Antennas
Introduction

An antenna gives the wireless system three fundamental properties. They are gain, direction, and polarization. Gain is a measure of increase in power. Direction is the shape of the transmission pattern. A good analogy for an antenna is the reflector in a flashlight. The reflector concentrates and intensifies the light beam in a particular direction similar to what a parabolic dish antenna would to a RF source in a radio system.

Antennas are rated in comparison to isotropic or dipole antennas. An isotropic antenna is a theoretical antenna with a uniform three-dimensional radiation pattern that is similar to a light bulb with no reflector. Unlike isotropic antennas, dipole antennas are real antennas. Dipole antennas have a different radiation pattern compared to isotropic antennas. The dipole radiation pattern is 360 degrees in the horizontal plane, and 75 degrees in the vertical plane assuming the dipole antenna is standing vertically, and resembles a donut in shape.

Antennas generally fall into two categories. They are directional and omnidirectional. Figure provides examples of the two types. A good antenna transfers power efficiently. Efficient power transfer depends on correct antenna alignment or polarization, and proper impedance match. Achieving an impedance match involves electrically matching the transmission line to the antenna. This means that the transmission line transfers all of the power to the antenna and does not radiate energy itself. Figure shows the directional versus omnidirectional antennas and some of the typical applications.

The antennas used for WLANs have two functions:

  • Receive: This is the sink or terminator of a signal on a transmission medium. In communications, it is a device that receives Information, control, or other signals from a source.
  • Transmit: This is the source or generator of a signal on a transmission medium.

Some knowledge of antennas is essential in order to understand how to set up and optimize wireless networks for best performance. Antennas are available with different gain and range capabilities, beam widths, and form factors. Coupling the right antenna with the right AP or bridge allows for efficient coverage in any facility, as well as better reliability, at higher data rates. The Cisco antennas for APs are shown in Figure . A variety of antennas are available for bridges, depending upon the required distance and mounting possibilities. All Antennas sold with a spread spectrum product must comply with t he U.S. FCC Antenna Regulations shown in Figure . Onmidirectional antennas are generally used for point-to-multipoint implementations. Figures and show the Cisco antennas for wireless bridges.

This module will cover some of the basics of antennas and antenna operation. These fundamentals are needed when choosing antennas for a WLAN installation.