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How to Define and Differentiate dB, dBm and dBi?

Date:2021/4/28 17:36:30 Hits:

"What is dB, dBi, dBM and dBW in antenna gain theory? What is the difference between dB, dBi, dBM and dBW? The following contents are the basic theory about antenna, it might help you further your recognition of RF technology. ----- FMUSER"

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Commonly Used Decibel Units in Electrical Engineering

What's the Difference between dB and dBM?

What's the Difference between dB and dBi?

5 Commonly Used Decibel Units (Click to visit)

Gain - Antenna Gain
dB - Decibel 
dBM - Decibel Millwatts
dBi - Decibel Isotropic
dBW - Decibel Watts

1. What is a Gain?
When the power coming out of an equipment is greater than the power coming into the equipment, it is said to have a gain in power. When you add a signal booster to your home or business, the device takes the existing signal and amplifies or boosts the power, thus making it possible for a stronger internet signal or connection. Measuring gain allows you to choose the perfect device for your needs. The amount of gain is measured in decibels.

2. What is a dB (Decibel)
1) Definition to Decibel

 A decibel or dB is a logarithmic and dimensionless unit used to indicate the level of acoustic waves and electronic signals in terms of ratio or gain, simply speaking, dB is the unit used to measure the intensity of a sound and sound pressure level, it is the symbol and the abbreviation of Decibel. dB is also the forward gain of an antenna, measured in decibels (dB),  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.

We all know that sounds are the energy that travels in waves that are made through the vibration of a subject. They are measured in two aspects: Amplitude and Frequency

● In terms of Amplitude

The Amplitude, which reported in the dB (decibel) scale, is used to measure and indicate its forcefulness or pressure. Simply speaking, if with more amplitude, a sound can be as louder as it could. For this, the sound is referred to as a pressure of 0.0002 microbars which equates to the standard for the threshold of hearing.

● In terms of Frequency
The frequency, which reported in Hz (Hertz), is used to measure and indicate the specific number of sound vibrations per second.

According to its characteristics, the decibels or dB are widely used in scientific measuring applications such as electrical engineering measurements (electronics, defining amplifier gains, component losses e.g. attenuators, feeders, mixers, etc.), sound engineering measurements (acoustics, noise figure, signal to noise ratio, etc), the control theory (Bode plots, etc), signals, and communication, etc.


Also read: Know RF Better: The Advantages and disadvantages of AM, FM, and Radio Wave

2) When and How the Decibels are Discovered?
The term bel, which originated in the measurement of transmission loss and power in telephony of the early 20th century (1928) in the Bell System in the United States, actually comes from a logarithmic unit of measurement called a "Bel" which is created by Bell Telephone Laboratories and named after its' founder Alexander Graham Bell.

Comparing with the seldom-used "bel", the decibel is the proposed working unit because a one-decibel difference in loudness between two sounds is the smallest difference detectable by human hearing, and decibel is only one-tenth of the bel, which is used for a wide variety of measurements in science and engineering (as mentioned above).

3) 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. 

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. 


Also read: Simple and Budget DIY - How to Make An FM Transmitter?

3. What is a Decibel-Millwatt (dBm or dBmW)?

dBm or dBmW (decibel-milliwatts) is a unit of level used to indicate that a power level is expressed in decibels (dB) with reference to one milliwatt (mW). dBm is an absolute unit for it is referenced to the watt, the dBm is also a dimensionless unit, just like dB, but since it compares to a fixed reference value, the dBm rating is an absolute one. It is used in radio, microwave and fiber-optical communication networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form compared to dBW, which is referenced to one watt (1000 mW). A dBm is relative to a 50-ohm impedance in RF (radio frequency), while In wireless communication, dBm is relative to a 600-ohm impedance. 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. 

(DeciBels at 1 Milliwatt) A measurement of power using one milliwatt as the reference point (0 dBm). For example, a signal at .1 milliwatt (100 microwatts) is a loss of 10 dBm. A radio station transmitting 50,000 watts of power may wind up attenuating to only a couple milliwatts by the time it is picked up by a radio receiver.

Tips: How to Convert dBm to Watts?

  +40 dBm = 10 watts        10.0
  +30 dBm = 1 watt           1.0
  +20 dBm = 100 milliwatts    .1
  +10 dBm = 10 milliwatts     .01
    0 dBm = 1 milliwatt       .001
  -10 dBm = 100 microwatts    .0001
  -20 dBm = 10 microwatts     .00001
  -30 dBm = 1 microwatt       .000001
  -40 dBm = 100 nanowatts     .0000001

Also read: What is Printed Circuit Board (PCB) | All You Need to Know

4. What is a Decibel Isotropic (dBi)? 
An isotropic antenna is a theoretical antenna that radiates power uniformly in all directions. The decibel Isotropic (dBi) is the unit of the gain when the gain of an antenna is calculated and compared to an isotropic antenna pattern (not a real antenna but rather a hypothetical antenna model). You may also consider dBi as a ratio, which is used by the antenna manufacturers to measure whether an antenna performs well. An Isotropic antenna has no gain/loss when compared to itself which means it has a 0 dB power rating.

For low-power systems, such as those used in mobile communications, the dBm (decibel-milliwatt) scale is a convenient reference power level, in which power is referenced to a level of 1 mW:


So if an antenna has a gain of 5 dBi in a particular direction, that means when compared to an Isotropic antenna (which will have a gain of 0 dB in that direction), that antenna has a gain of 5 dB.

You may even consider dBi as a measurement that compares an antenna’s gain with respect to an isotropic radiator (a theoretical antenna that radiates energy evenly in a spherical pattern.)

For your own, it is important to know that the signal booster comes with an antenna with a dBi value.


5. What is a Decibel Watt (dBW)

The decibel watt (dBW) stands for decibel with respect to 1 Watt, it is the unit for the measurement of the strength of a signal expressed in decibels relative to one watt. The power dBW is equal to 10 times the base 10 logarithms of the power in watts. It is very useful as it can express a large range of values in a short range of numbers.

For high-power systems, such as those used in satellite communications, the dBW (decibel-watt) scale is commonly used, in which power is referenced to 1 W:


Also read: What is VSWR and how to measure VSWR?

What's the Difference between 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. 


Also read: What's the Difference between AM and FM?

What's the Difference between 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.

-10 dBi
One tenth , 1/10, or "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)
0 dBi
No gain, "same", 100% (no gain, no loss)
+1 dBi
12% higher, 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
Times 10 (% 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)


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