The difference (or ratio) between two signal levels. Named after Alexander Graham Bell. (That is why the “B” is capitalized.) Used to describe the effect of system devices on signal strength. For example, a cable has 6 dB loss or an amplifier has 15 dB of gain. This is very useful because system gains and losses can be calculated by adding and subtracting.
dBm (dB milliWatt)
A signal strength or power level. 0 dBm is defined as 1 mW (milliWatt) of power into a terminating load such as an antenna or power meter. Small signals are negative numbers (e.g. -83 dBm).
For example, typical 802.11b WLAN cards have +15 dBm (32mW) of output power. They also spec a -83 dBm RX sensitivity (minimum RX signal level required for 11Mbps reception).
For example, 125 mW is 21 dBm and 250 mW is 24 dBm.
dBd (dB dipole)
The gain an antenna has over a dipole antenna at the same frequency. A dipole antenna is the smallest, least gain practical antenna that can be made. The term dBd (or sometimes just called dB) generally is used to describe antenna gain for antennas that operate under 1GHz (1000Mhz). The reason why the gain of many antennas, especially VHF/UHF antennas, is measured in dBd is that antenna manufacturers calibrate their equipment use a simple dipole antenna as the standard. Then replace it with the antenna they are testing. The difference in gain (in dB) is reference to the signal from the dipole.
dBi (dB isotropic)
The gain a given antenna has over a theoretical isotropic (point source) antenna. Unfortunately, an isotropic antenna cannot be made in the real world, but it is useful for calculating theoretical fade and System Operating Margins. The gain of Microwave antennas (above 1 GHz) is generally given in dBi. A dipole antenna has 2.14 dB gain over a 0 dBi isotropic antenna. Thus, a simple dipole antenna has a gain of 2.14 dBi or 0 dBd.
NOTE: If an antenna gain is just specified in dB from a manufacturer, be sure to ask if it is dBi or dBd. If they cannot tell you or do not know the difference, then you should consider buying from another vendor!
ERP (Effect Radiated Power)
Effect Radiated Power is defined as the effective power found in the main lobe of transmitter antenna. It is equal to sum of the antenna gain (in dBi) plus the power (in dBm) into that antenna. . For example, a 12 dBi gain antenna fed directly with 15 dBm of power has an Effective Radiated Power (ERP) of:
12 dBi + 15dBm = 27 dBm (500 mW).
NOTE: The ERP is found in the main lobe only. If you are using a high-gain omni-directional antenna, the radiation pattern is very flat and narrow (like a pancake). If the antenna is too high, the main lobe will actually shot over the heads of your customers. Yet, you oftentimes need great height to clear obstacle to your customers! The solution is to use down-tilt sector antennas. They have more gain then omni-antennas and the main lobe can be focused into the desired coverage area. Doing this also defines a “cell” that will prevent radio coverage all the way to the horizon. This has the benefit of not only minimizing interference at the WIPOP from distant signals, but also will enable you to re-use the frequency at another cell several miles away.
FSL (Free Space Loss)
Free Space Loss is defined as the loss a radio signal experiences when traveling through free space. The formula at 2.4 GHz is:
FSL = 104.2 + 20 log D
Where: D = Distance in miles
Example: At 5 miles FSL is 118 dB
RULE OF THUMB: Every time you double (or halve) the distance from the transmitter to the receiver, the signal level is lowered (or increased) by 6dB.
System Operating Margin (SOM)
System Operating Margin (also referred to as Fade Margin) is defined as the difference between the received signal (in dBm) and the receiver sensitivity (in dBm) needed for error free reception. For example, if the received signal level is 71 dBm and the receiver sensitivity is 85dBm (typical for a 2 Mbps WLAN), then the SOM is:
-71dBm – (-85 dBm) = 11 dB SOM
NOTE: If your WIPOP (Wireless Internet Point of Presence) is amplified and your customer’s WLAN card or AP is not, then the SOM needs to be calculated from the remote site back to the WIPOP. This is because this is the weakest signal in the system.
Thanks to Michael F. Young