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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