![]() ![]() Because dBm is compared to 0 mW, 0 dBm is a relative point, much like 0 degrees is in temperature measurement. Receiver sensitivity is defined as the minimum signal power level (in dBm or mW) that is necessary for the receiver to accurately decode a given signal. Antennas that can see each other without any obstacles between them are in line of sight.Ĭable losses between the receiver and its antenna This refers to how far apart the antennas are and if there are obstacles between them. The primary hardware factors that are involved are:Ĭable losses between the transmitter and its antenna The distance that a signal can be transmitted depends on several factors. In order to calculate EIRP, add the transmitter power (in dBm) to the antenna gain (in dBi) and subtract any cable losses (in dB). EIRP is the value that regulatory agencies, such as the FCC or European Telecommunications Standards Institute (ETSI), use to determine and measure power limits in applications such as 2.4-GHz or 5-GHz wireless equipment. Power that comes off an antenna is measured as effective isotropic radiated power (EIRP). The radiated (transmitted) power is rated in either dBm or W. Therefore, an antenna that is rated at 3 dBd is rated by the FCC (and Cisco) as 5.2 dBi. ![]() The power rating difference between dBd and dBi is approximately 2.2-that is, 0 dBd = 2.2 dBi. While some antennas are rated in dBd, the majority use dBi. Dipole antennas are more real-world antennas. All FCC calculations use this measurement (dBi). ![]() Isotropic antenna power is the ideal measurement to which antennas are compared. They are used only as theoretical (mathematical) references. Note: Isotropic antennas are theoretical antennas that transmit equal power density in all directions. You can also use the dB abbreviation in order to describe the power level rating of antennas: Note: You can find all values with a little addition or subtraction if you use the basic rules of algorithms. Then, subtract 3 dB again in order to drop the power by 50 percent again (14 dB = 25 mW). Subtract 3 dB from 100 mW in order to drop the power by half (17 dB = 50 mW). If 0 dB = 1 mW, then 10 dB = 10 mW, and 20 dB = 100 mW. This table provides approximate dBm to mW values: dBm These are commonly used general rules: An Increase of: If you use the formula above, you can calculate the power of 50 mW in dBs in this way: Power (in dB) = 10 * log10 (50) = 10 * log10 (5 * 10) = (10 * log10 (5)) + For example, you can apply this basic rule in order to calculate logarithms of large numbers: log10 (A*B) = log10(A) + log10(B) If you want to calculate the power in dB of 50 mW, apply the formula in order to get: Power (in dB) = 10 * log10 (50/1) = 10 * log10 (50) = 10 * 1.7 = 17 dBmīecause decibels are ratios that compare two power levels, you can use simple math in order to manipulate the ratios for the design and assembly of networks. Reference is the reference power (for example, 1 mW). Signal is the power of the signal (for example, 50 mW). This list defines the terms in the formula: You can calculate the power in dBs from this formula: Power (in dB) = 10 * log10 (Signal/Reference) The abbreviation dB is often combined with other abbreviations in order to represent the values that are compared. The dB measures the power of a signal as a function of its ratio to another standardized value. Refer to Cisco Technical Tips Conventions for more information on document conventions. ![]() This document is not restricted to specific software and hardware versions. Prerequisites RequirementsĬisco recommends that you have knowledge of basic mathematics, such as logarithms and how to use them. This information can be very useful when you troubleshoot intermittent connectivity. This document defines radio frequency (RF) power levels and the most common measure, the decibel (dB). ![]()
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