# Forum

## Difference between dBi and dBd?

13 posts by 6 authors in: Forums > CWTS - Enterprise Wi-Fi Fundamentals
Last Post: September 19, 2011:
• Page 121 of the Sybex book talks about the difference between dBi and dBd. They both pretty much sound the same.

The real world scenario on page 121 seems to suggest that dBi is stronger than dBd. Both show relative power values, not absolute.

If both are the same, why not just pick 1 and stop using the other?

• By (Deleted User)

Here are the two definitions from the CWNP Dictionary of Wireless Terms & Acronyms:

[b]dBi [/b]- decibel gain or loss referenced to an isotropic antenna. Commonly used to measure antenna gain. The greater the dBi value, the higher the gain, and the more acute the angle of coverage.

[b]dBd [/b]- decibel gain or loss referenced to a dipole antenna. A measurement of RF signal strength compared to a practical dipole antenna. dBd is 2.14 dB stronger than dBi.

dBi is a measurement of [i]gain[/i], whereas dBd is a measurement of [i]signal strength[/i].

• The latter part of the definitions from the dictionary can be a bit confusing. The original goal of all decibel and decilog calculations was ( and still is ) to compare one value to another or reference value. Originally this was done with sound waves. Comparing the intensity of one sound to that of another, standard reference sound. Later, this method was applied to electrical engineering where voltages were compared across the same value of resistance ( for example dBmV or dBV?.decibel ratios compared to a reference of one millivolt and decibel ratios compared to one Volt ). Power values were also considered, with dBm ( decibel ratios compared to one milliwatt ) and dBW ( decibel ratios compared to one Watt ).

Antennas can be compared, one with another. As mentioned, we can use isotropic antennas as a reference or dipoles, or anything we please. We could use the antenna on Mrs Smith?s Wi-Fi Router as a reference, if we wanted to, the only stipulation being that we would have to mention it in our formula: 10 Log base 10 ( Gain of antenna being measured/Standard gain of Mrs. Smith?s Wi-Fi Router Antenna ). Provided that the gain of that antenna did not vary and was kept under ?standard conditions?, that gain measurement could be used , but for obvious reasons would probably not.
Comparing gain values to a dipole antenna is a valid option, so I think that the statement that it is normally used for absolute signal strength measurement as oposed to dBi gain measurements could be a bit misleading.

Many antenna manufacturer?s sheets refer to gain values relative to a dipole without any mention of signal strength.

http://searchmobilecomputing.techtarget.com/definition/decibels-related-to-dipole-antenna

Dave

• This post will contradict things in the previous posts. I am an old school electronics tech who got into computers, not a computer guy who got into radio. Different perspective, more detailed training in the arcane art of RF.

An isotropic radiator is a theoretical construct. A thing that can be made only in your mind. An isotropic radiator is an infinitely small point in infinitely empty space that radiates equally in all directions.

dBi is decibels related to an isotropic radiator. dBi measurements are specified by the FCC in radio regulations because they reference a defined, if unobtainable, standard.

dBd is decibels referenced to a 1/4 wave vertical dipole, AKA Marconi antenna. the simplest and most commonly used antenna in most radio services.

A 1/4 wave vertical dipole exhibits 2.14 dB gain over an isotropic radiator, because the radiation pattern is shaped like a bagel, compared to the spherical radiation pattern of an isotropic radiator. Less radiation going up; more going out in the horizontal plane.

The dBi standard is used because it is a better engineering choice. It references a defined standard. This is necessary for defining power limits in regulations.
The dBd standard is used because it gives valid comparisons to a practical standard. My 9 dBi omnis return 7 dB gain over standard dipoles.

• Your post doesn't contradict anything I've written above. What you have written is correct. I'm an RF engineer by profession who later on his career got involved with Wi-Fi/IT.

Dave

• Different branches of RF engineering utilize different reference values. For example in satellite communications, dBd values are rarely used ( I?ve been doing satellite work for nearly thirty years and only saw it used once in an obscure document about a VHF experimental system . dBi values are used in all day to day calculations. There is nothing wrong with using a theoretical isotropic radiator as a reference, as the main issue we are concerned with in link engineering ( whether in satellite communications or terrestrial microwave links or Wi-Fi point to point parabolics ) is that of PFD or Power Flux Density when performing comparisons. Absolute signal strength values are calculated in link budgets, which show the overall effect of all gains and losses in the system. Signal to noise ratio calculations become much more complex than in Wi-Fi, as we have to take intermodulation noise etc into account. When a spacecraft antenna is looking down at the earth, we have to take into account the electronic noise caused by the motion of electrons in objects above absolute zero. In other words, the "warm Earth". When an earth station on the ground "looks" at an orbiting satellite, with the exception of some minor thermal noise caused by atmospheric gases, the antenna basically looks at "cold space" with a subsequent reduction in thermal noise.

