TABLE OF CONTENTS Bels were named after Alexander Graham Bell. They were simply the logarithm of the ratio of power out and power in. The bel was too small to be useful, so the decibel quickly emerged. The definition of a decibel (dB) is contained in any radio engineer s handbook, as given below.
where:
dB = decibel, no units
V o = voltage out, volts
V i = voltage in, volts
P o = power out, watts
P i = power in, watts
In colloquial usage, decibels are used to express any ratio or often just an absolute value. Decibels are just ten times the 1og 10 of that ratio or value. A factor of two results in about 3 dB, while a factor of one-half results in about 3 dB.
Because radar engineers often deal in power, decibels relative to a watt or decibels relative to a milliwatt are used. Another common use of decibels is for RCS, decibels relative to a square meter. Some approximate decibel values usually memorized by radar people are shown in Table A1.1.
| Absolute | dB | Absolute | dB |
|---|---|---|---|
| 0.000001 = 10 6 | 60 | 1,000,000 = 10 6 | 60 |
| 0.00001 = 10 5 | 50 | 100,000 = 10 5 | 50 |
| 0.0001 = 10 4 | 40 | 10,000 = 10 4 | 40 |
| 0.001 = 10 3 | 30 | 1,000 = 10 3 | 30 |
| 0.01 = 10 2 |
