On Fri, 25 Sep 2009 10:13:59 +0100, Andy GD0TEP wrote:
>Can someone tell me what the formula is for converting Watts to DBM please,
>or DBM to Watts.
It's actually pretty simple if you know logs. All dB values are ratios to
some specified reference. The dBm means dB with 1 mW as a reference, and dB
is 10 times the log of the ratio. Adding dB is like adding logs, so adding dB
is like multiplying the fundamental quantity.
SO -- 10X the power is 10 dB, 10X the voltage is 20dB. Every time you
multiply by 10 you add 10 dB. since 1mW is 0 dBm, 1 watt (10 x 10 x 10 x 1mW)
is +30dBm, 10W is +40dBm, 100W is +50dBm, 1kW is +60dBm. Some other easy
numbers -- double the power is 3dB, 4X the power is 6dB, 5X the power is 7dB.
To divide the power, simply subtract the dB numbers.
Note, BTW, that it's dBm. dB is the abbreviation for a deci Bel (one tenth of
a Bel, that log) and named for Alexander Graham Bell, and the "m" means one
milliwatt. dB are used in MANY disciplines, including acoustics (sound
levels), audio, and telephony. Every discipline has its own reference for
what "0dB" is. 0dBV is the voltage with resect to 1 volt. 0dBu is the voltage
equivalent to 1mW in 600 ohms, which is 0.78 volts. To convert from a VOLTAGE
dB to a power dB, you must know the impedance of the circuit. :) But dBm is
impedance-blind. That is, it talks ONLY about power, and the circuit
impedance can be anything that works for the circuit in question. Likewise,
dBV is impedance-blind -- it tells us only about the voltage.
73,
Jim Brown K9YC
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