The decibel is a logarithmic value commonly used for radio propagation calculations. Radio propagation works using logarithmic numbers.
Why? Radio propagation involves complicated multiplication at varying points along a radio path. A directional antenna multiplies the signal by its gain. Feed line cable divides the signal by its loss. The equations become very complex.
Taking logarithms of all gain and loss values allows them to be simply added and subtracted. When all gains and losses are identified, they can simply be added up to determine the final receive signal level. Overall mathematics is simplified by using the decibel, abbreviated to dB.
To keep the mathematics simpler, antennas, feed line losses and insertion loss of duplexer's etc. are all specified as dB values.
The Radio Mobile program works mostly in decibels. Where input is required from a normally linear device, such as transmitter power output or receiver sensitivity, the program provides the ability to enter number in either Watts or µV (linear) or dBm (logarithmic). The program will convert these numbers to a dB value for program use.
Decibel Mathematics
The decibel is logarithmic number that is a ratio, in this case, between 2 power levels. The decibel is defined by the following equation:
Note that the units in the above equation must cancel out, meaning that if the top is specified in Watts, the bottom must also be specified in Watts.
For example, linear amplifier gain may be specified in decibels. If input power = 75 Watts and output power = 300 Watts, the gain of this amplifier is:
Portions of this text are provided by Brian Henderson and can also be found in his Program Operating Guide.
