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Directivity Results Too Low

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.

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Torben Voigt

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

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"?>

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

Thanks again.

Torben Voigt

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.

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bouvy

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"?>

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

Torben Voigt

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

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"?>

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

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.