Some papers report gain in dB while some in dBi. So what is the difference between the two? Because gain is always measured with respect to some reference antenna for example dBd is used when gain is measured with respect to a dipole antenna. Its really confusing me with the reference level taken while mentioning it in dB.
dbi :means db isotrope its the same as db just difference in notation between references
Per definition gain is related to an isotropic radiator, so if you read dBi you now it is gain. Concerning my point of view always use dBi as you express the gain of an antenna. dB can mean anything!
Dear Pragati,
"dBi" stands for "antenna gain in dB above an isotropic radiator"; "dBd" stand for "antenna gain in dB above a resonant half wave dipole antenna". An isotropic radiator is a fictitious radiator that radiates evenly in all spatial directions. For comparison: 0 dBd = 2.15 dBi.
If antenna gain is specified without any reference, just "dB", you should be suspicious of the correctness of the value. A real antenna manufacturer or antenna measurement expert will always specify the reference antenna. There is a lot of cheating with antenna gain values anyway; apparently many manufacturers believe that nobody will check their values.
For completeness: there are two more antenna gain references, one to a monopole antenna over lossless ground, mainly used in long wave and medium wave broadcasting. It is generally denoted as "dB(monopole)". And there is another one that expresses antenna gain for circular polarization, sometimes denoted as "dBi(c)".
Hope this is of some help. Kindest regards, Ben.
Ben has answered nicely with what commercial people does. I fully agree with him. One point I would like to put here is, dB is generalised word. In real sense, if you have referred earlier as dBi or dBd, then you can use word dB to refer earlier reference.
db refer to ratio of a block gain but dbi refer to ratio of a point to certain reference gain
dbi refer to gain with respect to isotropic radiator, which is the one radiate in all directions. usually db gain is a absolute one. dbi gain value is always more than the db gain. (i.e if gain in db is 3db the the gain in dbi is more than 3db. because isotropic power is equal in all directions therefore designed or tested antenna have power or radiation pattern in one particular directions therefore the gain is high in dbi)
Dear Pragati,
Antenna gain is then a measure of the amount of focus that an antenna can apply to the incoming signal relative to one of two reference dispersion patterns. Digi specifies all antenna gains in dBi.
dBi is the amount of focus applied by an antenna with respect to an "Isotropic Radiator" (a dispersion pattern that radiates the energy equally in all directions onto an imaginary sphere surrounding a point source). Thus an antenna with 2.1 dBi of gain focuses the energy so that some areas on an imaginary sphere surrounding the antenna will have 2.1 dB more signal strength than the strength of the strongest spot on the sphere around an Isotropic Radiator.
dBd refers to the antenna gain with respect to a reference dipole antenna. A reference dipole antenna is defined to have 2.15 dBi of gain. So converting between dBi and dBd is as simple as adding or subtracting 2.15 according to these formulas:
dBi = dBd + 2.15
dBd = dBi - 2.15
Specifying antenna gain in dBd means that the antenna in question has the ability to focus the energy x dB more than a dipole.
Usually Antenna Gain is expressed in dBi with respect to isotropic radiator. Consider Antenna gain=10dBi means it will radiate 10dB more when compared to isotropic radiator in a particular direction. dB ( decibel ) is a ratio so using the term without relating it to anything is not very meaningful. When quoting the gain of an antenna it must always be related to something, usually isotropic dBi or dipole dBd.
dB is antenna gain in decibels reference to how much times gainincrease there is with respect to 1(0dB) . dBi is antenna gain with respect to isotropic antenna. Since isotropic antenna has gain 1 ( 0 dB) thats why here dB and dBi are same.
Gain of 3 dB means 2 times increase in gain.
Gain of 6dB means ( 3dB+3dB =2X2=4) TIMES increase in gain
gain of 9dB means( 3dB+3dB+3dB)=( 2x2X2=8) times increase in gain
gain of 10dB is 10 times increase in gain and so on.
@ Kifayatullah Bangash Your statement that 'dB' and 'dBi' is the same is incorrect. 'dB' is the abbreviation for 'deciBell' and provides a logaritmic expression of any ratio between two powers. It is used e.g. to express the power gain of an amplifier.
If a power value relative to an well-defined level is expressed, the 'dB' will be followed by one or two letters to indicate that reference level. For example 'dBm' expresses the power in dB relative to 1 milliWatt.
In the case of antennagain, the radiated power is expressed relative to the maximum radiation of a well-defined reference antenna.
This reference antenna may be an isotropic radiator (indicated with 'dBi') or a halfwave dipole (indicated with 'dBd').
Antennagain therefore always needs an indication of the reference antenna and should not be indicated with 'dB' only.
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