Remember: dB is a ratio of Power levels, so when you need to do a ratio of Voltages, the formula is different. He doesn't mention that until late in the video. However, he gives a much clearer explanation of why the formula is different (20 x Log10() vs 10 x Log10()) by doing the
math to demonstrate where the difference comes from.
(Remember P=I x E or P = (V 2)/R ?)
There is a table at the beginning of the video of the few things you should memorize both for your amateur radio test and for general use later:
- 3 dB represents a ratio that is a doubling. Of the two things being compared, one has twice as much power as the reference.
- -3 dB represents a ratio that is a half.
- 10 dB is 10 times greater.
- 20 dB is 100 times greater.
Note: W2AEW has posted a whole bunch of good video tutorials on electronics and radio.
A couple that might be relevant to prepping for your General Class amateur radio exam:
- Back to Basics: Voltage and Current in Capacitors and Inductors
- Basics of RF Mixers in Radio Receivers / Mixer Tutorial / Frequency Conversion