- [Instructor] In the modern world,
we humans are completely surrounded
by electromagnetic radiation.
Have you ever thought
of the physics behind
these traveling electromagnetic waves?
The great scientist, Heinrich Hertz,
was the first man to transmit
and detect electromagnetic waves.
In his famous experiment,
a high voltage current was
applied to the two ends
of two metal wires,
which generated a spark
in the gap between them.
This spark resulted in the radiation
of electromagnetic waves.
Those electromagnetic waves
traveled through the air
and created a spark in a metal coil
located over a meter away.
If you had placed an LED in that gap,
the bulb would have glowed.
This was a clear case of
electromagnetic wave propagation
and detection.
However, before Hertz, the
brilliant mathematician,
James clerk Maxwell, had already laid out
the foundations for
electromagnetic radiation
by formulating for mathematical equations.
However, these equations
and the Hertz experiment
raised a question,
how do electromagnetic
fields detach themselves
from wires and propagate through a space?
More specifically, what we need
is a traveling electromagnetic wave
and not a fluctuating one.
Let's explore this logically.
Consider an electric charge,
which is moving at a constant speed.
The electric field around it is shown.
Now imagine for a fraction
of a second it accelerates
after that, it continues
its uniform motion
at a higher speed.
What we need to understand
is the effect of this acceleration
on the electric field.
The interesting thing
is that the information
does not travel at an infinite speed,
instead, it travels at the speed of light.
Similarly, the information
about the sudden variation
of velocity of the charge
does not get conveyed
to the whole electric field region.
The field near it knows about it,
but the field far away still has no idea
that the charge has accelerated
and it is still in the old state.
Let's separate out these regions
with the help of two circles.
Since the electric field cannot break
the field between these
distances must transition.
This transition field is known as a kink.
The kink moves or radiates
outwards at the speed of light.
To show the kink animation in a clear way,
let's move the camera
along with the charge.
We can say here that the
acceleration of the charge
has caused an electromagnetic disturbance
or electromagnetic radiation.
Based on this understanding,
we will be able to understand
the most important experiment
in the field of antenna technology,
the oscillating electric dipole.
The interesting fact about
this simple oscillating dipole
is that it produces
electromagnetic radiation
in a perfectly sinusoidal manner.
Let's see how it is achieved.
Before getting into the electromagnetics,
let's understand how velocity
and acceleration vary in this simple case.
It is clear that at both ends
the velocities should be zero
and in the middle the velocity
should be at the maximum.
This means that this is a case
of continuous acceleration
and deceleration.
The electric field pattern is drawn here
when the chargers are far apart,
and when the velocity is zero.
In order to have a better understanding,
let's examine one of the
electric field lines.
Let's observe the electric
field line at t by eight.
You can see that the electric
field line is deformed.
The reason for this deformation is simple.
This time period is the region
with the highest acceleration.
As we saw earlier, accelerating
or decelerating charges cause
kinks in the electric field.
In short, the old electric field
does not get adjusted to
the new field very well.
This deformation is continuous
since there is continuous
acceleration in the charge.
When two charges meet
at the central point,
the deformed line also meets there.
After that, it detaches and radiates.
This radiation travels
at the speed of light.
If you applied an electric
field intensity variation
with respect to length,
you can see that the
radiation we have produced
is perfectly sinusoidal in nature.
Please note that this
varying electric field
will automatically generate
a varying magnetic field
perpendicular to it.
Now let's have a look at how
this applies to an antenna.
A time varying voltage is applied
to the metal wire is shown
due to the effect of the voltage
the electrons will be
displaced from right to left
and create positive and negative charges.
With a continuous variation of voltage,
the positive and negative charges
will shuttle back and forth in the wire.
The simple arrangement is
known as a dipole antenna.
The dipole antenna
produces the same radiation
as we saw in the previous section.
In this case, the antenna
works as a transmitter.
The frequency of the transmitted
signal will be the same
as the frequency of the
applied voltage signal.
The same antenna can act as a receiver
if the operation of the
antenna is reversed.
When propagating electromagnetic
waves strike the antenna,
the oscillating fields
of waves create positive
and negative charges at
the ends of the antenna.
The varying charge accumulation
means a varying voltage signal is produced
at the center of the antenna.
This voltage signal is the output
when the antenna works as a receiver.
We can note here that for perfect
transmission or reception,
the length of the antenna should
be half of the wavelength.
This is the first antenna design criteria
for proper reception or transmission.
The second most important design criteria
is a term called impedance matching.
Perfect impedance matching
will make sure that the waves are radiated
in the most efficient way.
When an alternating current
passes through a circuit,
it faces opposition from the
combined effects of resistance,
inductance and capacitance.
This combined effect
is known as impedance.
According to the maximum
power transfer theorem,
to transfer the maximum amount of power
the load impedance should match
with the source impedance.
For further understanding,
let's take an example of a circuit
containing an alternator as a source
and a motor bulb, et cetera, as a load.
In this setup to achieve
maximum power transfer
from alternator to the load,
the impedance of the load must match
with the impedance of the alternator.
A similar impedance balance is required
in the case of an antenna system.
Since an antenna works on
high frequency signals,
the impedance of the transmission lines
also becomes important.
Hence to achieve maximum power,
the impedance of an antenna should match
to the impedance of the source
and transmission line as well.
If the impedances do not match,
some portion of the power
would be reflected back to the source
instead of radiating
outwards from the antenna.
A free space has an
impedance value of 377 ohms.
In a parabolic antenna,
a wave guide is used
as a transmission line,
which has a different impedance
value from the free space.
That's why a feedhorn is also included
in a parabolic antenna.
This way, the impedance of the wave guide
is matched with the
impedance of the free space
so that the EM waves can
be received properly.
We hope the concept
of such an important
engineering phenomenon
is clear for you from this video,
and please don't forget
to support us, thank you.
