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What's the difference between "dB", "dBm", and "dBi"?

Date:2021/1/28 12:21:41 Hits:

"dBi is the forward gain of an antenna. dBm is a comparison against watts / milliwatts. dB is a method - not a measurement standard. The following contents are the basic theory about antenna, it might help you further your recognition of RF technology. ----- FMUSER"

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1) Definition to dB, dBm and dBi

2) dB vs. dBm

3) dB vs. dBi

4) A dB is a RELATIVE measure of two different POWER levels

5) Intersting posts in 2020

What is dBi? What is dB? What's dBm and what's dB(mW)?

dBi  = dB(isotropic): The forward gain of an antenna, nmeasured in decibels (dBi),  The dBi value reflects the antenna's directional / beamwidth characteristics, i.e., directional as opposed to omnidirectional:  Generally, the hIgher the gain (dBi), the narrower the beamwidth - the more directional the antenna. 

dB refers to the decibel, which is the unit of measurement of sound though it is also a relative measure of the power between two levels. Therefore dB is not an absolute measurement but rather a ratio.

dBm dB(mW):  dBm is is an expression of power in decibels per milliwatt.  We use dBm when we are measuring power emitted from amplifiers. We measure that power in milliwatts which is typically abbreviated as mW. 


See Also: >>dB, dBm, dBW, dBc Basics: Can You Clearly Tell Their Difference?

What's the difference beteen dB and dBm?

● Decibel (dB) and dB relative to a milliwatt (dBm) represent two different but related concepts.

A dB is a shorthand way to express the ratio of two values. As a unit for the strength of a signal, dB expresses the ratio between two power levels. To be exact, dB = log (P1/P2).

Using the decibel allows us to contrast greatly differing power levels (a common predicament in radio link design) with a simple two- or three-digit number instead of a more burdensome nine- or 10-digit one.

For instance, instead of characterizing the difference in two power levels as 1,000,000,000 to 1, it's much simpler to use the decibel representation as 10*log (1,000,000,000/1), or 90 dB. The same goes for very small numbers: The ratio of 0.000000001 to 1 can be characterized as -90 dB. This makes keeping track of signal levels much simpler.

The unit dBm denotes an absolute power level measured in decibels and referenced to 1 milliwatt (mW). To convert from absolute power "P" (in watts) to dBm, use the formula dBm = 10*log (P/1 mW). This equation looks almost the same as that for the dB. However, now the power level "P" has been referenced to 1 mW. It turns out that in the practical radio world, 1 mW is a convenient reference point from which to measure power.

Use dB when expressing the ratio between two power values. Use dBm when expressing an absolute value of power.

In many descriptions about FM products, we keep seeing people using the terms "dB", "dBm", and "dBi" interchangeably, when they actually mean very different things. So, here's a little background on the correct usage of the terms. 


See Also: >>Field Intensity Units

What's the difference beteen dB and dBi?

● Imagine an antenna that radiates energy equally in all directions, much like our sun does. In scientific lingo, this is said to be an “isotropic radiator”, because it has no preference for radiation in any direction …  in other words it has no “directivity”. 

● This type of isotropic antenna is said to have “no gain”. “No gain” can be expressed in linear terms like x1 (times 1). That simply means that all directions have the same energy radiation, and are all equal to the average energy radiation.Antenna engineers like logarithmic terms, and we say this no-gain situation is 0 dBi (pronounced “zero dee bee eye”). Imagine a giant stellar sized mirror beside our sun. Imagine how it would change this energy distribution and give the sun directivity. With such an imaginary mirror, one half of our solar system would be dark (behind the mirror). 

● The other half would be twice as bright (seeing the direct sun plus it’s reflection). Mirrors or lenses have the appearance of intensifying energy in some preferred directions by stealing and redirecting it from disadvantaged directions. Antennas do the same thing. 

● Mirrors don’t create light, they only divert, direct, or concentrate it in some direction. Antennas don’t create radio energy, they also only divert, direct, or concentrate it in some direction. This is directional feature is called gain. Please remember, no new energy is created, it is simply redirected or given directionality (directivity). The amount of intensification in a preferred direction is quantified as gain. Thus a mirror can redirect half of the energy from the sun (or a candle), and make it look twice as bright (i.e. two candles). It is said to have a gain of 2x (times two) or doubling.

See Also: >>Tips on Antenna Gain Measurement 

-10 dBi
One tenth , 1/10or "10 % of" (loss, not gain)
-6 dBi
One quarter, 1/4, or "25 % of" (loss, not gain)
-3 dBi
One half, 1/2, or "50% of" (loss, not gain)
No gain, "same", 100% (no gain, no loss)
+1 dBi
12higher, times 1.12, or 112%
+2 dBi
58% higher, times 1.58, or 158%
+3 dBi
100% higher, times 2, "double", or 200%
+6 dBi
300% higher, times 4
+9 dBi
Times 8 (% scale in not useful for large multiples)
+10 dBi
Time10 (% scale in not useful for large multiples)
+13 dBi
Times 20 (% scale in not useful for large multiples)
+20 dBi
Times 100 (% scale in not useful for large multiples)


See Also: >> Understanding Antenna Gain Basics

A dB is a RELATIVE measure of two different POWER levels

There's also dB relative to VOLTAGE levels, but I won't go into those, as we're mostly concerned with POWER levels in our discussions here. 3dB is twice (or half) as much, 6dB is four times, 10dB is ten times, and so on. The formula for calculating gain or loss in dB is: 10log P1/P2. It's used for stating the gain or loss of one device (P1) IN RELATION to another (P2). Thus, I can say that an amplifier has 30 dB of gain, or I have 6dB total feedline loss. I CANNOT say, My amp puts out 30 dB, or I have a 24dB antenna, as you must state what you're referencing it to, which is where the subscript comes in. The dB by itself is not an absolute number, but a ratio.

● For amplifiers

A common reference unit is the dBm, with 0dBm being equal to 1 milliwatt. Thus, an amp with an output of 30dBm puts out 1 Watt. How much gain it has is a different matter entirely, and you can have two different amps, each with an output of 30dBm (1Watt), that have different gains, and require different levels of drive power to achieve their outputs. You can also have two different amps with the same gain that have different output powers. 

There's also dBW (Referenced to 1 WATT), but you generally only use those when dealing with Big Stuff, as 30dBW is 1000w, and way beyond what we deal with here!

● For antennas

A common reference unit is the dBi, which states the gain of an antenna as referenced to an ISOTROPIC source. An Isotropic source is the perfect omnidirectional radiator, a true Point Source, and does not exist in nature. It's useful for comparing antennas, as since its theoretical, its always the same. It's also 2.41 dB BIGGER than the next common unit of antenna gain, the dBd, and makes your antennas sound better in advertising. The dBd is the amount of gain an antenna has referenced to a DIPOLE antenna. A simple dipole antenna has a gain of 2.41dBi, and a gain of 0dBd, since we're comparing it to itself. If I say I have a 24dB antenna, it means nothing, as I haven't told you what I referenced it to. 

See Also: >>dB and dBi and Antenna: What does Antenna Gain Actually Means?

It could be a 26.41dBi antenna (24dBd), or a 21.59dBi (also 24dBd!) antenna, depending on what my original reference was. The difference is 4.81dB, a significant amount. Most antenna manufacturers have gotten away from playing this game, but the reference will be different in different fields. 

Commercial antennas tend to be rated in dBi, as the people buying them understand it, and Amateur Radio antennas tend to be dBd, as Hams are very familiar with dipoles. 


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