View Full Version : Converting Db/dbi

04-28-2003, 01:49 AM
Does anyone know how to convert dbi to db? I have looked yet cannot find the conversion for it.

04-28-2003, 01:54 AM
dBi is dB over an isotropic radiator--a unity antenna (perhaps best described as a 1/4 wave vertical). This is 0 because there is no gain.

dBd is dB over a dipole. This is equal to (correct me if I'm wrong) 2.15dBi, or 2.15dB gain over an isotropic radiator.


Brent, W1NCH

04-28-2003, 04:49 AM
There is actually no conversion of dB to dBi. #A dB is a dB is a dB!

If what you are talking about is the rating of gain, front-to-back, etc., of various antennas, then you have to say what the antenna is being compared to. #The standard reference for antennas has been the dipole for many decades in the amateur radio field. #But, some references are made to "dBi". #An isotropic antenna is a theoretical antenna (it cannot be built in a practical sense) that radiates equally in all planes and all directions (is a "perfect" sphere in its pattern). #A dipole has a gain of about 2.1 dB over the isotropic antenna. #This can be referenced as 2.1 dBi.

The notation for gain over a dipole is dBd whereas for an isotropic radiator dBi. #Now, there is no reason why antenna gain cannot be expressed as dB over a wet noodle! #I guess that would be dBwn for the notation!

dB is a logrimithic expression of power or voltage ratios. #This has nothing to do with anything else. #For power you use the formula 10(log10 P1/P2). #For voltage you use the formual 20(log10 V1/V2). #The reasons for these formulas are given in a short article on two of my websites: #http://home.attbi.com/~zcomco and http://home.attbi.com/~k9sth . #

Anyway, if what you are looking for is the difference between a dipole and an isotropic antenna, then that is 2.1 dB.

By the way, the correct notation for a decibel is dB (not Db or DB). The lower case "d" indicates "deci" which means 1/10th. The upper case "D" indicates "deca" which is 10 times. Thus, the difference between a dB and a DB is 100 times in favor of the DB.

Glen, K9STH

04-28-2003, 07:18 AM
Just a further note as Glen pointed out an Isotropic radiator is a perfect radiator with a gain of one as it radiates equally in all planes, so consequently it is only a theoretical antenna, and is used as reference, as it never changes gain. It radiates as a total sphere but has nothing to do with a 1/4 wave vertical antenna. In real life a sphere would have a null at the feed point, so it would not be perfect, so only a theoretical antenna can be used. #
ENJOY!! Learning is what Ham Radio and Life is all about.......73, # ORV

04-28-2003, 02:49 PM
The closest physical radiator to being isotropic is a star, and the closest one is our sun. Even it isn't a perfect, but it as close as I can think of in the real world.

04-29-2003, 09:43 PM
Darn! I just ordered and received a case of isotropic antennas off of eBay, at a pretty good price. $2.00 apiece if you order a case of 50 antennas. They claimed that if you connect this thing to your existing ham antenna, you will increase the gain by 10 times the dbi rating and no one will even know you have one. Pretty cool.

I thought they felt kinda light for a case of fifty, but the box is from the Acme Antenna Corporation, real pretty. Says to besure to inspect the merchandise before making any claims for refund. Why would they say something like that?

Any way, even if they don't work, I've got this neat conversion chart for converting cps to hertz. I bet those are hard to find!

04-29-2003, 11:35 PM

By some chance was the seller of the isotropic antennas Wy Lee Coyote? He seems to have had a very good working relationship with the Acme Corporation and its affiliates for many years.

Glen, K9STH

04-30-2003, 08:58 AM
I bought a case of those Isotropic antennas too.. The instructions said "works best when attached to the driver's door of a motorcycle"... Now if I can just locate a source for post holes.. http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif http://www.qrz.com/iB_html/non-cgi/emoticons/biggrin.gif http://www.qrz.com/iB_html/non-cgi/emoticons/tounge.gif


04-30-2003, 01:09 PM
wy lee coyote .......genious pure genious!

04-30-2003, 04:41 PM
Is Wy Lee another one of the importers from the Far East ? #http://www.qrz.com/iB_html/non-cgi/emoticons/confused.gif
ENJOY! It now is only a FUN HOBBY.......73, # ORV

04-30-2003, 05:14 PM
Well, now, in the vernacular of the people and region of Glen (STH), Hold on there pardner, y’all are leadin me on, or at least not bein straight. I’m fixin’ to do bidness with y’all on the that there isotropic antenna.

Mr. Beers , or mb, or what ever your call sign, says he set about to build one of these critters. The word hypothetical means “ supposed, not real, imaginary, notional, speculative, abstract, conjectural, and academic” and my theory teacher said not real, in your dreams, etc. My theory book defines it as an imaginary antenna(radiator) constructed in free space. The formula for this dream world antenna is : A=(1/4pi) wavelength squared; which is a number approx. equal to 0.08 x wavelength squared. This also is assuming no heat loss.

Now, given the assignment to build one of these, by the above definition, would be ludicrous. Maybe NASA might take on this project to measure in free space ( I don’t know how free space is ) but getting the test equipment, personnel, power, antenna fit for operation at – 80 degrees of more, RF source to drive this antenna, and the test set-up for measuring an imaginary antenna without disturbing the radiation patterns, and the parameters go on, IS a monumental, if not impossible, assignment.

In other words, this little sucker only exists on paper. No one has ever seen one, made one, or sold one. Hence my little parody, that Glen so viciously shot down in a few sentences. Anyway, it is a calculated device that can be used for relating other “real” antennas. If you were to use a directive antenna in place of the isotropic antenna, the transmission equation is:

Where Gt & Gr are the power gains due to the directivity of the transmitting and receiving antennas respectively.

The apparent power gain is equal to the ratio of the effective area of the antenna, which is equal to 0.08xwavelength squared. The aperture of the antenna is not always something you would normally require, but is applicable to all antennas. Yagi’s, log-periodic, paraboloidal reflectors, etc. have an aperture dimension, which measurable and/or calculable.

That’s my schpeel, and I’m done.

Now, I need to check the snake oil level on my computer, maybe add some electrons, because when I shake it, it rattles. Maybe an electron has come loose, or more disastrously, fallen out.

04-30-2003, 06:56 PM

So you are telling me that you actually got the 50 isotropic antennas from the Road Runner and not Wy Lee Coyote!

Glen, K9STH

04-30-2003, 07:19 PM

You got it. Or maybe I got it......what ever.

That Acme Antenna Road runner sold me those empty boxes full of air, and not ether as he told me. Well, try and prove that one.

Dad burned wy-le is still trying to catch that varmint, maybe because he got ripped by the RR. Maybe he can set up a radio tower that would fall on his head when he passes...............
nah, the thing would most assuredly fall back, and hit wy-le.

So hows biz in the lone star? I'll be coming to El Paso this week.......but I think you will still be past the radio horizon where you live.


04-30-2003, 09:41 PM
CAW: #How about coming to New Orleans, St. Louis, or Birmingham? #They are all closer to me than El Paso!

It would be a "toss up" with Omaha, but the north - south propagation is usually a bit better on 6 and 2 meters! The "outskirts" of Atlanta might be closer as well.

Glen, K9STH

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