EM Wave Radiation Mechanisms
EM Wave Radiation Mechanisms
Radiation can be thought of as process of transmitting electrical energy. Radiation is also the flow of energy away from the source. An antenna is a metallic conductor system capable of radiating and capturing EM energy. The radiated energy propagates away from the antenna in the form of TEM waves.
An antenna can be a length of wire, a metal rod, or a piece of tubing. Many different sizes and shapes are used. The length of the conductor is dependent on the frequency of operation. Antennas radiate most effectively when their length is directly related to the wavelength of the transmitted signal.
Most antennas have a wavelength that is some fraction of a wavelength. ½, ¼ wavelengths are most common. The important criteria for radiation are that the length of the conductor be approximately one-half or one-quarter wavelength of the ac signal. Very little radiation of EM filed will occur if antenna wires are less than λ/2 or λ/4 length.
As frequency is increased, wavelength decreases. Note that wavelength (λ) should be within the range (1 MHz to 100 GHz) of practical conductors and wires. It is within this range that long-distance radiation occurs. For example, a 300 MHz UHF signal has a λ wavelength of 1 m, a very practical length.
An open transmission line can be made into an antenna simply by bending the conductors out at right angles. Such a line has maximum voltage at the end of the line and the current is minimum. By bending the conductors at right angle to the transmission line at the quarter wave point, an antenna is formed.
Antenna is a radiator or sensor of EM waves. It is a transition device or transducer between a guided wave and a free space wave or vice versa. It is an electrical conductor that radiates EM energy into or collects EM energy from free space.
Principle of radiation
Under time varying conditions, Maxwell’s equations predict the radiation of EM energy from current source (or accelerated charge). This happens at all frequencies, but is insignificant as long as the size of the source region is not comparable to the wavelength. While transmission lines are designed to minimize this radiation, radiation into free space becomes main purpose in case of antennas. Note that a good transmission line is a poor antenna.
Acceleration is the increase in the rate of change of velocity of charges. De-acceleration is the decrease in the rate of change of velocity of charges. Acceleration and de-acceleration of charges produces electric and magnetic fields.
In order to know how an antenna radiates, let us first consider how radiation occurs. A conducting wire radiates mainly because of time-varying current or an acceleration (or deceleration) of charge. If there is no motion of charges in a wire, no radiation takes place, since no flow of current occurs. Radiation will not occur even if charges are moving with uniform velocity along a straight wire. However, charges moving with uniform velocity along a curved or bent wire will produce radiation. If the charge is oscillating with time, then radiation occurs even along a straight wire.
Regardless of antenna type, all involve the same basic principle that radiation is produced by accelerated (or decelerated) charge. The basic equation of radiation may be expressed simply as:
