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Question 15: How many dBm is 40 mW?
A. 10 dBm
B. 16 dBm
C. 20 dBm
D. 22 dBm
E. 40 dBm
Correct Answer: B
Explanation:
The dB measures the power of a signal as a function of its ratio to another standardized value. The
abbreviation dB is often combined with other abbreviations in order to represent the values
that are compared. Here are two examples:
You can calculate the power in dBs from this formula:
Power (in dB) = 10 * log10 (Signal/Reference)
This list defines the terms in the formula:
Here is an example. 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 dBm
Because 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. 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 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)) + (10 * log10(10)) = 7 + 10 = 17
dBm
Reference: http://www.cisco.com/c/en/us/support/docs/wireless-mobility/wireless-lan-wlan/23231-
powervalues-23231.html
