[evlatests] Bandpass issues

[Michael Rupen]

Is it clear all this is antenna (rather than baseline) based?  and that
this cannot be some residual of the delay rocking?  Also, is there any sign of 
this in phase, or is it all just amplitude?

          Michael

> 3-bit tests at K band showed the bandpasses of ~half the antennas to
> be variable by 1-2% on timescales of minutes. This is x10 worse than
> we typically see (though careful tests were done a while ago, in A-
> array).  All current tests were done on 3C84.
> Best antennas 4,12,17,27,5RCP, 22RCP; worst 2, 5LCP, 22LCP;
> rest in between.
>
> Stacking all the 8 samplers on the same frequency (4R, 4L) shows that
> the bandpasses are independent of sampler. 8-bit bandpasses agree with
> the 3-bit at the same sky frequency, so its not the samplers (whew!)
>
> C band (8-bit) was done next, and shows curious ~80MHz ripples which
> are modulated with a ~5minute period. Again these are x10 worse than
> the 0.1% bandpass amplitude stability we have seen before. The delay
> steps are a few seconds apart, so I dont think that could be the cause.
> Best: 21, 15, 17, 27, 28, 2  Worst: 1, 6, 10, 11, 20, 22, 23.
> Is there any correlation with radome material on the C-band horns?
>
> X-band (8-bit) looks much better on most of the antennas. 10, 21 look
> just fine. ea1 looks less fine, but the ripples are smaller than at C,K.
>
> I'm concluding the problem is upstream of the T304s (i.e. not including
> them). Some issues are clearly front-end related (K5-LCP, K22-LCP) but
> are too small to see on the bench. Generally I find it hard to accuse
> the front ends and converters, since they are unchanged since previous
> tests.
>
> Any other ideas?
>
> Are other people seeing this?
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