Directivity Results Too Low

bouvy
bouvy Altair Community Member
edited November 2020 in Community Q&A

Hi,

 

I am currently simulating some antennas in the frequency range 3 to 30 MHz. When examining the far field results, it appears that the directivity values are too low. Because my simulation has a ground plane, the maximum directivity should be at least be twice that of an isotropic radiator, or 3 dBi. However, The maximum directivity that I am obtaining is around 0.25 dBi. In addition to this, the directivity is in other places very low (around -15 dBi). This would be okay by itself, but coupled with the low maximum directivity, this also does not make sense to me. 

 

Can you help? I have attached my project file and postFEKO file. My thoughts are that there are some settings within FEKO that I must have set incorrectly for this to be happening. I've had the same issue with other EM solvers, but cannot find the cause of this within FEKO.

 

Thanks.

 

 

Unable to find an attachment - read this blog

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Answers

  • Torben Voigt
    Torben Voigt
    Altair Employee
    edited July 2018

    Hi bouvy,

     

    The attached model gives a maximum Directivty of 2.49 dBi. Please note that for the Reflection Coefficient Approximation a minimum distance of lambda/10 should be between ground and segments/elements. That's why warnings are given:

     

    WARNING   667: A segment is close to the ground

    WARNING   668: A triangle is close to the ground

     

    I compared with Exact Sommerfel Integrals where the maximum Directivity is slightly higher (3.19 dBi). I assume that the lower the frequency, the worse the results, because the assumption of segments/elements being > lambda/10 away from the ground not fulfilled.

     

    Due to the small distance geometry and ground (1 cm) the Reflection Coefficient Approximation should only be used for frequencies > 30 GHz. I would recommend using Exact Sommerfeld Integrals instead.

    image.png.829250cf3fee056097072e2727a5a4a6.png

     

    Please also note, that for dielectric halfspaces the loss in the dielectric is unknown and therefore only Gain will be computed (Directivity = Gain).

  • bouvy
    bouvy Altair Community Member
    edited July 2018

    Hi Torben, 

     

    Thank you for the feedback. 

     

    When you refer to the attached model, is that in reference to the model I attached? In that case, I am not sure how you are finding a maximum directivity of 2.49 dBi. When I plot the directivity on a polar plot, I obtain the following:

     

    <?xml version="1.0" encoding="UTF-8"?>image.thumb.png.56d6046cb50c67024c0f54d7da5a65c2.png

     

    which shows a maximum directivity of 0.253 dBi at 0.11 degrees. Am I misunderstanding something?

     

    Thanks again.

  • Torben Voigt
    Torben Voigt
    Altair Employee
    edited November 2020

    Hi bouvy,

     

    Sorry, it seems I forgot to attach the models. It's a reduced test model, antenna only. I asume the more faces break the > lambda/10 rule for the RCA ground, the less acurate the results get. Comparing to Sommerfeld ground, the test model is quite close.

    Model is attached.

    Unable to find an attachment - read this blog

  • bouvy
    bouvy Altair Community Member
    edited August 2018

    Thanks Torben.

     

    Unfortunately, I am unable to open the attached file, as I get the following Error 18008: 'The file was written by a newer CADFEKO and is not compatible with this CADFEKO'. I should have mentioned earlier that I am running FEKO version 7.0.1.

     

    image.png.41cd0091ec93add9b829aba7eb380175.png

     

    I appreciate your remarks regarding the lambda/10 rule for the RCA. I was not aware of this requirement. Having learned that, I ran my original simulation using the exact Sommerfeld integral setting. In this case, I am observing the same problem as with the RCA: directivity values are very low. 

     

    <?xml version="1.0" encoding="UTF-8"?>image.thumb.png.f65c4735e5c45dbc2ba69815e102bd4d.png

     

    Do you have any suggestions for the case which includes the full model? 

     

     

  • Torben Voigt
    Torben Voigt
    Altair Employee
    edited August 2018

    Hi bouvy,

     

    Coming back to your original assumption that the Directivity should be ~3 dB larger than for an isotropic radiator. Could you please clarify why you expect this? And please remember: Directivity will not be calculated when using infinite dielectric ground. Only Gain will be calculated.

     

    Apart from the results, FEKO 7 is quite outdated. If possible you should install the current version of FEKO (2018.1).

  • bouvy
    bouvy Altair Community Member
    edited August 2018

    Torben,

     

    I previously misunderstood your point about gain and directivity. When you said that directivity = gain, I took that the other way to mean that gain = directivity. I see now what you mean. I re-ran my simulation with a PEC ground plane with the following results:

     

    <?xml version="1.0" encoding="UTF-8"?>image.thumb.png.b0ff49291454df4b4dd7d565624404d9.png

     

    These are much more consistent with what I would expect.

     

    I do think it is a little misleading the FEKO refers to the 'directivity' when a lossy material is used although that nomenclature is apparently inaccurate, if only the gain is being calculated.

     

    Regardless, thank you for your feedback. Very helpful!

  • Torben Voigt
    Torben Voigt
    Altair Employee
    edited August 2018

    Hi bouvy,

     

    I thhink that the Directivity in that case should not be available in POSTFEKO at all, but on the other hand

    WARNING  3976: Directivity cannot be computed for far field calculations involving real ground with losses, gain will be computed instead

    makes it quite clear.

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