dB to Watts, A Primer

dB is defined as ten times the base 10 logarithm of a ratio. As such, decibel is essentially a dimensionless number which only represents a ratio. Hence the chart should read:

3 dB = 2, 6 dB = 4, 10 dB = 10, 20 dB = 100, etc.

If you want to make these ratios with respect to some reference level such as 1 milliwatt of power into a 600 ohm load, then you have a decibel-milliwatt which carries the units, dBm. Hence, now the table would read:

3 dBm = 2 milliwatts, 6 dBm = 4 milliwatts, 10 dBm = 10 milliwatts and etc.

Remember that like all logarithms, they add when you are trying to multiply the source numbers or in this case the ratios. That's what makes them so powerful. You can work out very complicated gain and power ratios by simply adding and subtracting the logarithms. Some other examples are:

dBi is the ratio of antenna gain as compared to an isotropic dipole.

dBd is the ratio of antenna gain as compared to a real dipole antenna.

dBrnc0 is the ratio of reference noise to a C weighted psophometric factor. and so on.

Remember that ERP is exactly that, the effective radiated power and is the simple sum of the power into the antenna and it's gain.

For the case illustrated below, multiple antennas are known as an array. Panel antennas are nothing more than arrays of little dipoles in some configuration depending on the pattern desired. The larger the number of elements in an array means higher gain but narrower beamwidth in either the horizontal or vertical direction. If you stack elements vertically, then the vertical beamwidth gets narrower and if you stack them horizontally, then the horizontal beamwidth gets narrower. If they are in a square pattern then both beamwidths get narrowed. So for two 13 dBi antennas coupled together horizontally pointing in the same direction, the combined gain in the forward direction will be 16 dBi and the beamwidth in the horizontal plane will be half as wide at the 3 dB roll off points. The power is not changed presumably except by the increased loss of connectors which if done right should be less than 0.2 dB. Total gain should then be 15.8 dBi.