- [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.
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