What does dbi mean, in context of antenna performance?
Brice Wightman
In a previous posting, David Jenkins (djen...@jetson.uh.edu) writes:
> I have seen ads for antennas that quote "dbi" specs. Is dbi the same as
> db? If not, what's the diff?
dBi = dB (gain) of an antenna over an "isotropic radiator" There is no
such beast --- and there cannot be --- but this gain falls out of field
calculations for an antenna so it is used. The gain of a half-wave dipole
is 2.i5 dBi. The half-wave dipole is a more practical reference --- this
gain can be *measured* by calibrating an antenna range with a half-wave
dipole. Also it is something the average ham has direct experience with.
Antenna manufacturers like to use dBi because the number is bigger (by
2.15 dB) than dBd. Some of them conveniently neglect to specify the
reference.
BTW, what hams call gain, antenna engineers call "directivity". Power
gain is directivity multiplied by efficiency.
--
========================================================================
Brice Wightman am...@freenet.carleton.ca
Ottawa, Canada VE3EDR
========================================================================
David Jenkins
db? If not, what's the diff?
TIA es 73,
David F. Jenkins
Decision and Information Sciences
University of Houston
KC5JRR
Ron Cole
< Good dB Explination >
: A dipole has about 2.14 dB gain over isotropic. That is, the gain
: of a half-wave dipole is 2.14 dBi. Antenna manufacturers like to
: state their gains in dBi because it gives a bigger number than dBd.
Note that Antenna ERP is calculated based on dBi. IE: 10 Watts into an
antenna with 10dBi Gain = 100 Watts ERP.
Though I think a better number would be Field Gain. Easier to calculate
Power gain but not much use in path calcs.
Ron
N5HYH
Tom Bruhns
(Other good reference material deleted...)
: A dipole has about 2.14 dB gain over isotropic. That is, the gain
: of a half-wave dipole is 2.14 dBi. Antenna manufacturers like to
: state their gains in dBi because it gives a bigger number than dBd.
So, subtract 2.14 from a dBi figure to see how that antenna compares with a
half-wave dipole. A 3dBi antenna will only be .96dB better than a dipole.
73, K7ITM
Alan Bloom
: I have seen ads for antennas that quote "dbi" specs. Is dbi the same as
dB is a measure of the difference between two power levels. If you increase
your power from 1 to 100 watts, that's a 20 dB change. Referring to a
power of "20 dB" means nothing unless you specify what it is being
compared to.
dBw means dB compared to 1 watt. 20 dBw = 100 watts.
dBm means dB compared to 1 milliwatt. 20 dBm = 100 mw = .1 Watt.
dBd means dB antenna gain compared to a 1/2 wave dipole.
dBi means dB antenna gain compared to an antenna that radiates
equally well in all directions, at all take-off angles.
A dipole has about 2.14 dB gain over isotropic. That is, the gain
of a half-wave dipole is 2.14 dBi. Antenna manufacturers like to
state their gains in dBi because it gives a bigger number than dBd.
AL N1AL
Gordon B. Slater
In article <3folrl$k...@masala.cc.uh.edu>, David Jenkins (djen...@jetson.uh.edu) writes:
>I have seen ads for antennas that quote "dbi" specs. Is dbi the same as
>db? If not, what's the diff?
>
>TIA es 73,
>
>
>David F. Jenkins
>
dB = decibels, a *comparative* logarithmic unit
dBd = decibels compared/relative to a dipole
dBi = decibels compared to an ISOTROPIC RADIATOR, that is, a
radiator that radiates equally well in ALL directions (even
up/down, not just in a planar donut-type shape that some books
show. The actual plot of the radiation pattern of an isotropic
source is a sphere for points of constant field strength.)
0dBd = dbi + 2.15dB (ie a dipole *in the direction of its maximum
radiation* has a gain of 2.15dB.)
However, antenna manufacturers sometimes try to con you.....
"gain 6dB" = 6dB relative to what??? 6dBi or 6dBd??? Chances are
they mean 6dBi, because 6dbi is only (6 - 2.15) 3.85dBd which is
*much* less impressive.
"gain 8 dBi" ---- there's nothing wrong with this... 8 deciBels
relative to an isotropic source..... BUT is it in the direction
you want???
eg a vertical dipole in free space has a gain of 2.15dBi.
A commercial antenna has a gain of 7dBi , BUT at 32 degrees
elevation from the horizontal. So, all things being equal,(they
aren't), if you used the commercial antenna it may well show
a gain of MINUS 4dBi for example at 0 degrees. IF the direction
you want to communicate in is at 0 degrees elevation then you
would be better off with the dipole.
IF you wanted to talk to a mountain-top several miles away at an
angle of 32 degrees elevation then the commercial antenna is
better.
The moral of the story:-
"xxx dB relative to WHAT ???"
"is max radiation where I want it ???"
ps
dBc is dB relative to the carrier, used in some interference or
performance specs for transmitters etc (this often gets asked when
you talk about antenna references)
Gordy's the name..SIGINTs the game.
RF SpOps WW.
Offshore ops on a fast response basis.
Corporate only - no private or illegal work carried.
NNNN
Roy W Lewallen
> . . .
>dBd means dB antenna gain compared to a 1/2 wave dipole.
>dBi means dB antenna gain compared to an antenna that radiates
>equally well in all directions, at all take-off angles.
