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 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 0.40 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