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electromagnetic wave i.e. electric and magnetic field that relative to each other.
so electric field can represent voltage and magnetic field rep. current. so that alternative current mean that alternative magnetic field and therefore electric field so that electromagnetic field.
antenna is a radiating element.it is a metal conductor. when current flowing through it it converts electric current in to magnetic filed and it radiates out as a elctromagnetic field when aproper phase matches.
by current flow in the antenna, there is created the magnetic field around that and as by maxwell's equations this changing magnetic field creates elcectric field and then again by this elctric field , magnetic field is created.
this goes on till this changin fields reach an antenna and this changing field again creates flow of electron in the receiving antenna
According with electromagnetics theory, a particle that changes its velocity vector, generates radiation. This may be view when a particle with initial velocity, reaches the surface of a conductor. This happens between antenna-air interface.
As a conclusion, when a particle changes its momuentum due inteface, there will be radiation. Intensity varies with other physical parameters (e.g. permitivity, conductancy, etc).
When electric current of high frequency is passed through a conductor, the electrons are forced to oscillate at the signal frequency (which is to be radiated !!).
1.
During the + ve half cycle of the signal, an electron is accelerated and during the following -ve one, it is retarded.Thus the field (static field at dc signal level) around the electron is perturbed continuously and periodically.
2.
An accelerated electron is shifted to a higher energy level in the conduction band .When it is retarded, the excess energy is emitted (in order to come back to the original state)in the form of electromagnetic radiation at the applied the signal frequency.
3.
When an electron moves at a constant velocity, field lines do not bend (reactive field) while when it is accelerated (reactive + radiating field), they bend. At each position of the accelerated electron, the field around it updates to the new value. The difference in field value around the electron of the present and the immediate past positions is radiated at the speed of light.
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