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How does an Antenna work? | ICT #4 - YouTube
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- [Narrator] Antennas are widely used
[1]
in the field of telecommunications,
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and we have already seen
many applications for them
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in this video series.
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Antennas receive an electromagnetic wave
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and convert it to an electric signal,
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or receive an electric signal
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and radiate it as an electromagnetic wave.
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In this video, we are going
to look at the science
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behind antennas.
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We have an electric signal,
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so how do we convert it to
an electromagnetic wave?
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You might have a simple
answer in your mind.
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That is to use a closed conductor
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and with the help of the principle
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of electromagnetic induction,
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you will be able to produce
a fluctuating magnetic field
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and an electric field around it.
[42]
However, this fluctuating
field around the source
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is of no use in transmitting signals.
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The electromagnetic field
here does not propagate,
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instead, it just fluctuates
around the source.
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In an antenna, the electromagnetic waves
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need to be separated from the source
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and they should propagate.
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Before looking at how an antenna is made,
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let's understand the physics
behind the wave separation.
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Consider one positive
and one negative charge
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placed a distance apart.
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This arrangement is known as a dipole,
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and they obviously produce
an electric field as shown.
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Now, assume that these charges
are oscillating as shown,
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at the midpoint of their path,
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the velocity will be at the maximum
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and at the ends of their paths
the velocity will be zero.
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The charged particles undergo
continuous acceleration
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and deceleration due to
this velocity variation.
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The challenge now is to find out
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how the electric field
varies due to this movement.
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Let's concentrate on only
one electric field line.
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The wavefront formed at time zero
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expands and is deformed as shown
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after one eighth of a time period.
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This is surprising.
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You might've expected a
simple electric field as shown
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at this location.
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Why has the electric field stretched
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and formed a field like this?
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This is because the accelerating
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or decelerating charges
produce an electric field
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with some memory effects.
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The old electric field
does not easily adjust
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to the new condition.
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We need to spend some time to
understand this memory effect
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of the electric field or kink
generation of accelerating
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or decelerating charges.
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We will discuss this
interesting topic in more detail
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in a separate video.
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If we continue our analysis
in the same manner,
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we can see that at one
quarter of a time period,
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the wavefront ends meet at a single point.
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After this, the separation
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and propagation of the Wavefront happens.
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Please note that this
varying electric field
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will automatically generate
a varying magnetic field
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perpendicular to it.
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If you draw electric
field intensity variation
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with the distance, you can
see that the wave propagation
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is sinusoidal in nature.
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It is interesting to note
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that the wavelength of the
propagation so produced
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is exactly double that of
the length of the dipole.
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We will come back to this point later.
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This is exactly what
we need in an antenna.
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In short, we can make an antenna
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if we can make an arrangement
for oscillating the positive
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and negative charges.
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In practice, the production
of such an oscillating charge,
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is very easy.
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Take a conducting rod
with a bend in its center,
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and apply a voltage signal at the center.
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Assume this is the
signal you have applied,
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a time-varying voltage signal.
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Consider the case at time zero.
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Due to the effect of the voltage,
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the electrons will be displaced
from the right of the dipole
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and will be accumulated on the left.
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This means the other end
which has lost electrons
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automatically becomes positively charged.
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This arrangement has
created the same effect
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as the previous dipole charge case,
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that is positive and negative
charges at the end of a wire.
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With the variation of voltage with time,
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the positive and negative
charges will shuttle to and fro.
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The simple dipole antenna also
produces the same phenomenon
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we saw in the previous section
and wave propagation occurs.
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We have now seen how the
antenna works as a transmitter.
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The frequency of the transmitted signal
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will be the same as the frequency
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of the applied voltage signal.
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Since the propagation travels
at the speed of light,
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we can easily calculate the
wavelength of the propagation.
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For perfect transmission,
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the length of the antenna should
be half of the wavelength.
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The operation of the antenna is reversible
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and it can work as a receiver
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if a propagating
electromagnetic field hits it.
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Let's see this phenomenon in detail.
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Take the same antenna again
and apply an electric field.
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At this instant, the electrons
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will accumulate at one end of the rod.
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This is the same as an electric dipole.
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As the applied electric field varies,
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the positive and negative charges
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accumulate at the other ends.
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The varying charge accumulation
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means a varying electric voltage signal
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is produced at the center of the antenna.
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This voltage signal is the output
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when the antenna works as a receiver.
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The frequency of the output voltage signal
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is the same as the frequency
of the receiving EM wave.
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It is clear from the
electric field configuration
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that for perfect reception,
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the size of the antenna should
be half of the wavelength.
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In all these discussions,
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we have seen that the
antenna is an open circuit.
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Now let's see a few practical
antennas and how they work.
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In the past, dipole antennas
were used for TV reception.
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The colored bar acts as a
dipole and receives the signal.
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A reflector and director
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are also needed in this kind of antenna
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to focus the signal on the dipole.
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This complete structure is
known as a Yagi-Uda antenna.
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The dipole antenna converted
the received signal
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into electrical signals,
and these electrical signals
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were carried by coaxial
cable to the television unit.
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Nowadays we have moved
to dish TV antennas.
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These consists of two main components,
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a parabolic shaped reflector
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and the low-noise block downconverter.
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The parabolic dish receives
electromagnetic signals
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from the satellite and
focuses them onto the LNBF.
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The shape of the parabolic
is very specifically
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and accurately designed.
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The LNBF is made up of a feedhorn,
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a waveguide, a PCB, and a probe.
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In this animation, you can
see how the incoming signals
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are focused onto the probe via
the feedhorn and waveguide.
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At the probe, voltage is induced
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as we saw in the simple dipole case.
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The voltage signal so
generated is fed to a PCB
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for signal processing such as filtration,
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conversion from high to low
frequency and amplification.
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After signal processing,
these electrical signals
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are carried down to the television unit
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through a coaxial cable.
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If you open up an LNB,
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you will most probably find
two probes instead of one.
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The second probe being
perpendicular to the first one.
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The two probe arrangement
means the available spectrum
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can be used twice
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by sending the waves
with either horizontal
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or vertical polarization.
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One probe detects the
horizontally polarized signal
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and the other, the
vertically polarized signal.
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The cell phone in your hand
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uses a completely
different type of antenna
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called a patch antenna.
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A patch antenna consists of
a metallic patch or strip
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placed on a ground plane
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with a piece of dielectric
material in between.
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Here, the metallic patch
acts as a radiating element.
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The length of the metal patch
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should be half of the wavelength
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for proper transmission and reception.
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Please note that the
description of the patch antenna
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we explained here is very basic.
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Please show your support by
clicking the support button,
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and thank you for watching the video.
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