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.
Electronic – How antenna radiates(how currents flows through wire)
antenna
Related Solutions
Indeed it can be a very good antenna. Look no further than the transistor radios and AM band receivers. In those ubiquitous consumer goods the antenna consisted of a piece of very low loss ferrite with a very high permittivity. This was wrapped in many amp*turns of very fine copper wire. The high permittivity gave the antennas an effective cross-sectional area -due to the permittivity- (If I recall correctly) of a square mile or so, thus bringing the antenna's electrical size up to the dimensions of the wavelength that it was receiving.
On a technical bent, you could consider that the antennae interacted with the magnetic field portion of the radiating Poynting vector.
For reference, check out books by Balanis. They are THE reference on antenna design/theory.
This is a very good question and is key to fundamental EE principles.
To help visualize this, let's picture a quarter wave center-fed dipole antenna, which resembles a 'T' shape. When an incident electromagnetic wave of the same frequency at which the antenna is resonant, there is a standing wave generated at either end of the "T". This is generated because the charges generated by the incident wave on the antenna are not continuous along the antenna. So in theory you could take a voltmeter with both probes touching the same end of the antenna and gradually move one probe to the other end, and you will see a difference in voltage.
If you take a look at the book Electromagnetics Explained, it will show this distribution and explain (fairly well) the linkage between Maxwell's Equations and a very basic but accurate idiom on why the charges behave the way they do.
I know this is a somewhat vague answer, but a simple forum post like this doesn't do justice to really explain the interaction of EM waves and their behavior.
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:
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.