Now looking at specific relationships between milliwatts, dBM and dBW we see in Table 2, we see the conversion from power to dBm and dBW. Note that to convert from dBm to dBW, you must subtract 30 dB. That is because milliwatts are 1/1000 of a watt.  The log of 1000 is 3 and ten times that is 30.

Table 2: Milliwatts Referenced to dBm and dBW

The notation dB is for decibel. As a stand-alone notation, it is a ratio of gain, which can be added or subtracted to any referenced number. However if there is a letter to the right of the dB, like "m" or "W", it is an entirely different beast – it then becomes a calibrated measurement referenced to power in a milliwatts or watt scale

You can add or subtract dB to dBm and dBW for gains or losses. For example, if you have 30 dBm and have a gain of 20 dB, then you have a total power of 50 dBm. But you cannot simply add dBm to dBW – you must first convert one to the other by a factor of 30.

FCC Regulations and Antenna Selection

Now that we have discussed the mathematics involved in transmission, let’s look at requirements. The FCC has ruled that for an Omni-directional antenna, the total EIRP of the antenna could be 36 dBm or 6 dBW. This can be achieved with 1000 milliwatts Final Transmit Output Power (TFO) and an antenna gain of 6 dB for a total of 30 dBm + 6 = 36 dBm.

Remember 36 dBm as the critical number here. If the gain of an antenna is 7 dB, the final transmit power must be lowered by 1 dB to service 36 dBm. With an Omni-directional antenna, output can never exceed 36 dBm EIRP.

The real signal-strength gains, however, come from the gift that the FCC has given to directional antennas. Directional antennas have less chance of interference, so they have been granted a substantial advantage: for every dB gain over 6 dB for the antenna, the final stage of the transmitter must only be lowered by 1/3 of a dB.

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