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• - [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?

• 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.

• and thank you for watching the video.

- [Narrator] Antennas are widely used

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# How does an Antenna work? | ICT #4

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OolongCha posted on 2021/03/14
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