Antenna gain: dB,dBi,dBic ??

jor...@convax.iar.nrc.ca

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Feb 18, 1994, 10:04:27 AM2/18/94

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Hello,

Can anyone help me. I am trying to interpret various antenna gain specs for
a variety of circularly polarized antennas. The specs usually use dB, dBi
or dBic.

What is the definition of these units and how are they related to each
other ?? Does dBi mean the gain relative to an isotropic radiator ?
Does dBic mean the gain relative to an isotropic circularly polarized
radiator. Can one meaningfully convert from one to the other ?

For instance, a 4 ft. parabolic dish is spec'd at 24 dB. Should this be dBi
? I have some information on a patch antenna which has a gain of 16-17
dBic. How do I compare the two gains ??

Any help or pointers to reference books which discuss this would be
appreciated.

Thanks,
Jim

Fred McKenzie

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Feb 18, 1994, 6:33:29 PM2/18/94

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In article <0097A3CD.E4...@convax.iar.nrc.ca>,

jor...@convax.iar.nrc.ca wrote:
> Can anyone help me. I am trying to interpret various antenna gain specs for
> a variety of circularly polarized antennas. The specs usually use dB, dBi
> or dBic.

> For instance, a 4 ft. parabolic dish is spec'd at 24 dB. Should this be dBi
> ? I have some information on a patch antenna which has a gain of 16-17
> dBic. How do I compare the two gains ??

Jim-

I have asked questions like yours, and found an industry that is loaded
with confusion. I have seen catalog pages that had both dBi and dBd
(dipole) figures, with no clue as to which they meant. In some cases, I
suspect the method that gives the highest number is used, even though the
industry has standardized on the lower-valued method.

In the case of the antennas you referenced, your assumptions sound
reasonable. However, the parabolic dish probably needs more information
about the feed, before over-all gain can be established. In that respect,
gain in pure dB, is probably correct.

I have never encountered the dBic term before. If circular can be
generated by radiating half of your power in one plane, half power at 90
degrees, and delaying the phase of one, then dBic may be related to dBi, by
3 dB. A manufacturer might use dBic, if the resulting number was higher
than the published gain of his competitor's identical antenna.

Does it sound like I have a bad attitude?

73, Fred, K4DII

Cecil Moore

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Feb 19, 1994, 2:18:02 PM2/19/94

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jor...@convax.iar.nrc.ca wrote:
: The specs usually use dB, dBi or dBic. Thanks, Jim

Hi Jim, dB is a relative power or voltage level
comparison of one condition to another condition. Since it is relative,
one needs a reference point. I am running 3dB more power than you if I
am running 2 watts and you are running 1 watt. I am also running 3dB more
power than you if I am running 200 watts and you are running 100 watts.
Don't bother buying an antenna that advertises 10 dB gain... it could be
gain over a ground rod.

dBi is the comparison of something (antenna) to a theoretical
point source (isotropic) radiator (antenna). The gain of a half-wave
dipole in free space is 2.14 dBi and when an antenna gain is specified
in dBd it is usually the gain over a dipole in free space. But don't
expect your antenna to be that much better than your trusty dipole. A
half-wave dipole at a reasonable height over a reasonable ground has a
gain of around 7 dBi or around 5 dBd. What??? Yes, a half-wave dipole
at a reasonable height over a reasonable ground has around a 5 dB gain
over a dipole in free space so watch out for antenna gain claims. Most
of us don't have a lot of free space around us.

dBc is a measurement of something (like spurs or harmonics) compared to
the carrier of a signal. I don't know what dBic is.

73, Cecil, kg...@indirect.com

Alan Bloom

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Feb 21, 1994, 12:55:22 AM2/21/94

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Cecil Moore (kg...@indirect.com) wrote:

: ... The gain of a half-wave

: dipole in free space is 2.14 dBi and when an antenna gain is specified
: in dBd it is usually the gain over a dipole in free space. But don't
: expect your antenna to be that much better than your trusty dipole. A
: half-wave dipole at a reasonable height over a reasonable ground has a

: gain of around 7 dBi or around 5 dBd. ...

A reputable antenna manufacturer will compare apples to apples. It is
always assumed that if you specify dBd you are comparing the antenna under
test to a dipole mounted at the same location.

AL N1AL

Karl Beckman

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Feb 21, 1994, 8:35:29 AM2/21/94

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In article <0097A3CD.E4...@convax.iar.nrc.ca>,
jor...@convax.iar.nrc.ca wrote:

> Hello,
>
> Can anyone help me. I am trying to interpret various antenna gain specs for
> a variety of circularly polarized antennas. The specs usually use dB, dBi
> or dBic.
>
> What is the definition of these units and how are they related to each
> other ?? Does dBi mean the gain relative to an isotropic radiator ?
> Does dBic mean the gain relative to an isotropic circularly polarized
> radiator. Can one meaningfully convert from one to the other ?
>

DARN, you engineering students always figure out this technical stuff,
destroying the job security us old-timers got with our secret alphabet
soup!
dBi is the gain relative to an isotropic radiator;
dB (sometimes dBd) is the gain relative to a dipole, which is always by
definition 2.15dB less than dBi;
dBic is the gain relative to an isotropic radiator circularly polarized,

valid when the dish is equipped with a circularly polarized feedhorn.

> For instance, a 4 ft. parabolic dish is spec'd at 24 dB. Should this be dBi
> ? I have some information on a patch antenna which has a gain of 16-17
> dBic. How do I compare the two gains ??

The gain of a dish is given at a SPECIFIC FREQUENCY, since the bigger the
capture area in wavelengths squared, the more gain you get. It is spec'd
either as dB or dBi, the difference as noted above.

