9B1 Use the equations for decibel power, dB, dBW, dBm and voltage ratios dBV
The Decibel is a unit used to measure the intensity of a sound or the power level of a signal by comparing it with a given level of signal using the logarithmic scale. Note that the log scale is NOT equidistant between numbers as in the linear scale.
Now let's put that into a more understandable English !!
The Decibel is not a unit of anything. It is a RATIO.
It compares the value of one number to the value of another.
So what is dBW, dBm and dBV ? From above as these are not a unit of anything what do they mean? The part to look at is the W m and V.
The W is indicating a ratio of Watts
The m is indicating a ratio of milli units
The V is indicating a ratio of Volts
This
is the equation used to calculate gain of Yagi over a dipole
hence the "d"
This equation is used to calculate the Gain of a Yagi over an
Isotropic antenna hence the "i"
This equation is used relating to decibel power ratios which
could be "W" watts or "m" milli watts.
This equation is used for relating Voltages hence dBV as given in
the syllabus above but that on the equation sheet it is just
dB not dBV )So what is dBW, dBm and dBV ?
dBW is associated with power in Watts
dBm stands for decibel (power) milliwatts
dBV is associated with power in Volts
Previously you have learned that a gain or loss of 3dB was a factor of 2 so 20W with a gain of 3dB resulted in 40W out and vice verse a loss of 3dB on 20 W gave a result of 10W. This holds true for all dB measurements whether dBw dBm dBV so if this is the case why do we need to use log10.
Well most of the time you do not need to use log tables nor a log calculator just so long as you have the basis of dB in your head as shown above.
As dB may be added together and also subtracted one from another a Positive Result give a gain a Negative Result give a loss.
So consider yourself either a mathematician and
use logs or a hobbiest radio amateur and use the basics.
If you are a mathematician then you need not more explanation
but for the hobbiest take a look in 
on the section about
measurements, page 102 and forward from that, and you can
read a lot more about how to do the calculation easily.
The origin of some of the text on this page is from the RSGB with additions by the web master