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Marcus Chang

something wrong in far field pattern result when using near field source

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Dear all,

 

I have an exercise which I would like to extract near field from a horn antenna and import to another simulation (all in FEKO platform), then there is a weird result shown in the video as below link I just uploaded:

https://drive.google.com/file/d/1-NGAV75rxH2zKfahMF4qN6I2l8BpKN8c/view?usp=sharing

 

 You can see the far field pattern with near field source I imported is completely opposite to original one. I would appreciate if anyone can show me how to simulate correctly.

 

BR/Marcus

horn_X band_190309.cfs

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Hi @Marcus Chang,

 

In the model where the near field is requested, the Start and End values of U are defined from positive to negative:

image.png.6dbb3158c574d50e0f26c63fbfe0b65a.png

Therefore when trying to use as near field source, you should rotate the workplane of the source (or of the field data) so that the orientation agrees with the original model.

Marcus Chang likes this

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Hi @Torben Voigt,

 

Thanks for you immediately support, and it works now.

 

Here is another question for another exercise, I would like to use a horn antenna to radiate on a metallic sphere, then try to use the same horn antenna to receive power reflected from the metallic sphere.

237466573_2019-03-1811_24_41.png.e6bec379d61ac7b799f34f257e15ff2a.png

 

First, I extract near field data from a horn antenna I just run (please check file named horn_X band_190309.cfs).

Then in simulation 1 (please check file named FF source_PEC.......cfs), I use this near field source as TX and radiate to a metallic sphere, then I can get the results of the near field from a surface of sphere, and extract its as the near field source for simulation 2 (please check file named temp_190313.cfs)

In simulation 2, I use the near field source from previous results (on surface of sphere) and radiate to a near field receiving antenna (the same horn antenna) and try to get the result that how much power it can receive.

However, I get two different results from different method, one is using far field receiving antenna, another one is near field receiving antenna as below.

And also, it is weird that the power is negative, could you please help us check again what's happening to this simulation?

 

1138839619_2019-03-1900_29_19.thumb.png.bdb7b5038653dd4595ee0f7d08795b0f.png

 

And the last question is, how could we get results of the far field pattern from receiving antenna, not just power calculation?

 

BR/Marcus

onlyonechancy@gmail.com

 

temp_190313.cfs

horn_X band_190309.cfs

FF source_PEC sphere_sphere surface NF source_190312.cfs

NF for 10GHz horn.efe

NF for 10GHz horn.hfe

metallic_sphere_NF_for_10GHz.efe

metallic_sphere_NF_for_10GHz.hfe

FF_for_10GHz_horn.ffe

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Hi @Marcus Chang,

 

Let check what you did:

  1. In horn_X band_190309.cfx you calculate and save the near field and the far field of the horn antenna. 
  2. In FF source_PEC sphere_sphere surface NF source_190312.cfx you replace the horn by the near field data from 1. and to illuminate a PEC sphere. Here's mistake #1: The orientation of the near field is wrong (the horn is looking downwards):
    image.thumb.png.4c01926d5a4cc1d10cfe591298b39e22.png
    This would be correct:
    image.thumb.png.ec0cb31c3894408f4e2a1b3d7bc733c1.png

    Mistake #2: The near field data is placed at z=0, while in model 1. it is placed at z=0.2138m.
    Around the PEC sphere you request a new near field to capture the radiated field from that sphere. Here's the mistake #3: The near field request has the same radius as the sphere, so the near field point are calculated exactly on the PEC surface. For acurate near field results a field point should not be closer than 1.5 x the edge length of the surface mesh.
  3. In temp_190313.cfx you compare the received power of an RX near field antenna and an RX far field antenna. The results are different because of mistake #2 and also because of warning WARNING 39270: A receiving antenna described by a far field pattern is positioned too close to a scatterer, far field condition not met. It says that that a source (the field of the PEC sphere) is too close to the RX far field receiving antenna. In general for far field sources (or receiving antennas) a far-field radius should be considered.
    I also decreased the increment beween the near field points of model 1. to ensure accuracy. The final results agree quite well:
    image.thumb.png.79a957b72f3096bf1721205d5d445251.png
    Note, with larger distance between source and RX antenna (or plane wave instead of the near field of the PEC sphere) the difference would be around 0.2 dB, because of omitting Warning 39270.

The three new model files are attached: NF_vs_FF_RX_antenna.zip

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Hi @Torben Voigt,

 

Thanks for you support.

 

About Mistake #2, the location of receiving antenna (RX) should be the same as transmitting antenna (TX).

