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# Significance of negative dBi

#### fparveen

##### Junior Member level 3
Can an antenna have negative dBi in the direction of peak radiated power ?

dBi is the measure of radiated power of an antenna in any direction w.r.t. the radiated power of an isotropic antenna radiating the same total power, right?
So, in the direction of the maximum radiated power, the dBi value should be positive.
And, in the direction of the lowest radiated power, the dBi value should be negative, right?

Conservation of energy law applies, so yes you are right.
If it is a real antenna and real world losses are included then the antenna can have a lower gain than isotropic.

So, in the direction of the maximum radiated power, the dBi value should be positive.
This is true for a lossless antenna. If you have built your antenna on a lossy substrate like FR4, the peak gain can be lower than 0 dBi.

One useful measure here is the antenna efficiency, many EM simulators can calculate that number. Lossless antenna has 100% radiation efficiency.

can antennas have loss, while being "tuned" to the right frequency? sure. electrically small antennas often have high loss.

#### volker@muehlhaus​

What is the definition of dBi then? Is the following definition correct?
" dBi is the measure of radiated power of an antenna in any direction w.r.t. the radiated power of an isotropic antenna radiating the same total power "

If the total radiated power of the isotropic antenna being compared with, is the same as the total radiated power of the antenna under test (AUT), then shouldn't the AUT's gain toward the direction of peak radiation be less than the gain of that isotropic antenna ?

Because, the isotropic antenna has same radiation intensity in all direction. So we can get the radiation intensity of that isotropic antenna in any direction, by calculating the average (over all directions) radiation of the AUT. Then the maximum is always greater than the average, isn't it? That means, shouldn't the peak gain of the AUT be positive in dBi ?

Conservation of energy law applies, so yes you are right.
If it is a real antenna and real world losses are included then the antenna can have a lower gain than isotropic.
Yes, definitely it can have lower gain than isotropic.

But my question was whether it can have lower gain (in dBi) than isotropic (that radiates the same total power) in the direction of peak radiation?

If the total radiated power of the isotropic antenna being compared with, is the same as the total radiated power of the antenna under test (AUT), then shouldn't the AUT's gain toward the direction of peak radiation be less than the gain of that isotropic antenna ?
You describe a different parameter, directivity.

Reference for directivity is the power radiated by the antenna.
Reference for gain is the input power into the antenna (1)

So if the antenna has internal loss, this will reduce gain, but it will not reduce directivity.
Radiation efficiency can be calculated from the difference between gain and directivity.

(1) assuming matched source
--- Updated ---

But my question was whether it can have lower gain (in dBi) than isotropic (that radiates the same total power) in the direction of peak radiation?
Sure, see my description above.

Let's assume your antenna has 6dBi directivity, but only 10% radiation effiency. The peak gain is then (6-10)dBi = -4dBi.
This antenna has a strongly directed pattern, but radiates only 10% of the input power, and 90% is dissipation inside the antenna.

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Embedded (built-in) antennas were first used in mass production in 1996 by Hagenuk MT-2000 GSM mobile phone.
Before that the most used term of an antenna was "antenna gain" in dB. Then, when mobile phones became smaller and smaller with bigger circuit complexity, the embedded antenna's gain reduced dramatically, and negative antenna gain in dB was common. So, mainly for marketing reasons, the "antenna gain" term was replaced by "antenna efficiency" in %.
Antenna efficiency as a percent may be confusing when speak about "antenna efficiency change". For example a 5% efficiency change from 2% to 7% represent 5.5dB change, when the same 5% efficiency change but from 92% to 97% is only 0.3dB change.