The Basic lessons · Basic electronics and theory · B-005-008

Understanding Decibels and Power Ratios

Learn how to calculate power changes using the decibel scale, focusing on the critical 3 dB and 10 dB rules. This lesson explains why doubling power adds 3 dB and how to use these ratios to adjust signal strength.

This lesson is part of SignalPrep Pro. The subsection's 11 exam questions are free to practise: practise B-005-008.

What this subsection asks

• A two-times increase in power results in a change of how many dB?
• What change in transmitter power results in a 3 dB decrease?
• What change in transmitter power results in a 6 dB increase?
• If a signal transmitted with a power of 200 watts is received with an S-meter reading of "10 dB over S9," what would be the new reading if power was reduced to 20 watts?
• If a signal transmitted with a power of 150 watts is received with an S-meter reading of "20 dB over S9," what would be the new reading if power was reduced to 15 watts?
• What is the "decibel" used for?
• The power output from a transmitter increases from 1 watt to 2 watts. How many decibels does that increase represent?
• The power of a transmitter is increased from 5 watts to 50 watts by a linear amplifier. The power gain, expressed in dB, is:
• You add a 9 dB gain amplifier to your 2-watt hand-held. What is the power output of the combination?
• The power of your transmitter is 100 watts and your transmission line introduces a loss of 6 dB. How much power is delivered to the antenna?
• A local amateur radio operator reports receiving your 100-watt 2-metre simplex transmission with an S-meter reading of "30 dB over S9." What power could you use to reduce that reading to S9?