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Antenna directivity and gain

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kuanfu

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Hi,everyone

We usually can see on Paper some antenna the design is High gain and high
directivity.

But I do not understand the antenna Gain and directivity have any to
be different.

I knew these two relational type is G(Gain)=radiation efficiency*D(directivity).

But I did not understand to this formula significance.

The meaning is directivity if high gain can be relatively high?
(The premise is the radiation efficiency must be very good)

gain high that directivity also is relatively high?

If the antenna radiation efficiency not good has the possibility to
have directivity the high gain also high situation?

If radiation efficiency not good that were may let this antenna
directivity change high so can let gain change high?

Because the directivity definition is not said so long as let the
energy radiate such directivity toward some direction to be able to be
high,No matter the radiating capacity many or are few.

Or said so long as the radiation efficiency not good is impossible to
have directivity the high situation?

Asks fellow seniors to help me to illustrate

Thanks.

kuanfu
 

Azulykit

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If you are in marketing you are interested in directivity.

If you are buying an antenna you are interested in gain.



Gain is always less than directivity. Think of efficiency as an insertion loss.

gain = directivity - insertion loss (or alternately efficency)
 

alqasim

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could anyone tell me what is the definition of radiation efficiency?

what is the unit of " directivity " for the following formulas mentioned in the above post ?



gain = directivity - insertion loss (or alternately efficency)

G(Gain)=radiation efficiency*D(directivity)
 

dot4

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In general, I would talk about antenna efficiency instead of radiation efficiency. Antenna efficiency is a parameter that accounts for losses in your antenna. This is dimensionless parameter that is given by multiplication of three efficiences: reflection efficiency, conduction efficiency and dielectric efficiency (by multiplication of the last two you get radiation efficiency).
So if the losses in your antenna are high, e.g. your antenna is poorly matched, or the power is lost in conductor or dielectric, it doesn't matter high directivity of the antenna, you will get low gain.
Directivity is also a dimensionless quantity.

Hope I helped :)
 

drkirkby

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So if the losses in your antenna are high, e.g. your antenna is poorly matched, or the power is lost in conductor or dielectric, it doesn't matter high directivity of the antenna, you will get low gain.
Directivity is also a dimensionless quantity.

Hope I helped :)

Poor match does not reduce the gain. For a given input power the poor match will reduce the power absorbed by the antenna. But gain is defined by the IEEE in terms of the power absorbed by the antenna, so polarisation loss and mismatch loss don't effect the gain.

As for units, both directivity and gain are dimensionless.
 

alqasim

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What Dot4 has said :
1) radiation efficiency = conduction efficiency and dielectric efficiency
2) antenna efficiency = reflection efficiency, conduction efficiency and dielectric efficiency

What drkirkby has said :
polarisation loss and mismatch loss don't effect the gain.

So as a result :
G(Gain)=radiation efficiency*D(directivity) ( i believe not in a logarithmic values )

And i think the formula "gain = directivity - insertion loss (or alternately efficency) " is not correct . Am i right ?
 

drkirkby

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In general, I would talk about antenna efficiency instead of radiation efficiency. Antenna efficiency is a parameter that accounts for losses in your antenna. This is dimensionless parameter that is given by multiplication of three efficiences: reflection efficiency, conduction efficiency and dielectric efficiency (by multiplication of the last two you get radiation efficiency).
So if the losses in your antenna are high, e.g. your antenna is poorly matched, or the power is lost in conductor or dielectric, it doesn't matter high directivity of the antenna, you will get low gain.
Directivity is also a dimensionless quantity.

Hope I helped :)


I disagree. The gain of an antenna is totally independant of how good the match is. Check the IEEE definition in the document with a title similar to "Standard Antenna terms". I forget the title, but there's an IEEE standard which defines the terms.

There is also a term "Realized gain" which does take into account mismatch of impedance. There's also another term which takes into account polarisation mismatch.

- - - Updated - - -

What Dot4 has said :
1) radiation efficiency = conduction efficiency and dielectric efficiency
2) antenna efficiency = reflection efficiency, conduction efficiency and dielectric efficiency

What drkirkby has said :
polarisation loss and mismatch loss don't effect the gain.

So as a result :
G(Gain)=radiation efficiency*D(directivity) ( i believe not in a logarithmic values )

And i think the formula "gain = directivity - insertion loss (or alternately efficency) " is not correct . Am i right ?

Myself and dot4 disagree over the definition of gain. He mentions mismatch loss - yet I know this is not true - at least according to the IEEE standard.

gain = directivity - efficency

is correct, as long as the you are taking about dB. Since 10*log_10(a)+10*log_10(b)=a*b

So if the directivity is 40, and the efficiency 0.5, then

Gain=40*0.5 = 20

But in dB

Directivity = 16 dBi
Efficiency = -3 dB
Gain = 16-3=13 dBi

which is the same as a gain of 20, since

10*log_10(20)=13.
 
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dot4

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drkirkby is right!, according to the IEEE, gain has nothing to do with matching of the antenna. But in my defence, some books e.g. Balanis book, state the same as I wrote :)
 

davenn

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drkirkby is right!, according to the IEEE, gain has nothing to do with matching of the antenna. But in my defence, some books e.g. Balanis book, state the same as I wrote :)

If that is what is in Balanis then its very misleading, are you sure you havent misread/misunderstood what was written ?

think about this....
You have a 10 element yagi with 10dBi gain. its gain is still going to be 10dBi regardless of the matching. If the losses due to mismatch are large, then all that is going to happen is that the antenna is not going to see full power from the TX, as there is significant reflected power.

and to look at the OP's original Q as titled in the topic....
directivity and gain... basically the higher the gain the higher the directivity. The directivity of an antenna is its measured beamwidth in degrees for a 3dB drop in signal.
So one would say that the 3 dB beamwidth of antenna XXX is 10 degrees

Dave
 
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dot4

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If that is what is in Balanis then its very misleading, are you sure you havent misread/misunderstood what was written ?

think about this....
You have a 10 element yagi with 10dBi gain. its gain is still going to be 10dBi regardless of the matching. If the losses due to mismatch are large, then all that is going to happen is that the antenna is not going to see full power from the TX, as there is significant reflected power.

and to look at the OP's original Q as titled in the topic....
directivity and gain... basically the higher the gain the higher the directivity. The directivity of an antenna is its measured beamwidth in degrees for a 3dB drop in signal.
So one would say that the 3 dB beamwidth of antenna XXX is 10 degrees

Dave


So I opened the book :) and Balanis introduce two gains, GAIN and ABSOLUTE GAIN, the first one is defined regardless matching losses and the latter one takes into consideration the mismatch.
I see the point.
But I'd say this absolute gain is very handy, you find out a lot about antenna behaviour in just one parameter (from system point of view)
 

drkirkby

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So I opened the book :) and Balanis introduce two gains, GAIN and ABSOLUTE GAIN, the first one is defined regardless matching losses and the latter one takes into consideration the mismatch.
I see the point.
But I'd say this absolute gain is very handy, you find out a lot about antenna behaviour in just one parameter (from system point of view)

I know the IEEE defines "Realized gain" which includes mis-match loss. HFSS also uses the same name.

Dave
 

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