>A dipole has about 2.14 dB gain over isotropic. That is, the gain
>of a half-wave dipole is 2.14 dBi. Antenna manufacturers like to
>state their gains in dBi because it gives a bigger number than dBd.
am...@FreeNet.Carleton.CA (Brice Wightman):
>dBi = dB (gain) of an antenna over an "isotropic radiator" There is no
>such beast --- and there cannot be --- but this gain falls out of field
>calculations for an antenna so it is used. The gain of a half-wave dipole
>is 2.i5 dBi. The half-wave dipole is a more practical reference --- this
>gain can be *measured* by calibrating an antenna range with a half-wave
>dipole. Also it is something the average ham has direct experience with.
> . . .
Almost. dBi is gain relative to an isotropic source *in free space*, and
dBd is gain relative to the field of a dipole *in free space* in its
best direction. BOTH THESE ANTENNAS ARE FICTITIOUS! I'm not an expert in
antenna measurements, but suspect that making a "free-space" measurement
of a dipole on an antenna range would be tricky, since all ground
reflection would have to somehow be eliminated. Raising the antenna many
wavelengths above ground isn't adequate, since very significant ground
reflection still occurs. (The pattern simply has many lobes.) It could be
done in an anechoic chamber.
A dipole placed over real ground can easily have gain exceeding 0 dBd;
an example in the ELNEC manual illustrates an ordinary back-yard dipole
with 4.6 dBd gain. This means that the field is 4.6 dB greater than that
of a dipole in free space, no great trick since the energy is concentrated
in one hemisphere rather than two, as is the case for the free-space
dipole.
You'll be making a big -- but common -- mistake if you assume that "dBd"
means gain relative to a *real* dipole. It doesn't!
Roy Lewallen, W7EL
roy.le...@tek.com
Alan Bloom
: al...@sr.hp.com (Alan Bloom):
: >dBd means dB antenna gain compared to a 1/2 wave dipole.
: >dBi means dB antenna gain compared to an antenna that radiates
: >equally well in all directions, at all take-off angles.
: Almost. dBi is gain relative to an isotropic source *in free space*, and
: dBd is gain relative to the field of a dipole *in free space* in its
: best direction. ...
: A dipole placed over real ground can easily have gain exceeding 0 dBd;
: an example in the ELNEC manual illustrates an ordinary back-yard dipole
: with 4.6 dBd gain. This means that the field is 4.6 dB greater than that
: of a dipole in free space, no great trick since the energy is concentrated
: in one hemisphere rather than two, as is the case for the free-space
: dipole. ...
It makes no sense to compare the gain of an antenna over earth to a
dipole in free space. Even a dipole has "gain over a dipole" using
that definition. It makes far more sense to assume dBd means gain over
a dipole mounted the same way as the antenna under test. Using that
definition, a 1/2-wavelength dipole has 0 dBd gain, a 5/4-wave dipole
has 3 dBd gain, etc.
"dBd" means "gain relative to a dipole in free space" only if the antenna
under test is also in free space.
: I'm not an expert in
: antenna measurements, but suspect that making a "free-space" measurement
: of a dipole on an antenna range would be tricky, since all ground
: reflection would have to somehow be eliminated. Raising the antenna many
: wavelengths above ground isn't adequate, since very significant ground
: reflection still occurs. (The pattern simply has many lobes.) It could be
: done in an anechoic chamber.
Setting up an accurate antenna range can be fairly tricky, due not only to
ground reflections, but reflections from nearby buildings, power lines,
etc. One partial solution is to use a gain antenna (with a known,
well-calibrated gain) as the reference antenna instead of a dipole.
That reduces reflections in other than the forward direction.
Professional antenna ranges use other techniques to reduce reflections.
One is to intentionally place plane reflectors on the ground between the
two antennas. The planes are angled to direct the reflections away from
the antennas under test.
AL N1AL
Alan Bloom
: >It makes no sense to compare the gain of an antenna over earth to a
: >dipole in free space. Even a dipole has "gain over a dipole" using
: >that definition. It makes far more sense to assume dBd means gain over
: >a dipole mounted the same way as the antenna under test. Using that
: >definition, a 1/2-wavelength dipole has 0 dBd gain, a 5/4-wave dipole
: >has 3 dBd gain, etc.
: >"dBd" means "gain relative to a dipole in free space" only if the antenna
: >under test is also in free space.
: When someone quotes gain in dBi, I don't have to make any assumptions. ...
Sure you do. Does it mean an isotropic antenna in free space or over earth?
It's the same problem as with dBd, you get up to a 6 dB difference in your
answer, depending on which one you pick.
: I don't buy the premise that a person can "assume" dBd to mean one thing
: or another. A reference that requires the user to make assumptions is
: no reference at all. If you do insist on this vague definition, though,
: you'll certainly have to abandon the common assertion that dBd is 2.15
: dBi, since dBd fitting the above description could have a wide range of
: values relative to a fixed-field-strength reference like dBi.
But dipole gain is NOT 2.15 dBi if you assume a dipole over earth and an
isotropic antenna in free space (or vice versa). This is exactly my point:
for the concept of antenna gain to make any sense, the reference antenna
should be situated the same as the antenna under test. To speak of
the gain of an antenna over earth referenced to a dipole in free space
is to compare apples to oranges.
Now, it certainly can be useful to see what happens to the radiation
pattern when an antenna is mounted over earth. But to base your
"antenna gain" figure on the resulting maximum field strength makes
no sense, unless it is compared to a reference antenna at the same
height.
AL N1AL