in the modern world we humans are
completely surrounded by electromagnetic
radiation have you ever thought of the
physics behind these travelling
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 travelled through
the air and created a spark in a metal
coil located over a meter away if you
had place 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 four
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 it's 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
similarity 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 electro magnetics 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 8 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
caused 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 plot 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 as 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 this 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 criterion for proper reception or
transmission the second most important
design criterion 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
etc 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 waveguide 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 waveguide is
matched with the impedance of the free
space so that the e/m 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