Hm. I was taught that the egotistical SOB named it after himself, no honor
involved.

Jim



Actually what I was taught was the standard unit is the BEL, named in
honor of Alexander Graham Bell. This is a large value hence the DECIBEL or
1/10 of a BEL is generally used.

Dave WD9BDZ

RST Engineering wrote:
Hm. I was taught that the egotistical SOB named it after himself, no honor
involved.

Jim




Actually what I was taught was the standard unit is the BEL, named in
honor of Alexander Graham Bell. This is a large value hence the DECIBEL or
1/10 of a BEL is generally used.

Dave WD9BDZ




For a (fairly) comprehensive write up on the "decibel" go to
https://ewhdbks.mugu.navy.mil/decibel.htm .

Also:

Alexander Graham Bell
From Wikipedia, the free encyclopedia


Bell and decibel

The bell is a unit of measurement invented by Bell Labs and named after
Bell. The bell was too large for everyday use, so the decibel (dB),
equal to 0.1 B, became more commonly used. The dB is commonly used as a
unit for measuring sound intensity.

Dave WD9BDZ

If Roy's answer works for you, skip mine. I think an answer of mine on this
must be in the archives.

Rule number one:
The Decibel is a measure of the ratio of two power levels ONLY, ALWAYS and
NEVER anything else.
Rule Number two. We like to violate rules and drive newbies crazy.


However, I'll go back to Rule Number One first.


The so called "Voltage dB", "Current dB" and "Power dB' are all figments of
your (well some people's)imagination. I used to have fun with new Engineers
tripping them up on this. "Why are Voltage dBs twice as big as power
dB's?". This is a false-reasoned question.

While, it is a fact that there is only one "kind" of dB, there are, however,
three ways to _calculate_ them depending on what quantities you have at
hand. Because Power is Volts times Amps, there is something special
happening when we do the math. Lets call this V x A for this discussion.

For a simple case of doubling the power - or a power ratio of two to one:
In order to get twice the power, we need to change the Voltage AND Current
enough so that VxA is twice what we started.
1 Volt and 1 Amp gives 1 Watt
For 2 Watts we'll need:
1.414 Volts and 1.414 Amps 1.414 x 1.414 = 2

Looking, we see that the voltage did NOT double. It went up less that
twice.

However, we can calculate the decibel difference from the voltage difference
OR the power difference. Using the "Power dB" formula for dB we get the
correct 3.whatever dB and using the "Voltage dB" formula we get the VERY
SAME amount of 3.whatever dB.

So the dB are not different, only the starting quantities and, therefore the
formulas.


Rule Number 2:
We can, and, we do use the voltage formula ( if it's there, use it... just
like the mountain climber's answer to why does he climb the
mountain...because it's there) in cases where we are not actually doing
power ratio comparisons. Why? Because it is easy to compare large
differences this way..that is, in the log format. Frequently, voltage gain
in OP amp circuits is shown in dB even though there is practically an
INFINITE power gain. This is because the input resistance is very high and
the output resistance is very low and ignoring the resistance and ONLY
looking at the voltage gives erroneous dB numbers. HOWEVER, those in the
field know and understand this and all is well in the universe, so to speak.
By measuring the voltage gain, and using the dB formula, it is easier to
show 50 dB "Voltage" gain than showing a gain of 100,000. Silly Engineers!

73, Steve, K9DCI



"dansawyeror" wrote in message
...
There are three terms, idb, pdb, and vdb. They are not the same. idb and
pdb
have the same value: 10*log10 (i/i0), while vdb is 20*log10(v/v0).

Dan


Roy Lewallen wrote:
Robert11 wrote:

Hello:

When a table gives the attenuation (at some freq.) for a length of a
particular coax in db,
are they referring to db in voltage or db in power ?


Both. When voltage and power ratios are measured across the same single
impedance, the ratio of voltages in dB is the same as the ratio of power
in dB. That is, dB in voltage equals dB in power if only one impedance
is involved. That's why there are different formulas for dB voltage and
power ratios, to make this happen. And it's the case if a coax cable is
terminated in its characteristic impedance so the input and output
impedances are the same. It's under this condition that cable loss is
specified, so the dB loss represents the loss of both voltage and
current.

But when you compare the power at the output of a coax cable with the
power at the input and the cable isn't terminated in its characteristic
impedance, output and input impedances can be very different. The power
loss will be somewhat higher than the specification (probably not much,
when the matched loss is only 2 dB), but you can have much less or more
voltage at the output than the input. So the power and voltage loss in
dB can be very different, and you can actually have voltage gain --
although it can be argued that defining dB for a ratio of voltages
across two different impedances is a bit shaky and perhaps not too
meaningful.


Is is correct to ask a question as simply as the following:

If the attenuation is given as, e.g., 2 db, what Percentage therefore
of a received signal is "lost"
going thru the coax length ?


100 * (1 - 10^(-dB/10)) ~ 37% is the fraction of power lost.

100 * (1 - 10^(-dB/20)) ~ 21% is the fraction of voltage lost.

These assume that the coax is terminated with its characteristic
impedance.

And you don't need to put "lost" in quotation marks. It is truly lost as
a signal, having been turned into heat.

Roy Lewallen, W7EL

"Richard Harrison" wrote in message
...
In the telephone industry, the most common reference level is one
milliwatt.

Ditto the RF industry.

Signal loss is common.

Pretty ubiquitous... something about the three laws of thermodynamics... ;-)

Other dB units are easily contrived. There are dBw (dB referenced to one
watt), dBk (dB referenced to one kilowatt), dBRAP (dB above reference
acoustical power which is defined at 10 ro the minus 16 watts), etc,

The cable TV industry likes to use dBuV, for whatever odd reason.