Tutorial on Wireless LAN Antennas

Rick Gallaher is course director for CCI, President of Telecommunications Technical Services Inc., and author of  Rick Gallaher's MPLS Training Guide

April 4, 2003

Antenna Gain and Propagation

A wireless LAN does no good if it cannot cover its required distance.  Wireless LANs operate on radio waves that are subject to the laws of propagation. One of these laws states that signal strength decreases by the square of a given distance.  At a distance of two feet, for example, a signal becomes four times weaker; at four feet, the signal is 16 times weaker; and at a distance of 20 feet, the signal is 400 times weaker.

It stands to reason that one way to increase the distance covered by a wireless LAN is to increase power – much like turning up the power amplifier on your car stereo. When you want to hear the radio a block away, you simply turn up the power to 100 watts.

However, FCC regulations and rules restrict the power that can be sent – they limit the volume control, so to speak. In order to overcome this problem, network designers have learned to focus the radio energy by using sound antenna theory

Vocabulary 

• DB  - a logarithmic ratio calculation
• DBm - DB reverence to the milliwatts standard
• DBW - DB reference to the watt standard
• EIRP -  Estimated Isotropic Radiated Power
• Isotropic antenna - a theoretical antenna
• mw/cm2 - power density measurement
• TFO -  Transmit Final Output power
• XMIT - transmit

Radio Theory

In a theoretical antenna called an isotropic antenna, radio waves are transmitted equally in all directions.  To get a visual image of an isotropic antenna and its transmitting pattern, imagine an antenna that is shaped like a BB. The transmission pattern from this BB is a sphere, which would equate to the BB positioned in the exact center of a basketball.  From any location on the basketball, measure the thickness of rubber over one square inch. In radio transmission, this is called power density. Power density can be measured as milliwatts per centimeter squared, or mw/cm². Let’s call this value x.

In practice, no antenna works like an isotropic antenna, because no antenna gives a perfectly spherical transmission pattern.  Further, in most cases, you would want to focus the energy of an antenna like you would focus the light of a flashlight. By focusing an antenna's energy into a beam, you would make an Omni-directional antenna into a directional antenna and achieve an amplification effect. For example, if you took the isotropic antenna, cut its transmission area in half, and directed that power to the remaining half, you would have an mw/cm² of value 2x. 

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