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How to read signal strength in dbm's

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Anonymous
a b F Wireless
August 4, 2004 2:37:17 AM

Archived from groups: alt.internet.wireless (More info?)

Hello,
I googled around and can't find anything on this. Anyway, I see people
online mention that they are getting a -87dbm signal strength. As a
amateur radio operator I know about db's and antenna gain, but I use
S-Units for signal strength, not dbm's.
So, is -100dbm better than -1dbm?? How does that scale work? I am not
used to better signals having a negative number.

Thanks,
Scott
Anonymous
a b F Wireless
August 4, 2004 9:57:42 AM

Archived from groups: alt.internet.wireless (More info?)

/dev/scott0 <outsideworld@scottbrown.us> wrote in message news:<10h0tggh112et49@corp.supernews.com>...

> So, is -100dbm better than -1dbm?? How does that scale work? I am not
> used to better signals having a negative number.

-1 dBm is significantly better than -100 dBm.

Absolute power of a signal is measured in wattage. The bel or decibel
system can only describe relative power- a gain of 3 dB means your
signal is 2 times as strong as it was before, but the dB scale doesn't
define where you're starting from or what your 'zero' is. So, we
specify dBm, indicating that our scale is relative to 1 milliWatt of
power. 0 dBm = 1 mW.

The reason you see negative values is that you're representing small
but positive numbers, on a logarithmic scale. In logarithms, the value
indicated represents an exponent... for example, under a log 10 scale,
a value of -2 represents 10 to the -2 power, which equals 0.01.
Likewise, a negative dBm means that you're applying a negative
exponent in your power calculations; 0 dBm equals 1 mW of power, so
-10 dBm equates to 0.1 mW, -20 dBm equates to 0.01 mW, and so forth.
It's a lot easier, and more useful in some calculations, to describe a
weak signal as -100 dBm as opposed to 0.0000000001 mW.
Anonymous
a b F Wireless
August 4, 2004 1:04:50 PM

Archived from groups: alt.internet.wireless (More info?)

In article <10h0tggh112et49@corp.supernews.com>, /dev/scott0
<outsideworld@scottbrown.us> writes
>Hello,
> I googled around and can't find anything on this. Anyway, I see people
>online mention that they are getting a -87dbm signal strength. As a
>amateur radio operator I know about db's and antenna gain, but I use
>S-Units for signal strength, not dbm's.
> So, is -100dbm better than -1dbm?? How does that scale work? I am not
>used to better signals having a negative number.
>
>Thanks,
>Scott

Without a lecture on the subject a dBm is a simple reference level and
then you can go up in level say 10 dBm, meaning 10 dB UP from 0 dBm or
down from that say -10 dBm so -80 dBm is 70 dB lower than -10dBm its all
"relative" levels as such!. So -1dBm is a "higher" level than -100dBm!

Go to the Jack Daniel's site www.rfsolutions.com and download his RF
calculator which runs under excel and you can do lots of useful
conversions i.e. dBm to dBw etc!.....
--
Tony Sayer
Related resources
Anonymous
a b F Wireless
August 4, 2004 3:16:27 PM

Archived from groups: alt.internet.wireless (More info?)

On Tue, 03 Aug 2004 22:37:17 -0700, /dev/scott0
<outsideworld@scottbrown.us> wrote:

>I googled around and can't find anything on this. Anyway, I see people
>online mention that they are getting a -87dbm signal strength.

0dbm is equal to 1 milliwatt into 50 ohms.
-3dBm is half a milliwatt
-6dBm is 1/4 of a milliwatt
-9dBm is 1/8 of a milliwatt.
For every 3dB below 0dBm, you get half your power.

>As a
>amateur radio operator I know about db's and antenna gain, but I use
>S-Units for signal strength, not dbm's.

Gaaak. Turn in your ham license and reread the study guides. S-Units
are an ancient anacronism left over from the stone age of thermionic
valve (tube) receivers. The characteristic curve between receiver
input signal level and indicated AGC (automagic gain control) DC level
was anything but linear or even consistant. Someone decided to
establish a reference signal level. S-9 magically became 50
microvolts because either Collins of Hammarlund (I forgot which)
managed to make receivers that would consistantly read S-9 with 50 uv
of RF. I dunno what frequency was used for testing. It's anything
but a standard and suffers from serious errors (and guesswork).

See:
http://www.ac6v.com/sunit.htm
for more details and why you should avoid using S-units.

Most of todays cell phones have genuine logarithmic amplifiers to
detect IF signal strength (before limiting). The output is fairly
linear (well, fairly logarithmic) and can be used as an accurate
measure of signal stength (assuming you don't attach an external RF
amplifier stage). If you hack your cell phone into the test mode,
you'll get a display of receive signal in -dBm (without the - sign).
Anyway, -dBm and sometimes uv are the standard for receive signal
strength.

