How Antennas Work
We can't see them but radio and television waves are just another form
of light. They have a much longer wavelength than visible light but
both are electromagnetic radiation.
To generate radio and TV waves we typically make
electrons oscillate up and down on an antenna. This is done by
applying a variable voltage or alternating current to the antenna.
Antennas are generally made of metals and metals act like containers
filled with a liquid made of electrons. Metal atoms have one or more
weakly held electrons in their outer shells which can "float" from atom
to atom.
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Figure 1. Electric
Field Around a Positive Charge |
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When a negatively charged electron moves it leaves
behind what is generally referred to as a positively charged hole. The
hole is simply an atom with more positive protons than negative
electrons.
The electrical fields for the two types of charges are
shown in Figures 1and 2. These are ray diagrams. The arrows show the
direction of the force that would be exerted on a unit of positive
charge.
Unlike a vector diagram, the length of a ray does not indicate
the magnitude of the force. Instead, the space between rays indicates
magnitude. Both diagrams in Figures 1 and 2 show that the magnitude of the
field decreases with increasing distance from the charge because the
space between the rays increases. |
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Figure 2. Electric
Field Around a Negative Charge |
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We can use a simple analogy to help understand how
electromagnetic waves are produced by moving charges. Imagine for a
minute that the rays or electric field lines shown in Figures 1 and 2
are like very long springs attached to a circular frame with the
charge at the center, almost like a trampoline. If the charge is
bounced up and down waves will propagate outward along the springs.
Yes, the world of electromagnetic radiation is far more complex than
our simple analogy but hopefully it gives you some idea of how a
moving charge could create a wave.
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The waves and variation in the
electric field account for the "electro" part of the term
electromagnetic waves.
A moving charge is essentially a current and
currents create circular magnetic fields. In
Figure 3, a positive charge moving straight out of the page would
produce a magnetic field represented by the blue dashed line. The
direction of the field can be determined using the right hand
thumb rule. The thumb is pointed in the direction of the current
and the fingers of the right hand wrapped into a loose fist. The
fingers point in the direction of the magnetic field.
Note that the magnetic field lines are perpendicular
to the electric field lines. This is one of the famous characteristics
of electromagnetic waves.
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Figure 3. Magnetic Field
(shown in blue) Created by a positive Charge Moving Straight
out of the Plane of the Page |
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Okay, you're probably wondering why we use an
example of a positive charge when we just got finished saying that
it's the electrons which move. It turns out that all the conventions
in electricity and magnetism are set up for positive charges. Much of
this can be traced back to the work of Benjamin Franklin.
Unfortunately, the electron had not even been discovered in
Franklin's time.
When we talk about current we pretend the positive
holes are actually moving in the opposite direction as the electrons.
It may seem pretty silly but it does work as a concept and so we're sticking with
the tradition.
If a variable voltage is applied, it will send an
electrical wave up an antenna. Free electrons in the antenna act as
the media for propagating the wave. The situation is similar to
longitudinal sound waves propagated in a metal rod. The sound wave is
carried by alternating regions of tension and compression. In the
compressed areas the rod's molecules are pushed a little closer
together. In the tension areas they are pulled a little further apart.
Although the molecules barely move, the sound wave can be transmitted great
distances.
The very slight motion of electrons up and down an
antenna is enough to cause electromagnetic waves to radiate out the
sides of the antenna at the same frequency as the variable voltage
applied to it. These are used for transmitting radio and television
signals as well as other forms of wireless communication.
Like sound, when electrical waves at a defined
frequency hit the end of an antenna they are reflected backwards and
form a standing wave in the antenna. Antenna waves move at the speed
of light (3 x 10 8 m/s) and so the travel time from one end
of the antenna to the other is pretty quick.
The electrical waves created on antennas typically
have a fixed wavelength. If the length of the antenna is wisely
chosen it's possible to make it resonate. The free end of an antenna
acts like an open circuit. Voltage drop is maximum across an open
circuit and zero across a short circuit. Hence the end of an antenna forms an anti-node or area of maximum
voltage or e-field strength. A node is a point which has zero e-field. The distance
between an anti-node and node is a quarter of a wavelength.
The wavelength of an electromagnetic wave is
calculated as follows:
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l |
= |
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Where |
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l
=
wavelength |
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C = speed of light (3 x 108 m/s) |
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f = frequency |
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Figure 4 shows a dipole antenna
which is generally considered the simplest form of antenna. In
this case each half of the antenna is roughly 1/4 wavelength long with
the antenna fed from its center. Hence, the total antenna is 1/2
wavelength long. The ends of the antenna correspond to anti-nodes
and the center to nodes. This configuration causes the antenna to
resonate. An antenna will still transmit even if the length is
not ideal for resonance. However, less of the power input to the
transmitter will actually show up as useful output signal. In other words, the
efficiency of the system will be significantly lower. |
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Figure 4.
Dipole antenna |
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Dipole antennas are considered balance devices
because they are symmetrical and work best when they are fed with
a balanced current. In other words, the current has to be of equal
size on both halves. This is usually accomplished with a balun
when the antenna is fed with a coaxial cable. Coaxial cable is
considered unbalanced, hence the word balun is formed from parts of
the words BALanced and UNbalanced. A balun is basically a small
transformer.
The optimum size of a dipole antenna is slightly
different than would be expected based on wavelength alone. This is
due to the interaction of the balun and antenna. However, the
predicted resonance length is usually very close to the length
for optimum broadcast efficiency.
Electromagnetic waves emitted from an antenna
are generally modeled as transverse waves. Since the waves have both
electric and magnetic field components and are emitted in three
dimensional space, the transverse wave model drawn in text books is a
bit over simplified but the full picture is almost impossible to
draw.
Waves emitted from simple monopole and dipole antennas tend to be
polarized. In other words, if the emitting antenna is vertical the receiving antenna
also has to be vertical for best reception. If
the receiving antenna is horizontal the signal it picks up will be
greatly attenuated.
Antenna design is very complex and requires a lot of
time and study to master. However, any antenna will have to oscillate
charged particles in order to transmit radio signals and will tend to
do this best if the antenna is resonating.
For more information about wireless communication and the electromagnetic spectrum visit The Hidden World of the Electromagnetic Spectrum.
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