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Question about "db"

Apr 24th 2016, 11:38


Joined: Sep 23rd 2008, 13:42
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In reading articles relating to "db", how can it be determined if the author is referring to the "db" as a voltage or a power specification?
Apr 24th 2016, 14:12


Super Moderator

Joined: Apr 4th 1998, 00:00
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Normally what I do is to ask myself--which seems most reasonable given the numbers the author is quoting? In some cases, it may be necessary to do some calculations with the numbers provided. Keep in mind if the numbers seem to good to be true, the numbers are usually too good to be true. One of the great things about ham radio is that we are allowed to experiment and see for ourselves whether claims are accurate--if you can afford it you don't have to convince someone else to do the work!

Zack Lau W1VT
ARRL Senior Lab Engineer
Apr 24th 2016, 16:04


Joined: May 20th 2014, 01:06
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The decibel (symbol: dB, note capitalization of the B) is used to express a ratio. A common application is gain. To express power gain, A_p, in decibels, the following formula is customarily applied:
A_p(dB) = 10 log_10 (A_p)
For voltage gain, A_v, the formula customarily applied is:
A_p(dB) = 20 log_10 (A_v)
which yields power gain in decibels, under the assumption that the load is matched to the output. If this formula is used, and the assumption is met, it yields power gain.
Unless it is clear that this formula was used, and that the assumption is correct, it is prudent to check for yourself.
Apr 24th 2016, 21:28


Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
The decibel always relates to the logarithm of a ratio of two quantities. Usually, it is a ratio of two power values, such as dB gain for an amplifier [10 times log10(output power divided by input power)]. But it is possible to use dB for a ratio of anything (volts, dollars, weights, etc.) If it's something other than power (the usual situation in radio systems), the writer needs to specify that very clearly.

If you see a power level referred to by "dB", it is only correct when referred to a specific reference power. E.g., 0 dBm equals 1 milliWatt, +10 dBm is 10 milliWatts, and +20 dBm is 100 milliWatts. Sometimes, you see dBw, for power relative to 1 Watt, but mostly we use dBm.

Check for the hoary details.

73 Martin AA6E

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