What is a dB, dBm, dBu, dBc, (Tutorial Videos from W2AEW on Youtube)

If the concept of decibels (dB) is confusing, this Youtube video from W2AEW has a very clear explanation with a demonstration using a spectrum analyzer:

• What is a dB, dBm, dBu, dBc, etc. on a Spectrum Analyzer?

https://www.youtube.com/watch?v=1mulRI-EZ80#aid=P-LGfIJuuH8

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.

https://www.youtube.com/user/w2aew

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

I've also found his oscilloscope and spectrum analyzer tutorials to be very useful.  He's very good at explaining and documenting things.  Compared to a normal text presentation, the demonstrations / experiments really drive the concepts home and show that it's possible to try these things for yourself.

Enjoy.