Why use PFD ? Consider an outdoor Wi-Fi link using parabolics at both ends. The antennas will have side lobes and a main lobe. It is the main lobe we are concerned with in a properly aligned link. In the case of a practical parabolic, we should know the actual amount of physical RF power emanating from the final part of the transmitter chain prior to entering the antenna proper. For the isotropic, we simply have to calculate the PFD that would occur when the same level of RF power as the practical antenna is injected into the antenna. When tests are done on near field and far field of the antenna boresight on a test range, we can physically measure the amount of RF power and utilize that value in calculating Power Flux Density down the boresight main lobe. Measurements can then be made of PFD. We can also simply calculate the PFD on an isotropic antenna using that same value of physical RF power. One can be compared with the other. That is how antenna gain is measured in that case.

http://en.wikipedia.org/wiki/Near_and_far_field

Refering to signal strength values and dBd values in the ?same breath?can cause confusion, as the received signal strength of a radio link is a composite of many factors involving antenna gains, frequencies etc. This applies to both links utilizing dBd values and dBi values. It really doesn?t matter what is used, as long as the calculations utilized are consistent throughout. I.e. all dBi or all dBd. In microwave links and satellite links, dBd values could be used, however that would be just an extra unnecessary step. In Wi-Fi we often see that dBd values are used due to the ubiquity of the half wave dipole reference. In parabolic antenna links you are more likely to see dBi values. Again, a theoretical antenna model like an isotropic is fine is to use ( and is used every day by scientists working on NASA?s Deep Space Antennas as well as by the link budget engineers of the satellite companies who keep a huge percentage of the world?s international communications going ).

Indoor Wi-Fi antennas are mainly made up of dipole type structures. These work in a different manner to parabolics. Now "surface area of reception" becomes more theoretical. For example, one might think that by doubling the width of a dipole, you would get double the signal at the antenna output compared with previously. It doesn't work like that however. There is no main reflector to reflect energy to a focal point or subreflector. That is why we tend to use dBd values for those types. Often practical measurements are made by looking at received signal level.

Dave

• From a non-expert point of view, and almost a side comment to the thread, ad5mb and dave1234 seem to have enough knowledge to explain the difference between dBi and dBd, the different is a matter of writing style.

ad5mb explanation is simple and doesn't go very deep in parallel explanations to help (or have the reverse effect) the reader understanding his/her message. Thus the shorter message is easier to graps.

Follow-up question for both,

ad5mb mentions [code][b]1/4 wave vertical dipole[/b][/code] as the reference, while dave1234 mentions [code][b]ubiquity of the half wave dipole reference[/b][/code]. Are we talking about the same reference dipole antenna?

Thanks

• Thank you for your comments. Much appreciated. You should note that the forum is not just for beginners. There are many people taking the CWDP exam, and the comments I made refer mainly to them. You can find all the information that a beginner needs to know about isotropic antennas and dBd values in the CWNA handbook. I presume you have already checked that ? If you are looking for more beginner information, I suggest you go back and look at the many posts I have put up in the past for beginners and advanced users alike.

Bye the way, none of us get paid for taking the time and effort to put these posts up.

The term ?di? in the word ?di-pole?means two. The term ?pole? refers to an antenna element. This would imply two elements with a center feed. Many antennas only have one ?pole?, yet operate as if they had two.

The Marconi antenna utilizes ?reflected virtual elements?. Even though the physical element is a quarter of a wavelength long, the combination of the physical and reflected elements effectively gives us a ?half wave device?.

I put references to satellite communications and microwave communications in these posts because that is exactly where Wi-FI engineers took about 90% of the circuitry and antenna systems from. The electronic circuitry ( for the most part ) of mixers, oscillators, demods etc found in modern Wi-Fi cards APs nearly always is based on pre-existing systems in satcom and microwave. From parabolic antennas to forward error correction, same story. That is what the ?confusing parallelism? was about.

In order to properly understand this, you need knowledge of transmission line theory and antenna design. This is not a beginner?s topic. The issue I was commenting on was not simple ?isotropic versus dipole?, but the note in the dictionary about ?SIGNAL STRENGTH?. If I wanted to illustrate the difference between isotropic and dipole reference, then that is what I would have done, as I have done in the past many times.

It would take me ten seconds to cut and paste some ?standard definition? from Wikipedia to illustrate some of my comments, but I prefer not to do that, and try to give some information that I have learned from years of working with antennas of all shapes and sizes, during the over three hundred installs that I have done.

Dave

• Those were quite complicated answers to what I thought was a simple question! :) I think I get the gist, kinda.

Just glad dBi vs dBd won't be on tomorrow's CWS exam.

Michael

• Don?t worry about it. The information that you will get in the CWNA book is more than adequate for practical Wi-Fi engineering. CWDP has a section on antennas for more in-depth info. Antennas are strange things. If we look at something like Wi-Fi security ( with the exception of number and probability theory for codes etc ), we can pretty much explain most of it without any math, and using analogies ( doors, keys etc ).

With antennas, it?s a bit different. We can quite easily explain something like an isotropic radiator, but what about a dipole ? We know that with a parabolic antenna, that the larger the antenna, the larger the gain. But what about a dipole ? If we make the dipole ?thicker? and ?longer? than a standard Wi-Fi antenna, won?t that make it have more gain ? The answer is no ( That?s why you can?t just go and get a double-size dipole for your AP ).

There are two knowledge ?points? in antenna theory. The first ?point? is where we can physically imagine what is going on without the use of mathematics. For example, an isotropic antenna?..a balloon being filled with air etc. But we reach a point where some of the issues with antennas can only really be understood using some fairly detailed mathematics ( near field, far field etc ). There really is no middle ground. I have seen some ?physical descriptions? of some issues that can really only be explained using math, and they have left ?wrong ideas? in the minds of many, by ?oversimplifying?.

Dave

Page 1 of 2