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 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.

 WATTS dBW 0.1 -1.0 1.0 0.0 2.0 3.01 3.0 4.77 4.0 6.02 5.0 6.98 10 10.0 15 11.76 20 13.01 30 14.77 40 16.02 50 16.98 100 20.00 200 23.01 500 26.98 1000 30.00

 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 0.4 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 1.2 1.42 1.59 1.75 1.92 2.06 2.2 2.34 Andrews LDF2-50 (3/8"") 1.05 1.5 1.83 2.16 2.42 2.67 2.93 3.12 3.31 3.5 Andrews FSJ4-50B (1/2"" Superflex) 1.05 1.5 1.84 2.18 2.44 2.69 2.95 3.16 3.37 3.58 Belden 9913 (RG-8 type) 1.3 1.8 2.25 2.7 3 3.3 3.6 3.9 4.2 4.5 100% braid shield over foil shield Belden 9914 (RG-8/U type) 1.6 2.4 2.95 3.5 4 4.5 5 5.35 5.7 6 100% braid shield over foil shield Andrews FSJ1-50 (1/4"" Superflex) 1.85 2.68 3.3 3.92 4.4 4.89 5.37 5.78 6.18 6.59 Belden 8214 (RG-8/U type) 1.8 2.7 3.45 4.2 4.73 5.27 5.8 6.25 6.7 7.1 97% Belden 8267 (RG-213/U type) 1.9 2.7 3.4 4.1 4.9 5.7 6.5 7.05 7.6 8 97% Belden 8268 (RG-214/U type) 1.9 2.7 3.4 4.1 4.9 5.7 6.5 7.05 7.6 8 97% double silver shield Belden 8242 (RG-9/U type) 2.1 3 3.9 4.8 5.37 5.93 6.5 7.05 7.6 8.2 9% silver inner double shield Belden 9258 (RG-8/X type) 3.7 5.4 6.7 8 9.03 10.07 11.1 11.95 12.8 13.5 95% Belden 84142 (RG-142 type) 3.9 5.6 6.9 8.2 9.13 10.07 11 11.75 12.5 13.5 98% double silver shield Belden 9273 (RG-223/U type) 4.1 6 7.4 8.8 9.87 10.93 12 12.9 13.8 14.5 95% double silver shield Belden 8240 (RG-58/U type) 4.5 6.8 8.4 10 11.33 12.67 14 15 16 17 95% Belden 8259 (RG-58A/U type) 4.9 7.3 9.4 11.5 13.33 15.17 17 18.5 20 21.5 95% Belden 8216 (RG-174/U type) 8.4 12.5 15.75 19 21.67 24.33 27 29 31 34 90%

last updated on 05-Jan-2004