So remember that electromagnetic wave is
a wave where you've got an electric
field, and a magnetic field creating each
other travelling along. You'll note that
both the electric and the magnetic field
of travelling sideways to the direction
of the waves travelling. So
electromagnetic waves transverse waves,
you note the electric field and the
madding field are also running off to
each other. The direction of the trick
field we are going up down or side side,
is known as the polarisation of the wave.
So how do these things start or stop? 
Well, the best way to make an oscillating
electromagnetic field is to take some
charges and oscillate them. So fast I
have say a vertical antenna how else to
move the electrons up and down by
driving them with the voltage, then that
would cause the electric field to go up
and down in that direction, and so I tend
to start an electromagnetic wave to
radiate off from that with the same
frequency that I was oscillating
electrons and the polarisation of being
in the direction of an antenna.
And what happens when
wave hits a charge? Well, it's got an
electric field and make it feel so it's
going to apply forces to that charge, so
it's going to accelerate it. So if I want
to particularly receive the signal from
an electromagnetic wave, the best thing
to use would be an antenna and such that
the electrons or the charges are free to
move in the direction that
electromagnetic wave is oscillating. It
turns out to help for efficiency
purposes if the antenna has a size
that's roughly commensurate with the
wavelength that you're trying to admit
or receive, and so you need the same kind
of antenna to admit or receive and it
needs to be on the order of sort of a
millimeter to a meter for microwaves and
high-frequency Wi-Fi and often bigger
than that for TV and radio.
