Discussion Forum

Daniel,

By definition, far-field is the limit E_far=lim(r->infinity)|E|. Far-field is calculated from the near-field on a boundary using Strattot-Chu formula. Note that variable E_far actually represents field amplitude rather than physical electrical files. Strictly speaking, units of the far-field are [V], as follows from the definition.

Regards,
Sergei

Thank you,
The plot generated is the far field norm, with units V/m. Can you explain what this means exactly? I'm struggling to grasp the concept as the units aren't what I'd expect.

Daniel Frantz

Daniel,

Yes, Comsol adopts units [V/m] for the far-filed. I guess, this is done for convenience and to avoid confusion (although, units following from the definition are [V]).

Regards,
Sergei,

Hi Sergei

Could you explain a bit more around why you state an electric field, even at "infinity" (Far Field) should have units of [V] and not as of a field in [V/m] ?

For me the formula of the section Far-Field Calculations Theory (p 44, chap 2 Wave optics modelling (ed. v4.4) ) have "E" and "H" as field densities [V/m^2] and [A/m^2] respectively, hence the units tell me Ep remains in [V/m] because:
"k" has units as [1/m], "r0" is unit less as "n", hence [1], "eta" has units as [Ohm] and "S" has units of [m^2]

This is related, for me, to the unit change when you express field in the Fourier space

Or have I missed something ?

--
Sincerely
Ivar

Ivar,

Variables in the Stratton-Chu formula, as I understand it, have the following units:

[Ep]~[k][r0][n][E][dS]=[1/m][1][1][V/m][m^2]=[V]

i.e. far-field has units of [V].

Regards,
Sergei

I recommend you all also review the discussion at this thread:
www.comsol.com/community/forums/general/thread/20515

Hi Sergei and Robert

For Sergei: I agree one can look at the formula that way, but for me the "dS" in an integration over the direction, normalised just as r0 is normalised, so then (for me) E and H must also be normalised and that is probably where we have this [1/m] difference. Only COMSOL support can explain exactly what is behind.

For Robert: thanks pointing to that thread, I had forgotten about that one, a few years old already ...

And I agree, the Engineering "usual way" is power ratio at the far field

Anyhow this gives me another argument to add to my list:
if COMSOL have the best Scientists defining the different "physics" on their "physics builder", and they have the best mathematicians working on the solvers, as well as the best programmers defining the COMSOL workspace, they still lack the best ENGINEERS defining the content of the post processing (Results) section!
Efficient ways for those people who have to format the results to compare to Norms, and to the way people, from all the different fields, are used to see their data (I now spend 80% of my time in the results section, and at least half of it is what I consider lost time clicking repeatedly along to get recurrent data in the correct format).

I'll come with a short story (I just saw "Gravity" on the flight over, Mr Expresso had many stories to tell ;) Our company was once approach by a famous cosmetic company, they were looking for someone to make a new device to test the penetration of their cosmetic on the skin of their "patients". They had one developed >40 years earlier and it was starting to fail, and they had a huge data-base of all their measurements. After some analysis we came all to the conclusion (with todays knowledge) that the device was not the most appropriate, one could do much better, today.
But, and that is where our new High-Tech development story ended, if we were to develop a new device for them, how could they match the new results to their previous data, without re-starting from "0" ?

So even if COMSOL has done a great job to unify Physics and shown (at least to me) - that physics is far simpler than what History has given us: with many formulas of different names, but finally mostly all boil down to a few similar PDEs, with similar simulation behaviour, - they still have to provide the data in a format that can be understood and compared to older data-bases, even if these might be oversimplifications, and not strictly physically correct.
Only then will really the space and aerospace, civil engineering, nuclear engineering ... industry consider large scale purchase of their excellent scientific Product.

The Scientists have made the discoveries, but it's the Engineers who have build the items we use as our world appears today, and their methodology is to compare to older design and build delta's thereon

--
Have fun COMSOling
Ivar

Hi All,

I recently noticed that the 4.4 version has new far-field results available, among them emw.gaindBEfar and emw.gainEfar. So this now seems to be the geometrical directivity independent of feed power and matching.

Robert you had been elaborating on that years earlier.

The unit for both results is W/m^3 which is a little cumbersome and the dB value (being the logarithm of a ratio) should be dimensionless anyway. Unfortunately I couldn't find anything about it in the documentation.

Cheers
Edgar

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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com