Calculating Your Effective Radiated Power

Effective Radiated Power (ERP)

Probably the easiest way to calculate this is to first convert your actual power level to dBW, which is referenced to 1 watt.  0 dBW is therefore 1 watt.  Wattage LESS than 1 watt are expressed as negative values, so that 100 mw would be -1 dBW.  10 watts is 10 dBW, 100 watts is 20 dBW, 1000 watts is 30 dBW. (notice that this is simply the LOG of the wattage X 10.  (the formula is dBW=10 * log watts)

Using a slide rule, calculator, or your computer (using the calculator available from its menu) you can quickly determine your ERP IF you know all the dB losses and gains.  Your transmitter is of course a "gain", while the coax is a "loss", and the antenna is a "gain".

ERP will be the transmitter wattage, minus the coax (and other) losses, plus the antenna gain expressed as dBW .  Example.  Your 2 meter rig has an output of 50 watts.  Your coax has a loss of 1.7 dB.  Your vertical antenna has a gain of 6 dBd (expressed as a gain over a dipole.  If expressed as dBi which is gain over an isotropic antenna, then you need to subtract 2.14 dB before proceding.

50 watts is 10 * Log of 50 = 16.99 dBW.

Coax is a "loss" so its -1.7 dB

Antenna is a "gain" so its 6 dB

16.99   -1.7   +6   =  21.29 dB and working backword = 21.29 divided by 10 = 2.129 and the Inverse Log of that is 134.58 watts, your ERP.

Here is a small chart listing the dBW values for some various power levels.  Remember you are dealing with the "Log" of a number, so the scale is NOT linear.

WattsdBW.

0.1-1.0

1.0  0.0

2.03.01

3.04.77

4.06.02

5.06.98

1010.0

1511.76

2013.01

3014.77

4016.02

5016.98

10020.00

20023.01

50026.98

100030.00

Did you notice that interesting "3 dB" relationship?

We have all heard that "doubling" our power is like a 3 dB increase.  Well, take a look at the chart to the left.

Notice that the dB difference from 1 watt, to 2 watts, is 3 dB.   Likewise, going from 15 watts to 30 is also 3 dB, rounded of course.  "Doubling" the size of your  antenna system does the same thing.

Also note that going from 10 watts, to 20, then to 40, is a gain of 6 dB.  So is going from a single dipole, to 2 dipoles, then to 4 dipoles 6 dB to achieve the same gain.