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# a question about HFSS result

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#### ChowChen

##### Junior Member level 2
I simulate a horn antenna by HFSS.But i find the gain G is smaller than the directivity D. Usually ,G=ε × D , ε is efficiency ,0<ε≤1.

ChowChen said:
I simulate a horn antenna by HFSS.But i find the gain G is smaller than the directivity D. Usually ,G=ε × D , ε is efficiency ,0<ε≤1.

as you said:"the efficiency is smaller than 1 and so G must be lower than D" and it is clearly correct, also you said:"I find the gain is smaller than the directivity" and this is correct too, so
what is you problem exactly ?!

Sorry, there is a wrong in my question.
The result of hfss is that the gain is larger than the directivity.
The result of Hfss is wrong?

Sounds to me like your model has not converged.

Matt

ChowChen said:
Sorry, there is a wrong in my question.
The result of hfss is that the gain is larger than the directivity.
The result of Hfss is wrong?
Possible reasons:
1. in ur simulation model, metal loss is considered
2. mesh on the radiation boundary is too coarse, therefore the far field calculation is not accurate.

regards.

Hi,asdfaa
thank you for u
the material of model is pec,so it is not the first case.
how to solve the problem of 2.

set the airbox lambda/4 larger where the lambda is the min (not max) lambda of your interested band

ChowChen said:
Hi,asdfaa
thank you for u
the material of model is pec,so it is not the first case.
how to solve the problem of 2.

Try to increase the mesh density on the radiation box surface, you can do it by assign mesh operation.

Regards,

If You kwon, HFSS, and other softwares those solve the maxwell equations using numerical method, are dividing the Antenna and other mediums to diffrent pieses and will consider the wave on each pieces constant and fix. So if the length (area) of each picecs is long this consideration is wrong and you will receive wrong analysis and result.
solution is considering low area for each pices, thet is means increse the density of meshes, as asdfaaa Said.

To woshifanka:

Just want you to confirm that you want the airbox to be lambda/4 larger than the "MINIMUM LAMBDA".
I think it should be read: "lambda/4 larger than the HIGHEST LAMBDA (corresponding to the lowest frequency).
Am I correct ? The airbox has to be LARGE engough.
Thanks.
oxo

Dear costox : thank you for your advice. yes , its my fault.
rgz

Even if you mesh the radiation boundary very well, you can get numerical errors that will lead to gain > directivity. I've seen radiation efficiencies slightly greater than one from HFSS antenna models that are adequately meshed.

I believe this stems from the fact that the FEM solves for the E-fields directly and then calculates H-fields and the currents and far-field quantities from H-fields. So some small numerical error may be difficult to remove when using this method.

That's why you need to put in the setup analysis:
min convergence pass: 2

Asdfaaa said:

Possible reasons:
1. in ur simulation model, metal loss is considered

I don't understand what it means...
If I consider metal loss I have efficiency=Rrad/(Rrad+Rloss) <1. And no problem exists...
In the case of pec efficiency = 100% and so I would have G=D...
Isn't it right?

ifelix said:
Asdfaaa said:

Possible reasons:
1. in ur simulation model, metal loss is considered

I don't understand what it means...
If I consider metal loss I have efficiency=Rrad/(Rrad+Rloss) <1. And no problem exists...
In the case of pec efficiency = 100% and so I would have G=D...
Isn't it right?

There was my type error, it should be "metal loss is not considered".

Regards,

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