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IFA theory

 
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helix
Antenna Theory Regular


Joined: 29 Jan 2015
Posts: 64

PostPosted: Thu Jan 29, 2015 4:24 am    Post subject: IFA theory Reply with quote

Hi, the inverted F explanation on this site describes the IFA as a sort of slot antenna. However, the radiation pattern does not correspond (neither polarization or orientation of the "donut") to that of a slot... Any explanation? Judging from the radiation pattern, it seems to be more of a short dipole.
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admin
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Joined: 03 Jan 2007
Posts: 197

PostPosted: Thu Jan 29, 2015 10:50 pm    Post subject: Reply with quote

Are you sure the polarizations don't match? [they do]

IFA is a cross between a dipole and a slot
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helix
Antenna Theory Regular


Joined: 29 Jan 2015
Posts: 64

PostPosted: Fri Jan 30, 2015 3:08 am    Post subject: Reply with quote

Well, yeah, I'm pretty sure. I made a simple 300MHz IFA in FEKO (LITE) just now... Happy to send you the files if you want to have a look at what I've done.

Here's the 3D pattern that I think belongs to a z-directed electric current...

http://imgur.com/5gzGMnW

An x-directed magnetic current would not have a null in the z-direction, it would have a null in the x-direction.


Also, if we look at the field components I don't think the polarization is right for an x-directed magnetic current. The following shows the E-theta component in a polar-z coordinate system (shown on left, z-polar appropriate for z-directed electric current) as well as the E-phi' component in a polar-x coordinate system (shown on right, x-polar appropriate for x-directed magnetic current).

http://imgur.com/LhG7p0s

There is something just a very tiny bit strange (i.e., not due to a z-directed electric current) going on in the plot to the left but I expect that is due to the small amount of electric current flowing along the x-axis.

Anyway vector components in a polar coordinate system can be tricky, especially near the poles, but I'll say that an x-directed magnetic current on an infinite PEC ground plane would not have a null in the phi' component (again, phi' referenced to a polar-x system) along the meridian that the code results shows it does in fact have.


NOW I'll be the first one to admit that magnetic current is a very useful concept, and that because of their mathematical equivalence, it's always possible to (correctly) describe something in terms of M or the actual J's that give rise to the E x n fields that M is equivalent to.

I'm just saying that the model of an IFA as a slot, or an aperture of M, doesn't really fit the data (at least my understanding of it), at least for the IFA's I've designed. It's possible that I'm off in some other design regime where this effect isn't strong. Or maybe I don't fully understand the data I've shown. BUT this was the point of my asking the question.

Appreciate your ideas and input!
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helix
Antenna Theory Regular


Joined: 29 Jan 2015
Posts: 64

PostPosted: Thu Feb 12, 2015 5:34 am    Post subject: Reply with quote

Hey I was giving this more thought and one more piece of logic against the IFA being modeled as a slot mode/magnetic current...

Take a look at this graphic, viewing the "free" or open tail of the IFA end-on:

http://imgur.com/ZOvMK8X

I've drawn a surface around the IFA in red on which we want to represent the fields of the IFA with equivalent electric and/or magnetic current. Surface normal vectors are indicated with n-hat vectors.

I've sketched electric field vectors E in green. The equivalent magnetic current M is vector the cross product of E and n (M = E x n). From the right hand rule, M (blue) vectors are oppositely directed on the two sides of the IFA (arrow heads out of the plane of the page are blue circles, arrow tails into the plane of the page are x'es)... meaning the M currents will tend to annihilate/cancel. If one shrinks the surface small enough, they will cancel completely...

So that's my reasoning (besides looking at/thinking about my sim data) that the IFA does not at all behave like a slot.

Thoughts/comments?
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