# Antenna gain how to dB to dBi

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#### snkhan

##### Full Member level 6 convert db to dbi

Hi
How to convert antenna gain from dB to dBi (at 2.4 and 5.5GHz). I am sure its Quite simple but tricky #### balaguru thavamani

##### Full Member level 5 how to convert dbi to db

Hi snkhan,

Antenna gain is expressed in terms of dBi (ie with respect to isotropic radiator).in comparison with another antenna.

dB=decibel.
dBi= decibel with respect to isotropic.
dBm=milli decibel.
dBd=with respect to dipole.

0dBd=2.14dBi.

• nafah

### nafah

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#### snkhan

##### Full Member level 6 convert dbi

balaguru thavamani said:
Hi snkhan,

Antenna gain is expressed in terms of dBi (ie with respect to isotropic radiator).in comparison with another antenna.

dB=decibel.
dBi= decibel with respect to isotropic.
dBm=milli decibel.
dBd=with respect to dipole.

0dBd=2.14dBi.

I know the terminology but want to know if there any relation from dB->dBi as u mention from 0dBd=2.14dBi

Thanks

#### cuongtran

##### Full Member level 3 • nafah

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#### balaguru thavamani

##### Full Member level 5 friis transmission equation dbi

there is no such particular relation.

As i've mentioned earlier Usally 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.

for further clearance refer

www.cebik.com/a10/ant1.html

• nafah

### nafah

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#### G4BCH why antenna gain always in negative

dB ( decibel ) is a ratio so using the term without relating it to anything is meaningless. When it is used for amplifiers etc it is usually implied that the gain or loss is input power relative to output power.
For an antenna the gain if not related to anything, ie implying input to output power must always be negative, or laws of physics such as second law of thermodynamics & conservation of energy will be broken. The efficiency of an antenna power in to power radiated is always less than 1.
When quoting the gain of an antenna it must always be related to something, usually isotropic dBi or dipole dBd. This means that you can not convert dB dBm dBW dBV etc to dBi it is meaningless unless you know what the dB is referring to and the reference comparison makes sense. For example I could make an antenna that has 70 or 80 dB gain yet has less gain than an isotropic radiator. Because I have not told you what the reference is for my antenna the quoted gain is meaningless and you can't convert to dBi. It could be relative to an open circuit connector Hope this helps a bit
Peter

#### snkhan

##### Full Member level 6 dbi convert db

I agee with both of you but i scanned page of some book i donot know and below is exactly what written in that

Measuring the gain using two-antenna method is based on Friis transmission equation and it needs two identical samples of the tested antenna. One of the identical samples is the radiating antenna, and the other one is the receiving antenna. Assuming that the antennas are well matched in terms of impedance
and polarization (mutual alignment must be sufficiently good), the Friis transmission equation is
G(db)=1/2(20 log(4ΠR/lambda+10log(Pr/Pt)

One needs to measure accurately the distance between the two antennas R , the received power Pr , the transmitted power Pt , and the frequency f =c/λ

So if the method is right then how to dBi.

#### kyjackchan

##### Junior Member level 3 comparison of gain in db to dbi

The gain dB in the Friis formula is the same as the gain you mentioned in terms of dBi.

As you are already aware of isotropic antenna is an ideal antenna spread evenly across the sphere. This is the situation where 0dBi happens.

As you also aware in practical, 0dBi never happens due to assymetric of feeding structures, physical size of the antenna etc..

In Friis formula, it assumes the equation to be an ideal isotropic antenna, so ideally the equation should be ended up 0 on both sides, due to the conservation of energy.

So anything creating imbalance in the friis formula, it assumes to be the antenna gain (dB = gain of something unknown, or dBi = gain from the isotropic antenna).

Of course this is most likely not the case depends on your path loss models.

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