| ADVANCED LESSON 32 | |
| LEARNING OBJECTIVES and NOTES | |||||||||||||||||||||||||||||||||||||
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Decibels The decibel (dB) is used to express ratios. It is based on a logarithmic scale so that a large range of numbers can be expressed by a few small numbers. dB's have no units, just a number. Quadrupling the power is equal to a 6dB increase. Often in radio we use a reference value for example dBm which means the number of dBs compared to a milliwatt. For example if we quadruple the power from a transmitter from one milliwatt to 4 milliwatts we increase the power by 6dB. If we increase the power of our transmitter from 1 Watt to 64 Watts we have increased it by 1-4Watts = 6dBs 4-16 Watts = 6dBs 16-64 Watts = 6dBs Making a total of 18dBs |
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3o.1 Recall the equations for decibel power and voltage ratios. These are the two equations:   We have worked out the first couple of examples from a knowledge that quadrupling the power is an increase of 6dBs. At Advanced level we also need to be able to use the two equations for power and voltage ratios. So we can use the equation and a calculator or the knowledge that increasing the power by 4 is a 6dB increase. Increasing the power by 3dB doubles the power Likewise we can look at situations where the power decreases - for example when it runs along a length of poor quality coax. In this case the answer will be -dBs. A length of coax with a loss of half the power could be describe as having a -3dB loss. |
Example 1
 Example 2
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Recall (or determine) the power gain or loss of various dB ratios based
on ± 3, 6, 9, 12, 15 and 10, 20, 30dB. (This includes examples such as
25W ≡ 20-6=14dBW.) Without using the equation you will get on pretty well just using some knowledge about dBs and power. See the table opposite Example The input to an amplifier = 2 Watts. The output = 20 Watts. what is the amplifier gain in dBs? 20/2 = 10 times gain From the table an increase of 10 times = a 10dB gain Example The input to an amplifier = 3 Watts. The output = 12 Watts. What is the amplifier gain in dBs? 12/3 = 4 times From the table a 4 times increase = 6dBs Try Q 3 to 5 Example 3
 Example 5
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Example 4  |