Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KX4O, Feb 2, 2017.
Can anyone cite a pre-1957 publication using the term "dBi" for antenna gain?
I'm not going to research this for you, but have heard of isotropic radiators and dBi references since I was licensed in 1965.
Before that? Beats me.
I think I have a 1947 Handbook here, somewhere (I don't see it on my shelf) and if I find it, I'll look.
I have John Kraus's (W8JK, SK)book on Antennas from 1950. He uses an isotropic radiator as a reference for gain but the exact use of "dBi" doesn't appear to be used. 73, Pete
you cant BUY an isotropic radiator
you can't BUILD an isotropic radiator
you can't even create one in a NASA laboratory
so it is like saying. your antenna has 10 db gain over a UNICORN
its a totally useless reference used as a slick marketing ploy to sell antennas
Respectfully, please read Kraus's Antenna books or the ARRL Antenna Book or all comprehensive antenna engineering books and you'll discover why the isotropic radiator is used as a reference. 73, Pete WB2UAQ
Not at all - it's a convenient way of defining a reference field strength, against which real antenna field strengths can be compared. It doesn't need to be a characteristic of a real antenna to be useful.
To quote my 42 year old ARRL Antenna Book: " ... even though no actual antenna radiates with equal intensity in all directions, it is nevertheless useful to assume that one exists. It can be used as a "measuring stick" for comparing the properties of actual antenna systems."
But you can define one mathematically with precision and that's exactly what antenna mathematicians have done... quite some time ago.
The very definition of Directivity and hence Gain is mathematically based on, and referenced against, the 4 pi Steradian uniform radiator. All professional antenna engineers/techs state absolute Directivity or Gain in linear or dB (re isotropic) and have for a few decades now. Just as dBm - dB (re 1 milliwatt) - became a moniker for power, dBi - dB (re isotropic) - became a moniker for absolute antenna gain with respect to the actual mathematical definition of Gain. The reason for a specific moniker for isotropic antenna gain are derived from requirements stated in SI specs mandating the absolute level be specified. One example is NIST SP811...
"When reporting values of [power using Logarithmic quantities and units], one must always give the reference level."
Thus in the antenna/radio biz we have:
Relative power - dB,
Absolute Directivity (Gain) with respect to the isotropic radiator - dB (re isotropic) - aka dBi (based on the universal definition of Directivity/Gain),
Absolute Directivity (Gain) with respect to a dipole radiator - dB (re dipole) - aka dBd,
Absolute Directivity (Gain) with respect to a quarterwave monopole radiator over perfect ground - dB (re quaterwave-monopole-over-ground) - aka dBq,
Absolute power with respect to 1 watt - dB (re 1 watt) - aka dBW,
Absolute power with respect to one milliwatt - dB (re 1 milliwatt) - aka dBm.
There are others, but are becoming close to extinction thankfully. All readers should note the NIST document specifically prohibits the use of any modifying terms on the "dB" meaning things like dBm are in violation of the spirit of the SI. They are obviously being very anal about things, but I'm not holding my breath for the elimination of well known terms in the radio/antenna biz (dBi, dBd, dBm, dBV, etc.).
It is like saying your antenna has a 10 dB gain over a "Uniform" Radiator... 10 dB (re isotropic). There is no mystery here. In fact, gains stated with just "dB" are more like your Unicorn reference.
Your point of "slick marketing ploy" is well taken and is all too often when manufacturers describe gain using "dB" leaving the consumer wondering what reference is implied. Sometimes they flat out lie. An egregious example concerns the Diamond CP22E 2m base station antenna [accessed Feb. 3, 2017]. Some "marketing ploy" details with comments in brackets:
Gain: 6.5 dB [no reference to base this on - fake? - dBunicorn?]
Element Phasing: 2 5/8 [double collinear antenna - sounds good]
Nice gain pattern:
Note the lack of any absolute numbers on this graph. This is the pattern of what appears to be a real measurement of gain in the E-plane. Good of them to supply real measurements. Bad of them to not show the radial units.
More details from the CP22e Assembly Instructions [accessed Feb. 3, 2017]...
GAIN: 6.5 dB [consistent with web page]
ELEMENT PHASING: 5/8[wavelength] Two-element stacked groundplane [a good ole vertical collinear]
Then we read this in the Description...
"The CP22E is a 2-meter monoband, 2-5/8 wavelength groundplane antenna, optimized for the U.S. amateur band. Made from heavy-duty aluminum, the CP22E is easily assembled, yielding excellent performance with 6.5 dB gain over a 1/2 wave dipole."
Oh so now the gain is 6.5 dBd? Anyone with a basic understanding of antennas will immediately wonder how a double vertical collinear antenna can provide the omni-directional gain only available from a four element vertical collinear array.
Fortunately the CP22e is relatively easy to model. The resulting gain pattern from simulation is a very good fit and almost perfectly overlays the manufactuers' gain pattern above... including the minor lobes. However, the gain is 5 dBi, not 6.5 dB, not 6.5 dBi and certainly not 6.5 dBd!
You know what? 5 dBi is very very good. This is almost 3 dB over a dipole revealing the CP22e is about as good a 2m antenna of this size as is possible to make. Hence I own two. Yet Diamond still felt the need to confuse their customers with not only meaningless "dB" marketing gain hype, but flat out incorrect gain claims in their assembly instructions. They, and other manufactures, need to clean up their act and stop committing what amounts to a form of consumer fraud.
The term "dBi" has been around since at least the fifties and maybe earlier if I can find any proof. Regardless, it has been plenty long enough to not excuse any antenna manufacture of padding gain figures with misleading and sometimes flat out wrong gain hype. The problem is not with the isotropic gain definition, but the lack of its use in antenna specifications.
And you never ever get 3 db of gain when you double the size of an array except on a computer screen.
Since we have established the mfgr has no clue about dB's, we have to assume one of 2 things about everything else they claim.
1 they are simply ignorant.
2 they lie.
I doubt anybody actually ever measured one of these antennas for gain and pattern, but everybody will guess at a simulation and post the results as measured facts.
This whole bit about db dbi dbd dbmop (yea, its a real db Google it) ground gain, and simulated patterns of theoretical antennas serve one function.
To confuse newbies and separate them from their money.
Who wouldn't want a skyhook with more gain?
Who would want a single band dipole, when he could buy a "all band half wave end fed with octafilar transformer action"
Where did I put that credit card!
Englishese (or maybe Japanish) also.
Japanese engineers aren't stupid, but whoever writes a lot of their material might be.
My very first "imported from Japan" piece of amateur gear was made by FDK in Japan and imported by KLM in California. It was a 2m multimode (SSB-CW-FM) rig called a "Multi-2000." This was in the mid-70s. The first page of its instruction manual stated, "Congratulations on buying this almighty transceiver."
No kidding, it really stated that.
I didn't know whether to plug it in or genuflect.