distance from the antenna is
increased. Because the decrease is linear, the radiation field
reaches great distances from the antenna. Let's look at a half-wave
antenna to illustrate how this radiation actually takes place.
Simply stated, a half-wave antenna is one that has an electrical
length equal to half the wavelength of the signal being transmitted.
Assume, for example, that a transmitter is operating at 30
megahertz. If a half-wave antenna is used with the transmitter, the
antenna's electrical length would have to be at least 16 feet long.
(The formula used to compute the electrical length of an antenna
will be explained in chapter 4.) When power is delivered to the
half-wave antenna, both an induction field and a radiation field are
set up by the fluctuating energy. At the antenna, the intensities of
these fields are proportional to the amount of power delivered to
the antenna from a source such as a transmitter. At a short distance
from the antenna and beyond, only the radiation field exists. This
radiation field is made up of an electric component and a magnetic
component at right angles to each other in space and varying
together in intensity.
With
a high-frequency generator (a transmitter) connected to the antenna,
the induction field is produced as described in the previous
section. However, the generator potential reverses before the
electrostatic field has had time to collapse completely. The
reversed generator potential neutralizes the remaining antenna
charges, leaving a resultant E field in space.
Figure
2-3 is a simple picture of an E field detaching itself from an
antenna. (The H field will not be considered, although it is
present.) In view A the voltage is maximum and the electric field
has maximum intensity. The lines of force begin at the end of the
antenna that is positively charged and extend to the end of the
antenna that is negatively charged. Note that the outer E lines are
stretched away from the inner lines. This is because of the
repelling force that takes place between lines of force in the same
direction. As the voltage drops (view B), the separated charges come
together, and the ends of the lines move toward the center of the
antenna. But, since lines of force in the same direction repel each
other, the centers of the lines are still being held out.
Figure
2-3. - Radiation from an antenna.
As
the voltage approaches zero (view B), some of the lines collapse
back into the antenna. At the same time, the ends of other lines
begin to come together to form a complete loop. Notice the direction
of these lines of force next to the antenna in view C. At this point
the voltage on the antenna is zero. As the charge starts to build up
in the opposite direction (view D), electric lines of force again
begin at the positive end of the antenna and stretch to the negative
end of the antenna. These lines of force, being in the same
direction as the sides of the closed loops next to the antenna,
repel the closed loops and force them out into space at the speed of
light. As these loops travel through space, they generate a magnetic
field in phase with them.
Since
each successive E field is generated with a polarity that is
opposite the preceding E field (that is, the lines of force are
opposite), an oscillating electric field is produced along the path
of travel. When an electric field oscillates, a magnetic field
having an intensity that varies directly with that of the E field is
produced. The variations in magnetic field intensity, in turn,
produce another E field. Thus, the two varying fields sustain each
other, resulting in electromagnetic wave propagation.
During
this radiation process, the E and H fields are in phase in time but
physically displaced 90 degrees in space. Thus, the varying magnetic
field produces a varying electric field; and the varying electric
field, in turn, sustains the varying magnetic field. Each field
supports the other, and neither can be propagated by itself. Figure
2-4 shows a comparison between the induction field and the radiation
field.
Figure
2-4. - E and H components of induction and radiation fields.
The
complete United States Navy
Electricity and Electronics Training Series can be found at:
http://www.tech-systems-labs.com/navy.htm
Approved
for public release; distribution is unlimited.
The story of the
investigation of light and the other electromagnetic radiations
represents the work of brightest people, spans hundreds of years and
is still a work in progress. The descriptions are often complex
highly mathematical and almost impossible for most people grasp, but
is at the same time fascinating and alluring for many including many
radio amateurs.
For myself, I like
the humbling fact that despite building our modern technology and
economy to large degree on the manipulation of electro magnetic
radiation, when it comes to understanding exactly what in fact it
is, there still remains some uncertainty and mystery!
You will never look at
your antenna the same again!
For
more information on the electromagnetic spectrum see:
http://www.animations.physics.unsw.edu.au/jw/EMspectrum.html
For
more information on electromagnetic radiation see:
http://en.wikipedia.org/wiki/Electromagnetic_radiation
For
more information on the
Photon see: http://physics.about.com/od/lightoptics/f/photon.htm
The
complete United States Navy
Electricity and Electronics Training Series can be found at:
http://www.tech-systems-labs.com/navy.htm
Approved
for public release; distribution is unlimited.
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