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Posts posted by 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Apart from the results, FEKO 7 is quite outdated.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 likes this 
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.
bouvy likes this 
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.
Please also note, that for dielectric halfspaces the loss in the dielectric is unknown and therefore only Gain will be computed (Directivity = Gain).
bouvy likes this 
Hi FieldForcer,
You probably ran into insufficient memory. Could you please try to find out the memory usage during the FEKO run?

Hi FieldForcer,
 Did you get any warnings (I think there should be)?
 In general I would recommend to use continuous (interplated) range. FEKO will then catch all the resonces along the frequency range, whereas with discrete frequency points all the information betwwen the calcilated points is neglected.

Of course 0 Hz is impossible. Our recommendation for fmin and fmax for time analysis is
fmin = 1/sd and fmax = (Nt/sd)/2
with sd the signal duration and Nt the number of samples.
There is a video on Youtube about FEKO's time analysis (beginning at ~32:00). It's from 2014, but the basic rules still count: https://www.youtube.com/watch?v=YAruJMWRalI&t=8s

Hi FieldForcer,
I was referring to the length od the edges of the mesh on a surface. At 60 Hz and standard mesh settings, a sphere with 50 mm diameter will be meshed similar to this:
The edges on the surfaces should be around 1 cm in that case which would in principle allow you to request nar field points 1.5 cm from the surface.
Please note the mesh size of tetrahedra also depends on the dielectric properties of the medium.

For 100% accurate calculation of near field points we recommend a distance of 1.5 x edge length of the mesh on the surface. This goes for both metallic and dielectric faces.

Hi FieldForcer,
Sorry for being imprecise: With "properties are freespace" I meant a dielectric with properties of freespace (eps_r = 1, tand = 0). I was referring to
On 7/24/2018 at 4:17 AM, FieldForcer said:first model has two coils and second model has two coils with a sphere. Properties of sphere are free space. Ideally I should get same field patterns, but I get different results.
I assumed that you tried to solve the sphere with SEP, so I recommended VEP.
Regarding the Singular field errors, which version of FEKO are you using? In case it's the current version 2018.0.2 I would have to look into it. Is it posiible to attach the model?

Hi FieldForcer,
For very low frequencies / electrically very small models, the VEP should be much more stable for dielectrics than the SEP (even if the properties are freespace).

Hi Alberth,
Try the 3D view legend range settings:

Hi Alberth,
For StandardConfigurations you can find S11 similar to this (VSWR can be derived):
For SParameterConfigurations you find S12, S21,.. similar to this:

Hi Zobidi,
In case your model is not multiple wavelengths in size, i.e. you cant use MLFMM, I assume the geometry contains a lot of details. In this case you might want to try a different solver than MoM, e.g. FEM or FDTD.

Hi Kartik,
FEM modal ports must be assigned to dielectric boundary faces, contrary to MoM waveguide ports, which are assigned to PEC faces.
Also, there is a PEC face blocking the wave transmission:
Setting also this face to dielectric boundary should help.
Furthermore, the top part of the horn is isolator, shouldn't it be Air instead?
Last but not least, FEM modal ports between two dielectric regions are not supported (that's what the warning is referring to). I would rather cut the outer air box like this:
The model is attached: Anechoic_21_alt.cfx

Hi Pavlam,
One imoprtant addition: The calculation the Rx / Tx coefficients from the power flow through near field is only welldefined if the size of the near field requests is N * size of unitcell (N is an arbitrary interger value). In your case the size could be dy*dz or dy*2dz,...

Hi hykr,
One way would be using the "Project" function in CADFEKO (see attached video). However, the length of edges will change if they aren't orthongonal to the direction of curvature, It will depend on the degree of curvature, how much the length will change. For complex models there may be better solutions.
Regarding the best solver, this of course depends strongly on complexity and size of yor model. In case there are both small/complex parts and large parts, it may be feasible to decompose the mode, e.g. by replacing small/complex parts by equvalent near fields.
If you could attach an example model, I may be able to help more.
hykr likes this 
The files are now attached. Looking at the results I have to admit that they could look better (Rx+Tx =! 1 here and there). Most likely this can be improved by a larger distance between unit cell and near field requests. I chose 1 lambda at highest frequency, but looking at JIF's link, 10 lambda at lowest frequency probably would have been much wiser.

Hi Pavlam,
The reason for the runtime issue is still under investigation. It could be worked around by setting up the unit cell differently:
Note that your mesh setting of lambda/5 does reduce the memory requirement, but will most likely not lead to trustworthy results. The unit cell has a size of 1.6 x lambda and thus will demand some memory with a mesh size of lambda/10 (>20 GByte). I see from your *.out file that you have 64 GByte installed, so that should not be a problem.
BUT: Due to the size of the unit cell there are grating lobes for each frequency (Grating Lobes). They prevent an accurate calculation of the transmission / reflection coefficients. A simple workaround here is shown here:
Note, P_incident is calculated by 2nd model, with no geometry present, just calculating the power going through NF front.
I will add the files here once the simulation has finished.

Could you attach the *.out file here?
It seems that something is slowing down the simulation at the 6th frequency. We're currently investigating.
Directivity Results Too Low
in Altair FEKO
Posted · Report reply
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.