Menu

Discussion Forum

New Discussion
查找

Note: This discussion is about an older version of the COMSOL Multiphysics® software. The information provided may be out of date.

Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.

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

Posted 2014年2月7日 GMT-5 10:39 RF & Microwave Engineering, Results & Visualization Version 4.3 4 Replies

Bartek Chaber
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

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 2014年2月26日 GMT-5 13:29
Sergei Yushanov Certified Consultant
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago 2014年2月10日 GMT-5 08:27
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
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago 2014年2月26日 GMT-5 07:43
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
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago 2014年2月26日 GMT-5 08:54
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
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago 2014年2月26日 GMT-5 13:29
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.

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.

Suggested Content