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DIRECTIONAL ANTENNA?

By Admin on

Before we get into the black magic that is wireless signal propagation, we need to understand a vital part of the industrial wireless system: antennas. Antennas are the means for coupling the transmitter to the medium, in this case, free space. An antenna is an electromagnetic radiator; it creates an electromagnetic field that proceeds out from the transmitting antenna to the receiver's antenna, which then converts the electromagnetic wave into electrical signals that are applied to the receiver's input stages.

There are several different types of antennas in three broad categories: omni-directional, directional, and semi-directional.

- Omni-directional antennas propagate in all directions.

- Semi-directional antennas propagate in a constricted fashion, defined by a specific angle.

Directional antennas have a narrow “beam” that allows highly directional propagation; familiar types are the parabolic and Yagi. Each has unique characteristics and applications.

Propagation patterns are shown on a polar chart, the angle of propagation being limited to where the power level drops by 3 dB. the half-power beamwidth for a Yagi antenna is shown.




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.

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