Voltage and current distribution curves in radiation pattern of a dipole antenna

Basic action of an antenna

As per Maxwell’s theory, we know that the accelerated electrons always radiate energy in the form of EM waves. When a high frequency current is passed through a conductor, then the free electrons inside the conductor get accelerated through it and thus it produces alternating EM wave or a field.

Hence, the EM waves are characterized by the so called electric field (E) and the magnetic field (H) which are always perpendicular to each other.

Concept of distribution of electric current in an antenna

When an antenna is connected in parallel with the output of a transmitter, high frequency alternating current starts flowing through the antenna (along the entire length of the antenna). Now the distribution of current through the antenna can be understood with the help of following important points and the following figure -

1. Suppose at a particular instant of time, point A is negative and point B is positive.
2. Then the flow of charges will be from point A to X and from Y to B, as shown in the above diagram.
3. Now you may wonder that since the ends of the dipole antenna are open, how this flow of charges could be possible?
4. Actually the result is as such that the end point X becomes continuously positively charged due to loss of electrons.
5. At the same time, the end point Y becomes continuously negatively charged as it gains more and more electrons.
6. In this way, the charge is maximum at both the end points X and Y.
7. So the current is zero.
8. However, at the center the charge density is zero and thus current is maximum, as it is shown clearly with a RED curve in the above diagram. It is called as “CURRENT DISTRIBTION CURVE”.
9. Here it is assumed that the feeding of signals to the antenna is a sine wave, so the current wave is also sinusoidal in nature.
10. In this way, when a dipole antenna is fed with a high frequency sinusoidal wave, from the output of a transmitter, then both types of distributions that is current and voltage distribution are produced along the length of the antenna.

Note-1: The conditions explained above are only possible in ideal conditions. When we consider, practical conditions, then some losses take place which can be found out with the help of some formulae.

Note-2: When the length of antenna is equal to half of the wavelength of transmitted signal, then such antenna is called as “Dipole Antenna”.

 Have a look at the radiation pattern animation of a dipole antenna.