Power Conversion

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The following chart may be used as a starting point for converting Watts to dBm and dBm to Watts.

Note: The decibel is a logarithmic unit. DBm or decibels in milliwatts is referenced to 0 dBm equalling 1 milliwatt.

Memorize the dBm, mW & W columns and the 3 rules that follow and you’ll be able to do rough power calculations in your head in no time at all!!!

dBm	dBW	mW	W	kW
-20	-50	0.01	0.00001	***
-10	-40	0.1	0.0001	***
0	-30	1	0.001	***
10	-20	10	0.01	0.00001
20	-10	100	0.1	0.0001
30	0	1,000	1	0.001
40	10	10,000	10	0.01
50	20	100,000	100	0.1
60	30	1,000,000	1,000	1
70	40	10,000,000	10,000	10

Increasing power by 3 dB Doubles the power
Reducing the power by -3 dB cuts the power in Half
4.78 dBm = 3 mW. Increasing this value by factors of 10 (14.78 dBm, 24.78 dBm, 34.78 dBm, 44.78 dBm 54.78 dBm & 64.78 dBm will give you 30mW, 300mW, 3W, 30W, 300W & 3kW respectively.

Note: The 3dB values mentioned above are actually 3.0102999 dB.

Therefore, repeated additions or subtractions of 3db will allow small errors to

creep into your calculations!

Always remember and never forget.....

“dBm” is REAL Power!

“dB” refers to a change in levels (plus or minus...Gain or Attenuation)!

Don’t mix up the two!!! They are not the Same!!!


		The following Formulas will give you conversions between Watts & dBm

	Formula #1

	To Convert Watts to dBm:


	10 Log ( Watts x 1000) = dBm


	Formula #2


	To Convert dBm to Watts:


	10

(dBm/10)

	________________
	1000


	= Watts

10 raised to the dBm divided by 10 power over 1000 = Watts

Note: 10 to the (dBm/10) power is = the inverse log of (dBm/10)


	Formula #3*

	To Calculate decibels:

			P₂ (larger power in mW)


	10 Log


	__________________


	P₁(smaller power in mW)


	= dB


	* The bel named in honor of Alexander Graham Bell, is defined as the common logarithm of the ratio of two powers, P1 and P2. (Taken from Electronic Engineers Handbook 3rd Edition, McGraw Hill Copyright 1989)

Formula #4

To Calculate Gain:

dBm_out - dBm_in = Gain in dB

Formula #5

To Calculate VSWR:

VSWR = 1 plus (+) the square root of the reflected power (in Watts) divided by the forward power (in Watts) over 1 minus (-) the square root of the reflected power divided by the forward power.

Note on VSWR: If you think you are having a problem with VSWR, keep in mind that the HPA manufacturers typically spec their systems, at 2:1. If your VSWR is less than 1.5:1 Consider yourself to be better than normal. Instrument grade is 1.15:1 or better. Anything between 1.15:1 and 1.5:1 is optimal.

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