[evlatests] S-band Bandpass comparison: MeerKAT and VLA

[Rick Perley]

All:

I have obtained some MeerKAT data, three separate observations of the 
moon and nearby calibrators, at UHF, L, and S bands.

In calibrating these data, I was struck by the remarkable smoothness of 
their bandpass spectra.  See the attached files, which show their S-band 
bandpass and our S-band bandpass.  It's like 'night and day'.  The only 
processing done is to remove the delays via FRING.    RCP/Horizontal 
dipole are in the upper half, LCP/Vertical dipole in the lower.  
Amplitudes are dB power.   The scales both both plots are the same, to 
permit easy comparison.

The deep 'notches' in the VLA spectra are from the WIDAR subbanding -- 
this is a feature.  The very small, fine-scale oscillations in the VLA 
spectra (not easily visible for this antenna) are due to reflections 
between the subreflector and the OMT/polarizer.  Presumably, these are 
absent in the MeerKAT spectra due to their offset geometry.

But what about the other major features in our spectra (both amplitude 
and phase)?  My understanding is that these are due to the amplifiers. 
So far as I know, MeerKAT is using the same technology for their 
amplifiers, so why aren't similar fluctuations visible?  Perhaps the 
MeerKAT data I have have been 'preprocessed', but I was made to 
understand that these data are raw.  (I'll check on this).

Thoughts?  Any good explanations?

Rick
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