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Antenna basics

By Admin on

Antennas are an essential part of radio telecommunications equipment, bridging the gap between electronic and electromagnetic signals. The shape and size of an antenna is a strong clue as to its type, as the design dictates the antenna’s purpose. The antenna’s length, for example, corresponds to the length of the radio waves the antenna receives or transmits. The shape affects whether it receives radio waves from different directions or a single direction.

Passive gain amplifies the signal

All antennas exhibit passive gain, which serves to amplify the signal. Passive gain is measured by the quantity dBi, which is the gain referenced to a theoretical isotropic antenna; an isotropic antenna transmits energy equally in all directions, and does not exist in nature. The gain of an ideal half-wave dipole antenna is 2.15 dBi. It should also be noted that as directionality increases, so does gain.

EIRP, or equivalent (or effective) isotropic radiated power, is the measure of the maximum power a theoretical isotropic antenna would emit in the direction of maximum antenna gain. EIRP accounts for losses from transmission lines and connectors, and includes actual antenna gain. EIRP allows calculation of real power output and field strength values, if actual antenna gain and transmitter output power are known.

Directional antenna

Directional and semi-directional antennas focus radiated power into narrow beams, adding a significant amount of gain in the process. Antenna properties are also reciprocal. The characteristics of a transmitting antenna, such as impedance and gain, are also applicable to a receiving antenna. This is why the same antenna can be used for both sending and receiving. The gain of a highly directional parabolic antenna serves to amplify a weak signal; this is one reason why this type of antenna is frequently used for long distance links


Yagi

Building on the principles of the dipole antenna, the Yagi has several pairs of metal tubing elements laid parallel to each other on another long tube that serves as a backbone. One pair of elements functions as a traditional dipole antenna; the others reinforce the incoming radio signal, boosting its strength. Engineers select the lengths of each element and their relative spacing to produce the best radio sensitivity, in effect tuning the antenna to desired wavelengths. The Yagi is the familiar TV antenna you see on the rooftops of homes. It has more directional sensitivity than a dipole, so you aim it in the direction of the radio source.



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