What is dBi?

What is dBi?

For today’s blog, I was going to write about my plans for how to test my antennas and any new antenna against my “Standard” antenna.

Due to the blog about the new Antennas Direct DB8e (“17.4dBi gain” antenna at:
Solid Signal Blog – CEDIA Expo Report: Antennas Direct DB8e), I decided that the next blog should be about gain, dBi vs. dBd because it is something we need to understand. We may try to use it for judging one antenna against another.

dBi: dB gain measured against a theoretical Isotropic Radiator, a point source
dBd: dB gain measured against a dipole antenna cut specifically for the frequency used in the test of known wire and specified lengths.

NOTE: A good short on-line article about dBi vs. dBd can be found at:
http://www.digi.com/support/kbase/kbaseresultdetl?id=2146

One valid reason for using dBi rather than dBd is that it is a “perfect” 360/360 degrees radiator/receiver pattern. This means that every antenna, when measured against this theoretical antenna, will exhibit some gain because there is no known antenna that can radiate or receive in a perfectly uniform spherical manner to/from every direction.

The simple fact is that we never want this kind of pattern anyway, at least not yet. The one and only place I can see its having an advantage is in outer space where ships and planets can be at any orientation relative to your ship’s antenna. So in that regard, it is actually a bit of a disadvantage that there is no such antenna available to be used in outer space that will allow such uniform spherical coverage.

The one way to easily calculate your dBi value is to determine your dBd gain value and add 2.15 to this value. If we all use dBi, then we’ll all start from a standard location and this is always a good thing. If we all test using the same standard reference antenna, then, again, we have a common starting point. I like this idea.

We can measure both the power coming out of, or received by, a dipole antenna, our standard reference antenna, and then by the antenna under test (AUT) in the same situation in order to determine the dBd gain of the AUT. Add 2.15 to this value and we have our dBi gain value for the AUT. But why bother adding 2.15 if we’re all agreed on the standard reference antenna we’re going to use? It’s simply another step in the process which is not really needed. The other alternative is to model each antenna using a program such as EZNEC and let it do the calculations for us, just agree to use one or the other dBi or dBd. I own a copy of EZNEC but have yet to find time enough to become familiar with it let alone comfortable using it.

While I have not had time to figure out all of my proposed testing methods, I will say that thinking about this topic has at least clarified one proposed test. I was thinking that I would use one of my old UHF bow tie antennas as my standard reference antenna. But if I did this, you wouldn’t be able to buy the same antenna. We would then never know if your reference antenna was the same as mine and so we wouldn’t be able to duplicate each other’s test results.

The second idea I had was to build two dipoles for the two frequencies I wanted to test for a given, measured wire gauge so that anyone could build the same antennas. I like this idea since I cut wire and build antennas fairly often and am comfortable doing so. However, some of you may not feel so comfortable doing this kind of thing.

I’m going to assume:

1. That most of you are interested enough to read my blog but may not want to build or test antennas.

2. That some of you would not want to build a few antennas but might like to try your hand at testing and comparing antennas.

3. That there is probably a few of you who are willing to build antennas to use as the standard reference antennas to test against.

I propose to use a known quantity as our standard reference antenna. I propose that I will use a purchased bow tie UHF antenna. If Solid Signal sells one, I’ll use that. Otherwise, I’ll specify a known quantity from somewhere else such as Radio Shack, as long as I get clearance to do so.

I don’t want to appear to favor any particular seller but I want something everyone can get easily.

The bottom line may simply be that most of you will not want to do any of this kind of experimentation and will be happy to just see my results and the results of a few others willing to do the same kind of thing with their antennas. But, for the information to make any sense at all then all of us who will be building or buying antennas and testing them against a known reference MUST
(1) have the same reference and
(2) test all antennas in the same way.

So, we’re back to figuring out exactly how to test an antenna so we can see how they stack up to each other.

Until next time, good viewing & listening!
Phil Karras, KE3FL / KPC3FL
http://cs.yrex.com/ke3fl