Sometimes cell phones and 802.11 boxes use RSSI (receive signal
strength indicator) instead of -dBm. The scale is usually 0-100 and
there is usually no direct relationship to actual receive signal other
than perhaps a lookup table in the data sheet. This is where the
%signal and %s/N display on 802.11 boxes are generated. It's not as
useful, but much easier to understand.

Nobody but hams use S-units.

> So, is -100dbm better than -1dbm?? How does that scale work? I am not
>used to better signals having a negative number.

Better for what? -100dBm is a MUCH lower signal level than -1dBm. If
you're measuring receiver sensitivity, -100dBm is much better. If
you're measuring 3rd order IM overload characteristics, -1dBm is
better.



--
Jeff Liebermann jeffl@comix.santa-cruz.ca.us
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 AE6KS 831-336-2558
Anonymous
a b F Wireless
August 5, 2004 10:43:08 AM

Archived from groups: alt.internet.wireless (More info?)

Okay, first you say to "Turn in your ham license and reread the study
guides"
> Gaaak. Turn in your ham license and reread the study guides. S-Units
> are an ancient anacronism left over from the stone age of thermionic
> valve (tube) receivers. The characteristic curve between receiver
> input signal level and indicated AGC (automagic gain control) DC level
> was anything but linear or even consistant. Someone decided to
> establish a reference signal level. S-9 magically became 50
> microvolts because either Collins of Hammarlund (I forgot which)
> managed to make receivers that would consistantly read S-9 with 50 uv
> of RF. I dunno what frequency was used for testing. It's anything
> but a standard and suffers from serious errors (and guesswork).

Then you say: "Nobody but hams use S-units" If nobody but hams use
s-units than what good is the study guide? The only part I remember
about DB is in the antenna gain section, but hey, that was 5 years ago
and if you you don't use you lose it.
Every radio I ever had (Icom, Yaesu, Kenwood, and Alinco) all showed
signal strength in S-Units. Therefore, that is all I had to know about
till now.
> Nobody but hams use S-units.
>

--Scott
Anonymous
a b F Wireless
August 5, 2004 1:24:13 PM

Archived from groups: alt.internet.wireless (More info?)

On Thu, 05 Aug 2004 06:43:08 -0700, /dev/scott0
<outsideworld@scottbrown.us> wrote:

>Then you say: "Nobody but hams use S-units" If nobody but hams use
>s-units than what good is the study guide?

Well, I have my own opinions about the validity of the question pool.
For a long time, the question pool was not being properly updated.
You couldn't buy a commercial tube radio, but all the questions
related to tubes, dynamotors, Marconi antennas, and such. I
contributed some input on the question pool in the distant past, and
am gratified to see that the questions are currently being updated and
repaired. Unfortunately, most ham radios still have signal strength
meters calibrated in S-Units. Therefore, the questions remain
relevent. My guess(tm) is that the manufacturers use S-Units to save
them the effort of properly calibrating the meter.

>The only part I remember
>about DB is in the antenna gain section, but hey, that was 5 years ago
>and if you you don't use you lose it.

Ok, you're forgiven. Open book, insert face, read, think, etc.

>Every radio I ever had (Icom, Yaesu, Kenwood, and Alinco) all showed
>signal strength in S-Units. Therefore, that is all I had to know about
>till now.

You have embarked on the road to technological nirvana by taking a
detour. I suggest you abandon the dark side and immerse yourself in
decibels, for that is the way to the enlightenment, where your
presense among those that know shall be made welcome in the common
tongue of design, which be decibels, not S-Units. You must abandon
your worship of the S-Unit, for it only leads to an amateurish dead
end. However, the road to technological nirvana is a rough road and
full of miscalculations. The journey is not an easy one as it is
written that one must suffer before enlightenment. It will be a much
easier journey if you speak the common tongue, and that be decibels.
Learn the common tongue and you shall be known as technically literate
and astute. Continue to workship the cult of the S-Unit and it can
only lead to perhaps some DX and the absurdity of the 5-9-9 signal
report.


--
Jeff Liebermann jeffl@comix.santa-cruz.ca.us
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 AE6KS 831-336-2558
July 23, 2012 2:22:00 AM


Hi!! Could you please explain to me, how I can make the equivalence from negative dBm to positive dBm. I wanna use my computer to measure the SLP at my church and I donwloaded this free program http://minorshill.co.uk/pc2/testgen.html which is called audio test, but I don´t know how to interpret the scale because it shows negative values. Thanks in advance. God bless you.
a b F Wireless
July 23, 2012 4:04:41 PM

You don't need to convert anything. You just need to recognize that it's a continuous scale that contains both negative and positive numbers.

-99 -98 -97 ... -2 -1 0 +1 +2 +3 ...

Just like anything else involving such a scale, -1 is greater (louder) than -99.

That's why the absolute numbers don't really matter. It's their relative values that do. When your AV receiver (for example) shows you a volume knob of low to medium to high (or some other scale that’s not dBm), it’s completely arbitrary. The AV manufacturer just doesn’t want to confuse the customer, so he hides the true, underlying values (dBm). Of course, the audiophile hates this, he WANTS the raw data.
!