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Oscillations of E/H inside a sphere bounded with a PML

Posted 7 févr. 2014, 10:39 UTC−5 RF & Microwave Engineering, Results & Visualization Version 4.3 4 Replies

Bartek Chaber
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Hello,

I have some results but I'm not confident if they are correct. I am simulating an antenna which should generate very strong electric field, and I want to test it at the frequency of 200MHz (which is in the range provided by the producer).

I have attached two screenshots from COMSOL, depicting ratio between the norms of E and H in the plane XY cutting the antenna in half. The whole sphere has 3m radius and a PML 0.5m which is thick. Antenna's dimensions are 1m x 1m x 0.05m.

You can see that E/H looks like waves in this plane, but I would expect it to converge to the value of the impedence of the free space (~377 Ohm). Do you have any idea why it is this way? Is it a matter of mesh quality? Or rather PML settings?

Thanks,
Bartek Chaber
4 Replies Last Post 26 févr. 2014, 13:29 UTC−5
Sergei Yushanov Certified Consultant
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Posted: 1 decade ago 10 févr. 2014, 08:27 UTC−5
Bartek,

Impedance of the radiated wave is equal to the impedance of the free-space in the far-field zone only. You are plotting impedance in the reactive and near-field zone (Fresnel zone) where wave impedance is highly reactive and is not equal to the free-free space impedance.

Best regards,
Sergei
Bartek, Impedance of the radiated wave is equal to the impedance of the free-space in the far-field zone only. You are plotting impedance in the reactive and near-field zone (Fresnel zone) where wave impedance is highly reactive and is not equal to the free-free space impedance. Best regards, Sergei
Bartek Chaber
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Posted: 1 decade ago 26 févr. 2014, 07:43 UTC−5
Hi guys,
Inspired by your answer I've made a very simple dipole model. It is bounded with a sphere of 14 m radius.
As you can see in the screenshot below, we can see some oscillations. Am I correct that 12-14 m from antenna can be considered the far-field zone for the wavelength 4 m (freq. ~75MHz)?
Maybe the oscillations are meant to be there?

Thanks for any comments.
Hi guys, Inspired by your answer I've made a very simple dipole model. It is bounded with a sphere of 14 m radius. As you can see in the screenshot below, we can see some oscillations. Am I correct that 12-14 m from antenna can be considered the far-field zone for the wavelength 4 m (freq. ~75MHz)? Maybe the oscillations are meant to be there? Thanks for any comments.

Attachments:

Edgar J. Kaiser Certified Consultant
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Posted: 1 decade ago 26 févr. 2014, 08:54 UTC−5
Bartek,

sometimes you can read about 10 wavelengths for the beginning of the far field. It is a matter of the accuracy you want.
All in all, if you calculate an SWR from what you have I think it is not too bad. It is difficult to make a perfect PML. The dipole is not a spherically symmetric radiator and thus a spherical PML can't be perfect.
It might be interesting to make a cylindrical PML and see what happens. But again, you have to check if you really need the precision.

Cheers
Edgar

--
Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com
Bartek, sometimes you can read about 10 wavelengths for the beginning of the far field. It is a matter of the accuracy you want. All in all, if you calculate an SWR from what you have I think it is not too bad. It is difficult to make a perfect PML. The dipole is not a spherically symmetric radiator and thus a spherical PML can't be perfect. It might be interesting to make a cylindrical PML and see what happens. But again, you have to check if you really need the precision. Cheers Edgar -- Edgar J. Kaiser emPhys Physical Technology http://www.emphys.com
Bartek Chaber
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Posted: 1 decade ago 26 févr. 2014, 13:29 UTC−5
Thank you Edgar,
apparently I was wrong by thinking that about 2-3 wavelengths are always enough to reach the far-field zone. I just had no confidence in the results, but now I'm fine with them.

Thank you guys for clearing this for me.
Thank you Edgar, apparently I was wrong by thinking that about 2-3 wavelengths are always enough to reach the far-field zone. I just had no confidence in the results, but now I'm fine with them. Thank you guys for clearing this for me.

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