antenna-theory.com

[mega_blast]

Hello everyone!

First of all I wanted to noted that I begin to learn the bases of designing antennas. First steps have already had, I mean the theory of antennas. So I went to step two, the simulation.

My problem:
I design in hfss patch antenna for the UHF 868MHz (EU standard). I set myself a task - the patch antenna, which is to produce a beam with circular polarization.
I have read the book "Balanis - Antenna Theory" and based on the method for circular polarization:
- model:
https://picasaweb.google.com/lh/photo/HOEd7gGuCOWpgkIHUKXQyDo76djk1dXnqt3N-4Cw980?feat=directlink
- formula:
https://picasaweb.google.com/lh/photo/DsvT3ybsSa5PaZtMIFa1tjo76djk1dXnqt3N-4Cw980?feat=directlink

I created this model in hfss antenna. Final reports came out good:
- return_loss:
https://picasaweb.google.com/lh/photo/CHt3BmxrvgclAZtJg2tgaTo76djk1dXnqt3N-4Cw980?feat=directlink
- impedance:
https://picasaweb.google.com/lh/photo/MQoo5I8j6NXcY2SPbbhVHDo76djk1dXnqt3N-4Cw980?feat=directlink



For me, the only proof that the antenna radiates circularly (but more elliptically) - is changing the direction of the patch antenna surface J vector in time:
http://vimeo.com/30897400


And now my final question - I would like to know - Can I somehow get in the hfss -> coefficient of ellipticity (polarization beam) - ie the ratio of main and smaller semi-axis of an ellipse?


If I'm wrong in something, you can correct me,
I wait for any answers/suggestions.


Greetings from Central Europe.
Krs.

[Schubert]

Hi Krs,

Good post.

The bad news is that it is pretty difficult to get a patch antenna to radiate circularly polarized fields. If you can do it, remember it is only circularly polarized over a narrow beam width and narrow bandwidth.

Balanis' book does discuss obtaining circular polarization using the manner you described. However, I find it doesn't really work, when I've tried to do the simulations (albeit with FEKO, a different numerical code).

Can you set up some sort of imaginary surface (on the radiation boundary) and calculate the E-fields? Then animate them and see if they are rotating?

One method used in real systems is to measure the received energy with Left- and Right- Circularly Polarized antennas and compare. In your case, you would need to model one and then look at S12. But this may be a more difficult method than need be.

See also:
http://www.antenna-theory.com/phpbb2/viewtopic.php?t=74&sid=e455a68ab3f95e8a6dbbd56c4f650239

[mega_blast]

Hi Schubert! Thanks for your reply.

[Schubert]

Hmm, ok the air box results don't look great. The E-field vector has a significant z-directed component above the patch, which means we are not looking in the far field.

I would guess you probably have some degree of circular polarization, (really elliptical, with a major to minor axis ratio of at least 5.0). But it's not definitive from this view.

Can you move the imaginary surface farther away so that it is in the far field?

One more thing - don't look at surface currents on the patch. These aren't responsible for the patch radiation - it is the fringing E-fields on the side that give the radiation.

[mega_blast]

[Schubert]

For far field, you need all 3 conditions met, given here:
http://www.antenna-theory.com/basics/fieldRegions.php
(also need R>>lambda and R>2*D*D/lambda).

So you might not yet be in the far field.

Now, I believe you do have some circular polarization here. The E-field tip appears to move somewhat elliptically. This is generally characterized by the eccentricity of the ellipse (axial ratio). Also, the degree of circular polarization will vary as a function of bandwidth and angle (i.e. broadside or along the plane of the patch or whatever).

[mega_blast]

Thanks for all the answers
really helped me to clarify my knowledge about the polarization of the patch antennas.


Krs.