Dear Josh et al.,

I have made the obvious comparison of this Far Field data set with the one that Andrey sent. Our understanding is that Andrey's data set is just a direct fourier transform of the Near Field whereas the set Josh sent has been through the more elaborate NSI package.

First point was that I found I had to invert the x and y axis to get any sort of match. This just means that the opposite meaning for the sign of the phase in the NF has been assumed. This also means that the signs of phase in far field is reversed and that the signs of some of the fitted quantities come out reversed.

I then made a simple spreadsheet to calculate the real and imaginary parts from the NSI data and (taking account of the reversed sign of the phase) fitted for the scaling factor on the amplitude and a constant offset phase such that the differences are minimized.

I then made the crude plots in "comp-plots" attached. Clearly the amplitude patterns are similar. The Amp-diff x 4 plot shows that there are differences and in fact these mostly correspond to a difference in the beam position ~0.15 degrees in each coordinate.

When one includes the phase one finds substantial differences. This shows up when one takes the length of the vector difference or the plots of the differences in the real and imaginary parts.

It would be easier to see what is going on with false colour plots or the like, but it seems to me that the largest differences occur where the phase is changing most rapidly. This suggests that the problem is some sort of smoothing or in the interpolation onto the requested grid.

Things that I notice in the listing that may be of relevance are:

1) In the "Near-field display setup: Measurement type: NF Planar XY Scan options: CV Off, CP On, Bi-dir Off, H-scan Beamset smear: 0.00016 m Scan plane compensation: On"

I think scan plane compensation is adjusting the Z-position for known inaccuracies in the scanner meachanism, is that right?

What does the Beamset smear do? What are CV Off and CP On doing?

2) In the "Near-field setup: Data - Preprocessed
Off Amplitude tapering: Off Network correction: Off Probe/AUT Z-axis: On,K-correction: Off
Off, MTIphase: Off"

What does "Probe/AUT Z-axis On" do?

3) In the "Far-field transform setup FFT size: 512, 512 X/Y/Z shift= 0.000 m, 0.000 m, 0.000 m Filter Mode: Max FF, Zoom: Off Probe setup: Non-acquired Probe model: None"

What does "Filter Mode: Max FF" do?

I note that the data has apparently already been zero-padded to 512 to 512. This should mean that the data coming back from the transform will have a fine gridding and so the interpolation should have rather a small effect.

Also attached is the output from my analysis sheet for these two data sets. The overall efficiency number (in red) differs by just over 1%. More significantly the derived phase centre (X,Y,Z) differs substantially even when the sign inversions are take into account - if I use the position of one to reduce the other, the phase efficiency is only 94% instead of 99.6%.

I am really going to have to stop playing around with this stuff now. The obvious things to do are read the NSI manual and do a third independent FT of this data as a check.

Best Richard

-- ToddHunter - 02 Sep 2008
Topic revision: r1 - 2008-09-02, ToddHunter
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