Focal fields of dish antenna

Hello,

I'm currently facing following problem:

We have slightly disturbed offset dish, trying to find its focus.
Dish's surface was scanned and physical optics was used to get E field in XY cuts near the estimated focus (Z). Our observation is that localization of E field amplitude isn't the best way for obtaining the focal region. However, interesting changes in phase distribution (out of focus/at focus) was found, but I have no explanation for that... (see picture)

It seems that in focal region phase of E field creates clear rings. Is there any mathematical reason?

Thanks!
eirp

Pl have a look on attached document available at
http://en.wikipedia.org/wiki/Offset_dish_antenna

If I understand what you are saying, you are plotting the electric field at the focal plane of this parabolic dish?

In general, if you look at a signal at the focal plane of something, like a dish or a lens, you are looking at the fourier transform of the object's shape. In this case, the fourier transform of a big circular dish is a sinc function in r, or in other words "rings" of intensity, that die out as (sinr)/r. And at each succeeding ring, the phase swings 180 degrees, just like a sinc function.

It should be a little distorted due to the offset feed.

Added after 9 minutes:

http://www.physics.gatech.edu/gcuo/l...actionI.ppt#24

Check out the slide "Diffraction from a circular aperture"

And this:
http://www.uwrf.edu/~eileen.korenic/...rTransform.pdf

Thanks very much, that looks very reasonable! Just a remark - if we're not in far field then we have to use Fresnel transform, does the phase change significantly?

E.

The details of how to do it are beyond my knowledge. Maybe someone who is an antenna engineer can post further comments.

A nice effort!
I have seen some methods on how to find a focus of an offset antenna by geometrical measurement but when I tried some, I failed. The "easiest" way is to find the original LNB mount with a LNB- this will directly indicate the designed position. If, as you wrote, your offset was "a bit disturbed", I used a radio-telescope method quite successfully. I made a radio telescope with a Ku-band LNB, and by installing it in the offset adjustably, I was able to optimize the antenna-radiometer by receiving solar noise. If you wish to have more details, please respond to "jpolivka@spaceklabs.com".

Thanks! It's very old story, now finished... this was not primarily satellite antenna, rather theoretical work on this topic.
Anyway, I'm corrent you have articles in "Slaboproudy obzor"?

Thank you, I was publishing in Slaboproudy Obzor, also now I expect my paper to be issued.
I am not an antenna expert, I used some antennas in my satellite and radio-astronomy projects. Your field mapping is interesting; I developed a noise-field method that needs no test range or an anechoic chamber. Please see "Noise Can Be Good, in Microwave Journal, March 2004.

Happy I was right !

Thank you..