Bredhurst Receiving and Transmitting Society

Decibels

9B1   57  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
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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 os 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 ?
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dBW is associated with power in Watts,
dBm stands for decibel (power) milliwatts

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  .



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