Electronic – How antenna radiates(how currents flows through wire)


I don't understand how antennas radiate a signal.

I understand basics antenna(wavelength, electron E field, …), but I simply don't understand how can current go through a wire that doesn't have negative pole.

Can you please explain that to me.

Best Answer

I'm guessing you don't understand how current can flow if there is no complete circuit. Let's take a simple quarter-wave dipole as an example:


simulate this circuit – Schematic created using CircuitLab

How can any current flow, since there is no complete circuit from "-" to "+" of V1?

Consider this: relative to the speed at which the waves in the electromagnetic fields propagate, the dipole is long. It's true that current can't flow, but it doesn't know that until it gets to the end of the wire. As the current approaches the end of the wire but has no place to go, the charges pile up until they are pushed back in the other direction. By the time it's back, it's travelled \$\lambda/2\$ or experienced a \$180^\circ\$ phase shift. The voltage at V1 has also changed by this point, and so the current is constructively adding to the new currents being produced by V1. If it were not for some of this energy being lost as radiation, the energy in this antenna would grow without bound.

Why the energy radiates is complicated. The long answer is "Maxwell's equations". If you don't want to understand all the gritty details of that math, then here's a simple, incomplete understanding: the current in an antenna is associated with a magnetic field, and the voltage is associated with an electric field. An antenna is an arrangement such that at some distance away from the antenna (the far field) these two fields are mutually perpendicular and in phase, and what you get is a self-propagating wave like this:

enter image description here

Red is the electric (E) field, and blue is the magnetic (B) field. This is the sort of wave that would be emitted by a dipole aligned with the Z axis.