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Nie, dziękuję Przekieruj mnie tamhow to convert dB to dBm
You cannot directly convert dB to dBm unless you know the reference power.
So:
\[ \text{dBm} = \text{dB} + \text{reference power in dBm} \]
If the reference is 1 mW, then:
\[ 0\ \text{dBm} = 1\ \text{mW} \]
and the dB value becomes a dBm value relative to that reference.
Example:
\[ 20\ \text{dB relative to 1 mW} = 20\ \text{dBm} \]
But just saying “20 dB” alone is not enough to determine dBm.
| Unit | Meaning | Type | Reference |
|---|---|---|---|
| dB | Ratio, gain, loss, attenuation | Relative | No fixed reference |
| dBm | Power level | Absolute | 1 mW |
A value in dB tells you how much larger or smaller one quantity is compared with another. For power:
\[ G{\text{dB}} = 10 \log{10}\left(\frac{P_2}{P_1}\right) \]
A value in dBm tells you the actual power level compared with 1 mW:
\[ P{\text{dBm}} = 10 \log{10}\left(\frac{P_{\text{mW}}}{1\ \text{mW}}\right) \]
Since the denominator is fixed at 1 mW, dBm is an absolute power unit.
If you know the power in milliwatts:
\[ P{\text{dBm}} = 10 \log{10}\left(P_{\text{mW}}\right) \]
Examples:
| Power | dBm |
|---|---|
| 1 mW | 0 dBm |
| 10 mW | 10 dBm |
| 100 mW | 20 dBm |
| 1000 mW = 1 W | 30 dBm |
| 0.1 mW | -10 dBm |
| 0.001 mW = 1 µW | -30 dBm |
If the power is in watts:
\[ P{\text{dBm}} = 10 \log{10}\left(P_{\text{W}}\right) + 30 \]
Example:
\[ P = 2\ \text{W} \]
\[ P{\text{dBm}} = 10 \log{10}(2) + 30 \]
\[ P_{\text{dBm}} \approx 33.0\ \text{dBm} \]
So:
\[ 2\ \text{W} \approx 33\ \text{dBm} \]
Although you cannot directly convert dB to dBm, you can add or subtract dB from a known dBm level.
\[ P{\text{out,dBm}} = P{\text{in,dBm}} + G{\text{dB}} - L{\text{dB}} \]
Example 1: amplifier gain
Input power:
\[ P_{\text{in}} = 0\ \text{dBm} \]
Amplifier gain:
\[ G = 20\ \text{dB} \]
Output:
\[ P_{\text{out}} = 0\ \text{dBm} + 20\ \text{dB} \]
\[ P_{\text{out}} = 20\ \text{dBm} \]
Since 20 dBm equals 100 mW, the amplifier output is 100 mW.
Example 2: cable loss
Input power:
\[ P_{\text{in}} = 10\ \text{dBm} \]
Cable loss:
\[ L = 3\ \text{dB} \]
Output:
\[ P_{\text{out}} = 10\ \text{dBm} - 3\ \text{dB} \]
\[ P_{\text{out}} = 7\ \text{dBm} \]
So a 3 dB loss approximately halves the power.
You can do this:
\[ \text{dBm} + \text{dB} = \text{dBm} \]
Example:
\[ 5\ \text{dBm} + 10\ \text{dB} = 15\ \text{dBm} \]
But you normally should not do this:
\[ \text{dBm} + \text{dBm} \]
That is not a valid way to combine powers. If you need to add two actual power levels, convert each dBm value back to milliwatts, add them, then convert back to dBm.
\[ P{\text{mW}} = 10^{P{\text{dBm}}/10} \]
Example:
\[ P = 23\ \text{dBm} \]
\[ P_{\text{mW}} = 10^{23/10} \]
\[ P_{\text{mW}} = 10^{2.3} \approx 200\ \text{mW} \]
So:
\[ 23\ \text{dBm} \approx 200\ \text{mW} \]
Use these shortcuts:
| Change in dB | Power ratio |
|---|---|
| +3 dB | approximately 2 times power |
| -3 dB | approximately half power |
| +10 dB | 10 times power |
| -10 dB | one-tenth power |
| +20 dB | 100 times power |
| -20 dB | one-hundredth power |
Common dBm values:
| dBm | Power |
|---|---|
| -30 dBm | 1 µW |
| -20 dBm | 10 µW |
| -10 dBm | 100 µW |
| 0 dBm | 1 mW |
| 10 dBm | 10 mW |
| 20 dBm | 100 mW |
| 30 dBm | 1 W |
| 40 dBm | 10 W |
| 50 dBm | 100 W |
You cannot convert dB to dBm by itself because dB is relative and dBm is absolute.
To get dBm, you need a reference power:
\[ \text{Power in dBm} = \text{Reference power in dBm} + \text{gain or loss in dB} \]
If the reference is 1 mW:
\[ 0\ \text{dBm} = 1\ \text{mW} \]
Then a gain of 20 dB gives:
\[ 0\ \text{dBm} + 20\ \text{dB} = 20\ \text{dBm} \]
So the key idea is:
\[ \boxed{\text{dB is a ratio; dBm is power relative to 1 mW.}} \]