So I place the near field data at z=0 rather than z=0.2138m. (horn aperture is placed at z=0).

Then I increase the distance between horn and metallic sphere to 72 times lengths of wavelength. (2*D^2/lambda=72 lambda) to satisfy far filed requirement, so that warning 39270 is not shown again.

Finally I can get the results shown as below, please check:

 

image.png.efc806bf131acb10c173f37455fc839e.png   

 

But... this results do not accord with the number calculated by MATLAB, the coding is shown as below, please check:

 

f=10*10^9;
c0=3*10^8;
lambda=c0/f; %units: m 
lambda
Pt=1;
Gt=18.3; % simulated in FEKO
Gr=18.3; % simulated in FEKO
r=3*lambda %units: m, radius for metallic sphere
RCS=pi*r^2
R=72*lambda %units: m, distance between sensor and object
Pr=Pt*1*Gt*Gr*RCS*(lambda^2)/((4*pi*R^2)*(4*pi*R^2)*(4*pi))

 

 so that I can get Pr=177.56e-09 (W), 177nW approximately, almost twice the power of the results (100nW) simulated in FEKO.

 

Second question is, is it possible to get far field pattern of the reflected field of sphere?

 

BR/Marcus

onlyonechancy@gmail.com

 

 

 

 

190326.zip

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Hi @Marcus Chang,

 

Unfortunately I'm not sure about the Matlab code. Maybe someone else (@mel, @JIF) could have a look?

 

You can get the far field pattern of the reflected field of sphere by doing this:

image.png.1661f04d5687f617c349a935a6cd70b5.png

 

This will then only use the contribution of the surface currents of the sphere.

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Dear @Torben Voigt @mel @JIF

 

MATLAB code is just a easy way to calculate it.

But it is a simple and fundamental formula as below:

image.png.1b72746255ef01f9d75400995c62ea12.png

while the RCS value I use is: 

 image.png.d2b27305b533f15bd3d7889c51d7519d.png

here Gt=18.3=Gr, lambda=0.03(m), r=3*lambda, R=72*lambda (=2D^2/lambda, D=2*r) and Pt=1W (in Feko),

then I can get the value of Pr is equal to 177nW, however, the results simulated in Feko is less than 100 nW.

 

And thanks again for your suggestion of receiving far field pattern of reflected signal, I will try it and get back to you if there is any further questions.

 

BR/Marcus

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Dear @Torben Voigt,

 

For the suggestion of the contribution of the surface currents of the sphere you provided, it seemed not work as below,

there is no differences when I use it, please check:

 

1124190090_2019-04-0314_44_51.png.80e95e0e0258af6004ab6983d24ec3cd.png

 

753695772_2019-04-0314_43_26.thumb.png.3df34e9d7d026887ec9499dbc02681e5.png

 

 

BR/Marcus

 

02_FF source_PEC sphere_sphere surface NF source_190325_sphere placed 72 lambda_alt.cfx

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Hi @Marcus Chang,

 

If you get identical far field results for both, then I assume you have in addition "Calculate only the scattered part of the field" activated:

 

image.png.dbaa997991182ece5dca91b1b6998a73.png

 

This will in this model of course give the same results, as the sphere is the only scattering object. So, in both cases you will get the scattered field from the sphere surface.

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Dear @Torben Voigt  @mel @JIF,

 

Recently, I try to run some simulations for metallic sphere and corner reflector respectively, the results for these simulation is weird, the received power bounced by metallic sphere is larger than corner reflector.

 

image.thumb.png.f7b03ac24a19d16ccf49f84ba2f02143.png

 

But actually, the RCS for metallic sphere should almost 10 times smaller than corner reflector, no matter it's calculated by formula or simulated in FEKO (I also upload these simulations as attached, please check)

 

image.thumb.png.a3db7d864a52ef0c2acd9705791ba6d4.png

 

So I would like to check with you again, to see what's happened to the setting in FEKO.

 

BR/Marcus

 

 

 

190406.7z

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Hi @Marcus Chang,

 

For the corner reflector you're using PO, but with PO no multi-reflections between PO faces are taken into account. A corner reflector is THE classical example where PO is not feasible. Furthermore, the asymptotic methods (PO, LE-PO, RL-GO and UTD) are only feasible if the model is electrically very large, e.g. 20 lambda, bigger is better, but the corner reflector is only around 4 lambda. You should definitele use MoM / MLFMM here. Same goes for the sphere, which is only 3 lambda in radius.

When calculating the scattered field of the reflector and the sphere correctly (MoM), the received power of the reflector is around 10 times higher than that of the sphere.

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