Antenna

An antenna is a metallic conductor system capable of radiating and capturing electro-magnetic energy. Antennas are used to interface  transmission lines to the atmosphere, the atmosphere to transmission lines , or both. In essence, a transmission line couples energy from a transmitter to an antenna to a receiver. The antenna ,in turn, couples energy from a transmitter to an anteena or from an antenna to a receiver. the antenna in turn couples energy received from a transmission line to the atmosphere and energy received from the atmosphere to a transmission line.

At the transmit end of a free - space radio communications system, an antenna converts electrical energy traveling along a transmission line into electromagnetic waves that are emitted into space.At the receive end , an antenna coverts electromagnetic waves in space into electrical energy on a transmission line. A waveguide is a special type of  transmission line that consists of a conducting metallic tube through which high frequency electromagnetic energy is propagated . A waveguide is used to efficient interconnect high frequency electromagnetic waves between an antenna and a transceiver.

Radio waves are electrical energy that has escaped into free space in the form of transverse electromagnetic waves. The escaped radio waves travel at approximately the velocity of light and are comprised of magnetic and electric fields that are at right angles to each other and at right angles to the direction of travel. The plane parallel to the mutually perpendicular lines of the electrical and magnetic fields is called the waveform. The wave always travels in a direction at right angles to the waveform and may go forward or backward, depending on the relative direction of the lines of magnetic and electric flux. If the direction of either the magnetic or the electric flux reverses, the direction of travel is reversed. However , reversing both sets of flux has no effect on the direction of propagation.

All electrical circuits that carry alternating current radiate a certain amount of electrical energy in the form of electromagnetic waves. However the amount of energy radiated is negligible unless the physical dimensions of the circuit approach the dimensions of a wavelength of the wave .For example, a power line carrying 60-hz current with 20 feet of separation between conductor radiates virtually no energy because a wavelength at 60hz is over 3000 miles long , and 20 feet is insignificant in comparison. In comparison an inductor 1cm long carrying a 6ghz signal will radiate a considerable amount of energy because 1cm is comparable with the 5-cm wavelength.

It is apparent that the size of an antenna is inversely proportional to frequency. A relatively samll antenna can efficient radiate high frequency electromagnetic waves, while low- frequency waves require relatively large antennas. Every antenna has directional characteristics and radiate more energy in certain directions relative to other directions. Directional characteristics of antennas are used to concentrate radiation in a desired direction or capture energy arriving from a particular direction.

For an antenna to efficiently receive radio signals it must abstract energy from the radio wave as it passes by the receiving point. Electromagnetic waves reception occurs in an antenna because the electromagnetic flux of the wave cuts across the antenna conductor , including a voltage into the conductor that varies with time in exactly the same manner as the current flowing in the antenna that radiated the wave. The induced voltage along with the antenna absorbs from the passing waves.

Basic antenna operation is best understood by looking at the voltage standing - wave patterns on a transmission line . The transmission line is terminated in an open circuit , which represents an abrupt  discontinuity to the incident voltage wave in the form of a phase reversal. The phase reversal results in some of the incident voltage being radiated, not reflected back toward the source. The radiated energy propagates away from the antenna in the form of transverse electromagnetic waves. The radiation efficiency of an open transmission line is extremely low. Radiation efficiency is the ratio of radiated to reflected energy. To radiate more energy, simply spread the conductor farther apart.Such an antenna is called a dipole.