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