[evlatests] Non-linearity of ea24 at C-band remains ...

[Bob Hayward]

Just a reminder. We had essentially already cleared the T302 of being 
the source of this problem when we found that our C-Band "Milhouse" 
total power measurements on Antenna 24 taken at the output of the T302 
were perfectly linear.

We might want to do a set of measurements on Antenna 24 looking at the 
Y-Factors at both the input and output of the T304. We can play with the 
various L301 & L302 settings to see how they affect the sensitivity.

-Bob


Rick Perley wrote:
>     I reported a couple weeks ago on the reduction in PDif when we 
> observe the strong source Cygnus A.  
>     At C-band, the antenna which has by far the largest reduction is 
> ea24, in RCP.  IF 'A' had a reduction of 30%, IF 'B' was reduced by 
> 12%.  (By contrast, IFs C and D had virtually no reduction at all).
> 
>     Because the T302 may be implicated in this behavior, Jim and Rob 
> replaced antenna 24's T302.  They asked yesterday if the behavior seen 
> last month has changed.  I repeated the on-off Cyg A test this morning. 
> 
>     The reduction in PDif when on Cyg A is *unchanged* on this antenna 
> (and indeed remains the same on all the others also).  It would appear 
> that the non-linearity seen on this antenna is independent of the T302. 
> 
>     Worth emphasizing is the spread in behavior of PDif.  For most 
> antennas, there is a reduction when going onto a strong source.  But for 
> some antennas, there is an enhancement!   Most notable are 4A (+10%), 
> 23A and B (+5%), and 28C (+12%). 
> 
>     The change seen is generally different from IF to IF (so that A and 
> B are generally different).  This would seem to exonerate the T302, and 
> implicate the T304.  But on the other hand, IFs that show severe 
> compression at one band show little or none at another band.  So for 
> example, 24A, which is easily the worst at C-band, is one of the better 
> antennas at S and L bands, and shows no effect at all at X-band.  Given 
> this, how can the T304 be responsible? 
> 
>     It's all very odd...
> 
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