#2 - Antenna, Towers, Transmission Lines

ANTENNAS AND THEIR CHARACTERISTICS

An antenna is a device specifically designed to transfer radio signals from a transmitter and radiate that signal into the atmosphere. How far the signal travels is affected by a number of factors, of which the antenna plays a key role.

OMNIDIRECTIONAL ANTENNAS

An Omnidirectional antenna radiates the radio signal in a 360° pattern. Omnidirectional antennas are used when the antenna can be located near the center of the coverage area. With this type of antenna, the range will be about the same in all directions.

DIRECTIONAL ANTENNAS

Because antennas are used in diverse geographic areas, it is often desirable to change their radiation pattern to make them more directional to best suit the user’s specific coverage area requirements. There are three common types of directional antennas: Cardiod Unidirectional Bidirectional A characteristic of directional antennas is referred to as gain. Gain is the antenna’s ability to increase its effective radiated power and expand the normal coverage area. Antennas that focus, or concentrate radiated signals in one direction are basically sending a stronger signal in that direction. By doing this, there is a resultant gain in signal strength in the direction of the concentration. There is also a loss in signal strength in all other directions.

CARDIOID ANTENNAS

The cardioid antenna produces a heart-shaped radiation pattern that is more concentrated in one direction, allowing a longer maximum range. Cardioid antennas are used when the antenna will be located near the edge of the coverage area. This concentration of energy in one direction reduces the coverage in the opposite direction. This tends to reject radio signals being received from that direction. Thus reducing the amount of interference and unwanted signals.

UNIDIRECTIONAL ANTENNAS

A unidirectional antenna generates an even more focused radiation pattern which aims the radio signal in a narrow band in one direction. Unidirectional antennas are used when a narrow strip, or corridor area, requires coverage.

BI-DIRECTIONAL ANTENNAS

A bi-directional antenna increases coverage perpendicular to the plane of the antenna. A long strip of highway or railroad right of way can be covered by a bi-directional antenna.

TOWERS

Towers may be used to raise antennas to the height necessary to cover the required geographical area with radio signals. Antennas also may be raised by mounting them on buildings or by placing the antenna tower on top of a hill or mountain. The location and height of the tower, along with the power of the signal, determine the range of coverage. Towers are often used to support multiple antenna systems. Government regulations may require that towers over a certain height be properly painted and have lights to avoid a possible air navigational hazard. User’s may also be required to notify government agencies of all proposed tower construction and obtain appropriate licenses and permits.

TRANSMISSION LINES

The base station is connected to the antenna using a transmission cable and is usually located as close to the antenna site as possible. The transmission line is designed to efficiently carry radio signals between a base station and its antenna. A transmission line is made of outer and inner conductors called coaxial cables. Attenuation, or signal loss, is one of the most important considerations in transmission lines. All transmission lines dissipate some power, which results in a partial loss of signal strength. Because of attenuation, the transmission line length also dictates its diameter since the larger the line diameter, the less signal loss and the longer the line can be. This means that the further the base station is from the antenna site the bigger the transmission cable should be in order to minimize the amount of signal loss.