Clarification on EIRP calculation
Last Post: February 13, 2006:
An AP emmits 1000mW signal is connected to connector of 10dB loss, with 3dB gian anteena
My answer is : 1000/10=100*3 =300
But how will it be 200mW
AM I missing some thing or my mehtod of calculation is right, since in method of 10's and 3's 10dB loss is 1/10 of power
If you are using Gain 10s multiply the power by 10
( 100mw AP with a 10dBi gain Ant. = 1000mw or 1W ).
If you are introducing loss 10s divide by 10 ( 100mw AP with 10dBm loss in cable = 10mw ). With the 3s gain doubles and loss cuts in half ( 100mw AP with 3dBi gain ant. = 200mw and 100mw AP with 3dBm loss in cable = 50mw )
Remember decibels are measurements of change not absolute power.
1mW = 0dBm ....... 1mW = 0dBm
10mW = 10dBm ....... 2mW = 3dBm
100mW = 20dBm ....... 4mW = 6dBm
1000mW = 30dBm ....... 8mW = 9dBm
combined 10s and 3s
20mW = 13dBm
200mW = 23dBm
Here is a simpler way to solve the problem:
Keep in mind that Watts should converted to dBm in order to determine the power ratio relative to 0 dBm (1mW), then convert back to Watts if that is what you need to know
1000mW = 1W = 30dBm
30dBm ?¡é?€?¡° 10dB(cable loss) = 20 dBm
We know that 20dBm = 100mW
Add 3dBi gain by doubling (x2) 100mW x 2 = 200 mW.
OR to just work with dB terms 20dBm + 3dBi = 23 dBm EIRP (200mW)
The only mistake you made was one that I see often with my students. When adding or subtracting 3dB, you multiplied by three or divided by three. Remember, the rule for three's is that if you add 3 dB, you times by TWO (double) ,if you lose 3dB you divide by TWO (one half).
Gene - good catch. I missed that the first time too...they should have made it the rule of 2's and 10's
Knowing the math behind conversions dB to Watts and vise versa helps to validate your work and knowledge. It also takes away the guess work and provides a more accurate and professional evaluation. I guess you can say it's more empirical and supportive when dealing with CTO's.
To convert watts to dB: 10 Log (watts/.001mW)
And to convert dB back to watts use this equation
((10x (dB/10))X .001mW
The 10x is the inverse Log function on your calculator. Try this to validate your 10 and 3's assumptions.