Because a dB has no particular defined reference, the dBx, where x represents a specific value, is often used instead of the dB. For example, the dBm is referenced to the milliwatt. Since the dBm has a defined reference, it can also be converted back to watts, if desired. The power gain or loss in a signal is determined by comparing it to this fixed reference point, the milliwatt. The following related terms must be understood in order to properly design and install WLANs:

• dB milliWatt (dBm) - This is the unit of measurement for signal strength or power level. If a person receives a signal at one milliwatt, this is a loss of zero dBm. However, if a person receives a signal that is 0.001 milliwatt, then a loss of 30 dBm occurs. This loss is represented as -30 dBm. To reduce interference with others, the 802.11b WLAN power levels are limited to the following:
• 36 dBm EIRP by the FCC
• 20 dBm EIRP by ETSI
• dB dipole (dBd) - This refers to the gain an antenna has, as compared to a dipole antenna at the same frequency. A dipole antenna is the smallest, least gain practical antenna that can be made.
• dB isotropic (dBi) - This refers to the gain a given antenna has, as compared to a theoretical isotropic, or point source, antenna. Unfortunately, an isotropic antenna cannot exist in the real world, but it is useful for calculating theoretical coverage and fade areas.
• A dipole antenna has 2.14 dB gain over a 0 dBi isotropic antenna. For example, a simple dipole antenna has a gain of 2.14 dBi or 0 dBd.
• Effective Isotropic Radiated Power (EIRP) - EIRP is defined as the effective power found in the main lobe of a transmitter antenna. It is equal to the sum of the antenna gain, in dBi, plus the power level, in dBm, into that antenna.
• Gain - This refers to the amount of increase in energy that an antenna appears to add to an RF signal. There are different methods for measuring this, depending on the chosen reference point. Cisco Aironet wireless is standardized on dBi to specify gain measurements. Some antennas are rated in dBd. To convert any number from dBd to dBi, simply add 2.14 to the dBd number.

Lab Activity Lab 3.2.3 - Wireless Mathematics In this lab, students will convert Radio Frequency (RF) signal ratings into a common decibel (dB) unit in order to calculate power gain or loss.

 
 

Interactive Activity Interactive Activity: Calculating Decibels This activity allows the student to enter values for Power final and Power reference, then calculates for decibels.

Interactive Activity Interactive Activity: Using Decibels This activity allows the student to enter a value for the decibels and a value for the reference power resulting in the final power.