# How do my successive diffraction patterns look?

## Homework Statement:

Do they actually look reasonable for a circular aperture? Going from the far field to the near field?

## Relevant Equations:

No equations, qualitative question.

I feel like I am "exactly wrong" ; In the far field I get more variation in the same xy-space and in the near field I get less variation. I feel like the opposite would be true.

I'm trying to create a diffraction pattern by replacing the aperture with a thin cylinder with a uniform volume current parallel to the axis. This approach may not be theoretically informed, I'm just trying random radiation distributions.

Is my flux pattern even right for a uniform volume current (thin cylinder filled with uniform volume current parallel to the axis and flux pattern in a plane parallel to the circular faces of the cylinder)? I feel like it is not; I think the flux pattern should be more spread out in the far field as opposed to the near field but I'm getting the opposite of that.

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I think this is a little better. Far field to near field.

tech99
Gold Member
Is the sequence top left is far field and bottom right is near field? In which case near field is narrower.
I cannot quite picture your radiating cylinder and which plane it is in.
Can you say how far each diagram is from the aperture in terms of wavelengths, and what are the dimensions of the aperture in wavelengths?
Apologies, I cannot quite work out your information at the moment. I think a sketch of the set up might help.
As far as I can see, far field main lobe gets progressively wider and has side lobes and near field does not, which is correct.

PhDeezNutz
Is the sequence top left is far field and bottom right is near field? In which case near field is narrower.
I cannot quite picture your radiating cylinder and which plane it is in.
Can you say how far each diagram is from the aperture in terms of wavelengths, and what are the dimensions of the aperture in wavelengths?
Apologies, I cannot quite work out your information at the moment. I think a sketch of the set up might help.
As far as I can see, far field main lobe gets progressively wider and has side lobes and near field does not, which is correct.
I just realized that my "correct looking solutions" are completely contrived I; I got rid of ##\rho## in the Jacobian of my cylindrical coordinates integration and retained the first two terms. When I include the remaining 10 my solution doesn't work.

I'll include the information anyway for future reference.

The aperture has a radius of 1.

The wavelength is 6283

The picture in the previous post is

1 2 3
4 5 6

1 being the furthest field and 6 being the nearest field.

1) 1592 wavelengths

2) 1256 wavelengths

3) 921 wavelengths

4) 586 wavelengths

5) 251 wavelengths

6) 83 wavelengths

Like I said it’s wrong/inconsistent for the aforementioned reasons. And maybe for more reasons, but if you’re willing to help me I’ll definitely try to provide necessary information.

tech99
Gold Member
I am a hands-on engineer so I am not sure I can be of much help.
If the aperture radius is 1, I presume that means 1 wavelength. In this case 83 wavelengths for case 6 seems too great for near field conditions.

PhDeezNutz
I am a hands-on engineer so I am not sure I can be of much help.
If the aperture radius is 1, I presume that means 1 wavelength. In this case 83 wavelengths for case 6 seems too great for near field conditions.

I agree 83 wavelengths is way too far to be getting near field behavior.