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whats the use of antenna having negative gain

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shahid78

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negative gain antenna

hi

i was reading one paper and the author was proposing an antenna for RFID system.
the antennas proposed was having gain of -7.5 dBd

now i dont understand the purpose or use of antenna having negative gain.

can such antennas be used in practical applications

i will appreciate any comments on this topic

thanks
 

negative antenna gain

Hello Shahid,
I was reading through the articles and found this

" Unity Gain Antenna.
If a square antenna is visualized and G=1, Ae = lambda^2 / 4*pi. When a dimension is greater than 0.28 lambda (approximately 1/4lambda ) it is known as an electrically large antenna, and the antenna will have a gain greater than one (positive gain when expressed in dB). Conversely, when the dimension is less than 0.28lambda (Approximately 1/4 lambda)(an electrically small antenna), the gain will be less than one (negative gain when expressed in dB). Therefore, a unity gain antenna can be approximated by an aperture that is 1/4lambda x 1/4 lambda.

Think its all about the electric size of an antenna which in the case of rfid is very small hence gain negative.

Regards
 
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    apeeet

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Hi parfuldeo

thanks for your comments on negative gain. i have some points to make.

we know gain is the relative term . and most of time our reference is isotropic antenna which radiates equally in all directions and thus having Gain=1 or 0dB.
which means having no gain as it radiates equally in all directions.
but such an antenna doesnt exist physically as we can only get such a gain from a point soucre which mean electrically very very small.

now my point is if such an electrically small antenna has positive gain although 0dB how come RFID antenna which must be electrically larger ( even if it is less than lambda/4 as mentioned by you) than point source has negative gain?
let me know what do u think after reading my views.

regards
shahidλλλλλλλλλλ
 

Re: negative antenna gain

prafuldeo said:
Hello Shahid,
I was reading through the articles and found this

" Unity Gain Antenna.
If a square antenna is visualized and G=1, Ae = lambda^2 / 4*pi. When a dimension is greater than 0.28 lambda (approximately 1/4lambda ) it is known as an electrically large antenna, and the antenna will have a gain greater than one (positive gain when expressed in dB). Conversely, when the dimension is less than 0.28lambda (Approximately 1/4 lambda)(an electrically small antenna), the gain will be less than one (negative gain when expressed in dB). Therefore, a unity gain antenna can be approximated by an aperture that is 1/4lambda x 1/4 lambda.

Think its all about the electric size of an antenna which in the case of rfid is very small hence gain negative.

Regards
Click to expand...

What is the reference that you used to explain about the negative gain of an antenna?
 

Re: negative antenna gain

I am also curious about the reference u read. In my opinion, if neglecting mismatch and radiaiton loss, the gain of electrically small antennas should still be larger than 0 dB. That's why we say electrically small antennas are meantime super-directive antennas.

prafuldeo said:
Hello Shahid,
I was reading through the articles and found this

" Unity Gain Antenna.
If a square antenna is visualized and G=1, Ae = lambda^2 / 4*pi. When a dimension is greater than 0.28 lambda (approximately 1/4lambda ) it is known as an electrically large antenna, and the antenna will have a gain greater than one (positive gain when expressed in dB). Conversely, when the dimension is less than 0.28lambda (Approximately 1/4 lambda)(an electrically small antenna), the gain will be less than one (negative gain when expressed in dB). Therefore, a unity gain antenna can be approximated by an aperture that is 1/4lambda x 1/4 lambda.

Think its all about the electric size of an antenna which in the case of rfid is very small hence gain negative.

Regards
Click to expand...


---------- Post added at 17:16 ---------- Previous post was at 17:14 ----------

I guess there is high loss encountered in the proposed RFID antenna. You could check the directivity of the antenna (which should be a tag antenna working in harsh environments, I guess).

shahid78 said:
hi

i was reading one paper and the author was proposing an antenna for RFID system.
the antennas proposed was having gain of -7.5 dBd

now i dont understand the purpose or use of antenna having negative gain.

can such antennas be used in practical applications

i will appreciate any comments on this topic

thanks
Click to expand...
 

THIS IS HIS SOURCE
**broken link removed**
 

Gain = Directivity - Loss

So there you go: the directivity is probably, lets say for a dipole around 2...3 dBi. Now, if the antenna consists of lossy material or is very small (so a small radiation resistance and a comparable or greater loss resistance in the equivalent circuit), the loss becomes significant, lets say 10 dB.

So: Gain = Directivity - Loss = 3 dB - 10 dB = -7 dB.

Don't confuse that with any directional relations, all that is due to the directivity. Hope that makes things clearer.
 

It will sound annoying, but most of the mobile phones antennas have a negative average gain related to a dipole antenna.
The reason is simple: you cannot make an antenna which is squeezed inside a box, near metals, having whatever fraction of wavelength aperture, to work like a half wavelength antenna placed in open air.
 

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