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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?

  • 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 clicking the support button,

  • and thank you for watching the video.

- [Narrator] Antennas are widely used

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B1 US antenna electric field signal electric dipole field

How does an Antenna work? | ICT #4

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    OolongCha posted on 2021/03/14
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