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VK3KHB

RF Power Loss / Gain Calculator
and formulae *



What is dBm?

A logarithmic ratio of power levels with a base reference set to 1mW = 0 dBm. With a known impedance value you can convert any voltage V to dBm and vice versa. The calculator / chart lets you find out only Z, L or V and once known W can also be given.
Remember to clear the inputs to get correct figures.


* dBm=log10 (mW)*10
* mW=10^(dBm/10)
dBm to watts conversion
Enter any two values; Z, V, L the third value will be calculated.
V= Voltage, L= level, Z= Impedance
W= Watts is calculated on 2nd click of the calculate button.

..
Impedance Z Ω ohms
Voltage V V RMS
Power level L dBm ref. to 1mW
Power Watts W
.
double click .

examples
30dB 7.07V 1.0W 50Ω ref 1mW
10dB 0.707V 10mW "
-10dB 0.0707V 100uW "
-80dB 0.000022V 10pW "

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RF Power - Loss Formulae, Calculator and Chart. The example below uses a 3dB loss in a transmission line of a 10 watt transmitter, resulting in ~5 watts being presented to the antenna for emission, follow the transposition steps to understand how it is calculated.

Power to dB and dB to Power:

(P1 = ref compare / P0 = ref base)
Transposed
Step 1 (divide b.s. by 10)
Step 2 (Anti-log)
Step 3 for P1 (multi b.s. by P0)
- or - 3 for P0 (divide b.s. by P1)
- or - 3b for P0 (invert b.s.)
dB = 10 log (P1 / P0)


dB/10 = log (P1 / P0)
Alog(dB/10) = (P1 / P0)

P0* (Alog(dB/10)) = P1
(Alog(dB/10)) / P1= 1 / P0
1 / (Alog(db/10) / P1) = P0
Loss of Power e.g.
-3.01 = 10 log (5 /10)
-0.301= log (5/10)
Alog(-0.301) = (5/10)
10 * (0.5) = 5
Alog(-0.301) / 5 = 1/10
1 / Alog(-0.301)/5) = 10
Gain of Power e.g.
+3.01 = 10 log (10/5)
0.301 = log (10/5)
Alog(0.301) = (10/5)
5 * 1.9998 = 10
Alog(0.301) /10 = 1/5
1 / (Alog(0.301) /10) = 5

73
Gary, VK3KHB


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