antennaselectromagnetic-radiationelectromagnetism
I have heard that in an antenna the electrons move back and forth and create an electric field which varies with time. This varying electric field in turn, creates a varying magnetic field. These two varying fields together create an electromagnetic wave.
How do these waves travel out from the antenna?
If the electrons are moving back and forth vertically, wouldn't there only be an electromagnetic wave near the antenna? What makes them spread out or expand?
It is a difficult thing to visualize and connected with the wave-particle duality of photons. I think what you are interested in is the Second Quantization. This is where an electromagnetic wave is decomposed into its Fourier modes and each Fourier mode can be interpreted as simple harmonic oscillator. The energy levels of such oscillators corresponds to $E = nh\nu$, where each electromagnetic mode with that energy is a state that has $n$ photons with energy $h\nu$.
How does an electro-magnetic waves get detached from an antenna and spread to the space?
There is the classical formulation of electromagnetism. In that, a varying electric field generates a varying magnetic field and the wave propagates because it has a directional vector that carries the power of the wave, the Poynting vector.
Dipole radiation of a dipole vertically in the page showing electric field strength (colour) and Poynting vector (arrows) in the plane of the page.
While an antenna receives an EM wave, which quantity of the EM wave (electric or magnetic) is used for converting the EM wave to electric energy. i.e, fluctuating magnetic field or fluctuating electric field produces the movement of electrons in antenna?
The electric field.
When an EM wave travels in the space how it can be directional?
Look at the figure. It is a directional solution of the boundary conditions of the electromagnetic problem, and it carries energy and in the quantum mechanical representation with photons, momentum too.
If fluctuating magnetic field produces electric field (and vice verse), each point of EM wave in space again act like a source and it spread from there in all direction?
No. The poynting vector , look at the arrows in the figure, is directional, and it is the direction in which the wave propagates. It is not continuous point sources.
Best Answer
Antenna is a device, consisting of horizontal stick with aplied current on both ends. The current is alternating and varies harmonically (for simplicity) $I \sim I_0 e^{i\omega t}$.
From Ampere's law we know that current will generate harmonically varying magnetic field $B\sim B_0 e^{i\omega t}$, which from Faraday's law will generate alternating electric field $\nabla \times B = \frac{1}{c^2}\frac{\partial E}{\partial t}$, where $E\sim E_0 e^{i\omega t}$.
From Maxwell's equations: $$ \nabla \times ( \nabla \times E) + \frac{1}{c^2}\frac{\partial^2 E}{\partial t^2} = 0 $$
This is a wave equation that tells you how your EM waves propagate. In case when the size of antenna is much larger than the distance between observer and antenna, you will have your wave propagates perpendicular to the antenna in horizontal direction. However, when you will go further and further from antenna, it could be viewed as a point source and your wave will be radially symmetric.