- [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.
Very nicely done video -- however, I don't feel the explanation is really that clear.
The mechanism of waves separating from their charges, and the state of the excited medium outside of the antenna, are not detailed.
The reason of impedance mismatches reducing power transmission efficiency is not explained at all. The mechanism of power being reflected back in case of impedance mismatch is hand-waved away (and the arrows that are intended to denote reflection point into space, making it look like another radiation mechanism).
And finally, the video unfortunately does not attempt to bridge the gap to a mathematical representation of the phenomena discussed, making it less useful to apply the knowledge gained to everyday work.