>
> Any help or pointers to reference books which discuss this would be
> appreciated.
>
> Thanks,
> Jim

--
Karl Beckman, P.E. < STUPIDITY is an elemental force for which >
Motorola Comm - Fixed Data < no earthquake is a match. -- Karl Kraus >

Some of the opinions expressed above aren't even claimed by the author!
Amateur radio WA8NVW @ K8MR.NEOH.USA.NA NavyMARS VBH @ NOGBN.NOASI

Jim Bromley, W5GYJ

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Feb 21, 1994, 8:51:58 PM2/21/94

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In article <0097A3CD.E4...@convax.iar.nrc.ca>,
<jor...@convax.iar.nrc.ca> wrote:

>Hello,
>
>Can anyone help me. I am trying to interpret various antenna gain specs for
>a variety of circularly polarized antennas. The specs usually use dB, dBi
>or dBic.
>
>What is the definition of these units and how are they related to each
>other ?? Does dBi mean the gain relative to an isotropic radiator ?
>Does dBic mean the gain relative to an isotropic circularly polarized
>radiator. Can one meaningfully convert from one to the other ?

Short answer:

It all depends on the gain measurement set-up.

Long Explanation:

dB doesn't mean a thing without a reference level.
The way it's slung around in antenna circles, it *usually*
means in reference to a linearly polarized, half-wave dipole.
And to clarify that distinction, it is often abbreviated dBd.

dBi means the reference level is the linearly polarized
isotropic level. This postulates a measurement antenna with
linear polarization that is continuously aligned with the test
antenna's polarization. One rule, rigorously derived in just
about all antenna texts, is that a 1/2-wavelength dipole has a
gain of 2.14 dBi. So 0 dBd = 2.14 dBi.

dBic means the reference level is the circularly polarized
isotropic level. If you postulate a circularly polarized
measurement antenna with a 1.0 axial ratio, then
Gain(dbi) = Gain(dbic)

However, a linearly polarized antenna will have 3db more gain
compared to this level if you postulate a linearly polarized
measurement antenna. The reason is that the measurement
antenna gains 3db by being insensitive to cross polarized
radiation. And most practical gain measurements use a
linearly polarized antennas. They do the measurement at the
peak of the beam with polarizations aligned, compare it to a
1/2-wave dipole, add 2.14 dB to reference it to the linearly
polarized isotropic level.

For an antenna that has a strong cross-polarized radiation
component, (an HF aircraft antenna come to mind) it is
impossible to determine a linear isotropic level by
integrating the total radiation over a sphere surrounding the
antenna in the far field. And, as a practical matter, most
antenna radiation distribution measurements are performed in
spherical coordinates, which lead to polarization ambiguities
at the poles. Consequently, the only isotropic level
available is the circularly polarized one.

So, the bottom line in converting units:

Gain (dBic) = Gain (dBd)
+ 2.14 dB (linear isotropic level/dipole level)
+ 3 dB (circular isotropic level/linear iso. level)

>For instance, a 4 ft. parabolic dish is spec'd at 24 dB.
> Should this be dBi ?

That figure is *probably* dBd. It would be nice if the manufacturer
could be more explicit. Whatever it is, if you use a circularly
polarized feed, Gain(linear) = Gain(circular).

>I have some information on a patch antenna which has a gain of 16-17
>dBic. How do I compare the two gains ??

Depends whether it is linear or circular polarized. If it is
circular, then G(dBi)=G(dbic).

If it is linear, then G(dBi) = G(dBic) - 3dB

>Any help or pointers to reference books which discuss this would be
>appreciated.

The "bible" is _Antenna_Engineering_Handbook_, Jasik and Johnson eds.

Also see _Antennas_ by John Kraus.

Scott Townley

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Mar 5, 1994, 3:45:43 AM3/5/94

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Karl Beckman <CSLE87> writes:

>> Can anyone help me. I am trying to interpret various antenna gain specs for
>> a variety of circularly polarized antennas. The specs usually use dB, dBi
>> or dBic.
>>
>> What is the definition of these units and how are they related to each
>> other ?? Does dBi mean the gain relative to an isotropic radiator ?
>> Does dBic mean the gain relative to an isotropic circularly polarized
>> radiator. Can one meaningfully convert from one to the other ?

As has been mentioned, dBi=dB relative to isotropic radiator.
Therefore, 0 dBd=2.14dBi
dB=God only knows. I
I've noticed (particularly in the cellular industry) that when an antenna gain
is quoted simply in 'dB' they usually mean dBd, since it's an easy (comparison)
measurement for the manufacturer.
And dBic=dB relative to isotropic circularly polarized radiator. But to get
from circular to linear...there is a way.
First, dBic will apply to any circularly polarized antenna, a reflector, helix,
spiral, what have you.
Second, ingrained in dBic is the concept of AXIAL RATIO. This is the ratio
between the magnitudes of the semi-major and semi-minor axes of the
polarization ellipse...or more simply put, the ratio of the amplitudes of the
dominant linear polarization to th
e amplitude of the polarization orthogonal to the dominant.
So a perfect circularly polarized antenna has an AR of 1 (or 0 dB), and a
perfectly linear dipole has an AR of 0 (or -infinity dB).
Once you know the AR of the circularly polarized antenna, you can convert
between dBic and dBi (sometimes you will see dBiL--dBi to linear radiator,
which is actually redundant--it just points out that the reference radiator is
purely linearly polarized ).

REFERENCE: See Ludwig, A.C. "A Simple Graph for Determining Polarization
Loss", Microwave Journal, Sept. 1976, p. 63.
This article is excerpted with editorial comments in the August 1983 IEEE
Antennas and Propagation Society Newsletter, p. 28.
Good Luck! Scott Townley NX7U