Negative Directivity in HFSS.

[eskim_urtigas]

Hello,

I'm simulating a very small antenna, with low radiation efficiency and gain. In some simulations (HFSS) I observed only negative directivity values.

Then I optimized the model, corrected some things such as radiation boundary box size and material, etc.
Now i observe positive and negative values. I would like to understand why I keep getting these negative directivity values.

(2 directivity plot in the attachments)

Can anyone comment on this?

Does anyone know how HFSS calculates directivity?

Thank you.

 

[reidintransit]

Look at the hfss help on directivity and the topic on computing antenna parameters.
Warning: The computed values of max U and peak directivity depend on the user-determined set of aspect angles chosen for the computation of the radiated fields. If this set does not encompass the actual peak intensity of the radiated pattern, the displayed results for these three parameters will be inaccurate.
directivity = 4*pi*U/Prad where
U is the radiation intensity in watts per steradian in the direction specified.
Prad is the radiated power in watts.
My guess is that the negative has to do with direction of the beam you want vs. average beam direction.

 

[eskim_urtigas]

I increased the size of the air box around the antenna. Now I see positive values. Still don't really understand why, but it works now. Any ideas why?

 

[flanello]

The directivity values given are in dB.
Look at the definition of dB:
x in dB = 10*log(x).
Any value smaller then 1 leads to negative dB value. Therefore you have a positive directivity but the representation in dB leads to negative values. Thus the directivity there is very small.
Almost every antenna has regions with very small directivity leading to negative dB values.