antenna gain

时间：04-08 整理：3721RD 点击：

Hi All,

Can anybody tell me what is the unit for the antenna gain? We are using friis equation to get the gain value ....what would be the unit called ?

dBi or dBd.............

Waiting for your reply.

Thanx.

Regards,
Saran

Antenna gain is measured in either dBi or dBd

http://www.digi.com/support/kbase/kb...tl.jsp?id=2146

but all antenna have in description "gain is x dB" - only dB, no dBi or dBd. So, can you tell me if is dB = dBi or dB = dBd (for antennas) ? :D

Because dBi is slightly larger number, data sheets almost always use dBi. dBi is simply dBd + 2.1 dB.

That′s OK.
However, perhaps somebody is interested in the meaning of this abbreviation "dBi".
Background: dB corresponds to a simple number. As an antenna does not amplify a signal it does not multiply it with a number (as an amplifier does). Therefore the unit cannot be "dB".
But a directional antenna forms a beam and concentrates the power into one direction - leading to a gain figure if compared with the (theoretical case of an) isotropic antenna. Thus, it has x dB more power in that direction than for the isotropic case. That′s the meaning of "dBi".

Quote mauloftin:
I think:
.............. and "dB" compared rad. intensity with antenna input power. But it's the same number of course

No, that′s definitely not correct.
You cannot radiate more power than available at the antenna input.

Of course. the correct is that is norm, exactly:

Gain (IEEE): Norm the plot to the total input power. (Formula from CST MWS)

btw if compared rad. intensity with antenna input power you cannot > 0 dB, of course. The formula is:

G = 4pi(rad.intensity/input power)

so there is constant 4pi (it is from sphere) - so that this number can be > 0 dB

Hi Saran,

Antennas are usually compared to a "standard" and described as having a gain some number of dB above or below the standard. The "i" refers to an isotropic antenna while the "d" is referenced to a dipole. You may also encounter other notation as well often indicating a particular polarization of the reference. I see dBi most often in my work.

10 dBi is for example 10 db above an isotropic antenna.

Think of this a little like power or voltage references like dBW, dBm, dBmV and the like.

the unit of antnna gain is dBi where the subscript i indicattes the isotropic antenna

Gain can be in both terms dBi and dBd
if u r measuring in refrence to istropic antenna(which is the norm) then its dBi

and if refrence with sm directive antenna then dBd

Data sheets show dBi reading

An isotropic radiator at the centre of a sphere will illuminate the surface of the sphere equally in all directions. If that radiator is replaced by, say, a dipole antenna then the surface of the sphere will not be illumnated evenly. Since the energy, assuming the same transmitted power, is distributed differntly in both cases then the dipole will have an apparent gain over the isotropic. This, numerically, is a gain of 1.64 or 2.16dBi. Some antenna manufacturers then reference their product against the radiation achieved from a dipole and then express this as a gain in dBd. A dipole has 0dBd gain. Hope this makes sense!

As some one has already said...
dBi is referencing the gain to an isotropic radiator

BUT
dBd is referencing the gain to a dipole radiator

NOTE >>>> ( AT THE FREQUENCY OF INTEREST)

Therefore, any antenna gain should be referenced to one of those 2
they are the 2 standards used
to just say that an antenna has a 20 dB gain is totally meaningless
... 20dB gain over what ? a wet piece of string ? my mouse cable ?
my granny's wooden leg?
it has to be referenced to something
if you see an antenna for sale/quoted (etc) without a dBd or dBi reference
ASK the dealer/user to clarify, if he cant, he shouldnt be working with
antennae ;)

Quote...but all antenna have in description "gain is x dB" - only dB, no dBi or dBd. So, can you tell me if is dB = dBi or dB = dBd (for antennas) ? Unquote

As I have already stated that is NOT correct it MUST have a reference dB DOES NOT = dBd ot dBi

dB ... is just a ratio measurement of power, voltage, whatever of output voltage etc Verses input voltage or watever what ever

eg .. you can say that an amplifier audio or RF has a gain 20 dB

10 log (P2/P1) dB where the log is to base 10. where P1 = input P2 = output

and to move away from antennae with dBd or dBi

you can move to power levels with the common references of
dBm (reference 0dBm = 1mW) or dBW (reference 0dBW = 1W)

cheers
Dave
VK2TDN
DC - DAYLIGHT

Added after 40 minutes:

what sort of antenna operating on what freq ?
is it a yagi ? how many elements
is it a dish antenna ? what diameter ?

you have to be specific?

Dave
DC to Daylight

I am not sure, but if it is gain than it is the ratio of two same quantities, which means their units cancel out, hence no unit.
dB means the ratio is logarithmic form multiplied by 10 for power or 20 for voltage.

YES for an amplifier that is correct, it is just dB. BUT as already said, for an antenna the gain MUST always be
referenced to something... and that something is ALWAYS to the 2 most basic antennae ... either a isotropic radiator
= dBi or to a dipole which is dBd

that is because you are NOT comparing input power/voltage to output power/voltage to get a plain dB gain...
instead you are comparing the gain of a given antenna, say a 5 element yagi, over a "standard radiator"
and comparing how much better (hopefully) that the yagi performs over a known standard antenna

cheers
Dave
VK2TDN

only in that they use the logrithmic scale of dB (deciBel) to measure loss or gain
power = Log 10, Voltage = Log 20

dB = could be a power or voltage gain or loss (loss if -dB (negative dB))

dBi = for antenna

dBm = is as I related a few reply's ago, purely a Power measurement....
my quote...
you can move to power levels with the common references of
dBm (reference 0dBm = 1mW) or dBW (reference 0dBW = 1W)

In RF work we like to work in dBm or dBW (in my case dBm) as I am generally working with power levels
less than 100 Watts. Using dBm for all calculations simplifies working out losses amd gains in a RF system eg

transmitter pwr out - cable loss + Antenna Gain - pathloss + Receiver ant. gain - receive cable loss + receiver gain
insert some examle figures....

30dBm (1W) - 3dB (cab Loss) + 35dBd (ant gain) - 150dB (pathloss) +35dBd (RX ant gain) - 4dB(RX cab loss) + 40dB RX gain

its much easier and practical to use every thing in dB rather than mixing actual power levels eg Watts or milliWatts
with other system gains and losses measured in dB

Also dont forget, the dB = deciBel = 1/10 (1 tenth of a Bel) is also a measure of sound level

Dave
VK2TDN