What does dBi and dBm mean?

For Beginners: A Few Words About Measurement Units Used in Antenna Technology and High-Frequency Radio Engineering That May Be Confusing to Many.

• dB — decibel. Generally speaking, it is a logarithmic unit for expressing ratios of various quantities. It replaces the concept of "times" or "fold." In other words, it is not an absolute unit like volts or watts, but rather a relative one, similar to percentages.
  

  N_p(dB) = 10 lg (P₁/P₂)

  For example, if the signal level increases 1000 times in terms of power, this corresponds to +30 dB (we say the signal has increased by 30 dB). Using this unit of measurement for ratios allows us to replace multiplication/division with addition/subtraction when calculating gain/attenuation. For example... In a feeder line, the signal was attenuated by a factor of 4, and an amplifier boosted it by a factor of 220. Then in the feeder-amplifier system, the signal is amplified by 220 / 4 = 55 times. In decibels, the calculation is simpler: 23 - 6 = 17 dB.

• dBm. Sometimes it is convenient to adopt a certain value as a reference (zero level) and measure levels relative to it in decibels. Thus, if we take 1 mW as the zero level and measure power relative to it on a logarithmic decibel scale, we get a unit of measurement called dBm (1 mW = 0 dBm). Unlike impersonal decibels, this unit has quite tangible physical meaning — dBm is a measure of power. It is used to measure weak signal levels (including in modem "signal meters"), receiver sensitivity, transmitter power, etc. For example, a level of 50 μV at a 50-ohm receiver input corresponds to a power level of 5·10⁻⁸ mW or -73 dBm. Measuring sensitivity in power units is more convenient than in voltage units, since we have to deal with signals of various forms, including noise. Moreover, we eliminate the need to specify the receiver's input impedance each time. For example, the threshold power for most "dongles" at which they can still connect to a base station is around -110 dBm. Transmitter power can also be measured in dBm. For example, a Wi-Fi router power of 100 mW equals +20 dBm. You can use our online calculator to convert mW to dBm and vice versa. In many devices, you will find the signal level in ASU. This is yet another unit for measuring signal level, designed to confuse people with its obscurity. It stands for "Arbitrary Strength Unit" — an averaged signal level unit. The fact is that in different bands we use channels with different modulation, different frequency bandwidths, etc. Therefore, equal dBm values in 3G and 4G are not equivalent to the same signal-to-noise ratio sensitivity in the channel. To bring sensitivity to a common denominator, ASU was invented. The relationship between ASU and dBm for different bands is as follows:

  • GSM: dBm = 2 × ASU - 113, ASU in the range 0..31 and 99 (network not detected).
  • UMTS: dBm = ASU - 116, ASU in the range -5..91 and 255 (network not detected).
  • LTE: (ASU - 141) ≤ dBm < (ASU - 140)

• dBi. A unit of measurement for antenna gain relative to a "reference" antenna. The reference antenna is the so-called isotropic radiator — an ideal antenna whose radiation pattern is a sphere, with a gain coefficient equal to one and an efficiency of 100%. Signal radiation by such a radiator occurs with uniform intensity in all directions. Such an antenna does not exist in nature; it is a virtual object, but very convenient as a reference for measuring parameters of real antennas. There is another unit: dBd — here a half-wave dipole is taken as the reference. However, using dBi is preferable, as it simplifies the calculation of the radio link energy budget. dBi is a relative unit, essentially no different from a simple decibel, except for the definition of the reference against which measurements are made. There is no fundamental difference between dBi and dBd — gain in dBi = gain in dBd + 2.15 dB. In old amateur radio handbooks and magazines, antenna gain is measured simply in decibels. In this case, it usually means gain relative to a half-wave dipole, i.e., it is equivalent to dBd. Measurement relative to an isotropic radiator was originally used only in the USA, but has recently spread worldwide, so to avoid confusion nowadays, when discussing antenna gain, it is considered good practice to use decibels with a suffix — dBi or dBd.

 In principle, any value can be taken as the "zero level." This is how such creatures as "dBμV" (voltage - ratio to one microvolt) and "dBW" (power - ratio to one watt) came into existence. In acoustics, the zero sound level is taken as a sound pressure of 2·10⁻⁵ Pa — the threshold of hearing. In this case, they didn't bother adding a suffix to "dB" and measure sound level directly in decibels. This happened historically because decibels were first used precisely in the field of acoustics. But it should be kept in mind — these are not "pure" relative decibels, but "sound" decibels — absolute ones. For example, the noise of a jet aircraft from a distance of 25 m equals 140 dB, while 0 dB is the threshold of hearing. Often you can encounter a unit called dBA. It was specially invented for measuring noise intensity. The dBA value is the sound pressure level measured in "sound" decibels using a sound level meter containing a correction circuit that simulates the sensitivity of the human ear, which makes it possible to obtain readings more consistent with the actual audibility of noise.

In general, people didn't start using decibels to measure various things just randomly. Back in the 19th century, psychophysiologists Ernst Weber and Gustav Fechner established that "the strength of sensation p is proportional to the logarithm of stimulus intensity S." This applies to sound, illumination, and tactile sensations. In wired communication engineering, another unit is used — the Neper. Nepers are defined not through the common logarithm, but through the natural logarithm. Perhaps this is more correct, since many laws of nature are based on Euler's number, which is the base of the natural logarithm. But historically, we use decibels. Although the difference is not essential in principle: 1 Np = 8.686 dB, and 1 Bel = 10 dB.

In calculations, all these dB, dBi, dBm are essentially decibels, i.e., they are added (if gain) or subtracted (if attenuation), but dBm has priority as a measure of signal power. For example:

Receiver input level (dBm) = Transmitter power (dBm) + Antenna gain (dBi) - Signal attenuation (dB)

An inexperienced person usually gets confused when seeing such an abundance of decibel varieties. But then comes the understanding that this actually simplifies calculations. For example, in calculating Wi-Fi communication range. Many find it difficult to visualize the "decibel" scale, especially in the negative range. In fact, this is easy to do by analogy with the familiar thermometer. The higher the power in dBm, the "warmer" the number. In other words, -75 dBm is greater (higher on the scale, "warmer") than -95 dBm. A more negative number in the sensitivity parameter means that the receiver is capable of receiving a weaker (colder) signal.

That's how tangled everything is in this decibel kingdom. And finally... Keep in mind that a decibel and an imbecile are completely different concepts.