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 than1 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 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 (relative gain over a half-wave 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.

WATTSdBW0.1-1.01.00.02.03.013.04.774.06.025.06.981010.01511.762013.013014.774016.025016.9810020.0020023.0150026.98100030.00

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.

**Coax Cable Loss in
dB per 100 ft.
**

Coax Style Frequency in Mhz | 100 | 200 | 300 | 400 | 500 | 600 | 700 | 800 | 900 | 1000 | Shield | Notes |

Andrews LDF6-50 (1-1/4"") | 0.28 | 0.40 | 0.49 | 0.58 | 0.65 | 0.72 | 0.79 | 0.85 | 0.91 | 0.97 | ||

Andrews LDF5-50A (7/8"") | 0.37 | 0.54 | 0.66 | 0.78 | 0.88 | 0.97 | 1.07 | 1.15 | 1.23 | 1.31 | ||

Andrews LDF4-50A (1/2"") | 0.68 | 0.98 | 1.20 | 1.42 | 1.59 | 1.75 | 1.92 | 2.06 | 2.20 | 2.34 | ||

Andrews LDF2-50 (3/8"") | 1.05 | 1.50 | 1.83 | 2.16 | 2.42 | 2.67 | 2.93 | 3.12 | 3.31 | 3.50 | ||

Andrews FSJ4-50B (1/2"" Superflex) | 1.05 | 1.50 | 1.84 | 2.18 | 2.44 | 2.69 | 2.95 | 3.16 | 3.37 | 3.58 | ||

Belden 9913 (RG-8 type) | 1.30 | 1.80 | 2.25 | 2.70 | 3.00 | 3.30 | 3.60 | 3.90 | 4.20 | 4.50 | 100% | braid shield over foil shield |

Belden 9914 (RG-8/U type) | 1.60 | 2.40 | 2.95 | 3.50 | 4.00 | 4.50 | 5.00 | 5.35 | 5.70 | 6.00 | 100% | braid shield over foil shield |

Andrews FSJ1-50 (1/4"" Superflex) | 1.85 | 2.68 | 3.30 | 3.92 | 4.40 | 4.89 | 5.37 | 5.78 | 6.18 | 6.59 | ||

Belden 8214 (RG-8/U type) | 1.80 | 2.70 | 3.45 | 4.20 | 4.73 | 5.27 | 5.80 | 6.25 | 6.70 | 7.10 | 97% | |

Belden 8267 (RG-213/U type) | 1.90 | 2.70 | 3.40 | 4.10 | 4.90 | 5.70 | 6.50 | 7.05 | 7.60 | 8.00 | 97% | |

Belden 8268 (RG-214/U type) | 1.90 | 2.70 | 3.40 | 4.10 | 4.90 | 5.70 | 6.50 | 7.05 | 7.60 | 8.00 | 97% | double silver shield |

Belden 8242 (RG-9/U type) | 2.10 | 3.00 | 3.90 | 4.80 | 5.37 | 5.93 | 6.50 | 7.05 | 7.60 | 8.20 | 9% | silver inner double shield |

Belden 9258 (RG-8/X type) | 3.70 | 5.40 | 6.70 | 8.00 | 9.03 | 10.07 | 11.10 | 11.95 | 12.80 | 13.50 | 95% | |

Belden 84142 (RG-142 type) | 3.90 | 5.60 | 6.90 | 8.20 | 9.13 | 10.07 | 11.00 | 11.75 | 12.50 | 13.50 | 98% | double silver shield |

Belden 9273 (RG-223/U type) | 4.10 | 6.00 | 7.40 | 8.80 | 9.87 | 10.93 | 12.00 | 12.90 | 13.80 | 14.50 | 95% | double silver shield |

Belden 8240 (RG-58/U type) | 4.50 | 6.80 | 8.40 | 10.00 | 11.33 | 12.67 | 14.00 | 15.00 | 16.00 | 17.00 | 95% | |

Belden 8259 (RG-58A/U type) | 4.90 | 7.30 | 9.40 | 11.50 | 13.33 | 15.17 | 17.00 | 18.50 | 20.00 | 21.50 | 95% | |

Belden 8216 (RG-174/U type) | 8.40 | 12.50 | 15.75 | 19.00 | 21.67 | 24.33 | 27.00 | 29.00 | 31.00 | 34.00 | 90% |

last updated on
05-Jan-2004